User Manual

Asset-Link 4XX User Manual May 10, 2004
970-00047 Rev. 1.0
CSI Wireless©2004 Specifications subject to change without notice Page 2-3
error, VDOP – vertical error and TDOP – time error) affects fix accuracy but not fix
update rate, only time to first fix (TTFF).
TTFF SiRF Definition
Cold Start
< 45 seconds (avg.)
A condition in which the GPS can arrive at a navigation
solution without initial position, time, current
ephemeris, and almanac data.
Warm Start
< 38 seconds (avg.)
Start mode of the GPS receiver when current position,
clock offset
and approximate GPS time are input by the
user. Almanac is retained, but ephemeris data is
cleared.
Hot Start
< 8 seconds (avg.)
Start mode of the GPS receiver when current position,
clock offset, approximate GPS time and current
ephemeris data are all available.
Table 2-2 Establishment of Position Fix
Upon initial startup the GPS almanac data is downloaded from the GPS satellites and
requires 12.5 minutes to download completely. The almanac is updated weekly and is
typically valid for up to a month. On subsequent startups the almanac data does not
have to be reloaded provided it is still valid. To achieve a hot start the receiver must
have current satellite ephemeris data (each satellite transmits its own orbital
parameters). The satellite ephemeris data is transmitted every 30 seconds, changes
every hour, and is valid for up to 4 hours. In Asset-Link the satellite elevation
threshold is set to 7.5 degrees above horizon, this cannot be changed by
configuration.
Stored GPS position information will be updated when the Asset-Link GPS is
powered ON and receiving enough satellite information to calculate position and
velocity information (nominally 4 satellites). An internal filter screens out low PDOP
(position dilution of precision) fixes. The GPS receiver is powered on in the Full
Power mode and can be independently powered on or off in the Active mode. A GPS
“warm up delay” uses a configurable timer to delay certain actions requiring a GPS
fix, such as a cellular “check-in”, until the GPS has an opportunity to get a fix
(nominally set to 45 seconds). When GPS receiver is on, it acquires GPS fix once a
second.
GPS information can be requested through PDI commands, information such as:
• Date/Time the request for position information was processed.
• GPS position (latitude, longitude, and altitude).
• GPS time of week and week number indicating when this fix information was
calculated (time of fix).
• GPS 3-D velocity vectors (east, north, and up velocity vectors) or plane speed
and direction.
• Fix type, providing status of the fix (available/not available,number of satellite
vehicles used for fix, etc).