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6 Message Formats
MiLLennium Command Descriptions Manual 35
RTCAREF TYPE 7
An
RTCAREF (RTCA Reference Station Position Information) message contains reference station position
information, and should be sent once every 10 seconds. Each message is 24 bytes (192 bits) long.
If RTCA-format messaging is being used, the optional station id field that is entered using the
FIX POSITION
command can be any 4-character string combining numbers and upper-case letters, and enclosed in quotation
marks (e.g. “RW34”). Note that the representation of this string in the log message would be a number within the
range of 266,305 to 15,179,385 as per RTCA notation. The lower bound of 266,305 represents “AAAA” and the
upper bound of 15,179,385 represents “9999”.
RTCA STANDARD LOGS
The RTCA (Radio Technical Commission for Aviation Services) Standard is being designed to support Differential
Global Navigation Satellite System (
DGNSS) Special Category I (SCAT-I) precision instrument approaches. The
RTCA Standard is in a preliminary state. Described below is NovAtel’s current support for this Standard. It is based
on "Minimum Aviation System Performance Standards
DGNSS Instrument Approach System: Special Category I
(SCAT-I)" dated August 27, 1993 (RTCA/DO-217).
RTCA
This log enables transmission of RTCA Standard format Type 1 messages from the GPSCard when operating as a
reference station. Before this message can be transmitted, the GPSCard
FIX POSITION command must be set. The
RTCA log will be accepted by a GPSCard operating as a remote station over a COM port after an ACCEPT port RTCA
command is issued.
The
RTCA Standard for SCAT-I stipulates that the maximum age of differential correction (Type 1) messages
accepted by the remote station cannot be greater than 22 seconds. See the
DGPSTIMEOUT command in Chapter 2
and Appendix C for information regarding
DGPS delay settings.
The
RTCA Standard also stipulates that a reference station shall wait five minutes after receiving a new ephemeris
before transmitting differential corrections. See the
DGPSTIMEOUT command for information regarding ephemeris
delay settings.
The basic
SCAT-I Type 1 differential correction message is as follows:
Format: Message length = 11 + (6*obs) : (83 bytes maximum)
➀ The pseudoran
g
e correction and ran
g
e rate correction fields have a ran
g
e of ±655.34 metres and ±4.049 m/s respec-
tively. Any satellite which exceeds these limits will not be included.
RTCAA
This log contains the same data available in the
RTCA SCAT-I message, but has been modified to allow flexibility
in using the
RTCA data. The RTCA data has been reformatted to be available in ASCII hexadecimal, utilizing a
NovAtel header and terminates with a checksum.
Field Type Data Bits Bytes
SCAT-I header – Message block identifier
– Reference station ID
– Message type
(this field will always report 00000001)
– Message length
8
24
8
8
6
Type 1 header – Mofdified z-count
– Acceleration error bound
(In the GPSCard, this field will report
000)
13
3 2
Type 1 data – Satellite ID
– Pseudorange correction ➀
– Issue of data
– Range rate correction ➀
– UDRE
6
16
8
12
6
6 *obs
CRC Cyclic redundancy check 3