Satellite navigation systems use radio time signals transmitted by satellites to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.
History & Theory
An early predecessor were the ground based Loran and Omega systems, which used terrestrial longwave radio transmitters instead of satellites. These systems broadcast a radio pulse from a known "master" location, followed by repeated pulses from a number of "slave" stations. The delay between the reception and sending of the signal at the slaves was carefully controlled, allowing the receivers to compare the delay between reception and the delay between sending. From this the distance to each of the slaves could be determined, providing a fix.
The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites traveled on a well-known paths and broadcast on a well known frequency. The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position.
Modern systems are more direct. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.
Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Civil and military uses
The original motivation for satellite navigation was for military applications. Satellite navigation allows for hitherto impossible precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons. (See smart bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war.
Satellite navigation using a laptop
and a GPS
In these ways, satellite navigation can be regarded as a force multiplier. In particular, the ability to reduce unintended casualties has particular advantages for wars being fought by democracies, where public relations is an important aspect of warfare. For these reasons, a satellite navigation system is an essential asset for any aspiring military power.
Satellite navigation systems have a wide variety of civilian uses:
Note that the ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires. Thus, as satellite navigation becomes an essential service, countries without their own satellite navigation systems effectively become client states of those which supply these services.
The same applies to the use of smart bombs: the operator of a satellite navigation system can effectively degrade the performance of smart bombs being used by other states using its satellite navigation system to that of gravity bombs, or even offset them from their targets in such a way as to render them useless.
Current and proposed satellite navigation systems
The best known satellite navigation system is the United States' Global Positioning System (GPS), and as of 2002 the GPS is the only fully functional satellite navigation system. This consists of 24 to 27 satellites that orbit in six different planes. The exact number of satellites varies as satellites are replenished when older ones are retired. They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees. The satellites are tracked by a world-wide network of monitor stations. The tracking data is sent to a master control station that continuously updates position and clock estimates for each satellite. The updated data is then uplinked to the satellitevia one of several ground antennas.
When it was first deployed, GPS included a "feature" called Selective Availability (or SA) that introduced intentional errors of up to a hundred meters into the publicly available navigation signals. Additional accuracy was available in the signal, but in an encrypted form that was only legally readable by military units. Using the encoded signal accuracies of about 10m horizontally and 30m vertically are possible. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern US area might read 30m off, but 30m off everywhere and in the same direction.
In order to improve the usefulness of GPS for civilian navigation, fixed GPS receivers started broadcasting a signal "fixing" the inaccuracy. Known as differential GPS (or dGPS), the signals could be received on an FM receiver and plugged into many civilian GPS receivers, at which point they too gained 10m accuracy. However this signal was available only at short ranges, making it useless for enemies guiding long range missiles into the United States.
In the 1990s the FAA started pressuring the military to turn off SA permanently. This would save the FAA millions of dollars every year in maintenance and manning of their own, much less accurate, radio navigation systems. The military resisted for most of the 1990s, but SA was eventually turned off in 2000.
The Russian counterpart to GPS is called GLONASS and was used as a backup by some commercial GPS receivers. However the GLONASS constellation is currently (as of 2001) in very poor repair, rendering it almost useless as a navigation aid.
The European Union and European Space Agency have agreed (March 2002) to introduce their own alternative to GPS, called Galileo, pending a review in 2003. At a cost of about $ 2.5 billion (2.5×109 dollars) the required satellites will be launched between 2006 and 2008 and the system will be working, under civilian control, from 2008. Galileo is expected to be compatible to the next-generation GPS system that will be operational by 2012. The receivers will be able to combine the signals from 30 Galileo and 28 GPS satellites to greatly increase the accuracy.
As a precursor to Galileo, the European Space Agency, the European Commission and EUROCONTROL are developing the European geostationary navigation overlay system (EGNOS). This is intended to supplement the GPS and GLONASS systems by reporting on the reliability and accuracy of the signals, allowing position to be determined to within 5 metres. It will consist of three geostationary satellites and a network of ground stations and is intended to be operational in 2004.
China has started to launch a series of satellites intended to form a system called the Beidou navigation system.
Topics to be covered
- differential satellite navigation
- phase-counting differential satellite navigation
- ESA information on EGNOS (http://www.esa.int/export/esaSA/GGG63950NDC_navigation_0.html)
- Information on the Beidou system (http://www.astronautix.com/craft/beidou.htm)