Configuring Rtcm Messages - u-blox C94-M8P User Manual

Figure 4: Base Station moving from Survey-in Mode to Fixed Mode
Users should be aware that the absolute accuracy derived from a survey in exercise is unlikely to be better than 1 m,
even with the best antenna and sky view combination. If the antenna location is known more precisely users should
use this and preset them using the fixed mode.
The rover location is derived from an offset from the base and hence any inaccuracy in the base coordinates will be
inherited plus the relative position uncertainty of typically less than 2 cm.

4.2.4 Configuring RTCM messages

The GNSS satellite configuration used will directly affect RTCM3 messages selection. The NEO-M8P module uses
GPS+GLONASS by default, otherwise other constellations can be selected if desired. A Base device should select the
RTCM Reference Station coordinates message and satellite observation messages for the configured GNSS
constellations. For example, using the default GPS + GLONASS configuration, RTCM messages 1005 (Reference
Station coordinates message), 1077 (GPS observations), 1087 (GLONASS observations) plus 1230 (GLONASS code-
phase biases) are required. For a GPS + BeiDou GNSS configuration, 1005 (Reference Station coordinates), 1077
(GPS observations) and 1127 (BeiDou observations) are required.
The throughput of the RTCM communications link must be considered with respect to the amount of bytes required
for the enabled RTCM messages and message rates. The C94 radio link has a fixed capacity for the amount of
messages sent through the radio link. If the Base receiver is set to run higher than 1 Hz the messages selected will
be sent over the radio link at higher than 1 Hz. If the message data rate exceeds the radio link capacity, delays will
increase and messages can be lost. This can negatively impact the RTK performance of the Rover to a point where it
will not reach Float/Fixed operation.
RTCM 1230 is required to make GLONASS ambiguity fixing possible. This can be sent at a low rate (eg once
every 10s) to save radio link capacity.
MSM4 observation messages (i.e. 1074 for GPS, 1084 for GLONASS, and 1124 for BeiDou) can be used instead
of MSM7 to reduce the communication bandwidth requirements. For moving baseline only MSM7 messages are
supported. These are 1077 for GPS, 1087 for GLONASS and 1127 for BeiDou.
UBX-15031066 - R07 Early Production Information
C94-M8P - User Guide
Configuration
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