======================================================================== Visual Satellite Observing F A Q Chapter-01 What Is "Visual Satellite Observing"? ======================================================================== ---- 1.0 What Is "Visual Satellite Observing"? Many readers probably have already, without knowing it, seen an artificial satellite moving across the sky. At first glance, there is nothing spectacular about watching "slowly moving stars", since that is what most artificial satellites look like. Yet, since the launch of Sputnik 1 in 1957, thousands of amateur astronomers have become fascinated by these artificial objects. The reasons are manifold, but the sometimes unpredictable behavior of satellites and the scientific usefulness of observations certainly play an important role in this fascination. Most certainly, viewing objects such as Mir and the Shuttle crossing the sky as points of light, makes one marvel that there are living beings aboard them. Anyone who has ever spent some time star gazing shortly after sunset has probably noticed one or two of these "stars" gracefully sailing across the sky. These are orbiting satellites of various types and ages, visible due to the reflection of sunlight off their surfaces towards the observer. The tasks of satellites cover fields such as communications, astronomy, military applications, remote sensing, meteorology, geology, geography, climatology, and so on. Furthermore, the orbits they trace can indicate the condition of the upper atmosphere, the structure of the Earth, and the nature of the solar cycle. The amateur observer can contribute to this field, even though satellite data is regularly generated and posted on the Internet by the OIG (Orbital Information Group) at the NASA (National Aeronautics and Space Administration) Goddard Space Flight Center. Observations of various satellites can provide insight into the rarefied upper atmosphere and subtleties of the Earth's gravitational field. Amateurs can also help supplement measurements of tumbling satellites, leading to better understanding of the near-Earth environment. Visual satellite observing is an interest in locating, viewing, analyzing and identifying those points of light that move across the sky. Other skywatchers may see them occasionally during their observations of the dark sky, but more than likely, they do not have a good understanding of their origins, identities, and functions. The tools used in pursuing this interest have changed dramatically, especially over the past 10 years or more. The advent of the personal computer, the rapid growth of the Internet, and free or low cost tracking programs have made it relatively easy for the casual observer to obtain the information needed to both track and identify these moving points of light. The tools available to the casual observer of 20 or 30 years ago were occasional newspaper articles, which described when a sighting might be made or when a satellite launch was scheduled and the planned inclination of its orbit. The more ardent observers who were "members" of the various professional observing programs such as Moonwatch and the English efforts under Desmond King-Hele and Pierre Neirinck, sent out predictions every week or so to fellow members via air mail. It took a deeper understanding of orbital mathematics then to observe a satellite one night and subsequently estimate when it might be visible again. As late as 1990, orbital elements issued by OIG, were very limited in scope and were mailed out only to subscribed individuals via the postal services. Nowadays, government agencies provide extensive orbital information for non-classified satellites, and some private individuals provide the orbital information for many of the classified satellites, via the Internet or Bulletin Board Systems (BBS). Observers can now input this timely information into sophisticated tracking programs on their home computers to predict when and where satellites may be sighted. The relative ease, with which satellites can be tracked now, does not diminish the excitement of observing them. Numerous satellites are launched every year, and many are visible to ground observers. Some are very bright, some have unusual or otherwise interesting visual characteristics, and finding some of them pose a challenge to even long-time observers, either because they are very dim or because their orbits are not well known. Government and private news sources on the Internet announce information about most upcoming launches and describe the various mission programs in detail, which enhances the excitement. The long-time presence of the bright Mir complex, visible to observers from about 85% of the Earth's surface, and the frequent presence of the highly visible Space Shuttle, make satellite viewing possible for the most casual interested observer. The Russians are scheduled to place the first element of the International Space Station, called the Functional Cargo Block (FGB), in an orbit similar to Mir's in mid-1998. Then a month or two later, the Space Shuttle will attach Node 1 to the FGB, which will allow additional modules to be attached over subsequent years. Once this construction has begun, the International Space Station will be another very bright satellite, easily visible to casual observers. Visual treats abound for the observer with periodic launches of especially interesting visual satellites that may have tethers, highly reflective surfaces, or unusual flashing behavior. There are also elusive dim satellites, sometimes in highly eccentric (non-circular) orbits, which challenge an observer's ability to locate and track. Much of the original excitement of this hobby remains in the location and identification of classified satellites. Since they are classified, orbital elements for these satellites are not readily available to the public. However, private individual observers make positional measurements and create estimated orbital element sets. These preliminary elements, distributed on the Internet, allow other hobbyists to search the sky to enable sightings. This usually leads to additional sightings and allows for the generation of even more accurate orbital elements. In other instances, however, a classified satellite may be observed over a short period of time then subsequently disappear from further observations because of maneuvers to a different and more elusive orbit. An interest in observing may be casual or it may be driven by a desire to make highly accurate observations, so that others can benefit in subsequent viewings. Whatever specific interest an individual has, visual satellite observing can be interesting and enjoyable with as little investment as a computer connected to the Internet or to a Bulletin Board System (BBS), a good viewing location, and maybe a relatively inexpensive pair of binoculars. You probably already have access to a computer that's connected to the Internet or a BBS, so what are you waiting for? Getting involved in tracking Earth satellites is easy. Tracking programs can be ordered through the mail from a provider on the Internet for a relatively modest cost. They can also be downloaded from Internet sites and BBSs, either for free or for trial use. There are tracking programs for all types of computer platforms written by individuals who want to provide a "better, more versatile" program for satellite observers. To keep the observer up-to-date on the orbital status of Earth satellites, there are satellite interest groups on the Internet, such as the SeeSat-L mailing list and the Usenet newsgroup, sci.space.shuttle. In addition, there is a multitude of satellite-related World Wide Web sites on the Internet that provide information regarding satellites. Most of these sites have links to other related sites. There are even satellite prediction services on the Internet so novice observers don't even need a tracking program. However, having one's own tracking program may be preferable as it allows the information to be displayed in a format that the individual finds more suitable. Also, with a tracking program, the observer can pick and choose which satellites are to be tracked, rather than being restricted to those provided by prediction services. Personal tracking programs are easily updated periodically by downloading up-to-date orbital elements from an Internet site or Bulletin Board System for the many satellites that orbit the Earth. Note: Measurements used in the following sections are metric. Use the following approximate conversions to obtain the English equivalent measurements: Meter to feet: m x 3.3 = feet Centimeter to inches: cm x 0.4 = inches Kilometer to miles: km x 0.6 = statute miles Kilogram to pounds: kg x 2.2 = pounds The members of SeeSat-L hope that this introduction will make it easier for any reader to locate and use information provided on the Internet and BBSs to track and view Earth satellites, as well as serve as a resource for acquiring knowlege and sharpening the skills needed by those who are interested in the more demanding aspects of visual satellite observation. Clear skies to all. ---- 1.1 How Many Satellites Are In Orbit? By the beginning of 1998, there had been over 3,830 successful satellite launches (since 1957). There are expected to be 80 to 100 launches a year for the next few years. Each launch not only delivers one or more payloads into Earth orbit, but also leaves other objects in space besides the payload. These secondary objects include third or fourth stages of the rocket, shrouds, kick motors, payload platforms, and similar objects. In addition, some satellites and rocket bodies have exploded, littering the near-Earth space environment with small orbiting fragments of debris. By the end of November 1997, 25,066 orbital objects had been cataloged (since 1957) and 8,660 cataloged satellites remained in Earth orbit. Over 16,000 objects had burned up in the Earth's atmosphere, landed on Earth or on another celestial body, or continued into the solar system and beyond. There is still an unknown number of very small debris fragments in orbit, which are too small to be detected by radar or optical means and so remain uncataloged. Orbiting objects are regularly tracked by means of sensitive radar and optical equipment and then cataloged. Both the USA and Russia have this capability. In the USA, the United States Space Command (USSPACECOM) assigns a unique sequential Satellite Catalog Number to every object. The World Warning Agency for Satellites (WWAS) assigns the International Designation (ID) to the payload. USSPACECOM assigns the same International Designation, with an appended letter A, to the payload. Any subsequent non-payload objects (e.g., platform, booster) from the same launch receive the same International Designation from USSPACECOM, only with the next higher letter(s) in the English alphabet appended. ---- 1.1.1 Payloads A payload provides the scientific or intelligence gathering information desired by the launching country or customer, either directly by radio communications or indirectly by observations made from Earth. By the end of 1996, there were close to 2,300 payloads in orbit. About one fourth of these payloads are still active. For identification purposes, payloads are normally assigned the first letter (and the next higher letter(s) in the case of multiple payloads) of the English alphabet in the International Designation (ID), e.g., 96-034 A. In this example, 96 refers to the launch year 1996, 034 is the sequential number assigned that year to an orbiting body, and the letter "A" indicates that the object is a payload. ---- 1.1.2 Rocket Bodies A satellite rocket launcher has multistage rocket boosters to place the payload (and platform, if used) into orbit. The final stage booster(s) go into orbit with the payload. After burnout, these spent stages are known as rocket bodies. They are normally larger than the payload and usually are more easily visible to the observer than the payload. A rocket body's orbit normally decays faster, causing it to reenter the Earth's atmosphere before the payload. There are two reasons for this. First, in most cases, a booster rocket will be in an elliptical orbit, bringing it very close to the upper atmosphere, where significant drag will be encountered at its low point in orbit (perigee). Second, because rocket bodies are relatively large, and much of the mass has been ejected to provide thrust, they have a low ratio of mass to surface area. A lower mass/area ratio means greater drag, causing the orbit to decay faster. Rocket bodies are assigned the next higher sequential English letter designation in the International Designation (ID), e.g., 96-034 B, unless a platform is utilized to launch a payload. In that case, the platform usually receives the designation after the payload, and the rocket body gets the next designation after the payload and platform. The Orbital Information Group (OIG) uses the acronym R/B for rocket body in their Two Line Element designations. Approximately 15% of the total 8,600+ cataloged objects are rocket bodies. ---- 1.1.3 Platforms A platform may be used to support a payload while it is being placed in orbit. Such support may consist of orientation or attitude control, electrical power supply, or propulsion for (usually in-plane) orbital maneuvers. Alan Pickup, a member of SeeSat-L, reviewed the Satellite Situation Reports between May 1987 and November 1997, which showed 146 occasions when both a main rocket and a platform have been catalogued in Low Earth Orbit. For these launches the rocket body decayed first only 11 times (8% of the total), while the platform decayed first 127 times (87% of the total). For the other 5% of the launches, both the rocket and platform decayed on the same day. The platform (if used) is normally the first object identified after the payload designation with the next sequential English letter designation in the International Designation (ID), e.g., 96-034 B. OIG uses the acronym PLAT for platform in their Two Line Element designations. ---- 1.1.4 Debris Debris presents hazards to present and future payloads due to the devastating amount of kinetic energy that can be released if debris collides with a payload. Debris is a scourge to present and future payloads, because of the large number of objects involved and the inability of the launching countries to detect small debris. Debris is placed in orbit when parts (such as covers, fasteners, explosive bolts, thermal covers) are separated from the payload, when rocket bodies or payloads disintegrate or explode (major contributor), or when tools or parts are placed into free space from manned orbiting spacecraft during operations. Above an altitude of 500 km (310 miles), where debris hazard is the greatest, knowlege of man made orbital debris 10-30 cm (4-12 inches) in diameter is incomplete. For debris smaller than 10 cm in diameter, our knowlege is virtually nonexistent. Unfortunately, it is in the altitude regime above 500 km that debris poses the most serious long-term problem. Below this altitude, the debris population is purged fairly quickly by natural decay (atmospheric reentry). Above 500 km altitude, decay can take hundreds or thousands of years. In an article on space debris in the August 1996 issue of Popular Mechanics, it was estimated that there could be 35 *million* pieces of debris in orbit around the Earth. Debris objects represent 58% of the total cataloged objects, even though only a tiny fraction of the total debris objects have been cataloged. Only near a size of 0.1 mm diameter does the sporadic flux of micrometeoroids prevail in numbers over that of man-made orbital debris. Only 6% of the cataloged orbit population are operational spacecraft, while 50% can be attributed to decommissioned satellites, spent upper stages, and mission related objects (such as launch adapters and lens covers). The remaining 44% has originated from 129 on-orbit fragmentations, which have been recorded since 1961. From these events, all but 1 or 2 of which were explosions of spacecraft and upper stages, it is estimated that a debris population of 70,000 to 120,000 objects larger than 1 cm have been generated. Debris larger than 1 cm in diameter presents a catastrophic hazard to orbiting payloads. There is no known shielding material available for debris of this size for present operational satellites or for future satellites, such as the International Space Station. Smaller size debris can also be a problem, as documented by pits found in spacecraft windows, including the Shuttle's, and similar damage found on one of Hubble Space Telescope's high gain antennae. In one instance, chemical analysis of a pit on the shuttle's window showed that it was caused by a chip of paint. The first *reported* collision between two cataloged objects (in orbit) happened in late July 1996. A French military microsatellite called Cerise (International Designation 95-033B / Satellite Catalog Number 23606) suddenly lost stability. It appeared that its stabilization boom had been impacted. After analysis it was concluded that the culprit was a piece of space debris from an Ariane booster (86-019RF/18208). Controllers were able to reprogram the Cerise payload and regain attitude control. For further details on this collision see URL: http://www.stk.com/cerise.html Detecting, cataloging, and tracking debris objects is the current best defense against catastrophic on-orbit collisions. The USA Shuttle has released radar calibration objects, called ODERACS, as has many Russian Cosmos series satellites. In April 1996, the MSX (Midcourse Space Experiment) satellite 96-024A/23851) was launched into a 900 km orbit. Detailed information about MSX can be found at URL: http://msx.nrl.navy.mil One of MSX's missions is to detect previously undetected orbital debris in known orbital debris fields, both in Low Earth Orbit (LEO - with a period of rotation around the Earth of less than 225 minutes) and in Geosynchronous Earth Orbit (GEO - with a period of rotation around the Earth of 1440 minutes or 24 hours), using optical instruments. In addition, MSX will release 2 cm diameter reflective reference spheres that will be tracked on a routine basis by the USA Haystack radar facility, to make precise measurements of atmospheric drag. The Haystack radar facility is located near Boston, Massachusetts and can reportedly track 1 cm objects at an altitude of 1000 km. Measurements with this radar have provided the best and most comprehensive picture available of the small debris population. These and other efforts are being made to improve upon the detection resolution of orbital debris. However, serious and well-funded efforts are still required to reduce the hazard of orbital debris to acceptable levels. Debris objects have the highest sequential English letter assignments in the International Designation (ID), e.g., 96-034 D, 96-034 E, 96-034 F. When the letter Z is reached, the scheme begins using double or triple characters, e.g., AA, AB, AC, ..., AAA, AAB, and so on. OIG uses the acronym DEB for debris in their Two Line Element designations. More information about orbital debris can be found at URL: http://www-sn.jsc.nasa.gov/debris/toc.html ---- 1.2 How Many Satellites Can Be Seen? ---- 1.2.1 How Many Can Be Seen With The Naked Eye? Depending on the observer's location upon Earth, there are normally hundreds of satellites above the local horizon at any one time. However, only several dozen satellites in total can be easily seen with the naked eye. Thus, on any given date, when the late evening or early morning conditions allow satellites to be seen from reflected sunlight under dark sky conditions, there may be one or two easily visible satellites above the observer's horizon during a 30 minute time period. The large Russian manned laboratory Mir can become as bright as a steady magnitude -2 (much brighter than the brightest star). The USA Space Shuttle can become as bright as a steady magnitude -4 (about as bright as Venus, and brighter than Mir). A list of "100 (or so) Brightest Satellites" can be found at the URL: http://www.grove.net/~tkelso The term "magnitude" refers to an object's brightness. It is a logarithmic (exponential) measurement of brightness. Extremely dim objects have large positive values, while extremely bright objects have large negative values. Objects can be observed with the naked eye in a dark sky down to magnitude +6. Thus, satellites visible to the naked eye can range in brightness magnitude values of from +6 to -2 and can sometimes become even brighter temporarily. The brightness of a satellite is a function of its size, surface reflectivity, how well and from what angle the Sun's light is illuminating the satellite, the satellite's height above the horizon, and the corresponding effects of atmospheric interference. Another factor in observing a satellite is that it has to be above the observer's local horizon. The Shuttle's orbit is normally confined to between 30 degrees north/south latitude, but it can be visible as far as 60 degrees latitude when it is placed into a 57 degree inclination orbit with respect to the equator. Thus, an observer's location on Earth plays a large role in determining what satellites can be seen. ---- 1.2.2 How Many Can Be Seen With Binoculars? Using binoculars, at least several hundred satellites have the potential to be seen. On average, a dozen or so satellites are visible at any given time to an observer using binoculars. These dimmer satellites are mainly smaller rocket stages, and active and dead payloads. Experienced observers have also reported seeing some of the debris near Mir using binoculars. Using 7X50mm (seven power magnification by fifty millimeter aperture) binoculars can allow one to see satellites under ideal viewing conditions as dim as about magnitude +8 or +9. Higher power and larger aperture instruments will allow one to spot even dimmer objects. ---- 1.2.3 How Many Can Be Seen With A Telescope? By using a telescope and knowing exactly where to look, through the use of prediction programs, thousands of additional satellites have the potential to be observed briefly in a stationary telescope with a relatively small field of view (2-3 degrees). A special tracking program interface for a computer-driven telescope would be needed to actually follow satellites in Low Earth Orbit (LEO). These tracking systems, along with image intensifiers, are needed to observe structural details of large and low orbiting satellites. A telescope can also allow the observer to see some of the larger pieces of debris, as well as some of the more distant satellites, such as the geostationary platforms, which are located 36,000 km above the Earth's surface. There are several amateurs who modify telescopes for tracking and who are imaging structural details of satellites such as the Russian space station Mir and the Space Shuttle. Alain Grycan and Eric Laffont in France have obtained some spectacular amateur-made images of Mir. In these images, the different Mir modules are clearly visible. Also clearly discernible is the Sofora mast structure and the Progress motor compartment. Another image of Mir, taken in April 1991 with a 2.3 m (90 inch) telescope, was produced by Dave Harvey at the Steward Observatory in Arizona, using the Comsoft commercial satellite tracking package. Marek Kozubal and Ron Dantowitz at the Boston Museum of Science Observatory are experimenting with a 30 cm (12 inch) reflector using the ArchImage mount to obtain images of satellites. Recently they reported observing the docked Mir/Atlantis pair, noting details such as the solar panels, and the Shuttle's tail and nose. Other images have been made by a ground based telescope at the USA Air Force Maui Optical Site (AMOS). The outline of the Shuttle is clearly visible, and there is a hint of detail. Images from frames in a video sequence were taken using a CCD (charge-coupled device) camera and a 1.2 m (48 inch) telescope at the USA Air Force Phillips Lab Malabar Test Facility, while the Shuttle flew over Florida during the STS-37 mission. Most of the images mentioned above can be found at the URL: http://www.satobs.org//telescope.html Possibly the most spectacular telescopic observations of any satellite were those rumored to have been made of the Space Shuttle Columbia during the STS-1 mission, by an orbiting Keyhole optical reconnaissance satellite. Supposedly to allay fears concerning detached thermal protection tiles on the underside of the Shuttle (crucial to determine whether the vehicle would survive the heat of reentry), the orbiting Keyhole satellite was used to examine the belly of Columbia after tiles were noticed to be missing from the Orbital Maneuvering System (OMS) pods at the rear of the craft. Subsequent analysis of the orbits of the Shuttle and the known Keyhole satellites in orbit at the time of the STS-1 mission indicate that only one possible photo opportunity arose. The two craft were several tens of kilometers apart at the time and traveling in different directions. Thus, any image would have more than likely suffered significantly from motion blur. It is debatable as to whether the use of suitable image restoration techniques could reclaim sufficient resolution, in order to identify individual tiles or groups of tiles. In any event, one is unlikely to see such pictures, if they exist, for many years yet, if at all. ---- 1.3 When Are Satellites Visible? Whether or not a satellite is visible to a given observer is dependent upon many factors such as observer location, time of day, satellite altitude, and sky condition. Knowing these details may aid an observer in determining the most favorable times for sightings and is most certainly necessary, in order to spot some of the more elusive targets that speed across the heavens. ---- 1.3.1 Factors Affecting Satellite Visibility ---- 1.3.1.1 Orbit Altitude And Inclination The visibility of a satellite depends on its orbit. The simplest orbit to consider is circular, in which the satellite remains at (approximately) a constant distance from the Earth. A circular orbit can be characterized by stating the orbital altitude (height of the spacecraft above the Earth's surface) and the orbital inclination (the angle that the satellite's orbital plane makes with respect to the Earth's equatorial plane). On a simple level, it is the values of these two parameters that dictate whether an orbiting satellite can be seen by a particular observer. Most orbits are elliptical, rather than perfectly circular. In an elliptical orbit, the satellite's height (above Earth) varies smoothly between the apogee (farthest point on the orbit from the Earth), and the perigee (closest point on the orbit to the Earth). The orbital inclination dictates over which areas of the Earth the satellite will "fly". For example, in an orbit of 25 degrees inclination, the ground track (the point on the Earth's surface directly below the satellite, which is traced out during its orbit) will never exceed 25 degrees North or 25 degrees South latitude. This satellite would never be visible from Northern Europe, for example, unless its orbital altitude were some 1500 km or so (and in that case would then appear considerably dimmer, than if it were in Low Earth Orbit or at higher elevation in the local sky). Orbital inclination is the measure of the angle between the plane of the Earth's equator and the orbital plane. It is measured counter-clockwise from East (0 degrees) to West (180 degrees). Based on inclination, we can place orbits into some general categories: * Prograde/Retrograde Orbits with an inclination less than 90 degrees are "prograde" or "direct" (their groundtracks move in the easterly direction, in the direction of Earth's rotation), while orbits with an inclination greater than 90 degrees are "retrograde" (their groundtracks move in the westerly direction, against the direction of Earth's rotation). Satellites launched in an easterly direction (prograde) can take advantage of the Earth's eastward rotation to assist the launch. This bonus can be used to either reduce the fuel requirement, or increase the payload capacity of the launch vehicle, or both. * Equatorial Equatorial orbits are of low inclination (within a few degrees of the Earth's equator), where the majority of satellites will travel from west to east in the sky if launched in an easterly direction (prograde) or from east to west if launched in a westerly direction (retrograde). * Geostationary/Geosynchronous These orbits are special cases of equatorial orbits. Here the orbital altitude is such (around 36,000 km) that it takes the satellite one day to orbit the Earth, and it thus "hovers" over the same point on Earth. Such orbits are suitable for communications or meteorological observation. Satellites in such orbits are, however, only observable with telescopes and binoculars, because they are so far away. * Polar A high inclination orbit (80-100 degrees) will take a satellite over the polar regions so that it covers the whole Earth's surface, as the Earth rotates below it. * Low-Inclination Orbit This is an orbit defined as having an inclination of less than 45 degrees (prograde) or greater than 135 degrees (retrograde). * High-Inclination Orbit Orbital inclinations between 45 and 135 degrees are considered high-inclination orbits. Thus far, we can see that for a satellite to be easily visible to an observer it should be in Low Earth Orbit at an inclination that is almost equal to or greater than the observer's latitude. ---- 1.3.1.2 Earth's Shadow The Earth's shadow cone is also a factor in whether a satellite is visible. When a satellite is eclipsed and no sunlight illuminates it, no sunlight can be reflected from it and the satellite is not visible to an observer. Eclipse events depend on the satellite's altitude and inclination, the time of year, and the observer's location. For example, during June, the Earth's shadow is "longer" or "higher" in the local sky for an observer at the equator than it is for, say, an observer in the northern polar region. At the same time, the shadow in the southern hemisphere is even higher than at the equator. Thus, in June, the fraction of the night available for observing Low Earth Orbit satellites is shorter in Ecuador than it is in Sweden, and even shorter in Australia. Conversely, in December, the situation is reversed. In fact, high latitude observers may seldom see satellites disappear into the Earth's shadow during their Summer as long as the sky is dark enough to observe. ---- 1.3.1.3 Ground Track Precession Of course it is not simply a question of watching for a given satellite at the same time each night. Few satellites have an orbital period which is a simple fraction of one day, the geostationary satellites being the obvious exception. The orbital period is dictated by the satellite's altitude. The higher the altitude, the further a satellite must travel to complete one revolution around the Earth and, thus, the longer it takes. Satellites in Low Earth Orbit (say 300 km) complete one orbit in around 90 minutes, whereas at geostationary altitudes (about 36,000 km) one orbit takes 24 hours. This is simple orbital mechanics. Thus, the satellite arrives later (or earlier) on successive nights. With each delay/advance in arrival time, the Earth will have rotated a little farther (or less) with respect to the satellite's orbit. The consequence of this is that each night the satellite will appear in a different portion of the sky during each pass, and the number of visible passes will vary. This shifting is called ground track precession. Ground track precession is also due to the non-spherical shape of the Earth, which can cause the orbital plane to be shifted by a few degrees. In the longer term (days to weeks) the passes will drift from evening to daylight hours, then into the morning before returning to the evening once more. Imagine trying to live a 22 hour day. As the days passed, one would gradually wake earlier and earlier until one was having breakfast when others were off to bed. With more time, one's waking hours would re-synchronize with everyone else's, before beginning this cycle once more. Thus, windows of satellite visibility are created. Consider the Russian space station Mir. It will be visible for a week or so in the evening sky, and the best passes (those of highest local elevation above the horizon) will occur earlier each day. Eventually it is lost in daylight for the next two weeks or so before emerging in the pre-dawn sky. After a series of early morning passes for a week or so, visible passes are again lost, due to Mir being eclipsed by the Earth's shadow at around midnight, before reappearing in the evening sky. Mir repeats this visibility cycle about every four weeks. Many satellites in Low Earth Orbit go through a similar cycle of visibility. The cycle varies with orbital inclination, altitude, and observer location. In the case of the Shuttle, due to the short term nature of the missions (typically 7-10 days) an entire mission can occur entirely outside of one of these windows of visibility. ---- 1.3.1.4 Other Factors The simple idea of circular/elliptical orbits presented here belies the complications, which arise from the fact that the satellite suffers greater air resistance the lower its orbit. This bleeds off the orbital energy, lowering the orbit yet further as the satellite begins to brush the upper atmosphere at perigee. The gravitational forces on the satellite due to the Earth (and Moon, Sun, and so on) vary throughout its orbit (the Earth is not a nice spherical shape!) giving rise to continual change in the orbit. Fortunately, advanced orbital models using SGP4 and SDP4 codes take into account terrestrial, lunar and solar effects. These models are the basis for many software packages for satellite tracking and predicting. When used with recent and accurate orbital data, these programs yield very accurate predictions, which are a great aid to observers. ---- 1.3.2 Times Of Satellite Visibility ---- 1.3.2.1 Evening Viewing Satellites viewed in the late evening and early night are more easily seen in the eastern half of the sky. As is the case with the Moon, one half of the satellite is always illuminated by the Sun, except when it's within the Earth's shadow. The relative position of the Sun, satellite, and observer determines whether the satellite will be more or less illuminated as seen by the observer. With the Sun in the west and a satellite located in the east, the angle formed by Sun-to-satellite-to-observer (the phase angle) will be small. This means a greater portion of the illuminated satellite will be facing the observer. Although "normally bright" satellites may be located in the western part of the sky for a particular evening's observations, most likely, the observer will have difficulty in locating them as the major portion of the illuminated satellite will not be facing the observer. Note that, by another convention, phase angle is alternatively measured as the angle formed by Sun-to-observer-to-satellite, in which case the phase angle will *increase* as the satellite appears to the observer to be more illuminated by the Sun. Many satellite prediction and tracking programs provide the phase angle and/or percent illumination of the satellite to the observer. Some programs can provide empirical magnitude (a value independent of the geometry of the pass) and/or the standard magnitude value (a value dependent upon the geometry of the pass). ---- 1.3.2.2 Morning Viewing Conversely, satellites viewed in the early morning hours before dawn are more easily seen in the western half of the sky. Also, morning observations can be subject to less light pollution as the general public is asleep and more building and area lights may be off. ---- 1.3.2.3 Other Times Most Low Earth Orbit satellites (LEO, having an orbital period of less than 225 minutes) cannot be viewed for the entire overnight period, because they eventually fly into the Earth's shadow. Exceptions can occur at the beginning of Summer in an observer's hemisphere, when the Sun is at its highest inclination to the Earth. At that time, it is possible for some LEO satellites having high inclination orbits to avoid the Earth's shadow, so that they may be viewed several times during the "whole night". On the other hand, an extremely high latitude observer may not be able to view satellites during early summer, as the sky never gets dark enough for observations. There are two other exceptions to these visibility constraints, though both are not exactly common methods of observation. The first is daytime viewing. This is not recommended, but only is mentioned, as a few individuals have reported viewing some of the brightest satellites, such as Mir and the Shuttle, during the daytime. It obviously helps to know exactly where to look (courtesy of one of the many prediction programs available) and to look under optimum lighting conditions, that is to say, when the Sun-satellite-observer angle (phase angle) is at a minimum, which occurs when either the satellite is quite low in the west just after sunrise, or low in the east shortly before sunset. Binoculars are a great help with such observations, but be wary of the Sun, as -- SEVERE EYE DAMAGE -- will occur if the Sun is inadvertently viewed with or without binoculars! One technique, which may be of some use, is the use of a polarizing filter to increase the contrast between the sky and satellite. Sunlight scattered in the atmosphere becomes polarized. Thus, some contrast improvement may be gained by using an appropriately aligned filter. Note that ABSOLUTELY NO protection against eye damage caused by viewing the Sun is afforded with the use of such filters. The second exception lies in the fiery death of an orbiting body reentering Earth's atmosphere. A few observers make public predictions on the decay of satellites. However, a prediction for decay is not an exact science. Many variables will cause a decay to occur earlier or later than predicted. However, lucky observers may find themselves in the right place at the right time to witness a reentry, as a satellite experiences frictional heating in the upper atmosphere, leaving a fiery trail across the night (or even daytime) skies. ---- 1.4 What Do Satellites Look Like? ---- 1.4.1 "Normal" Satellites The majority of satellites (normally payloads) have a steady (non-pulsating) illumination associated with them. A gradual brightening and dimming may be observed, but it is associated with the changing phase angle of illumination. As the satellite traverses from one horizon to another, the area illuminated by the Sun changes its orientation with respect to the observer. As a result, the amount of area illuminated (depending on the geometry of the satellite) smoothly varies, which causes a gradual change in brightness. "Steady" satellites have a stable orientation in orbit. They may not be rotating at all, either because they have an attitude control system of some type, or because they have become gravity gradient stabilized, or because their rotational energy has been dissipated by eddy current torques. Alternatively, they may be spin stabilized and have evenly reflective surfaces, so that their observed brightness is relatively stable. Most satellites appear white, others may be off-white. A few appear yellow, or even a somewhat reddish hue. These color differences can normally be attributed to the satellite's surface color and finish and can be very subtle. For example, a reconnaissance satellite called Lacrosse 2 has a reddish hue associated with it because of the red-colored kapton insulation used on the surface of this large LEO satellite. In addition, a brief color change can occur as a satellite enters or leaves the Earth's shadow. ---- 1.4.2 "Flashing" Satellites Flashing (pulsating) satellites provide additional interest to observers. The flashing is caused by the satellite body rotating and different parts of the satellite reflecting different intensities of brightness back to the observer. A satellite may rotate around more than one of its three principal axes, producing spectacular, irregular flashing. There can be several different observable types of light intensity pulsations associated with one satellite. The flashing characteristics can evolve over time as the satellite's rotation about one or more rotation axes changes. The changes can be the result of venting gasses, interaction with the upper atmosphere, and interaction with the Earth's magnetic field. ---- 1.4.3 "Flaring" Satellites A constellation of communication satellites, called Iridiums, was added to the skies in 1997. Although an Iridium is a relatively small satellite, it can provide a very bright reflected light to an observer lasting between 5-20 seconds. This brilliant reflected light or "flare" can be observed only within a relatively small local area (20-30 km in diameter) as the satellite passes across the sky at an altitude of 780 km. Some flares have been observed during the day by observers who knew exactly where to look. Not all Iridiums will provide this flare on every pass. The geometry has to be just right between the observer, satellite, and Sun for the brillant reflection to seen. The standard brightness for an Iridium satellite is around magnitude +6, or just barely visible to the naked eye. Flares have been observed much brighter than Venus, which has a peak magnitude of -4.7. Additional information on Iridium satellites (and flares) can be found at URL: http://www.satobs.org/iridium.html ---- 1.4.4 What Do The Mir Complex And Space Shuttle Look Like? The Russian space station Mir and the USA Space Shuttle (while in orbit during a mission) are the two inherently brightest satellites visible to the naked eye. They are very easy to spot by virtually anyone, regardless of equipment or experience. ---- 1.4.4.1 Mir Complex The Mir Complex has been in orbit since early 1986, in a high-inclination orbit of nearly 52 degrees, with an average altitude of approximately 390 km. This means that anyone between latitudes 61 degrees North and 61 degrees South can view this object quite easily with the naked eye. Over the years, the complex has grown in size from the initial Mir module to a combination of five additional laboratory modules, plus the Soyuz transport and Progress cargo vehicles. This combination makes the orbiting module complex approximately 32 meters long by 30 meters wide by 27 meters high. Factor in the solar arrays, and the result is a relatively bright object that can be viewed with the naked eye. Mir's color is a slightly off-white or yellow. It appears as a steady illuminated object, though occasionally bright glints can be viewed, probably from the various solar arrays as the sunlight reflects off of them. Depending on an observer's location on Earth, it is possible to periodically view the rendezvous of the Mir complex with supporting transport and cargo vehicles (Soyuz and Progress). Also, regular extravehicular activities (EVAs) are planned, to move and adjust experiments, solar panels, portable cranes and other equipment. Material discarded from these EVAs can sometimes be viewed (with the aid of binoculars or telescope) in the immediate area of the Mir complex. Much less frequently, the Space Shuttle rendezvous with the Mir complex may be viewed by some observers. Normally all rendezvous dockings of the Mir complex take place over Western Russia and Eastern China, in order to facilitate communications between Mir and the Russian ground control center via Russian communication satellites and ground stations. Further information on observing the Mir complex can be found in section 6.7 of this FAQ. ---- 1.4.4.2 Space Shuttle The USA Space Shuttle is also easily visible to the naked eye. The 37 meter long by 24 meter wide vehicle is sometimes observed to be brighter than the Mir complex. This can be attributed to the bright white upper surface wing area and the extension of the highly reflective Shuttle cooling radiators inside the opened cargo bay doors. Additionally, the Shuttle normally flies at a lower altitude of approximately 300 km, compared to Mir's altitude of 390 km. The Shuttle maintains various attitudes during its missions for experimental purposes and for cooling considerations. The attitude of the Shuttle, as well as its location over the Earth during a mission, can be found in real time on the NASA web page for the Shuttle at the URL: http://shuttle.nasa.gov/ Unique to the Shuttle is the periodic observance of water dumps. The water turns to ice crystals and until it subliminates to a vapor, can be visible as a hazy cloud around the immediate area of the Shuttle vehicle. Sub-satellites are sometimes launched from a Shuttle during a mission. These sub-satellites either trail or lead the Shuttle by 100 km or so while deployed, so as to not be influenced by contamination originating from the Shuttle. Most sub-satellites are recovered by the Shuttle before the end of the mission. Normally these objects, while deployed, can be viewed with the use of binoculars (or even naked eye) and can be seen keeping formation with the Shuttle. Further information on observing the Space Shuttle can be found in section 6.6 of this FAQ. ---- 1.5 What Equipment And Knowlege Are Needed To See Satellites? ---- 1.5.1 Equipment The only equipment that is absolutely necessary are eyes and a set of predictions indicating when and where to look to see naked-eye satellites. ---- 1.5.1.1 Binoculars Naturally, use of binoculars or a telescope improves the viewing over the unaided eye. Much fainter objects can be seen, but at the expense of a smaller field of view. Also, large binoculars become heavy and could require a mounting system in order to provide a stable view. As the aperture of the instrument increases, fainter satellites can be seen. As a rough guide, a decent 50 mm pair of binoculars (e.g., 7x50, which magnifies sevenfold and which has an objective diameter of 50 millimeters) will extend visibility from the naked eye limit of about magnitude +6 down to about magnitude +8 or +9, in dark skies with stable atmospheric conditions. The purchase and use of a relatively inexpensive ($100) pair of astronomical binoculars greatly increases the observability of satellites. For new purchases, an objective diameter of at least 50 mm with fully coated optics is highly recommended. ---- 1.5.1.2 Telescope With a 15-20 cm (6-8 inch) reflector telescope, satellites as faint as magnitude +14 can be viewed. With experience, a small telescope can be manually slewed to track a satellite during the pass. However, tracking a satellite with a large telescope requires a computer motor driven mount and use of accurate satellite coordinates during the pass. Even when using valid, up-to-date USSPACECOM orbital elements, the tracking error can amount to up to one degree. This is even without considering the maneuvering that the likes of the Shuttle and Mir will perform regularly. ---- 1.5.1.3 Tracking Programs And Internet Resources ---- 1.5.1.3.1 Home Computer Tracking Programs Tracking software is widely available for amateur satellite observers on the Internet or on BBSs, either commercially or as shareware or freeware. Most of these programs use Earth-centered orbital Keplerian Two Line Elements (TLEs). The TLE is a standard mathematical model to describe a satellite's orbit. TLEs are just one type of format for orbital elements. Another type is known as the AMSAT format and is mainly used for software that predicts amateur radio satellites. Two Line Elements (TLEs) are processed by a computer tracking software program, yielding predictions for viewing time and position. The program determines the location of selected satellites above the horizon from a chosen observing location. The satellite's celestial Right Ascension (RA) and Declination (Dec) coordinates and/or local coordinates of the satellite in terms of elevation (angle above the local horizon) and azimuth (true compass heading) during the pass are provided by the program at a frequency determined by the observer. Most of the tracking programs display these predicted coordinates and related information both graphically and in text format. Tracking program resources are at many URLs, including: * http://www.satobs.org/tletools.html * http://www.satobs.org/orbsoft.html * http://www.satobs.org/otherinfo.html * http://www.grove.net/~tkelso/software/satellite/sat-trak.htm * http://www.ozemail.com.au/~dcottle/ * http://www.amsat.org/amsat/ftpsoft.html * ftp://seds.lpl.arizona.edu/pub/software/ ---- 1.5.1.3.2 Orbital Element Sets For Tracking Programs Naturally, tracking programs need accurate and recent data in order to generate accurate predictions. This data comes in the form of Keplerian or Two-Line Elements (TLEs). Groups of TLEs are commonly called "elsets". ---- 1.5.1.3.2.1 TLE And Satellite Data On The Internet NASA's Orbital Information Group (OIG) is the primary public distributor of satellite orbital data on the Internet. OIG receives its information from the USSPACECOM (United States Space Command). OIG disseminates non-classifed information to other agencies and to the public on the Internet. OIG also disseminates classified information to certain government agencies on a "need to know" basis. The Jet Propulsion Laboratory (JPL) disseminates satellite orbital data to the public via their anonymous FTP site. In addition, there are private individuals and organizations not affiliated with government agencies that generate data on the Internet regarding Earth orbiting satellites. Positional measurements of some classified satellites are made from observations by private individuals around the world. More accurate orbital data derived from subsequent observations is again generated by private individuals and is disseminated on the Internet. Four such resources having Two-Line Elements (TLEs) generated by private individuals are: * SeeSat-L (Listserver) Subscribe via e-mail to Seesat-L-Request@satobs.org (in the subject line type "subscribe" without quotes) * SeeSat-D (Listserver, Digest format) Subscribe via e-mail to Seesat-D-Request@satobs.org (in the subject line type "subscribe" without quotes) * SeeSat-L Archives http://www.satobs.org/seesat/index.html * Mike McCants TLE files http://users2.ev1.net/~mmccants/tles/index.html Two Line Element sets (TLEs or elsets) can be found at several Internet locations. A few of the many Internet sites with TLEs are: * OIG http://oigsysop.atsc.allied.com * JPL ftp://kilroy.jpl.nasa.gov/pub/space/elements * SPACELINK ftp/http/telnet://spacelink.msfc.nasa.gov (via their Spacelink.Hot.Topics/Next.Shuttle.Mission.STS-xx/ directory) * KSC http://www.ksc.nasa.gov * Private Individual http://www.grove.net/~tkelso/ * Private Individual http://www.ozemail.com.au/~dcottle/ * VSOHP http://www.satobs.org/getkeps.html * OIG telnet://oig1.gsfc.nasa.gov Login: oig Password: goddard1 ---- 1.5.1.3.2.2 TLE And Satellite Data On Bulletin Board Systems (BBSs) The following are just a few of the BBSs available. Note that the telephone numbers provided are for placing calls from within the same country as the BBS. International calling requires the use of an International code and a Country code in addition to the provided telephone number. International callers should consult their telephone company's international access provider to obtain the proper calling codes. United States: * OIG's BBS 1-301-805-3251 or 1-301-805-3154 (8 bit, no parity, 1 stop bit) * David Ransom, Jr.'s RPV BBS 1-520-282-5559 * The NASA SpaceLink BBS 1-205-895-0028 * The Datalink RBBS System 1-214-394-7438 (8 bit, no parity, 1 stop bit) Canada: * The Canadian Space Society BBS 1-905-458-5907 Belgium: * Alphonse Pouplier's BBS 32 (0) 81 460122 (The BBS owner requests that users first obtain approval by voice using the same telephone number.) Japan: * The Space Board BBS +81-45-832-1177 (provides OIG TLE sets and online prediction service with a UNIX SatTrack, in addition to astronomy and space news) ---- 1.5.1.3.2.3 Brief Introduction To TLEs And Satellite IDs Keplerian or Two-Line Elements (TLEs) are distributed in the form shown in the example below: THOR ABLESTAR R/B 1 00047U 60007C 96198.95303667 -.00000008 +00000-0 +24803-4 005026 2 00047 066.6626 011.9766 0252122 190.4009 169.1818 14.34618735877842 A few of the fields in this TLE are described below. THOR ABLESTAR R/B ^^^^^^^^^^^^^^^^^^ 1 00047U 60007C 96198.95303667 -.00000008 +00000-0 +24803-4 005026 2 00047 066.6626 011.9766 0252122 190.4009 169.1818 14.34618735877842 Line 0 (the line above line 1) provides the common name of the satellite object. Not all TLEs have common names associated with them, but they are an additional enhancement provided by some TLE distributors to allow the tracking program to provide a common name for the satellite in addition to the Satellite Catalog Number and/or International ID. Note, "R/B" is the OIG acronym for "rocket body". THOR ABLESTAR R/B 1 00047U 60007C 96198.95303667 -.00000008 +00000-0 +24803-4 005026 ^^^^^ 2 00047 066.6626 011.9766 0252122 190.4009 169.1818 14.34618735877842 ^^^^^ The first field in line 1 (00047U) and in line 2 (00047) contains the unique, sequential Satellite Catalog Number assigned to the object by USSPACECOM. The official title for this identifier is "Satellite Catalog Number". However, many acronyms are used because of their brevity and past history of use. These include NORAD (North American Air Defense), NSSC (NORAD Space Surveillance Center), Cat # (Catalog Number), Object Number, USSPACECOM (US Space Command) number, and so on. "Satellite Catalog Number" comes from the early days of satellite identification done at Hanscom Field, Massachusetts, USA in the late 1950's, where they kept track of satellites they identified, by giving them the next ascending number in a log that began with the number 00001 for the Sputnik launch. When NORAD assumed the responsibility for tracking, they continued using the sequence. Now USSPACECOM continues the assignment. Thus the satellite in the example TLE was 47th satellite ever cataloged by USSPACECOM. THOR ABLESTAR R/B 1 00047U 60007C 96198.95303667 -.00000008 +00000-0 +24803-4 005026 ^^^^^^ The second field in line 1 (60007C) contains the International Designation (ID). In this example, the launch took place in 1960, and it was the 7th successful orbital launch for that year. The "C" designates the third object cataloged for that launch. The International Designation is also described by terms such as International ID, COSPAR (COmmittee for SPAce Research) number, and COSPAR/WWAS (COSPAR World Warning Agency for Satellites) number. The World Warning Agency (WWAS) is the body authorized by the United Nations to issue the International ID. WWAS issues the International Designation for the payload but not for any of the other objects placed in orbit as a result of the launch. Subsequent International Designations for non-payload objects are normally assigned by USSPACECOM using the same International Designation as the payload, but using the next higher English letter in the alphabet. 1 00047U 60007C 96198.95303667 -.00000008 +00000-0 +24803-4 005026 ^^^^^^^^^^^^^^ The third field in line 1 contains the epoch, which indicates how "fresh" or recent the elset is. The epoch is the day and time in a given year when the satellite is estimated to have made its last "known" crossing of the Earth's equator in an ascending (northerly) direction, based on observations made by ground-based tracking stations. In the above example, observations calculated the satellite made an ascending (northerly) equatorial crossing on day 198.95303667 in the year 1996. Note, Universal Time (UT), formally known as Greenwich Mean Time (GMT), is the time standard that is used. Specifically, the equatorial crossing (for the series of observations corresponding to the example elset above) was estimated to have occurred on day 198 of the year 1996 at 22:52 UT [24 (hours) x 0.95303667 = 22.87288 hours, and 60 (minutes) x 0.87288 = 52.3728 minutes]. Most tracking programs will inform the user how old the element is by using the epoch date element. This tells the user if an old and possibly unreliable TLE is being used. The determination and interpretation of epoch date is not always straightforward. Jim Varney, a SeeSat-L member, responded to a question posed by another subscriber as to what the epoch date referred to in the TLE. His response (in part) was: "Tracking stations measure the early/late error and the off-track (plane) error compared to predictions made from their own ephemeris software, not elsets. If the errors are acceptable, the satellite is considered 'correlated'. If the errors are too large or the object is completely unknown, it is considered an uncorrelated target (UCT). Once a UCT is found, ground stations attempt to obtain tracking data for a minimum of 5.5 percent of one orbital period. This is their rule of thumb for the minimum data needed to generate a good set of elements. Correlated objects are always observed from multiple ground stations to sample different parts of the orbit. The observed track is far less than is done for UCT's. The multiple observations from multiple stations are used to correct the previous element set using differential corrections. These working 'elements' are state vectors plus perturbation terms. Two-line mean elsets are made after the state vector elements are produced. The only exception to the use of multiple ground stations is for objects near decay. Then they use what observations they can get. For most objects in the catalog (provided by OIG) there is no correlation between the 2-line elset epoch and any given ground observation. In a sense you could say that most elsets are 'predicted' because the elset position at epoch is never the raw observed position. The near decay objects appear to be highly correlated to ground observations only because one or two stations are contributing to the analysis." ---- 1.5.1.3.3 Satellite Prediction Services On The Internet There are several satellite prediction services on the Internet. Each provides an ephemeris for one or more satellites, which indicates when and where to look to see the satellites. These ephemerides, which are a tabular collection of data points that include position and velocities as a function of time, are generally in a text format. The ephemeris service at Georgia State University: http://www.chara.gsu.edu/sat.html and at North Carolina State University: http://acsprod1.acs.ncsu.edu/scripts/HamRadio/sattrack have a database of over 800 satellites and allow the user to decide how bright a satellite must be in order to be included in the output ephemeris. Users must input their geographical location as latitude and longitude. A link to a geographic server is provided to obtain this information if it is not known by the user. The ephemeris service at Manfred Bester's web-site: http://www.bester.com/satpasses.html provides ephemerides for six preselected satellites. The geographic location is limited to one of 72 major cities. If the user lives further than 100 km from a selected city, the output viewing information will probably not be accurate enough to readily spot the satellite. However, one nice feature of this site is that it provides links to several other predictions services. The Visual Satellite Observer's prediction service: http://www.satobs.org/satpred.html provides satellite predictions for European cities. The German Space Operation Centre (GSOC): http://www.gsoc.dlr.de/satvis/ provides prediction services for satellites brighter than magnitude +4.5 for any location in the world, including the "flaring" Iridium satellites. ---- 1.5.1.4 Watch And Computer Time Settings An accurate watch (with low drift and set to a time standard) is necessary for making accurate positional and flash period measurements. A one-second error in timing will result in a location error of 8 km (5 miles) for a LEO satellite traveling at nearly 30,000 km/hr (18,000 mi/hr). Time standard signals are available by short wave radio, telephone, and the Internet. WWV in Boulder, Colorado transmits time signals on 2.5, 5.0, 10, 15, and 20 Mhz. WWVH in Hawaii transmits on 2.5, 5.0, 10 and 15 Mhz. Their radio service is for the Continental USA and Pacific area with a delay time of 1-10 milliseconds. Reception outside these regions will result in additional delay. WWV can also be accessed by telephone at 1-303-499-711 and WWVH can be accessed at 1-808-335-4363. However, be aware that some long distant telephone services use satellite connections with the associated transmission delays. In Europe the following radio time services can be used for an accurate timing reference. Freq (KHz) Station Location 75 HBG Neuchatel Observatory, Czech. 77.5 DCF77 Bundesantalt, FRG 2,500 MSF Rugby, UK 5,000 MSF Rugby, UK 10,000 MSF Rugby, UK The USA Naval Observatory (USNO) provides various time services on the Internet at several URLs, e.g.: * telnet://tick.usno.navy.mil:13 * telnet://tock.usno.navy.mil:13 * http://tycho.usno.navy.mil/what.html Most telephone companies provide a time service, but the accuracy varies considerably, and caution must be exercised. In the USA, AT&T provides (for a fee) a supposedly accurate time service over land lines (no satellite transmission delays) from the USA Naval Observatory at 1-900-410-TIME. Note that some telephone and Internet time services do not correct for network transmission delays. When using a tracking program in real time to locate a satellite, the computer's internal clock must be accurately set to a time standard before using the program, to provide useful real-time tracking information. Some tracking software allows the user to set the computer's internal clock from the tracking program. Most computers, unless they have specific hardware and software installed, will not keep accurate time over a relatively short period (several hours). The USNO provides links to various time services, including links to software that will allow a computer's internal clock to be more accurate with and without modem time synchronization. Further information can be found at the URL: http://tycho.usno.navy.mil/ctime.html ---- 1.5.1.5 Stopwatch A stopwatch can be used to time flash periods or to accurately mark a satellite's position. A stopwatch accurate to within 0.1 seconds is necessary for accurate positional and flash period measurements. A quartz watch with 0.1 seconds readout and the ability to store and retrieve at least 50 "lap" times is better. Having this capability is more convenient for a session of flash measurements, because the observer doesn't have to write down the elapsed time after each satellite pass. A "lap" stopwatch is also needed for positional measurements if the observer wants to take more than one measurement during a pass. Note that it is very important to synchronize the stopwatch to an accurate time standard, as discussed in the previous section. One method used in accurately determining the position of an observed satellite is to commence timing with a stopwatch when a satellite passes between two known stars, whose position can be determined from either a star catalog or astronomy program. Timing should stop on the announced full minute, utilizing a radio time signal. Subtracting the recorded duration from the announced reference time gives the time, at which the satellite passed between the two stars. The observer should always use Universal Time (UT) (also called Greenwich Mean Time - GMT) and date when an observation is reported. Also, the location of the satellite between the two known star positions must be interpolated. Determination of UT with respect to the observer's location (time zone difference) can be found at URL: http://tycho.usno.navy.mil/tzones.html One method used to time flash periods associated with rotating satellites is to count several dozen flashes and the duration between flash 0 and flash n, then dividing the duration by n. Note that the flash count must begin at 0, because the flash *period* is the time duration *between* flashes. This method is much easier than timing every single flash. The error of the calculated flash period is also much smaller than that resulting from timing only one flash period. ---- 1.5.1.6 Tape Recorder A portable battery powered tape recorder with fresh batteries is most useful for recording observer commentary and for later flash period data reduction. Running a WWV broadcast in the background can provide an accurate time standard reference for later analysis. ---- 1.5.1.7 Chair It's important to be comfortable during long viewing sessions. Since one is usually looking up, it's best to be in a reclined position. A lightweight reclining lawn chair is portable and quite suitable for remote observing locations. At home, an old beat-up swivel recliner works great. The next best thing would be any type of back supported chair. ---- 1.5.2 Knowlege It is not necessary to have much knowlege of astronomy to enjoy satellite viewing, but some basic knowlege does help in knowing where to look and how to make observation reports. Further detailed information on basic concepts described below can be found in "Basics of Space Flight Learners' Workbook", located on the Jet Propulsion Laboratory web site at the URL: http://www.jpl.nasa.gov/basics/ ---- 1.5.2.1 Celestial Coordinates If the satellite is not very bright, it is difficult to use only local azimuth and elevation as coordinates. In most cases, it is more practical to use the Right Ascension (RA) and Declination (Dec) to locate the track of the satellite in a star field. Celestial object positions specified in this coordinate system are convenient, because an object's RA/Dec coordinates remain the same regardless of the viewing location on Earth. ---- 1.5.2.1.1 Right Ascension (RA) Right ascension is the angular distance measured eastward along the celestial equator in hours, minutes, and seconds (sometimes measured in degrees) from a reference point called the vernal equinox. The vernal equinox is that point on the celestial sphere, where the Sun's path crosses the celestial equator, going from south to north, each year on or near March 21st. The celestial location for 0 hrs RA is located approximately in the constellation Pisces. One complete rotation on the celestial sphere around the equator is, of course, 360 degrees. This full rotation is also normally measured as an angular distance of 24 hours. Therefore, one hour in RA corresponds to 15 degrees eastward rotation (360 degrees / 24 hours). For example, a RA of 02:30:30 (2 hours, 30 minutes, and 30 seconds) corresponds to an angle of rotation of 30 degrees (for 2 hours), plus 7.5 degrees (for 30 minutes), plus 0.125 degrees (for 30 seconds), giving a total angle of 37.625 degrees eastward from the vernal equinox along the celestial equator. ---- 1.5.2.1.2 Declination (Dec) Declination, which is measured in degrees positive or negative, corresponds to the global coordinate of latitude on the Earth. It is a measure of how far north or south the object is located from the celestial equator. Thus a declination of -10 degrees would mean that the object is located 10 degrees south of the celestial equator. Similarly, a declination of +10 degrees would mean that the object is located 10 degrees north of the celestial equator. Most satellite tracking and prediction programs provide the celestial coordinates of a satellite in RA and Dec. This is very handy, as celestial coordinates are the same, regardless of where on the Earth's surface the observer is located. ---- 1.5.2.2 Local Coordinates Most tracking programs will also provide the local coordinates of a satellite for a particular viewing location in terms of azimuth and elevation (altitude) angles. Of course, the observer's local latitude and longitude coordinates must be known fairly accurately, in order for the program to generate accurate local coordinates. An observer can refer to a detailed geographical map to obtain a close approximation. A detailed geodetic map should provide adequate latitude and longitude information. In the USA, such maps are produced by the United States Geological Survey (USGS) and are generally available for purchase from specialty map stores. Also in the USA, one can use online services to determine latitude and longitude. Two such services, which both use place name keywords to perform a search, are at the URLs: * Census Bureau: http://tiger.census.gov * USGS Database: http://www-nmd.usgs.gov/www/gnis/gnisform.html An online service to determine latitude and longitude outside the USA can be found at URL: * USGS: http://edcwww.cr.usgs.gov/nfwebglis/ Another resource is to use a Global Positioning System (GPS) receiver at the observer's site to obtain the geographical coordinates. These systems have dramatically come down in price over the last few years and are particularly helpful when observing at a remote site. ---- 1.5.2.2.1 Azimuth (Az) Azimuth is measured in degrees, corresponding to the points on a compass heading on the local horizon. To accurately locate an object, the observer must become familiar with the location directions on the horizon in terms of compass heading, where both 0 and 360 degrees correspond to true North; 90 degrees corresponds to true East; 180 degrees corresponds to true South; and 270 degrees corresponds to true West. Azimuth angles are "true" (i.e., geographic) headings, not magnetic headings. For observers in the northern hemisphere, the star Polaris is currently less than 1 degree misaligned from true North and is therefore a useful guide for locating the four cardinal points of the compass heading on the local horizon. Using a magnetic compass and compensating for local magnetic deviation (the difference between True North and Magnetic North) can also be used to locate the true heading. The magnetic deviation at your location can be found using the online calculator at the URL: * http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html ---- 1.5.2.2.2 Elevation (Alt) Elevation (or Altitude or Alt) is measured in degrees above the local horizon. An elevation of 30 degrees would mean that the object is located 30 degrees above the local horizon. (Note, 10 degrees can be approximated by the width of one's closed fist held at arm's length, so an object at 30 degrees elevation would appear to be approximately three fist widths above the horizon.) An object having an elevation of 0 degrees would be directly on the observer's local horizon. An object having an elevation of 90 degrees would be on the observer's zenith (directly overhead). It takes continual practice to accurately estimate or locate an object's local coordinates. ---- 1.5.2.3 Brightness Of Stars In astronomy, the brightness of any star is measured using the magnitude scale. This method was devised originally by the ancient Greeks, who classified the stars that were visible to the unaided eye as being first magnitude (brightest) to sixth magnitude (dimmest). This rough method was altered in the 19th century, so that magnitude +1 stars were defined as being exactly 100 times brighter than magnitude +6 stars. Thus, the magnitude could be expressed as varying logarithmically (exponentially) with the star's brightness. With the advent of accurate modern photometry, the scale was extended in both directions. At one extreme, the bright Sun is magnitude -27. At the other extreme, some of the faintest observed stars are about magnitude +24. The full moon is magnitude -12.5. Sirius, the brightest star in the nighttime sky, is magnitude -1.5, while the faintest stars visible to the naked eye under good conditions are about magnitude +6. It is very useful to know some stellar magnitudes in order to estimate the brightness of a satellite during a pass. The advantage of this method is, of course, that the stars are readily available for comparison with a satellite. Knowlege of stellar magnitudes also helps in judging the current viewing conditions. It is useless to look for a magnitude +5 satellite, if atmospheric conditions limit the seeing down to only magnitude +3. Some tracking programs provide the magnitude value of stars on their display star field as an aid to the observer. A quick guide to atmospheric conditions and satellite brightness can be gleaned from examining a suitable constellation. In the Northern hemisphere, Ursa Minor ("Little Bear") is ideal. (Note, the "Little Dipper" asterism is only a part of the constellation Ursa Minor or Little Bear. An asterism is a group of easily recognized stars that are a part of one or more constellations.) Circumpolar, and thus usually visible to a Northern hemisphere observer, Ursa Minor contains stars ranging in magnitude from +2 down to +6. Brighter satellites can be gauged by comparing against some of the more brilliant stars, such as Sirius (-1.5), Vega (0.0), Altair (+0.8), and Deneb (+1.3). The closest equivalent to Ursa Minor in Southern hemisphere skies is the constellation Crux (Southern Cross). Similarly circumpolar, it contains stars ranging in magnitude from +0.8 down to +6.5. ---- 1.5.2.4 Tracking Considerations If the satellite is not very bright, it is difficult to use only azimuth and elevation as coordinates. In most cases, it is more practical to use Right Ascension and Declination coordinates to draw or locate the track of the satellite in a star field, as shown in an atlas or astronomy program. Some graphical tracking programs show the satellite's location in a star field. To locate and track the satellite, choose an easy reference point along the orbit, such as passage near a bright star, passage between two bright stars, and so on. The predicted track can deviate from the true one, if the input orbital elements are not very recent. The track can also change considerably due to the influence of the Earth's atmosphere, which in turn depends on varying solar activity. Any orbital maneuvering of an active satellite will also cause deviations from predicted track. Fortunately, for most satellites, such deviations are a matter of at most one minute in time and one degree in position. For satellites in an orbit lower than 300 km, however, or for active (maneuvering) satellites, the track deviations can reach half an hour in time and several degrees in position. Most rocket stages can be predicted fairly accurately for longer than a month. But for the Space Shuttle, which maneuvers frequently, predictions can become inaccurate very quickly. To locate and track a satellite, start watching the selected area of the star field a few minutes before the satellite is predicted to pass through that field. To anticipate deviations from the predicted track, "sweep" or "scan" with binoculars in a direction that is perpendicular to the predicted track. At about the predicted time, the satellite should appear in the field of view. The satellite can be tracked from that point, and flash period or positional measurements made. ---- 1.6 What Can Be Learned About Satellites By Visual Observation? Quite a lot can be learned about satellites by making visual observations, both from positional observations and from "visual appearance" observations. Positional observations are measurements, at a particular time, of the satellite's position in celestial coordinates, usually by noting the location of a satellite relative to known stars. "Visual appearance" observations encompass general observations of the satellite's appearance as well as specific measurements of flash phenomena, for those satellites that exhibit flash behavior. A great deal of useful and important scientific information can be gleaned from such observations. ---- 1.6.1 Positional Observations Most obviously, if the satellite does not appear at the predicted time or position, the observer knows that the satellite's orbital elements used for the prediction are inaccurate. Measuring the satellite's position, and comparing the measured positions with the predicted ones, can thus provide information about the satellite's orbit. There are many reasons why predictions can be inaccurate. The satellite could have used its onboard propulsion and maneuvered (e.g., Russian Mir space station or USA Space Shuttle). Alternatively, the satellite's orbit could have been influenced by external forces, such as air drag or resonances with the Earth's gravitational field. Some of these forces are poorly understood and difficult to include correctly in prediction models. Measuring positions of satellites thus affected can provide valuable scientific information about the underlying external forces. Taking positional measurements to an extreme (of usefulness) is done by observers tracking classified spy satellites, for which no orbital elements are provided by USSPACECOM. These observers and analysts use positional measurements to derive their own orbital elements for Keyhole, Lacrosse, NOSS, and other classified satellites. The orbital elements (and their evolution) can then be used to make educated guesses about the mission of such satellites. Most of these observers are active on the SeeSat-L mailing list. ---- 1.6.2 "Visual Appearance" Observations The visual appearance of a satellite can also provide a diverse set of information about satellites. The observed magnitude sheds light on the size and structural details of the satellite, taking into account the distance between observer and satellite, and the phase angle at which the satellite is observed. Generally, larger satellites will be brighter, but the surface composition (painted, polished, reflective, and so on) also plays a dominant role. Whether or not a satellite has a "steady" appearance can provide information about its mission, its functional status, and even the direction of its rotation axis. For example, spin-stabilized satellites will usually exhibit regular flashes. However, for certain geometries, satellites will appear steady, despite the fact that they're tumbling. If flashing is present, the nature of the flash pattern can shed light on the structure and composition of the satellite. The simplest type of flash pattern is one where all flashes (or maxima in the brightness curve) have the same magnitude, duration and brightness evolution. Often the flash pattern contains flashes of different types: some are bright and short, others are fainter and perhaps show a smoother rise and decay in brightness. The brighter flashes are usually called primary flashes, the others secondary or even tertiary, depending on their brightness relative to the primary flashes. Secondary maxima are usually caused by smaller substructures of the satellite. Roundish (i.e. smooth) maxima in the flash pattern can indicate a less polished, less reflective surface. Bright, short flashes can indicate the presence of antennae or solar panels. Measuring the time between flashes (the flash period) and tracking its evolution over time provides information about the rotational status of the satellite. Satellites are acted upon by torques, which alter a satellite's rotational axis vector and its rotational speed. Some of these torques are poorly understood, and flash phenomena measurements can improve our understanding of them. One particularly useful type of data is the flash period averaged over one satellite pass and the upward and downward trends in these averages over many passes. An even more useful type of data is precise measurements of all of the times at which flashes appear. This information can be used to determine the direction of the rotation axis vector. The (slow) evolution over time of the direction of the rotation axis vector provide more accurate information about the torques acting on the satellite. These torques can, in turn, provide information about events such as fuel leaks and collisions with debris, as well as electromagnetic composition and the mass distribution of the satellite. ======================================================================== This FAQ was written by members of the SeeSat-L mailing list, which is devoted to visual satellite observation. Members of this group also maintain a World Wide Web site. The home page can be found at the URL: http://www.satobs.org/ The information on the VSOHP web site is much more dynamic than that found in this FAQ. For example, the VSOHP site contains current satellite visibility and decay predictions, as well as information about current and upcoming Space Shuttle missions and Mir dockings. The VSOHP site also contains many images, equations, and data/program files that could not be included in this FAQ while maintaining its plain text form. This FAQ and the VSOHP web site are maintained asynchronously, but an effort is made to synchronize information contents as much as possible. The material in this FAQ chapter was last updated in February 1998. ========================================================================