I’ve submitted this ticket to Sierra, but I’m wondering if anyone within the forum community has any experience with this.
Internal gpsOne chip appears to be losing clock data when module is powered down. The loss of clock data is detrimental to hot start performance.
Our hardware configuration shuts down the SL module between GPS tracking sessions. Shutdown occurs by applying signal the external ON/OFF pin. Power remains applied to the module while turned off. When the next tracking session begins, module is powered back on via ON/OFF pin and a GPS hot start is initiated.
If elapsed time between tracking sessions is less than 1 hour, hot start performance is acceptable. If elapsed time is greater than 2 hrs hot start performance is no better than cold start and on occasion can be much worse (GPS lock-up occurs and fix is never achieved).
Some satellite data in the gpsOne system is retained over a module shutdown, but UTC time is lost.
Here are the first NMEA sentences of a hot start session for a device that was A. left on since the last GPS session and B. turned off since the last session. Both tests were run with a fresh GPS fix less than 1 minute before.
A. continuous power since last fix
B. module turned off since last fix
In A the UTC time is retained and allows for a very quick hot start fix (GGA sentence). In B satellite data is retained (GSV sentences) but UTC is lost. Hot start still works well for B only if the last session was very recent and hence the satellite data still relevant.
I’ve attempted to fix this two ways:
- Synching the RTC in the SL module prior to turning GPS on (AT+CCLK)
- Injecting the gpsOneXTRA engine with current UTC time (AT!GPSXTRATIME)
Neither method seems to help in any way.
Questions for Sierra Support
Why does the gpsOne system lose clock time but retains satellite info over shutdown?
Is there any hardware configuration for the SL module that will allow the QSC6270 chip (gpsOne) to retain the clock over a power cycle?
Typically GPS chips have a voltage input for maintaining the clock between sessions - hopefully this is the case for the QSC6270 and that Sierra made this pin available on the SL module.