GPSM001 Datasheet, PDF (16/20 Pages) rfsolutions.ltd – OEM GPS Receiver Module

OEM GPS Receiver Module
GPSM001
GPS DATA BACK-UP
The almanac data is the information transmitted by each satellite on the orbits and state (health) of the entire
constellation. The ephemeris is a list of accurate positions or locations of celestial objects as a function of time. So, the
availability of almanac and ephemeris data, in addition to time and approximate position, allows the GPS receiver to
rapidly acquire satellites as soon as it is turned on. There are 3 possible start conditions when the receiver is turned on:
a) the Cold Start, that is the start-up sequence of the receiver when no initialization data is available; b) the Warm Start,
that is the start-up sequence of the receiver when the last position, the time and the almanac information are available;
and c) the Hot Start, that is the start-up sequence of the receiver when the ephemeris, the last position, the time and the
almanac information are available.
In the XE1610-OEMPVT design the GPS data structure, including almanac, ephemeris and last position fix, is copied
into the on-board Flash memory. The data is stored the first time the almanac is complete and up-to-date, then every D
days, where D is defined in the Default Parameters Table *, parameter #13. Alternatively, the Flash can be programmed
with valid information during the manufacturing process. This is to avoid downloading it from satellites, which takes
approx. 12.5 minutes. Then, as long as the main power supply remains turned On, the GPS data structure is kept in
RAM. However, data in RAM is not maintained if the main supply is switched Off (or in case of a power failure). In this
case, upon power up, this data is uploaded from the Flash back-up memory into the GPS data RAM. Provided this data
is valid – 6 months for the Almanac, 4 hours for ephemeris – the TTFF will be shorter than Cold Start TTFF, since the
receiver will be in a Warm or Hot start condition.
(*) see the Default Settings section below
Note: with the current Flash technology embedded in this design it takes approx. 20 seconds to erase and update the
Flash sectors where the GPS data structure is stored.
REAL TIME CLOCK
The receiver board has a hardware Real Time Clock chip that operates independently from the MCU and the GPS
function. When the GPS receiver is active and as soon as the GPS time becomes available the RTC is synchronized
with GPS time. Then, as long as GPS time is available, the RTC is synchronized every 60 minutes.
If the main power supply VCC is turned OFF and provided the VRTCBK supply is available, the RTC operates and
keeps the RTC information up to date. By doing so, when both the main VCC supply and the GPS receiver are turned
ON again the time information will be immediately available.
DS031-2 Jan ‘04
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