Observing USA 193 - tips for beginners

From: Ted Molczan (ssl2molcz@rogers.com)
Date: Fri Feb 15 2008 - 19:14:15 UTC

  • Next message: Lutz Schindler: "Obs 15.02.08"

    Allen Thomson wrote:
    > The apparent divergence between USG estimates and SatEvo persists.
    > Hopefully enough tracking data will be collected to throw 
    > light on where the difference comes from.
    Visibility windows for various latitudes are available here:
    The object has entered a brief period of visibility near 30 N, which will
    gradually move northward over the coming days.
    Beginners can useful contribute to the tracking of USA 193, especially when it
    has not been tracked for several days, as this case now. It was last observed
    with precision on Feb 11, and its orbital elements were updated, but due to its
    rapid and uneven rate of decay from orbit, those elements are rapidly losing
    accuracy for predictions. Tonight, it could easily be early or late, by at least
    1 minute. 
    In these circumstances, an observation timed to within one or two seconds, and
    accurate in position to within one degree or so, would make a valuable
    contribution to tracking the object, by enabling us to approximately revise the
    orbital period and rate of decay.
    Here is how to make a simple, but reasonably accurate observation.
    1. Obtain a Prediction
    Heavens-Above provides predictions of nearly all satellites, including star
    charts showing the time and path. Due to the interest in USA 193, the site has a
    special link at the top of the main page.
    I recommend allowing for at least 60 s prediction time uncertainty, and 2 min to
    be safe.
    2. Observe
    Go outside at least 10 min before the satellite is due, to give your eyes time
    to adapt to the darkness, and to locate the predicted point of arrival of the
    satellite shown on the Heavens-Above chart. If you are not all that familiar
    with sky, or how to use a star chart, then you may need 30 min or longer.
    Wait for the satellite to appear, allowing at least 1 or 2 minutes, in case it
    is early or late. Beware of imposters! There are many satellites in orbit, so it
    is all too easy to follow the wrong one. The one you want will closely follow
    the predicted path.
    When you spot it, observe it as it crosses the sky, and when you see that it is
    about to pass close to a star that you can identify, get ready to make your
    observation. At the moment of closest approach, note whether the satellite was
    above, below, right or left of the star, and the approximate separation. For
    example, "one half degree below Procyon". (For reference, the moon's apparent
    diameter is 1/2 deg.) The closer the separation the better, but 1 or 2 degrees
    is acceptable for a rough observation. If you do not know the star by name, then
    circle it on the Heavens-Above chart, or other star chart.
    At the moment of closest approach you must also measure the time. 
    If all you have is a wrist-watch, then turn to read it as quickly as you can
    after noting the position of the satellite. You may wish to subtract a second or
    two to allow for the delay in taking the reading. The goal is to achieve 1 or 2
    sec accuracy.
    You will need to calibrate the watch to a precise time source. Perhaps the
    simplest method is to phone radio station WWV (303-499-7111), and note how many
    minutes and seconds your watch is early or late, and then correct your observed
    time accordingly. Or, you may wish to set your watch to synchronize as closely
    as possible to WWV. Keep in mind that watches tend to drift, so this should be
    repeated each day you observe.
    If you have a stop watch handy, then that is the preferred timing method. The
    usual approach is to hit the start button at the moment of the observation, then
    stop it at the start of a minute using WWV. Subtracting the elapsed time from
    the WWV time, yields your observed time.
    3. Report
    Your observation report can be as simple as "passed one half degree below
    Procyon" on 2008 Feb 15 at 7:12:43 PM CST. 
    If you are familiar with Universal Time, aka UTC or GMT, that is preferred, but
    local civil time is acceptable, as in the above example. If you know the time
    zone, that would help. If you can roughly estimate the accuracy of your position
    and time, please include it.
    Last, but not least, we need to know your location on Earth. Typically, we
    report latitude and longitude to 0.0001 deg accuracy, and height above sea level
    to within a few metres. If you have GPS, use it, else you can use Google Maps.
    For low-precision observations, 0.1 deg accuracy should suffice; height can be
    within 100 m.
    Happy Hunting!
    Ted Molczan
    Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive:  

    This archive was generated by hypermail 2b29 : Fri Feb 15 2008 - 19:21:34 UTC