ashtech Reliance FS/2, Operation Manual

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Survey P
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Survey Planning 21
The theory behind differential correction is simple. At least two receivers must
operate at the same time, with one unit located on a known point. Both receivers must
use the same set of satellites to derive positions. In post-processed differential, GPS
data collected on the known point (by a receiver usually referred to as a reference
station) is used, after the field survey is completed, to remove inaccuracies from the
field data.
Post-Processed differential corrects for SA after collection (post-processed), while
Real-Time differential applies corrections while you’re in the field.
Q: How far away from a control point can I be and still get accurate readings?
A: It depends upon the accuracy you’re looking for. If you’re concerned with a meter
or two, the usual requirement for natural resource applications, there’s no practical
limit to the distance as long as the satellites you work with in the field are available
from your reference dataset. Some people have tested equipment at 600+ kilometers
and found answers within a meter or two. Most people work much closer than that,
but you can work a good distance away from your home base and still achieve solid
results. Keep the baseline accuracy of your system in mind when you go away from
your reference station. For the Decimeter and Precision systems, this is 10 cm + 1 part
per million of distances between reference and rover system.
In addition, you should conduct a careful mission plan to ensure the satellites you’re
working with at the field point are visible at the reference station. Watch for low-
lying satellites visible at one end of the session, and obscured at the other end. Set the
elevation mask angle at the base station to five degrees, and 10 degrees in the field
unit. Atmospheric differences can affect your post-processing over long baselines too,
although for meters-accurate results it’s unlikely you’ll notice any substantial effect.
Q: What is a sampling rate, and what does it mean?
A: The sampling rate (or recording interval) of a GPS receiver describes how often
the unit records satellite data and how often position fixes are calculated. The system
can be set to sample from once per second to once every 999 seconds. The system
units are also in sync with the GPS second. This means they record pseudorange data
exactly on the second (or the interval is selected). By syncing the receiver to the
second, data files interface smoothly and efficiently with other GPS receivers that
follow this basic convention. Some systems on the market interpolate between
measured satellite data, and may produce an unreliable measurements.
Every post-processed GPS coordinate file owes its success to accurate timing. Fixes
and data taken in the field match up with fixes and data acquired at the reference
station. If there’s a timing difference between one end of the equation and the other,
your accuracy will suffer. This is why your receiver is exactly locked to GPS time,
and works only with actual measured data in the Processor software package.