Specifications

Differential and RTK Operations 33
Differential and RTK
setting is burst mode (9), which automatically adjusts the bit rate to the
fastest possible rate based on the serial port baud rate:
$PASHS,RTC,SPD,9
Serial port baud rate. This should be as high as possible.
RTCM message rate. This is the rate at which messages are generated.
RTK messages (RTCM 18 & 19, RTCM 20 & 21, Ashtech DBN) are
the most important. They should be generated as fast as possible, ideally
once per second. If they are generated slower then the effect on the
remote receiver depends on the mode. The slowest allowable setting for
type 18 and 19 is once per 5 seconds.
Fast RTK mode: accuracy will degrade by approximately 1cm for
each second of latency (example: type 18 and 19 generated every 5
seconds, fast RTK accuracy of 5cm, horizontal 1
σ
. Fast RTK
update rate is unaffected.
Synchronized RTK mode: accuracy is unaffected. Update rate is
limited to the update rate of messages 18 and 19.
Differential messages (1) are next most important, ideally once per
second. If the data rate does not support this, these messages may be
generated slower, with a corresponding decrease in differential accuracy
(Figure 3.2 to see the accuracy sensitivity to lower update interval).
RTK base station position (RTCM 3 & 22 or Ashtech BPS) are least
important. They affect the RTK initialization time following power on
of the remote receiver, (the remote receiver cannot provide an RTK
position until it has received messages 3 and 22 once or until receiving
the $PASHS,CPD,POS command), but the rate at which these messages
are generated does not affect RTK accuracy.
Required Differential Update Rates
For RTK operation there is a minimum radio baud rate that is acceptable. The
required radio rate depends on which messages are being generated at the base
station, and the message period. The slowest rate at which one should send RTK data
is once every 5 seconds. The remote receivers can fix integers with base station data
arriving once every 5 seconds or faster.