Position Reporting Technologies

All current position reporting systems rely is some way or another on the GPS constellation. A groups of very time stable satellites drift through space sending signals to receivers. The receiver collects data from the GPS signal, around 1575 MHz and can work out where it is within a reasonable range. DGPS or differential GPS can be used to issue corrections to GPS receivers. Most GPS receivers output NMEA sentences. For example:-

$GPRMC,014916,A,2734.4582,S,15306.2207,E,000.0,000.0,110807,011.2,E*62
$GPGGA,014916,2734.4582,S,15306.2207,E,1,05,,,M,39.2,M,,*56
$GPGSA,A,2,,,07,,,,,21,24,,29,30,,,*17
$GPGSV,3,2,12,12,24,006,00,16,02,216,00,18,28,330,34,21,42,250,00*7E
$PGRME,,M,,M,,M*00
$PGRMM,WGS 84*06

APRS or Automatic Position Reporting System is used by Amateur radio operators around the world. VHF is prodominate, as well as HF for long range tracking. APRS is a system designed by Bob Bruninga in the USA. It uses 1200 and 300 baud packet radio tones delivered over standard radio voice channels. It has the ability to send positions, objects ane even messages. Several good mapping solutions exist for APRS. The main program is UI-VIEW for windows and XASTIR for LINUX / MAC. APRS employs compressed and uncompressed position reporting. APRS world wide data servers also exist providing streams of data to see other APRS users.

ADS-B or Automatic Dependant Surveillance Broadcast is a system used my aircraft to send their current position, altitude, flight id as derived from on board navigation systems. ADS-B is a high rate (1Mb/s) data transmission on 1090 MHz. This frequency is also shared with classical SSR transponder replies and TCAS. ADS-B transmissions can be received by other Aircraft or Ground stations. ADS-B transmissions are random and at a rate of twice per second. This is a faster rate than say SSR which has an update rate of 4 seconds. ADS-B is gaining momentum world wide.

Multi-lateration is a system of ground based receivers that pick up signals on 1090 MHz from Aircraft and other objects such as trucks with 1090 MHz transmitters. The system relies on very accurately synchronised reciever stations and measures the time of arrival at each station. A central server then works out where the object is and displays it on a screen. A larger version exists called WAM or Wide Area Multi-lat.

Flarm is a system developed for gliders. It employs units mounted in the glider, which picks up GPS signals, works out where it is and transmitts to other FLARM units. The FLARM units can then work out where other gliders are and warn of possible issues. FLARM in Australia works on a licence free band around 900 MHz and exibits ranges of a few km's. Work is being done to display flarm positions on a ground based mapping system. FLARM has been accepted as a standard by NMEA and some FLARM data sentences have been addes to the NMEA spec.

I have been working on a tracking system using GPS, PIC chips and LIPD easy radio transmitters and receivers. During a recent glider field trial we got around 3/4 kms reliably and 7/8 kms in a burst. The module sends data at 19k2 on 434 MHz at a one second rate. FLARM also uses licence free modules. Please have a look at 434 MHz GPS Tracker for more information.

No position reporting system would be complete without some way fo shoing where things are. Google earth has stirred up a lot of interest recently. It has the ability to open data on the fly and show objects on maps. I did some work with APRS then ADS-B and then finally ADS-B in three dimensions. There are of course heaps of mapping programs around that will accept data in. APRS mapping programs are a good example. You can also get back to basics and show other object poisitions on a GPS screen using the NMEA $GPWPL sentence. PDA devices are also popular as display tools.

AIS is for large ships. They use a time slotting system to stop clashes. There are now some good display programs coming out for AIS. It operates around the VHF band. Again using audio channel data transfer.

With the wide spread availability of GSM networks these days it makes sense to use the existing mobile phone devices for position reporting. I started out doing some dial up and then SMS sending of positions from a GPS. You can get hold of PDA's with Mobile phones built in and send positions to a central server using GPRS.