RE: Accurate Lat/Long Info

Date: Wed Jun 07 2000 - 04:53:51 PDT

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    While this topic has taken on a life of its own, most SEESAT users do not
    need accuracies better than 100m for satellite observing applications.
    Certainly if you are even miles away from your actual position, you can
    compute satellite predicts good enough to see with binoculars. However, if
    there is a valid requirement for accuracies less than this, then there must
    be enough data taken over a long enough period to justify statistical
    statements for accuracy purposes. Ron's recommendation for this is correct
    as noted below, though perhaps the length of time does not nearly have to be
    so long. Based on my personal testing at known benchmarks here after S/A was
    turned off, this period of time does not have to be more than a few minutes
    to get to 5-7m provided the satellite geometry is giving a PDOP that is low
    enough to maximize accuracy (generally less than 4.00). One might even argue
    that in a matter of seconds you could reach this goal. 
    For newbies to SEESAT, one should not over emphasize GPS for making valid
    sat predicts. GPS simply is not necessary for successfully predicting most
    satellites. Last week I was in Hersonissos, Crete and on the island of
    Corfu; in both places I used a Garmin GPS III Plus to ascertain my
    instantaneous position to feed into to get Iridium flare
    predicts and CGRO look angles. I took data for 10 seconds and the position
    generated was perfectly OK for this. In April, I surveyed several sites west
    of Lusaka, Zambia in preparation for our NASA JSC Astronomical Society June
    2001 solar eclipse expedition and the errors using the same receiver were
    about twice that I am getting now after S/A was turned off. Of course, the
    GPS readouts were used to enter my location into heavens-above and to use
    QUICKSAT to generate low altitude sat predicts. All the predicts were right
    on target; I could have used a coarse map before I left on my trips and the
    results would have been the same. 
    I have conducted numerous such expeditions when differential correction was
    needed from a base station to reduce the single receiver error and I am
    extremely glad that this is no longer mandated. For general astronomical
    applications, a better than 30 m accuracy was required for and 15m or even
    less was a goal of ours.  With S/A off, these applications are truly
    achievable with inexpensive single frequency receivers. Before S/A was
    turned off, taking readings every 2 seconds, for example, and averaging them
    over 4 hours was enough to get us to a 30m accuracy (by collecting lots of
    data and averaging) with 95% confidence using a single standalone receiver
    under normal geometry from the GPS constellation. One has to be careful even
    now because the accuracy does change with time as the satellite geometry
    moves to a less favorable orientation. Some manufacturers such as Trimble
    have a web site program to predict this geometry, making it easier to know
    which periods of time at a particular location are best to avoid.
    I would caution folks not to accept all the low accuracy
    values unless you take a full day (or more) of data.
    Typically accuracy is mentioned as the data points 
    containing 95% of the observed points.  At this time I
    am accepting the 7-9 meter values I quoted earlier
    because these are 24 hour measurements.
    I have not yet "surveyed" my normal observing position 
    with my GPS but I suspect I am plenty close as it is.
    Certainly GPS now allows a person to observe from a new
    site without having to get topo maps, etc.
    Ron Lee
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