The Global Positioning System (GPS)

The Global Positioning System (GPS) is a space-based navigational and positioning system. The space segment of the system is a network of up to 24 orbiting satellites that continuously transmit radio signals that allow the user segment – a GPS receiver anywhere on earth – to calculate its own three-dimensional position, velocity, and time. A GPS satellite is also known as a space vehicle (SV). The control segment of the system involves a Master Control facility in Colorado that measures signals from the SVs which are then incorporated into orbital models that compute precise orbital data clock corrections for each satellite. The first and current GPS system made available for civilian use is called NAVigation System with Timing And Ranging (NAVSTAR), and is managed by the U.S. Department of Defense. Its Russian counterpart is the GLObal NAvigation Satellite System (GLONASS).

To compute location, the GPS receiver needs information on a) where the satellites are (satellite location), and b) how far away each satellite is from the receiver (satellite distance). The GPS receiver stores in memory the unique radio signal pattern of each satellite along with almanac data describing the orbit of all GPS satellites, satellite clock offsets, and atmospheric delay parameters. Every 30 seconds, each individual satellite also transmits ephemeris data (obtained from the Master Control facility) which includes more precise satellite position information and clock settings. Ephemeris data is more accurate than the almanac data but is applicable over a shorter four to six hour time frame.

GPS is a distance and ranging system based on radio signal travel time, which is converted by the GPS receiver into distance by solving the velocity equation (V = D/T, where V is the speed of light) for distance:

Distance (between GPS satellite and receiver) = radio signal Velocity * travel Time

Thus, the GPS receiver measures signal transmission time, which is converted into distance by the above equation. The intersection of the radio signal distances of at least three satellites are used to compute GPS receiver location. This calculation – called trilateration – uses the laws of trigonometry, and is conceptually similar to triangulation, but uses distances instead of angles.

The steps in position finding can be summarized as a) GPS receiver obtains the simultaneous signal of three satellites (and a fourth for clock synchronization and altitude calculation), b) the receiver then matches each signal to a unique satellite and its corresponding orbital position, c) using the velocity equation, the receiver computes the distance of each satellite, and d) using trilateration, it computes the position of each satellite relative to the GPS receiver, and finally e) it calculates the receiver’s geographic location (e.g. latitude and longitude).

GPS Sources of Error

The accuracy of the GPS measurement varies over time and location. Noise errors are associated with the radio signal itself, which results in decreased accuracy of about 1 meter, and within the receiver, which results in decreased accuracy of up to 10 meters. Bias errors in the past were predominantly due to Selective Availability (SA), an intentional, random degradation added to the SV signal by the Department of Defense to reduce accuracy by as much as 70 meters. SA is generally no longer employed. Other sources of bias can include errors in ephemeris data (reducing accuracy by 1 to 5 meters), troposphere delays (reducing accuracy by 1 to 30 meters if incorrectly modeled), and unmodeled ionosphere delays (reducing accuracy by up to 30 meters). Also, the internal clock on a GPS receiver is regularly synchronized with the atomic clocks in the SVs and Master Control, but tiny differences represent sources of error of up to 1.5 meters when unsuccessfully corrected by Master Control. Multipath error can reduce accuracy by up to 1 meter. It involves situations where SV signals reach the receiver by more than one path, generally through interference caused by nearby structures or other reflective surfaces.

Noise and bias errors are all influenced by satellite geometry relative to the location of the GPS receiver. If the satellites available are clustered together in a small area, the trilateration method used by GPS receivers to calculate a position is less effective. This GPS ranging error is measured as Dilution of Precision (DOP). The DOP factor is included in ephemeris data and is computed as a statistical estimation expressing the confidence factor of the position solution based on current satellite geometry. DOP values range from 1 to 6; the lower the value, the greater the confidence in the solution. The DOP factor is multiplied by the summed noise and bias errors resulting in total GPS accuracy.

In general, these combined errors amount to an accuracy of 20-30 meters on most recreational GPS receivers when SA is off and no additional corrections are provided. User blunders, the most common of which is not carefully setting and recording the location format (coordinate system, map projection and map datum), can result in errors of up to hundreds of meters.

Methods to Improve GPS Accuracy

The most basic fundamental way to improve accuracy is to time GPS data collection with more favorable SV conditions in your study area through mission planning. Mission planning is possible because periods of GPS signal degradation (including the use of SA), SV status/maintenance, and poor SV configurations for specific times and locations are generally forecast in advance by the Department of Defense. The U.S. Air Force publishes advisories to NAVSTAR users (NANU). Trimble®, a GPS manufacturer, provides a freeware mission planning software called QuickPlan and downloadable daily almanac files.

GPS accuracy can also be improved through differential correction, a process of using the errors measured by a stationary GPS receiver at a known location to improve the measurements of a GPS receiver being used for data collection. The stationary GPS residing at known geographic coordinates is called a base or reference station. Any positioning errors in the base station GPS positioning measurements are called pseudorange errors. A GPS receiver being corrected through the base station is called a roving unit or rover. The pseudorange errors identified at the base station can be used to correct the positioning data of any rover that has access to data from the same configuration of SVs. Differential correction can take place real-time or during post-processing.

There are a number of different strategies for differential correction, each with associated accuracy improvements and costs. Some inexpensive GPS receivers are capable of receiving real-time Wide Area Augmentation System (WAAS) GPS corrections that can reliably improve location accuracy to plus or minus 3 meters. WAAS is being created by the Federal Aviation Administration (FAA) to provide sufficient reliability and accuracy to permit GPS-based instrument approaches in aviation. WAAS is a system of approximately 25 ground-based Wide Area Reference Stations positioned across the U.S. that monitor GPS signals to detect errors. These errors are sent to the WAAS Master Station which generates augmentation messages containing information that allows GPS receivers to remove errors in the GPS signal. The augmentation messages are sent to geostationary communications satellites which broadcast them on a GPS-like signal which can be used by an enabled receiver to supplement the standard GPS calculation of the user’s position. A conceptually similar system called the Maritime Differential Global Positioning Service is useful in coastal areas. It is operated by the U.S. Coast Guard Navigation Center (NAVCEN). It consists of two control centers and over 60 remote sites which broadcast correction signals on marine radiobeacon frequencies to improve the accuracy of and integrity to GPS-derived positions to 1 - 3 meter positional accuracy in established coverage areas. Commercial differential GPS (DGPS) services such as OmniSTAR® can provide sub-meter accuracy.

A Local-Area Augmentation System (LAAS) can also be used for differential correction. This involves finding a local source for the calculation and transmission of correction data. This source might be an airport in the area, or might be as simple as a second GPS unit placed at a known location. A LAAS is typically useful up to 30-50 kilometer radius, depending on terrain and other physical obstructions. Real-time DGPS in a LAAS requires a base station which computes, formats, and transmits corrections through a data link (e.g. VHF radio or cellular telephone) and a rover that can receive and integrate the corrections with each GPS observation. An exceptionally accurate form of real-time DGPS called Real Time Kinematic (RTK) surveying is commonly used when accuracies of 5 cm or greater are required. The RTK rover requires a clear line of sight to 5 satellites to initialize and generally must be within 20 km of a base station.

Post-processing differential GPS also requires GPS receivers capable of producing DGPS data streams and software to integrate the base and roving unit data. Base station data for post-processing DGPS can be downloaded from the two networks of continuously operating reference stations (CORS) coordinated by the National Geodetic Survey (NGS). It is also possible to create your own base station if you have a second DGPS-capable receiver that can be placed at a geodetic control marker. These are permanently affixed points (often a brass, aluminum or concrete marker with a unique NGS identifier) at various locations all over the United States to enable land surveying, civil engineering and mapping to be done efficiently. It is essential that the base station be placed at known coordinates, which make the NGS benchmarks ideal.