Instruction manual

Differential GPS Page C-5
Differential
Selective Availability
Selective Availability (SA) is the intentional degradation of the GPS satellite signal
by the US Department of Defense. Like multipath, SA is strongly correlated; but
unlike multipath, it affects the base and rover stations about the same way. Therefore,
most of the SA effects can be removed by a differential GPS system . Note that small
position errors will still be present due to SA for any real-time differential GPS
system. This is because the rover may have to predict the SA effect for the time gap
caused by the radio link delay. A rule of thumb to predict the effect of SA is
SA Error = 0.5 * a * t
2
(meters)
Where a is the SA acceleration in meters /second
2
, currently it is about 0.01 meters /
second
2
and it is subject to change by DoD without notice; t is the delay in the radio
link. For example, for a 2 second delay the error due to SA is roughly 0.02 meters.
Differential Message
Table Table C.1 shows the differential message type supported in RZ Sensor.
Table C.1: Differential Messages
Differential Operation Differential Message Supported
Code Differenti al • RTCM type 1
• RTCM type 2
• RTCM type 9 (recei ver only)
Carri er Phase Di fferenti al
(CPD or RTK)
• RTCM type 18/19
•DBEN