What is GPS?

Date

Developed by the US Department of Defense, the GPS (Global Positioning System) is a satellite-based navigation system consisting of a network of 24 orbiting satellites, including 3 standby satellites. After the US Department of Defense opened the Coarse Acquisition Code standard measurement to civilians, a target (GPS receiver) can be located within 10m on average. Therefore, it is an accurate radio positioning system.

A GPS consists of 3 major sections: the satellite, signal and ground sections.

Satellite

Currently, the Block II GPS satellite has been completed and manufactured by Rockwell International. It is about 1,900 pounds in weight on the orbit. The length of solar energy receiver board is about 17ft long. The life of the satellite is about 7.5 years. After the 24th satellite was launched to the orbit in 1994, the full constellation of 24 GPS satellites was officially completed. Therefore, there are now 24 GPS satellites orbiting in the space for positioning. They are distributed on 6 segments, i.e. each segment has 4 satellites orbiting at a 55 angle about 10,900 knots (approx. 20,000km) above ground. That is to say, there are at least 4 GPS satellites orbiting above us anytime anywhere, including the poles.

Signal

The 2 sets of random codes generated by the GPS satellites are the C/A (Coarse Acquisition) and P (Precision) codes. While the C/A code is opened for civilian uses, its accuracy has been reduced intentionally. As the P code is reserved by the US Department of Defense for military purposes, it is much more accurate than the C/A code. Therefore, it is enciphered and difficult for civilians to decipher. In general, a GPS satellite sends carriers in 2 frequencies: L1 (Link 1) at 1575.42 MHz and L2 (Link 2) carrier at 1227.60MHz.

Ground

The GPS ground facility mainly includes control/monitoring stations and user receivers.

Control/monitoring stations

These include one master control station, 5 monitoring stations (located in Hawaii, Ascension Island, Diego Garcia, Kwajalein Island and Colorado State), and 3 ground control stations. A monitoring station tracks the orbiting position, time, meteorological data and ionosphere data, and calculates the smoothed data of every 15 minutes with data collected within 15 seconds. After receiving these data, the master control station will calculate the ephemeris of satellites, the clock modification and the ionosphere correction factor, and convert these data into navigation signals, in order to maintain the accuracy and normal operation of the GPS satellite system. The US Department of Defense will take charge of this work, while users will not need to care about this because they will never know how it works and how it is done.

User receivers

It is a satellite signal receiver containing different positioning capacities according to different purposes. Its basic function is to receive L1 carriers and filter the C/A code for the lowest level of virtual distance positioning. It is the commonest model for vehicle GPS systems. One should be noted that a GPS satellite generates 2 types of carriers to carry all codes and messages. The C/A code is only set to the L1 carrier, while the P code is found in both L1 and L2 carriers and identified as P1 and P2 codes. Yet, the US military opens for civilian uses only the C/A code. General user receivers can be positioned on an average of 15ft or better via the DGPS. Yet, users must pay for the DGPS, so they must consider the cost and if they need the accuracy. If you are using a GPS for locating a destination when you are driving, all you need is a GPS and an e-map, and the DGPS will be too accurate for that.