GPS Receiver User Guide

38 Promark2 User’s Guide
location for the given time. Pay particular attention to satellite availability (number of
satellites in view) and the satellite distribution. To assist in analyzing the quality of satellite
distribution, Dilution of Precision (DOP) values are presented. DOP is a quality analysis value
for satellite distribution. The most popular DOP value is PDOP, which stands for Positional
Dilution of Precision. The PDOP value estimates the impact on the precision of your GPS
observations due to satellite geometry. The smaller the PDOP value the better the satellite
distribution (geometry) and therefore the better the precision of your observations. The
manual and on-line help for Ashtech Solutions goes into more detail on Mission Planning,
satellite availability, and DOP values. Please take the time to read this additional information
to get a better understanding of the concepts.
With the current constellation of 26+ GPS satellites, it is uncommon to find periods in the day
when satellite availability and distribution are so poor that data collection should be avoided.
Time of poor availability and distribution are usually short in duration. When using the static
mode of GPS data collection, where observation times are usually 20+ minutes, short periods
of poor availability and distribution can be tolerated. When performing an observation during
which a period of poor availability and distribution appears, observation times will normally
need to be extended to compensate for this event. The ProMark2 receiver includes a feature
which estimates the observation time required to produce a quality solution. This feature is
called the Observation Timer and is discussed in more detail later on in this chapter. The
Observation Timer takes into account satellite availability and distribution when determining
the required observation time. If you are collecting data during a period of poor availability
and distribution, you will find the Observation Timer will take longer to inform you when your
survey is completed. It is automatically extending the observation period to compensate for
the poor availability and distribution of satellites. Analysis of the satellite constellation with
Mission Planning prior to data collection will give you an idea of the time periods when
extended observations will be required.
Note: The ProMark2 receiver is designed to store GPS data for only those satellites that
are at least 10° above the horizon. The receiver may lock onto a satellite between 0° and
10° but will not record this data. When using the Mission Planning software to analyse the
satellite constellation, be sure to set the satellite cut-off angle to 10°. This will ensure that
the satellite availability and distribution presented by Mission Planning matches what is
being used by the ProMark2 receiver for data storage.
Note: The ProMark2 will track up to 10 satellites simultaneously. If more than 10 are
available, ProMark2 will track the 10 satellites with the highest elevation.