User's Manual Part 1

82 OEMV Family Installation and Operation User Manual Rev 5B
Chapter 5 Positioning Modes of Operation
5.3.2 Position Solutions
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 computed at the rover station when the observation
information for a given epoch has arrived from the base station via the data link. Matched
observation set pairs are observations by both the base and rover stations which are matched
by time epoch, and contain the same satellites. The matched position solution is the most
accurate one available to the operator of the rover station, but it has an inherent latency – the
sum of time delays between the moment that the base station makes an observation and the
moment that the differential information is processed at the rover station. This latency
depends on the computing speed of the base station receiver, the rates at which data is
transmitted through the various links, and the computing speed of the rover station; the
overall delay is on the order of one second. Furthermore, this position cannot be computed
any more often than the observations are sent from the base station. Typically, the update rate
is one solution every two seconds.
2. The low latency position solution is based on a prediction from the base station. Instead of
waiting for the observations to arrive from the base station, a model (based on previous base
station observations) is used to estimate what the observations will be at a given time epoch.
These estimated base station observations are combined with actual measurements taken at
the rover station to provide the position solution. Because only the base station observations
are predicted, the rover station’s dynamics are accurately reflected. The latency in this case
(the time delay between the moment that a measurement is made by the rover station and the
moment that a position is made available) is determined only by the rover processor’s
computational capacity; the overall delay is in the order of a hundred milliseconds. Low-
latency position solutions can be computed more often than matched position solutions; the
update rate can reach 20 solutions per second. The low-latency positions are provided for
data gaps between matched positions of up to 60 seconds (for a carrier-phase solution) or 300
seconds (for a pseudorange solution, unless adjusted using the DGPSTIMEOUT command).
A general guideline for the additional error incurred due to the extrapolation process is
shown in Table 16.
Table 16: Latency-Induced Extrapolation Error
Time since last base
station observation
Typical extrapolation
error (RMS) rate
0-2 seconds 1 cm/s
2-7 seconds 2 cm/s
7-30 seconds 5 cm/s