ashtech GG Surveyor, Reference Manual

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Differential and RTK Operations 49
Differential and RTK
Message Rate
To improve Differential and RTK performance, minimize base station data latency by
using the highest possible data rates that your data link supports. There are three
different settings that affect data rates:
• RTCM message bit rate. $PASHS,RTC,SPD. This is the internal bit rate
used to generate the RTCM messages. This should be as high as possible
without exceeding the baud rate of the serial port. Recommended bit rate
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 (18 and 19) 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 and 31) 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 5.2) to see the accuracy sensitivity to lower update interval.
• RTK base station position (3 and 22) 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.