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Appendix C: RTCM Messages
RTCM Message Types
Background information
Radio Technical Commission for Maritime services. Commission set up to define a differential data link to relay GPS correction
messages from a monitor station to a field user.
RTCM 2.x
Differential GPS Corrections. Message Type 1 provides the pseudorange correction (PRC(t)) for any
user receiver GPS measurement time "t". The Type 1 Message contains data for all satellites in view of
Delta Differential GPS Corrections. Upon a change in ephemeris, the reference station broadcasts a
Type 2 message paired with a Type 1 message, and continues to broadcast Type 2 messages over a
period of several minutes following a change in satellite navigation data in order to accommodate users
coming on line. The message contains the difference in the pseudorange and range rate corrections
caused by the change in satellite navigation data. The general format is the same as that of a Type 1
GPS Reference Station Parameters. Message Type 3 contains reference station information. It includes
the GPS coordinates (Earth-Centered-Earth-Fixed (ECEF)) of the reference station antenna to the
nearest centimetre. WGS-84 is the recommended reference datum. If a datum other than WGS-84 is
used, Message Type 4 will be broadcast frequently to inform the users of the datum being used for the
reference station coordinates.
GPS Partial Correction Set. The Type 9 Message serves the same purpose as the Type 1 Message, in
that it contains the primary differential GPS corrections. However, unlike Type 1's, Type 9 Messages do
not require a complete satellite set.
RTK Uncorrected Carrier Phases. GPS/GLONASS Satellite constellation indicator are used to
differentiate between the satellite systems. Hence, there are individual messages for GPS and
GLONASS satellites and for each frequency. The GNSS time of measurement is referenced to GPS
time for GPS satellites and to GLONASS time for GLONASS satellites. Note that GLONASS satellites
are supported in v2.3 only.
RTK Uncorrected Pseudoranges. GPS/GLONASS Satellite constellation indicator are used to
differentiate between the satellite systems. Hence, there are individual messages for GPS and
GLONASS satellites and for each frequency. The time of measurement is referenced to GPS time for
GPS satellites and to GLONASS time for GLONASS satellites. Note that GLONASS satellites are
RTK Carrier Phase Corrections. Same handling as for Type 18. Note that GLONASS satellites are
currently not supported.
RTK/High Accuracy Pseudorange Corrections. Same handling as for Type 19. Note that GLONASS
satellites are currently not supported.
only)
Antenna Type Definition Record. Message Type 23 provides the information on the antenna type used
on the reference station. The RTCM commission adopted the naming convention from the IGS
equipment-naming table as supplied by the International GNSS Service Central Bureau (IGS CB).This
table provides a unique antenna descriptor for antennas used for high-precision surveying type
only)
Antenna Reference Point (ARP). Message Type 24 solves the problem of referencing the L1 phase
centre by utilizing the Antenna Reference Point (ARP), which is used throughout the International GNSS
RTCM 3.x
L1-Only GPS RTK Observables.
Extended L1-Only GPS RTK Observables.
Compact L1&L2 GPS RTK Observables.
Extended L1&L2 GPS RTK Observables. Contains Signal to Noise Ratio (SNR).
Stationary RTK Reference Station Antenna Reference Point (ARP).
Stationary RTK reference station ARP coordinates with Antenna Height.
Antenna Descriptor & Serial Number.
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