Our GPS Technologies – How They Work
GPS is based on a constellation of 24 U.S. government-owned satellites (plus several working backups) that circle 22,000 km (14,000 miles) above the Earth twice a day in precise orbits. The satellites constantly transmit coded information in the UHF band (1.575 GHz) back to GPS receivers on the ground.
There are two GPS services: the Precise Positioning Service (PPS) that the U.S. Department of Defense reserves for itself and authorized partners for security reasons, and the Standard Positioning Service (SPS) available free for all worldwide civilian users.
The Department of Defense, to maintain a strategic advantage, used to intentionally degrade the accuracies of the SPS or civilian GPS system. However, it stopped doing so in 2000 to encourage more development of GPS technology which in turn would yield economic and social benefits.
A GPS receiver knows where each satellite is in its orbit and compares it with the time required to receive each satellite’s signal. The receiver uses these measurements to calculate its specific position on Earth.
A GPS receiver can only track satellites orbiting above the horizon. Typically there are between six and twelve satellites visible above the horizon at any one time. The receiver will try to track all visible satellites. If some satellites become blocked or "shaded" by tall buildings or other major obstacles, the receiver automatically will try to reacquire the blocked signals. Although a GPS receiver needs at least four satellites to provide a three-dimensional solution (latitude, longitude and altitude), it can maintain a (latitude-longitude) position using three satellites.
GPS is designed to provide worldwide positioning services with an accuracy ranging from 10 to 15 meters. More precise accuracies are impossible with standard GPS due to minor timing errors and satellite orbit errors, plus atmospheric conditions that affect the signals and their arrival time on Earth. However, there are ways to improve GPS accuracy using additional services. There are four primary services available, each capable of improving position accuracies to better than one meter:
- Radio beacon Differential GPS (DGPS) corrections
- Space-Based Augmentation Systems (SBAS), including the United States' Wide Area Augmentation System (WAAS), Europe's EGNOS, Japan's MSAS, and India’s GAGAN
- Privately owned "L-band" satellites, including the OmniSTAR® system
- Real Time Kinematic (RTK) Positioning
Hemisphere GPS is an industry leader in designing and manufacturing products that are compatible with radio beacon, SBAS, and OmniSTAR to offer customers maximum flexibility. The innovation continues with our RTK-capable products available on both our Crescent and Eclipse product lines.
The U.S. Coast Guard and Army Corps of Engineers have established a network of radio beacons that constantly broadcast differential GPS (DGPS) corrections to receivers. The network covers the United States from coast to coast. The Canadian Coast Guard provides similar beacon coverage along its coast lines, the Great Lakes, and the St. Lawrence River. There are similar beacon networks in many other regions worldwide.
One of the main advantages of radio beacon DGPS is that the DGPS corrections are free to anyone with the appropriate equipment, and the equipment is relatively inexpensive. The long-range signals penetrate into valleys and urban canyons and travel around obstacles, providing service where other services cannot. The corrections are continuously monitored to ensure their integrity.
Space-Based Augmentation Systems
Whereas radio beacon stations broadcast DGPS correction signals from the ground, Space-Based Augmentation Systems (SBAS) broadcast correction signals from geostationary satellites.
SBAS corrections rely on networks of base stations on the ground to monitor the GPS satellites. Rather than broadcasting corrections directly to users, the networks send signals up to geostationary satellites, which broadcast the signals back to individual SBAS-capable receivers on Earth.
Like radio beacon differential signals, SBAS differential signals are free to anyone with appropriate equipment. Instead of calculating corrections at each base station applicable to the area surrounding each site, WAAS, EGNOS, MSAS, and GAGAN determine corrections for very large areas by using all base station data together. This facilitates more uniform corrections, often continent-wide.
For example, the WAAS network of 38 reference stations includes base stations in Canada and Mexico. It is able to provide relatively uniform accuracy and coverage from the Arctic Ocean to Hawaii to the mid-Caribbean to the mid-Atlantic and the shores of Greenland. By comparison, approximately 100 radio beacon DGPS stations operated by the U.S. Coast Guard, the Army Corps of Engineers, and the Canadian Coast guard provide coverage coast to coast in the U.S.A. and some of Canada’s coastal waters, but no coverage into Mexico.
Privately Owned Satellites
Privately owned satellite systems such as OmniSTAR provide differential correction signals to anyone subscribing to their services. OmniSTAR offers several different services with various accuracies. Their signals are available almost worldwide.
The highest level of accuracy for GPS navigation comes from a technique relying on a nearby, stationary GPS reference receiver, called a base, and a radio link. The base provides more data to the user’s receiver, called the rover, than other correction methods such as SBAS or beacon corrections. The additional information is called carrier phase information and is the basis for high accuracy positioning.
RTK positioning requires the GPS base data to be sent to the rover every second. Typically a digital radio link is used to transmit from the base to the rover. The rover then solves for a carrier-phase solution. This phase solution is accurate to about one centimeter in most situations. For RTK to work, the base typically has to be within 30 to 50 km (20 to 30 miles) of the rover.