We are a municipal government with about 200 of these mobile data terminals deployed and are having trouble tracking our units when they are parked under cover in their bays. Our public safety software that manages our dispatch of units calculates the closest unit to the reported 911 call and is often not dispatching units that are available and close because their location is not being reported. I understand that these devices use a GOBI 5000 chip and that they support Assisted GPS. Can anyone tell me if the assisted gps using tower triangulation is enabled by default or is this something we could enable or adjust to help resolve this issue and maintain a fix. When leaving the bay we usually have a gps fix within 15 seconds or so but somehow we need to maintain a gps fix or even continue reporting the last known location even if we cant see the satellites. Any ideas? Thanks in advance for any advice or help you can offer.
Hi just curious if you ever found a fix for this issue?
You seem to have a misunderstanding what A-GPS means.
A cold started GPS unit needs two different data tables to be able to work - the ephemeris data and the almanac. Updates for these tables are interleaved with the regular navigation data. As GPS data rate is really low (50 bit/s), it takes 12.5 minutes do receive the almanac, longer if you had reception problems during this time span.
Fortunately, the almanac is valid for quite some time (up to 180 days). The ephemeris data is repeated every 30 seconds, but specific for each satellite.
Assisted GPS works around this long start times by downloading the tables not from the satellite, but using a regular mobile data connection. It helps with startup times, but still requires satellite visibility.
Assisted GPS does not use triangulation from cell towers. There are other services like location.services.mozilla.com/ which use Location Area Codes (LAC) resp. Traffic Area Codes (TAC, for LTE) and Cell IDs and sometimes also consider RX power to guess an approximate position. For GSM, the Sierra modules report also neighbor cells (AT!GSMINFO), for UMTS and LTE there is only the currently used cell, thus accuracy is very limited, from a few 100 meters to several kilometers (radius).