Shorten TTFF by sending time data to NEO-M8M without RTC

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We are using the AssistNow Autonomous feature on a EVA-M8M without external flash and without an RTC (if possible). Every half an hour we switch on the M8M to get a position and then switch off again (V_BCKP is always high). With an RTC we get very short TTFF of around 5s. Without RTC, TTFF is around 15s. In case of no RTC, can I get shorter TTFF by sending the time with UBX-NAV-TIMEUTC? What else can I do to shorten TTFF and get more precise positions?
asked Oct 12, 2016 by gugc

1 Answer

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Possible solutions from most effective to least effective. If battery preservation is a concern, compute the tradeoffs with power consumption/duty cycle for each alternative - your present implementation may be the best approach:

Note that AssistNow Autonomous helps TTFF only for those satellites that will appear 24 hours from now. With a half-hour update period, subsequent short TTFFs use ephemeris data that was previously downloaded and will expire 4 hours after it was first broadcast by that satellite requiring a fresh download.

a) Provide an RTC and allow enough "on" time for the RTC to be calibrated. (Calibrated RTC maintains approximate frame synch with SVs.  In self-managed On/off Power Save mode, the accurate RTC can schedule turn-on times just in advance of the next 30-second ephemeris cycle to minimize on time).  Otherwise allow enough time every half-hour or so for ephemerides to be updated on visible satellites and downloaded for newly-risen satellites (just as is done in self-managed power save mode operation except you may not have the benefit of knowing when the next ephemeris cycle will start).

b) Provide current time hardware strobe and message accurate to within a few hundred microseconds of UTC "truth", (last known position should still be valid in BBRAM). Allow enough time every half-hour or so for ephemerides to be updated on current satellites and downloaded for newly-risen satellites.

c) Provide current time message accurate from "truth" to within 2 seconds (last known position should still be valid in BBRAM). Allow enough time every half-hour or so for ephemerides to be updated on current satellites and downloaded for newly-risen satellites.
answered Oct 12, 2016 by grampy Senior Principal Expert