Data Sheet

NEO-M8P - Data Sheet
UBX-15016656 - R06 Early Production Information Functional description
Page 8 of 30
1.6 RTK operation
Figure 2: The M8P modules work as a pair, where the Base provides a stream of RTCM messages to the Rover
Under RTK operation, the M8P modules operate as a pair consisting of a Rover and a Base. The Rover needs
access to a stream of RTCM 3 messages before it can enter RTK mode and before centimeter level accuracies can
be reached. The various concepts are explained in detail below.
1.6.1 Rover navigation modes
In its default configuration the NEO-M8P Rover will attempt to provide the best positioning accuracy dependant
on the received correction data. It will enter RTK Float mode as soon as it receives an input stream of RTCM 3
messages. Once the Rover has resolved the carrier phase ambiguities it will enter RTK Fixed mode. It is when the
Rover is in RTK Fixed mode that the relative accuracies can be expected to be correct to the cm-level.
It will typically take less than 60s before the Rover has been able to solve the carrier ambiguities and go from
RTK Float mode to RTK Fixed mode. The length of this time period is referred to as the Convergence time.
The Rover will attempt to provide RTK fixed mode when 5 or more ambiguities can be estimated. For single-
constellation receivers, this means that at least 6 satellites with continuous phase lock need to be visible above
the elevation mask (default 10°). Adding satellites from an additional constellation requires at least 2 satellites to
form the double difference measurement. Hence with dual GPS and GLONASS operation a minimum would
consist of 7 satellites (e.g. 5 GPS + 2 GLONASS). However, use with additional BeiDou satellites would require 8
satellites (e.g. 5 GPS + 3 BeiDou) owing to the two different BeiDou satellite variants.
The Rover will drop back to RTK Float mode if is looses carrier phase lock on the minimum amount of signals
needed to maintain RTK Fixed mode. The Rover will continue to attempt to resolve carrier ambiguities and go
back to the RTK Fixed mode once the minimum number of signals has been restored.
If RTCM 3 corrections become unavailable, the rover will run as a standalone standard precision receiver.
The command UBX-CFG-DGNSS can be used to specify that the receiver should stay in RTK Float mode and that
it should not attempt to fix integer ambiguities.
The current operation mode is indicated by relevant NMEA and UBX-NAV messages; see the u-blox 8 / u-blox
M8 Receiver Description Including Protocol Specification [2].
1.6.1.1 Relative and absolute position
In RTK mode the Rover calculates its position relative to the location of the Base position. The relative accuracy
can at best be correct to the centimeter level. To get an accuracy that is optimal in an absolute sense the
RTCM
stream