ashtech Reliance FS/2, Руководство по эксплуатации

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20 Reliance with FS/2 Field Operations Manual
the carrier phase of the L1 satellite signal. Centimeter processing requires the P and V
options installed on the receiver, and is available for point features collected while
remaining stationary for 30 minutes to fix ambiguities. All other features are
processed to decimeter level.
Q: What sort of accuracy can I expect from a single GPS receiver?
A: The accuracy is about 100 meters, give or take a few tens of meters. The DoD has
intentionally degraded the signals. No matter what type of receiver you have, a single
GPS receiver produces about 100 meter accuracy, depending on the nature of the SA
degradation for the period of time you’re using the unit. 100 meters or so is the best
accuracy you can ever hope to get with a single unit. (It may be a $200.00 recreational
unit or a $20,000.00 top-of-the-line geodetic receiver.)
The official DoD policy on autonomous (single receiver) GPS accuracy is 95% of the
time you’ll be within 100 meters, and you’ll be 99.99% certain to be somewhere
within 300 meters.
You will not get good results by averaging uncorrected positions. You’ll have to
average hours (days) of measurements to get a number that converges on truth. SA
does not produce steady-state disturbances. SA is a random degradation, and it varies
constantly, but it does not vary with any consistency. For example, if you averaged
ten minutes of measurements, your result may show a spherical error of probability
(SEP) of 15 meters. You might think, judging from your statistics, that you’ve gotten
a valid average and you’re inside a 15-meter sphere, when in fact you’re probably
well off the mark. Remember, SA is constantly affecting your results. The tight little
group you averaged may well be 89 meters away from your actual point. Five hours
(or five minutes) later, your readings will likely indicate an entirely different location.
The only sure method to get reliable, accurate position fixes better than 100 meters
RMS is with differential GPS. Anyone who tells you otherwise is either
misrepresenting the reality of GPS and the SA environment, or does not fully
understand what GPS can, and cannot do.
In most cases you’ll probably find you’re within 35-50 meters of your actual position if you compare
the system reading with a known point. However, SA fluctuates so much you could be 35 meters one
direction one minute, then 50 meters the another direction 10 minutes later.
Q: What is differential GPS, and how do I get it?
A: Differential GPS is a straightforward method to overcome the effects of SA. There
are two basic ways to achieve differential results. The most common uses a computer
to process data collected and stored while in the field, then downloaded later to a
processor system. Another method is by receiving radio signals that provide
corrections in real time. The first is known as Post Processed Differential, the latter as
Real-Time Differential. Each is useful, although post-processing is the most common,
reliable, and proven method now available. Ashtech receivers operate in either mode.