GPS

Global Positioning System

and how to use it

• The System and theory

The US military developed the system for guiding missiles and troops. After a civilian aircraft got lost and was shot down the system was seen to have civilian uses. Now personal and embedded devices are common and inexpensive.

A set of about 30 satellites are in orbit carrying accurate atomic clocks. These are corrected for relativistic effects and are checked and updated by ground stations. The satellites also know exactly where they are at any instant - also updated by ground stations. They can also use GPS to calculate their positions from other satellites.

Each satellite broadcasts a message saying where it is and what the time is. If a receiver knows the time, it can calculate its distance from the satellite by the time it takes for the message to arrive. This gives it a sphere around the satellite on which it is located. The signal from a second satellite gives a second sphere. The intersection of these two reduces the possible locations to a circle. A third, if the geometry is suitable, reduces this to a point. Unfortunately the cost of atomic clocks is prohibitive, so the receiver has a secondary clock which is synchronised with the satellites using the signal from a fourth one. The suitable geometry is arranged by having the satellites on orbits that give a favourable geometry. And the receiver can use a fifth or more satellites if they are available to refine the calculation. The mathematics are horrendous (4D complex Newton-Raphson) - see the detailed Wikipedia article.

In practice there are several errors that can be estimated and corrected - like the slowing of the radio signal in the ionosphere. Some errors can't be readily corrected - such as those caused by the wavelength of the radio carrier. Many errors can be corrected by WAAS technology which measures the location of a known nearby base station by GPS and broadcasts corrections - although this is not yet fully implemented (not available here).

In general the location accuracy is about 10m and possibly better. In practice my results plotted in Google Earth^® or Google Maps^® are quite accurate enough to place my position correctly within 5m - e.g. at road intersections or geographic features. This is more than adequate for my needs.

• Practical considerations

The above accuracy assumes good conditions for reception - clear line-of-sight to at least 4 of the satellite and no radio interference.

In urban areas the radio interference is beyond your control. And in high-rise zones the satellites are easily obscured by buildings. This has three main effects reducing accuracy.

Multipath reception and reflections.

A satellite signal may be received by two routes - direct and reflected. This can confuse the receiver, but better receivers can reject the slower and weaker reflected signal. A bigger problem is when the direct signal is not received (obstructed by a building) but the reflected one is. This results in the position being calculated as further from this satellite than it really is, causing an error in the calculated position. A weird effect can be observed as the satellites move, one coming out of shadow and another going into a reflected path - the position of a stationary receiver seems to jump about.

Restricted view

In a building with a view only to one side through a window all the satellites are on one side. In this case the 'suitable geometry' is not provided as all the satellites used are not distributed around the receiver. Thus the calculation has an inbuilt bias. Again this results in the apparent position jumping about as satellites move into and out of view.

Too few satellites in view

With less than 4 (ideally more) satellites in view the equations are unsolvable. However the receiver can guess which possible solutions are correct if it had previously made an accurate fix. So if you were in Sydney 5 seconds ago, you are probably not in New York now. If the receiver had previously synchronised its clock to GPS time it can assume that it hasn't drifted too much, so can dispense with the time calculation. These measures help give you a relatively continuous location readout, but at reduced accuracy.

These problems relate to the 'cold start' and 'warm start' times quoted for receivers. At a cold start the receiver has no idea where or when it is, so it must locate 4 or more satellites and compute its clock and location. If your receiver reports a location during this time it may be several hundred meters out! Most modern devices do not report a position until the lock is established. At a warm start it knows roughly its location and time, so can do a quickie calculation with fewer satellites.

The above are 'hardware' errors - errors in the reported position. Other errors ('software') relate to the interpretation of the data, which we look at later.

• Information provided

You probably all expect latitude and longitude. The decoders also provide altitude (at lower accuracy) and a highly precise time. Computed data (heading and speed) are provided. The number, identity and signal strength of the satellites in view is also sent out. Most applications only use latitude and longitude.

NOTE - The latitude and longitude are based on the WGS84 datum, Australia used the AGD66 or AGD84 'geocentric' system (didn't Copernicus shoot that one down centuries ago?) but now has been moved by 200m to use GDA94 which is close enough for GPS. So if you are using an old map you could be out by 200m. Check the setting on your GPS and the datum of your map before your trip if using a paper map. See http://www.ga.gov.au/geodesy/datums/cosys.jsp. Google Earth/Maps uses WGS84. GDA94 was where Australia was on 1 January 1994, but moves 7cm NE annually! So now things are about 1m out, within the GPS error.

• Devices

Devices available range from the 'chip' to embedded devices like the 'cruise missile'. The GlobalSat site gives a good selection of devices available to civilians. Expansys sells a good selection online.

• The chip or module is for embedding in another application, and simply sends the location data as a serial data stream to the rest of the circuitry. Several chips are available, the 'SiRF Start III ' is the best currently, so look at the device specification to check.

• A packaged chip, with antenna and interface circuit is called a 'GPS Mouse' and is designed to connect to a laptop computer with mapping (or other) application software. (e.g. OziExplorer). Initially by a serial port, but now with USB or Bluetooth serial emulation.

• A mouse with built-in memory is a 'GPS Datalogger'. It is used for recording your path for later analysis. Also works as a mouse.

• Handhold devices with a small LCD screen to display location, waypoints etc. Used for bushwalking, locating your fishing spot etc.

• 'SatNav' devices with colour screen and included mapping software and a nice lady telling you where to go.

• Embedded in consumer devices - mophos, cameras, car security.

• Uses

  • General navigation - e.g. outdoor recreation like bushwalking, fishing, confluence search

  • SatNav - more below

  • Safety - emergency beacons

  • Security - car theft systems, mopho locators for your kids

  • Monitoring - where did your sales rep REALLY go

  • Embedded - cameras for recording location of photos

  • Military - I could tell you more, but the they would have to kill me

• SatNav

This is the most commonly known application. A GPS receiver is combined with mapping software and maps in a dashboard mounted system. It shows your location on a map, can locate petrol stations, hotels and your destination. A nice lady then tells you where to go.

The usefulness of this system is strongly linked to the map database and software included. The 'hardware' errors above are of the order of 10m, usually 3-5m. So your position will be quite accurate. BUT if the map is missing a road, or has a road that isn't real you can get lost. Also the routes suggested are for (usually) normal cars, not semi-trailers. There are stories of truck drivers getting stuck in small lanes and cars getting stuck on roads suitable for 4WDs only by blindly following the directions of the nice lady.

The mapping software and maps are the major part of the price of these systems - an annual update costs about 50% of the initial price, so electronic specifications are secondary to those of the software. "Choice" did a review a few months ago. 'TomTom' came out on top.

Another consideration is ease of use. You don't want to spend 15 minutes setting the thing up to save you 5 minutes travelling. Also, please, pull over to the side to set it up, not while driving.

• PC Software

Your device should come with software to down/up-load data to the computer. Most manufacturers have their own incompatible data system. A common format is GPX, an xml data format. GPSBabel will read and convert just about any format, and interface to most devices to download the data. Winders and Linux versions free.

OziExplorer ($) is a mapping application for the PC.