iSXblue/SXBlue II GNSS Series Technical Reference Manual
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3.1.2
Binary Messages
Both binary and NMEA 0183 data messages may be output from the iSXBlue/SXBlue II simultaneously.
Binary messages have a proprietary definition and would likely require custom software support if you
wish to use them. Binary messages are inherently more efficient than NMEA 0183 and would be used
when you require maximum communication efficiency. Use of binary messages for most users is not
recommended - the NMEA interface (commands) allows you to control the operation of the
iSXBlue/SXBlue II and also receive most types of information regarding status and positioning
information.
Note – If you wish to log binary data, please ensure that your logging software has opened the file
as a binary file, otherwise you may lose data.
3.1.3
RTCM SC-104 Protocol
RTCM SC-104 is a standard that defines the data structure for differential correction information for a
variety of real time differential correction applications. It has been developed by the Radio Technical
Commission for Maritime services (RTCM) and has become an industry standard for communication of
correction information. RTCM is a binary data protocol and is not readable via a terminal program. It
appears as ‘garbage’ data on-screen since it is a binary format and not ASCII text. The following is an
example of how the RTCM data appears on-screen:
mRMP@PJfeUtNsmMFM{nVtIOTDbA^xGh~kDH`_FdW_yqLRryrDuhcB\@}N`ozbSD@O^}nrGqk
eTlpLLrYpDqAsrLRrQN{zW|uW@H`z]~aGxWYt@I`_FxW_qqLRryrDCikA\@Cj]DE]|E@w_ml
roMNjkKOsmMFM{PWDwW@HVEbA^xGhLJQH`_F`W_aNsmMFM[WVLA\@S}amz@ilIuPqx~_IZhT
CpLLrYpdP@kOsmMFM[kVDHwVGbA^P{WWuNt_SW_yMsmMnqdrhcC\@sE^ZfC@}vJmNGAHJVhT
CqLRryrdviStW@H_GbA^P{wxu[K
RTCM has various levels of detail; however the highest level is the message. RTCM defines numerous
messages that contain specific information. The iSXBlue/SXBlue II receiver supports more advanced
methods of differential positioning, such as real-time kinematic (RTK) that uses different standard RTCM
message types. The iSXBlue/SXBlue II uses standard (RTCM2.x and RTCM 3.x) and proprietary (ROX,
CMR, CMR+) real time correction messages to achieve cm-level RTK.
In very simple words here are the various version of RTCM SC-104 and their usage:
•
RTCM 2.0 - Pseudorange corrections for GPS L1 C/A code only. Usually used for submeter positioning.
•
RTCM 2.1 – Added GPS-only L1 carrier phase message to v2.0 for single frequency RTK.
•
RTCM 2.2 – Equivalent to 2.0 with the addition of the GLONASS pseudorange corrections.
•
RTCM 2.3 – Equivalent to 2.1 with single frequency carrier phase corrections for both GPS and
GLONASS.
•
RTCM 3.1 – Sends dual-frequency corrections for both GPS and GLONASS for the highest level of
performance for RTK work.
Note - RTCM is a local area data standard. This means that when positioning with external
correction, performance will degrade as a function of distance from the base station. The
additional degradation will depend on the difference in observed orbit and ionospheric errors
between the reference station and the remote unit. This error is often seen as a bias in positioning,
resulting in a position offset. The scatter of the receiver is likely to remain close to constant. RTK
Networks overcome the limitations of relatively short baseline length (distance between rover and
base) with sophisticated interpolation techniques.