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TDC GPS Seminar

Basingstoke 13th April 1999

Notes relevant to wearable use by Cliff Randell. Full documentation available in 1.15 MVB

Presentation given by Simon Taylor and Andy Barnes. David Taylor also a useful contact for RadioBeacon dGPS.

Rockwell now renamed Conexant - the market leader in GPS receivers. First GPS satellites launched in '80s, fully operational since 1995. Only 3 satellite fixes needed to find a 'reasonable' solution (altitude restricted to between -20,000' and 60,000'!).

Receiver uses GaAs front end to give good noise characteristics enabling use of passive antennae.

Satellite transmission power occasionally increased e.g.during Gulf War. Gives improved accuracy. Normal accuracy is within 100m 95% of the time; and within 300m 99.9% of the time - in practice within 30m 'most' of the time. Height accuracy 1.5x worse than Lat/Lon

UTC (GMT) accurate within 1microsec n.b. indicator led locked to UTC.

Software available from TDC to convert Lat/Lon to Ordinance Survey GB co-ords.

Mapping products are available - e.g. Autoroute Express.

Products range now includes low power version (3.3v, 140mA); miniature version (2 x 15mm sq.+memory - min quantity 1,000); and programmable flash version.   PCMCIA version problematic so discontinued. Pluto is low cost solution but needs extra power - not recommended.

TimeToFirstFix <20secs when initialised with date, time, position and ephemeris(valid for 4 hours). Date is transmitted by the satellites every 20 secs.

TTFF <40secs when warm - no ephemeris.

TTFF ~2mins when cold - initialised with date/time only.

TTFF ~20mins if frozen - no initialisation. 

If implementing software best to use Binary messaging NOT NMEA - this gives more control options and can improve TTFF.

Jupiter pin out and conrol switches explained - see hard copy.

NMEA messages preceded by $GP - General Purpose; or $PR - Proprietary Rockwell.

$GPRMC includes magnetic variation (in degrees).

$PRWIILOG enables message rate to be set.

Not currently relevant to wearable use.

Jupiter should run for 5 hours on fully charged batteries.

If not using battery back up - and using capacitor instead, CN5 should be shorted (Capacitor holds data for 2 to 4 hours).

SW1-5 sets cold start. Switch off after first fix (ever) as it stores data about the state of the crystals (they age and need to be corrected).

Conexant Dev. Kit also available - but TDC's is better!

TTFF ~90secs from cold with only date/time info - all switches off except active antenna selection (on ~12m of cable).

Demo used Binary controls to demonstrate full range of facilities - useful if we get deep into GPS. NMEA sufficient for our current purposes. 

Mostly about trains, boats and 'planes, so not a lot in this section for us - combined GPS/GSM antenna interesting though - perhaps for helmet mounting!

A GPS covert antenna is available -  8cm sq, ~1mm thick, 1oz - a bit large for shoulder mounting but may be useful somewhere else (on head again?).

Siemens M20T interesting - black box GSM modem - a bit bulky though, modern mobiles have better ergonomics.

Mapping products described, and a car was tracked using GPS/Autoroute while it drove five miles up the A30 - it worked.

Three techniques available - Relative - using your own base station as a reference; Differential - using Classic(Focus)FM in UK, or Radiobeacon worldwide; and Post Processing - store data and analyse later.

  1. Relative is good for aircraft landing on carriers and gives accuracy better than 1m.
  1. Differential is most commonly used. Accuracy typically 1-5m (subscription needed for FocusFM acccuracy better than 10m) but differential data degrades by 1cm for every km away from base station and is only valid for a maximum of 60 secs.

The dGPS RadioBeacon operates at 300kHz and is very susceptible to noise.  The OEM card is easily connected to Jupiter - RS232 serial. It is a two channel device monitoing two transmitters simultaneously though only one is needed. An active antenna (1.5"dia x 15") is essential, requiring an adaptor lead to connect to the board.

There is spare 5v power capability on the Jupiter serial board which should be sufficient to power either dGPS solution.

  1. Post Processing good for surveying - accuracy to between 1 and 2 metres.

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