Novatel Software Version 4.45 OM-20000026 Rev 1 3.2 DIFFERENTIAL CORRECTIONS DATA

3 Special Data Input Commands

Use this special data input command to quickly update the GPSCard Universal Time Coordinated (UTC) parameters

following a system restart (always appended to $ALMA records unless intentionally stripped). The UTC data is

required before the GPSCard can accurately compute UTC time. If not input with $UTCA, it may take up to 12.5

minutes after a reset for the GPSCard to receive current UTCA data. In order to comply with NMEA standards, the

GPSCard will null NMEA log data fields until valid UTC parameters are collected or injected by the $UTCA input

command. This command is generated from a GPSCard ALMA log and is accepted as the following format:

$UTCA,-1.769512891769409E-008,-1.776356839400250E-015,552960,744,755,9,10,5*03

3.2 DIFFERENTIAL CORRECTIONS DATA

NovAtel MiLLennium cards can utilize the special data input commands $RTCA and $RTCM. These special data

input commands are utilized by a GPSCard operating as a remote station to accept NovAtel ASCII format

differential corrections. The data is generated by a GPSCard operating as a reference station with intent to be

received by remote stations. To correctly interpret these commands, the remote GPSCard must have its ACCEPT

command option set to "COMMANDS" (default). See Appendix A for further information on differential positioning.

The $PVAA and PVAB data messages contain the receiver’s latest computed position, velocity and acceleration.

These quantities are in rectangular ECEF coordinates based on the centre of the WGS 84 ellipsoid.

When a GPSCard receives this data message, it uses the information to update its own position, velocity and

acceleration parameters. This would only be needed if the GPSCard could not compute its own position, velocity

and acceleration due to signal blockage. This data message helps the receiver reacquire satellites after loss of lock.

The data would "steer" the receiver channels to be in the correct state to receive satellites ag ain; thus, the receiver

could “follow” the blocked satellites and re-acquire them much more quickly when they become visible again.

The position, velocity and acceleration status fields indicate whether or not the corresponding data are valid. Only

those messages containing valid data are used by the GPSCard.

NOTE 1: This command is intended for applications involving very high dynamics - where significant

position, velocity and acceleration changes can occur during a signal blockage. This data

message helps the receiver reacquire satellites after loss of lock.

NOTE 2: This is a highly complex function, to be used only by advanced users.

The ASCII $PVAA data message is generated from a PVAA log, and the binary PVAB data message is generated from

a PVAB log. For descriptions of these data messages, please see the description of the PVAA/B logs in Chapter 4

and Appendix D. An example of a $PVAA data message is as follows:

$PVAA,845,344559.00,-1634953.141,-3664681.855,4942249.361,-0.025,0.140,

0.078,0.000,-0.000,0.000,1,1,1*02

In cases where the receiver does not have an ephemeris for a newly-viewed satellite, these data messages can be

used to reduce the time required to incorporate this satellite into the position solution

The $REPA and REPB data messages contain the raw binary information for subfram es one, two and three from the

satellite with the parity information removed. Each subframe is 240 bits long (10 words - 25 bits each) and the log

contains a total 720 bits (90 bytes) of information (240 bits x 3 subframes). This information is preceded by the

PRN number of the satellite from which it originated. This message will not be generated unless all 10 words from

all 3 frames have passed parity.

The ASCII $REPA data message is generated from a REPA log, and the binary REPB data message is generated from

a REPB log. For descriptions of these data messages, please see the description of the REPA/B logs in Chapter 4 and