ICC-ASIA Co., Ltd. - The Leading GPS Monitoring Systems Provider in Thailand

What is GPS?

GPS is the acronym for Global Positioning System, a satellite-based radio-navigation system developed and operated by the U.S. Department of Defense. GPS allows users to determine their geographic position anywhere in the world. The Global Positioning System reached full operational capability on July 17, 1995.

The GPS consists of 24 operational satellites in six circular orbits above the earth. These satellites continuously broadcast position and time data and are spaced in orbit so users anywhere in the world may, at any time, receive data from a minimum of 6 satellites.

There is a master control station in Colorado Springs, with five other monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite via the ground antennas, which also transmit and receive satellite control and monitoring signals.

Most users of the GPS employ receivers that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position (latitude and longitude). Sophisticated users may also calculate altitude and even velocity.

Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range to the satellite. The satellites act as precise reference points and users figure their position on the earth by measuring their distance from the group of satellites in space.

How accurate is GPS?

While today's GPS receivers are extremely accurate, even in dense foliage or urban settings with tall buildings (though they transmission signal does follow line-of-sight), certain atmospheric conditions and other sources of error can occasionally affect the accuracy of GPS receivers. Unaided by DGPS or WASS, most GPS receivers are accurate to within 10 yards on average.

• The first GPS satellite was launched in 1978.
• A full constellation of 24 satellites was achieved in 1995.
• Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
• A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
• Transmitter power is remarkably low, only 50 watts or less.
• Intentional degradation of the satellite signal - Selective Availability (SA) was an intentional degradation of the signal once imposed by the U.S. Department of Defense. SA was intended to prevent military adversaries from using the highly accurate GPS signals. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers.

WAAS

Simply put, WASS provides GPS signal corrections, generating enhanced accuracy.

The Federal Aviation Administration and the Department of Transportation developed the WAAS program for use in precision flight approaches, since GPS alone did not meet the FAA's navigation requirements for accuracy, integrity, and availability. Essentially, WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors.

WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.

Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. WAAS provides extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. WAAS does not require additional receiving equipment, while DGPS does.

Other governments are developing similar satellite-based differential systems. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS), while Europe has the Euro Geostationary Navigation Overlay Service (EGNOS). Eventually, GPS users around the world will have access to precise position data using these and other compatible systems.