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D Logs summary
MiLLennium Command Descriptions Manual 203
VLHA/B Velocity, Latency, and Direction over Ground
This log is similar to the SPHA/B message. As in the SPHA/B messages the actual speed and direction of the
GPSCard antenna over ground is provided. The
VLHA/B differs in that it provides a measure of the latency in the
velocity time tag and a new velocity status word which gives the user more velocity quality information. The
velocity status indicates varying degrees of velocity quality. To ensure healthy velocity, the position sol-status
must also be checked. If the sol-status is non-zero, the velocity will likely be invalid. Also, it includes the age of
the differential corrections used in the velocity computation. It should be noted that the GPSCard does not
determine the direction a vessel, craft, or vehicle is pointed (heading), but rather the direction of motion of the
GPS
antenna relative to ground.
VLHA
Structure:
➀ Velocity Latency
The velocity is computed using Doppler values derived from differences in consecutive carrier phase
measurements. As such, it is an average velocity based on the time difference between successive
position computations and not an instantaneous velocity at the
SPHA/B time tag. Under normal operation
the position's coordinates are updated at a rate of two times per second. The velocity latency compared
to this time tag will normally be 1/2 the time between position fixes. The default filter rate is 2 Hz, so
this latency is typically 0.25 second, but if, for example, the
POSA records were to be logged ontime 0.2,
then the velocity latency would be one half of 0.2, or 0.1 second. The latency can be reduced further by
the user requesting the
POSA/B, the SPHA/B, or the VLHA/B messages at rates higher than 2 Hz. For
example, a rate of 10 Hz will reduce the velocity latency to 1/20 of a second. For integration purposes,
the velocity latency should be applied to the record time tag.
Example:
$VLHA,640,333111.00,0.250,3.500,0.438,325.034,2.141,0,0*02[CR][LF]
$VLHA week seconds latency age hor spd trk gnd
vert spd sol status vel status *xx [CR][LF]
Field # Field type Data Description Example
1 $VLHA Log header $VLHA
2 week GPS week number 640
3 seconds GPS seconds into the week 333111.00
4 latency ➀ A measure of the latency in the velocity time tag in seconds. It
should be subtracted from the time to give improved results.
0.250
5 age Age of Differential GPS data in seconds 3.500
6 hor spd Horizontal speed over ground, in metres per second 0.438
7 trk gnd Actual direction of motion over ground (track over ground) with
respect to True North, in degrees
325.034
8 vert spd Vertical speed, in metres per second, where positive values
indicate increasing altitude (up) and negative values indicate
decreasing altitude (down)
2.141
9 sol status Solution status as listed in Table D-1 0
10 vel status Velocity status as listed in Table D-12 0
11 *xx Checksum *02
12 [CR][LF] Sentence terminator [CR][LF]