Satellite navigation systems take their location cues from 30 GPS satellites that circle the Earth twice a day transmitting status, date and time, and orbital information. Soon there will be around 100 satellites to lock on to as GPS is joined by global constellations from Europe (Galileo), Russia (GLONASS), and China (Compass).
GPS wasn't built to help us find our way to the shops - it was a Cold War project funded by the US Department of Defense to ensure that nuclear submarines could surface and target their missiles accurately. There are strategic rumblings about the new satellite constellations too, but the current consensus is that civilians have most to gain from more accurate and reliable location and tracking applications. That's if receiver designers can get the power consumption under control.
Russia's GLONASS system used to be famous for its satellites failing faster than they were launched, but since last month it has had 24 functioning satellites in orbit. Meanwhile, Europe's much-delayed Galileo system will have 14 satellites operating by 2014, according to the European Commission, with the full 30 available by 2017. The US GPS system is being modernised to become GPS III by 2013, with additional navigation signals for both civilian and military use. Information about China's Compass system is sketchier - it was going to be a regional system but is now understood to be global.
'All this activity is great news because whatever the application, there will potentially be multiple constellations to get a position fix from, which will help with signal integrity in safety-critical environments such as maritime, aviation or rail, and accuracy for mobile phone users in urban areas,' says Andrew Sage, director of Helios, a consultancy specialising in satellite navigation.
A GPS receiver should be able to 'see' at least four GPS satellites anytime, anywhere on the globe and establish three position coordinates (latitude, longitude, and altitude). But in city streets hemmed in by tall buildings, a receiver is unlikely to be able detect more than two satellites and the signals will often have bounced off structures.
'For the average pedestrian, the position fix can be a long way out and very unpredictable,' says Sage. 'Most users don't see that today because GPS receivers match us to maps and smooth the errors out. But if you are walking around a city and not on a road in a car, multi-path reflections are a problem.'
The more satellites visible from within these 'urban canyons', the easier it is to carry out consistency checks on the received signals. 'Even when you can't isolate the multipath-contaminated signals, the more signals you have, the more your errors average out,' says Dr Paul Groves, lecturer in global navigation satellite systems (GNSS), navigation and location technology at UCL.
Better GNSS integrity would enable new applications, such as road-user charging, enforcing bail conditions and pay-as-you-drive insurance. 'Clearly, if position information might be used as legal evidence, it has to be reliable,' says Groves.
The delayed arrival of Galileo and the resurrection of GLONASS have complicated matters for receiver makers. Galileo was designed to offer the simplest possible upgrade path from GPS to a dual-constellation system. Agreements were made to put the carrier frequencies of the main open services in the same part of the spectrum as GPS, at around 1575MHz, so receivers could share the same radio, analogue components and antenna. Both systems also send their signals using a spread-spectrum code-division multiple-access (CDMA) approach. GLONASS uses a frequency-division multiple-access coding technique (FDMA) and a main open-service carrier frequency of 1602.2MHz.
