Novatel OM-20000053 Rev 2 4MESSAGE FORMATS

4 Message Formats

46 MiLLennium GPSCard SW Version 4.503/4.52 Command Descriptions Manual Rev 2

4 MESSAGE FORMATS

In a NovAtel RTK positioning system, the observations transmitted by a NovAtel reference station to a NovAtel

remote station can be in either a proprietary RTCA Type 7 or a proprietary RTCM Type 59N message format. A

NovAtel Rover station is also able to receive CMR-format messages, Section 4.3, from a non-NovAtel base station.

Table 4-1 illustrates the various combinations of hardware and message formats, together with the positioning

mode (RT-20 or RT-2) which will result when using all-NovAtel devices:

Table 4-1 Positioning Modes

The following information can be used to calculate the minimum data throughput required of the communications

data link. Keep in mind that manufacturers of communication equipment add extra bits to each message (e.g. for

error detection), forming an “overhead” that must be taken into account; also, radio transmitting equipment may

have a duty cycle which must also be factored into the calculations. Thus, a “4800 bits per second” radio modem

might actually sustain only 2000 bits per second. Consult the documentation supplied by the manufacturer of your

communications equipment.

NovAtel has defined two proprietary RTCA Standard Type 71 binary-format messages RTCAOBS and

RTCAREF, for reference station transmissions. These can be used with either single or dual-frequency NovAtel

receivers; existing users of RT-20 wishing to switch from RTCM to RTCA message formats will require a software

upgrade. The RTCA message format outperforms the RTCM format in the following ways, among others:

•a more efficient data structure (lower overhead)

•better error detection

•allowance for a longer message, if necessary

RTCAREF and RTCAOBS, respectively, correspond to the RTCM Type 3 and Type 59 logs used in single-

frequency-only measurements. Both are NovAtel-proprietary RTCA Standard Type 7 messages with an ‘N’

primary sub-label.

RTCAOBS TYPE 7

An RTCAOBS (RTCA Reference-Station Satellite Observations) message contains reference station satellite

observation information. It is used to provide range observations to the remote receiver, and should be sent every

1 or 2 seconds. This log is made up of variable-length messages up to 255 bytes long. The maximum number of

bits in this message is [140 + (92 x N)], where N is the maximum number of satellite record entries transmitted.

Using the RTKMODE command, you can define N to be anywhere from 4 to 20; the default value is 12.

Reference station:

RTCM Type 59N

Reference station:

RTCA Type 7

Reference station:

L1 & L2

RTCM Type 59N

Reference station:

L1 & L2

RTCA Type 7

Remote station: L1 RT-20 RT-20 RT-20 RT-20

Remote station: L1 & L2 RT-20 RT-20 RT-20 RT-2

1. For further information on RTCA Standard Type 7 messages, you may wish to refer to:

Minimum Aviation System Performance Standards - DGNSS Instrument Approach System: Special

Category I (SCAT-I), Document No. RTCA/DO-217 (April 19,1995); Appendix A, Page 21.

4MESSAGE FORMATS