GPS (Global Positioning System) is a satellite-based technology. It allows users to determine positions at points in time by utilizing navigational signals broadcast by multiple satellites, known as a satellite constellation.
Currently, this constellation consists of 32 active satellites, which orbit the earth at an altitude of approximately 11,500 miles. Each satellite completes an orbit every 12 hours. The constellation includes some in-orbit spare satellites which can be activated to replace any satellites which may fail.
The GPS system (also called NAVSTAR) was developed by the United States and is owned and operated by the United States Department of Defense. The initial satellites were launched in 1978, and by 1994 a full constellation of 24 satellites was available. Satellites typically last 8 to 12 years and new satellites are periodically launched to replace older satellites. Enhancements have been made over the years and currently there are a number of new technologies, including new signals that are being planned.
In the current GPS satellite constellation, each satellite broadcasts two different navigation signals, known as L1 and L2. The L1 signal is broadcast on a frequency of 1575.42 MHz, and the L2 signal is broadcast on a frequency of 1227.60 MHz. The L2 signal is encrypted andis available only to authorized users – typically military applications. The L2 signal provides a highly reliable and accurate time and location solution. The L1 signal is not encrypted and is available to users worldwide, 24 hours a day,without charge or subscription. GPS navigation signals are broadcast with circular polarization. See Figure 1.
For a given signal (L1 or L2), all satellites broadcast on the same frequency. The signals are differentiated by the different codes that are
transmitted by each satellite. This is called “code domain multiple access” or CDMA.
Two different code rates are used. The first code is the Coarse Acquisition code, or C/A code, which has a code rate of 1.023 MHz and it repeats every 1 millisecond. The second code rate is 10.23 MHz, it is called the Precise, or P code and it repeats every week. Typically the P code is encrypted to form a code called P(Y). Each satellite is assigned a unique code sequence for the C/A and P codes, respectively. These sequences are identified by a number called the PRN (pseudo-random) ID.
The navigation signals convey information to the GPS receivers including precise timing information as well as system status, orbit descriptions of each satellite, and satellite health information. Using this information a receiver can determine the distance to each satellite and, using a triangulation approach, a position and time can be determined.
Navigation signals are broadcast with a fairly low power and appear at a minimum level of about -155 dBm at sea level. Signal levels may be even lower inside of buildings or under tree cover. These signal levels are extremely low and require significant amplification and baseband processing to recover.
Although GPS receivers have been available for many years, initial implementations were large, expensive, and consumed considerable power.
Because of this, the GPS application was limited to high-end commercial and military applications.
In recent years, the cost for GPS receivers has declined significantly for commercial technology. The result is that GPS receiver technology has recently become increasingly important in consumer products such as handheld receivers, automotive receivers, mobile phones, and other tracking devices.