IDP 100 Modem Series - Developer Guide
©
SkyWave Proprietary 13 T203, Version 04
2.7.1.1 Satellite Modem
The IDP modem initiates a GPS fix in following cases
On startup or reset to determine its operating beam
At least every 12 hours to confirm any IsatData Pro transmit timing offsets due to
the position of the satellite
If a GPS fix is obtained due to an AT command or other trigger, the satellite modem may
delay its request for a GPS fix.
2.7.1.2 AT Command
An AT command for a GPS fix does not necessarily turn on the GPS. If the modem
previously obtained a GPS fix within the specified stale time that GPS fix is used. If there
is no previous fix with the stale time, the modem turns on the GPS and starts to get a GPS
fix. The stale time is the maximum acceptable age for a GPS position. For example, if the
stale time is 5 minutes, the modem uses a previous GPS fix if it was obtained within the
last 5 minutes.
The AT command's wait time parameter specifies the time for the AT command to wait
for a GPS fix - it does not specify the time the GPS stays on waiting for a fix. Once an
AT command initiates a GPS fix, the AT command cannot turn off the GPS. The GPS
remains on until one of the following occurs:
The modem gets a GPS fix
A 3-minute timeout (if the wait time is shorter than 3 minutes)
The modem reaches the wait time as specified (if the wait time is longer than
3 minutes)
2.7.1.3 Over-the-Air Request
A GPS fix can be initiated by a modem message that requests a position report. When
this modem message is received, the modem acknowledges the message. The modem
turns on the GPS; attempts to get a GPS fix and transmit a modem message containing a
GPS report. If a GPS fix is not obtained, the returned modem message contains an invalid
GPS position.
2.7.2 GPS Fix Type
If the modem requests a GPS fix to get a position for system purposes, the GPS fix must
be a 3D fix.
If a user requests a GPS fix using an AT command, the GPS normally waits for the first
3D fix. However, if the modem only has a partial view of the sky, only a 2D fix may be
possible. In this case, a 2D fix report is used if a 3D fix is not available after the wait time
expires. The wait time is the time specified by the AT command to wait for a GPS fix.
2.7.3 GPS Fix Times
The GPS can perform both warm and cold start GPS. A warm fix is always attempted.
Summary of Contents for IDP 100 Series
Page 106: ...www SkyWave com ...