Novatel OM-20000041 用户手册

4

Message Formats

46

 MiLLennium GPSCard Software Version 4.50 Command Descriptions Manual Rev 1

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”.

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).

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 

 command in Chapter 2,

Page 23 and Appendix C, Page 90 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 

 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)

1

The pseudorange correction and range rate correction fields have a range of ±655.34 meters and ±4.049 m/s respec-
tively. Any satellite which exceeds these limits will not be included.

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

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

–

Modified z-count

–

Acceleration error bound

(In the GPSCard, this field will report 
 000)

13
3

2

Type 1 data

–

Satellite ID

–

Pseudorange correction

1

–

Issue of data

–

Range rate correction 

 

1

–

UDRE

6
16
8
12
6

6 

*

obs

CRC

Cyclic redundancy check

3