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RGPSM002

GPS Receiver

5.5

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 RGPSM002, the GPS data structure, including almanac, ephemeris and last position fix, is copied into a specific

sector of the on-board Flash memory, that’s the back-up Flash sector, or BBFlash. The data are stored the first time the

almanac is complete and up-to-date, then every M minutes, where M is defined in Default Parameters Table *, parameter

#37. 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 are not maintained if the main supply is

switched OFF (or in case of a power failure). In this case, upon power up, these data are uploaded from the Flash back-

up memory into the GPS data RAM. Provided these data are valid – 2 months for the Almanac, 2 hours for ephemeris –

the TTFF will be shorter than Cold Start TTFF, since the receiver will be in Warm or Hot start condition.

(*) see the Default Settings section below

With the Flash technology embedded technology, it takes approx. 0.5 seconds to erase and update the Flash sectors

where the GPS data structure is stored.

5.6

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 seconds.

If the main power supply VCC is turned OFF and provided the VRTCBK supply is available, the RTC operates and keep

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 available.

5.7

SPLIT SEARCH MODE

This feature is useful when the initial position is incorrect, for example when the receiver is powered down, put on a

plane, flown 10,000 km, and then warm-started. If the user does not activate the DELPOSN I/O or does not sent the

proprietary PXEMSTR,SSDI NMEA command, the receiver will start to search for the satellites it believes are above it

(warm start condition). However, after approximately 5 minutes, it will free up some of its channels to search for other

satellites in the constellation. When it finds one it will free up more channels and recover from a Warm start with an

inaccurate initial position.

5.8

HARDWARE RESET AND SYSTEM WATCHDOG

The receiver has an embedded microcontroller voltage supply supervisor. This device will automatically generate a

hardware reset if the voltage supply goes under the specified value. In this case, the receiver will restart from a known

and stable state in case of a power shortage on the main supply while in operation.

In addition, the receiver firmware includes a system watchdog reset procedure. Should a system exception occurs, the

receiver will execute a software reset to restart operation in a safe mode. In this case the receiver will not have position

fixes for 15 seconds on average.