Specifications

Understanding RTK/CPD 59
Understanding
Fast CPD: $PASHS,CPD,FST
Fast CPD off achieves the ultimate in GPS accuracy. With Fast CPD off, sub-
centimeter position solution accuracy can be obtained with fixed integer ambiguities.
However, it suffers from solution delay. This delay is caused by measurement and
radio link delays. The measurement delay is about 1 second. Typical radio data link
delays are about 1 second also. DLf and Tf are not shown in $PASHR,CPD message
when Fast CPD is off.
For surveying application where accuracy has higher concern over the latency, fast
CPD should be turned off, especially when collecting data for static points.
Turning Fast CPD on (default) reduces the solution delay to about 50 millisecond.
Because Fast CPD computes the position as soon as Rover measurement has been
collected, it does not suffer from radio link delays. However, the position accuracy is
only 2-3 centimeters.
Turning FAST CPD on also allows the solution to be available when there is a
temporary data drop-out from the base station.
Multipath: $PASHS,CPD,MTP
Set this parameter to the expected GPS signal multipath environment according to the
list below:
SEVERE Forest, urban canyon
HIGH Water surface, nearby buildings
MEDIUM(default) Cropland, occasional tree or building
LOW Flat terrain, no nearby objects
NONE No multipath, for testing purpose only
DBN Message Interval: $PASHS,CPD, PED and CPD Update
Rate: $PASHS,CPD,PER
In some application where the data link bandwidth is not wide enough to transmit the
DBN or RTCM message at 1Hz rate, you can slow down the DBN or RTCM output
rate in the base side and slow down the CPD update rate in the rover side.
To change the DBN message interval at the base, use $PASHS,CPD,PED command.
To change the CPD update rate between 1 and 5 seconds, using $PASHS,CPD,PER
command. This will affect the CPD solution update rate when fast CPD is off, but not
with the fast CPD on. The fast CPD update rate is controlled by $PASHS,RCI
command for recording on a PC data card or raw data output (CBN, MBN,...), and
$PASHS,NME,PER for real-time NMEA output.
It is important to set the rover’s update rate to match the base’s DBN message output interval.