Instruction manual

Differential GPS Page C-3
Differential
measurement data over a radio link to multiple rover stations which use the raw
measurement data to compute their positions more accurately. This operational mode
is widely used in vehicle navigation, land and aerial surveying.
However, real-time Z is not constrained to operate in this mode alone. For
applications such as robot monitoring and remote machine control, you may want to
know the rover’s position at the base station. This is called Reverse Vector Processing
(RVP). When the real-time Z system is configured in RVP mode, the raw
measurement data from the rover station is transmitted. The base station, at a known
site, uses the data from the rover station to compute the rover’s position.
Sources of Error
Error source affecting the performance of the real-time differential GPS can be
categorized into two groups - those that affect the Z-12 sensor and those that affect
the data link.
Sensor Related Errors
The major sources of error affecting the accuracy of DGPS solution are SV orbit
estimation, SV clock estimation, ionosphere, troposphere, sensor noise and multipath
in the measurements, carrier phase integer ambiguities in carrier phase differential.
The first four sources of error are almost totally removed using differential GPS.
Their residual error is in the order of one millimeter for every kilometer of separation
between base and remote sensors.
Total position error (or error-in-position), is a function of the range or carrier phase
errors (or errors-in-range) and by the PDOP (three-coordinate position dilution of
precision). The PDOP is a measure of the SV geometry.
GPS Signal Multipath
Multipath is by far the most unpredictable error source affecting differential GPS
position accuracy. Multipath is also one of the largest error sources, and because it
has strong correlation, it requires a longer time to average out its affect. While most
error sources can be canceled out by a differential GPS system, multipath can not
because it is a local effect. That is, multipath affects the base station and rover station
differently. To avoid large multipath effects, do not place the GPS antenna near any
metal objects, vegetation, or water, since the GPS satellite signals reflect off these
objects very easily and corrupt the reception of the direct signals from the satellites.
Also, try to mount the antenna higher to reduce the effect of ground multipath
reflections.
Carrier phase has relative smaller multipath error as compared to the code-phase.
Thus for code differential, the carrier phase is used to smooth the pseudo-range