86 MPC User Manual Rev 0D
Appendix B GPS Overview
B.4 Differential Positioning
There are two types of differential positioning algorithms: pseudorange and carri e r pha se . In both of
these approaches, the “quality” of the positioning solution generally increases with the number of
satellites which can be simultaneously viewed by both the reference and remote station receivers. As
well, the quality of the positioning solution increases if the distribution of satellites in the sky is
favorable; this distribution is quantified by a figure of merit, the Position Dilution of Precision
(PDOP), which is defined in such a way that the lower the PD OP, the better the solution.
Due to the many different applications for differential positioning systems, two types of position
solutions are possible. NovAtel’ s carrier-phase algorithms can generate both matched and low-latency
position solutions, while NovAtel’s pseudorange algorithms generate only low-latency solutions.
These are described below:
1. The matched position solution is co mputed at the remote s tation when the observ ation in-
formation for a given epoch has arrived from the reference station via the data link. Matched
observation set pairs ar e ob servations by both the ref erence and remo te stations which ar e
matched by time epoch, and co ntain the same satell ites. The matched position solution is
the most accurate one available to the operator of the remote station, but it has an inherent
latency – the sum of time delays between the moment that the reference station makes an
observation and the moment that the differential information is processed at the remote sta-
tion. This latency depends on the computing speed of the reference station receiver, the
rates at which data is transmitted through the various links, and the computing speed of the
remote station; the overall delay is on the ord er of one second. Furthermore, th is position
cannot be computed any more often than the observations are sent from the reference sta-
tion. Typically, the update rate is one solution every two seconds.
2. The low latency position solution is based on a prediction from the reference station. Instead
of waiting for the obser vations to arrive from the reference station, a model (based on pre-
vious reference s tation obser vatio ns) is used to est imate wh at the obs ervations wil l be at a
given time epoch. These estimated r eference station ob servations are com bined with actual
measurements taken at the remote station to provide the position solution. Because only the
reference station observations are predicted, the remote station’s dynamics will be accurate-
ly reflected. The latency in this case (the time delay between the moment that a measure-
ment is made by the remote station and the moment that a position is made available) is
determined only b y the remote process or’s computation al capacity; the overall d elay is of
the order of a hundred milliseconds. Low-latency position solutions can be computed more
often than matched position solutions; the update rate can reach 10 solutions per second.
The low-latency positions will be provid ed for data gaps between matched pos itions of up
to 30 seconds (for a carrier-phase solution) or 60 seconds (for a pseudorange solution, un-
less adjust ed using the DGPSTI MEOUT command) . A general guid eline for the additi onal
error incurred due to the extrapolation process is show n in Table 4.
Table 4: Latency-Induced Extrapolation Error
Time since la st reference
station observation Typical extrapolation
error (CEP) rate
0-2 seconds 1 cm/sec
2-7 seconds 2 cm/sec
7-30 seconds 5 cm/sec