About Our Technology waas_diagram Our GPS Technologies – How They Work
GPS is based on a constellation of 24 U.S. government-owned satellites (plus several working back-ups) that circle 22,000 kilometres above the Earth twice a day in precise orbits. The satellites constantly transmit coded information in ultra-high or UHF frequencies (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. But it stopped doing so in 2000 to encourage more development of GPS technology and of related economic and social benefits.
A ground-level GPS receiver, using pre-programmed data, knows where each satellite is in its orbit. The receiver takes this information and compares it with the time required to receive each satellite’s signals at ground level, to calculate a specific geographic position on Earth.
A GPS receiver can "observe" only those satellites orbiting above the horizon, which normally totals from four to 12. The receiver selects the satellites offering the best "view" to do its calculations. If some satellites become blocked or "shaded" by tall buildings or other major obstacles, the receiver automatically uses alternate satellites to maintain the position fix.
Although a GPS receiver needs at least four satellites to provide a three-dimensional fix (latitude, longitude and altitude), it can maintain a (latitude-longitude) fix 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.
There are four primary services for refining GPS to overcome the impact of these errors, and achieve position accuracies of better than one meter:
* Radio beacons; * Space-Based Augmentation Systems (SBAS), including the United States' Wide Area Augmentation System (WAAS), Europe's EGNOS and Japan's MSAS; * Privately owned "L-band" satellites, including the OmniSTAR system.
Hemisphere GPS Inc. 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 new RTK-capable products available on both our Crescent and Eclipse product lines.
Radio Beacons
The U.S. Coast Guard and Army Corps of Engineers have established a network of radio beacons that constantly broadcast DGPS corrections to receivers. The network serves both coast lines, the Great Lakes and most of the Eastern U.S. 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 other parts of the world.
The advantages of radio beacon DGPS include the fact that Differential signals and correction information are freely accessible to anyone with the appropriate equipment, that the equipment is relatively inexpensive, that long-range signals penetrate into valleys and urban canyons and travel around obstacles, and that the corrections are continuously monitored to ensure their integrity.
Space-Based Augmentation Systems
Whereas radio beacon stations broadcast Differential GPS correction signals from the ground, Space-Based Augmentation Systems (WAAS, EGNOS and MSAS) broadcast correction signals from satellites.
WAAS, EGNOS and MSAS have networks of stations on the ground that receive signals from the U.S. government's 24 GPS satellites. These ground stations, rather than sending corrected positioning signals to users via beacon stations, instead send signals up to their own proprietary satellites, which transmit the signals back to individual WAAS/EGNOS/MSAS-capable receivers on Earth.
Like radio beacon Differential signals, SBAS Differential signals are freely accessible to anyone with appropriate equipment.
Instead of calculating range corrections, which are applicable to only local geographic regions, WAAS, EGNOS and MSAS determine the value of each separate source of error. This facilitates more uniform corrections. Although WAAS was intended to work only in the U.S., it can be “stretched” into Canada and Mexico using a complex extrapolation technique.
Privately Owned Satellites
Privately owned satellite systems such as OmniSTAR provide Differential correction signals to anyone subscribing to their services. OmniSTAR signals are available almost worldwide. RTK Positioning
The highest level of accuracy for GPS navigation comes from a technique relying on a nearby, stationary GPS reference receiver and a radio link. The reference receiver provides more data to the user’s system 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 reference station data to be sent to the user’s system, often called the rover, every second. Typically a digital radio link is used to transmit from the reference to the rover. The rover then can solve for a carrier-phase solution. This phase solution is accurate to about one centimeter in most situations.
Footnotes: OUTBACK™, OUTBACK Guidance®, OUTBACK Guidance Center™, OUTBACK S™, OUTBACK S Lite ™, OUTBACK S2 ™, OUTBACK AutoMate ™, OUTBACK 360™. OUTBACK Steering Guide™, OUTBACK Hitch™, eDrive®, eDriveTC™, Just Let Go™ and BaseLineHD™ are proprietary trademarks of Hemisphere GPS. The OUTBACK S™ and S2 ™automated navigation and steering guide system is protected under U.S. Patents No. 6,539,303 and No. 6,711,501. The OUTBACK Hitch™ automated hitch control system is protected under U.S. Patent No. 6,631,916. The OUTBACK eDriveTC™ GPS assisted steering system is protected under U.S. Patent No. 7,142,956. Other U.S. and international patents pending.