Intro to GPS Apps
How GPS and GNSS works

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Contents

What Is GPS?

GPS stands for Global Positioning System. Users of a global positioning system can calculate their location anywhere on the earth. There are currently two "public" GPS systems. The NAVSTAR system is owned by the United States and is managed by the Department of Defense. The GLONASS system is owned by the Russian Federation. While both NAVSTAR and GLONASS systems are global positioning systems, the NAVSTAR system is often referred to (in the U.S., anyway) as the GPS because it was generally available first. Nevertheless, both systems are GPS systems and the reader should keep this in mind.

The NAVSTAR system is managed by the NAVSTAR GPS Joint Program Office, located at Los Angeles Air Force Base. The civilian point of contact for NAVSTAR information is the U.S. Coast Guard's Navigation Center (NAVCEN). The NAVCEN provides system status information in the form of a document called a Notice Advisory to Navstar Users (NANU). Civil users have input to NAVSTAR system issues through the Civil GPS Service Interface Committee (CGSIC), which is organized by the Coast Guard.

The point of contact for GLONASS information is the Russian Space Forces' Coordinational Scientific Information Center (CSIC). The CSIC presents system status information in the form of a document called a Notice Advisory to Glonass Users (NAGU).

How GPS Works - The Short Explanation

A global positioning system uses the characteristics of radio transmissions for location determination. Unlike previous navigation systems using ground based transmitters, satellite based transmitters are used to cover earth with higher accuracy that that available from the land based systems. The satellites transmit timing information, satellite location information and satellite health information. The Space Segment is technical term for the satellites that belong to the system.

The user requires a special radio receiver - a GPS receiver - to receive the transmissions from the satellite. The GPS receiver contains a specialized computer that calculates the location based on the satellite signals. The user does not have to transmit anything to the satellite and the satellite does not know the user is there. There is no limit to the number of users that can be using the system at any one time. The users with their receivers are known as the User Segment.

The satellites are controlled and monitored from ground stations (the Control Segment). The control stations monitor the satellites for health and accuracy. Maintenance commands, orbital parameters and timing corrections are uploaded from the ground on a periodic basis.

Both NAVSTAR and GLONASS provide two sets of positioning signals. The higher accuracy system is reserved for each country's military use. The lower accuracy system is freely available to civilian users.

What is GNSS?

GNSS stands for Global Navigation Satellite System. There are currently no GNSS systems in operation; however, several are planned. These systems will use either or both NAVSTAR and GLONASS to provide positioning, along with other components to improve accuracy and provide fast warning of problems.

How GNSS Works

A GNSS uses satellite positioning techniques to provide users with accurate and timely navigation information. A global positioning system is a subset of a global navigation satellite system because a GPS system need only provide the ability to determine position information. A GNSS must include:
Real time navigation information
It must be possible to generate real-time navigation information fast enough for safe navigation. The TRANSIT satellite system required several satellite passes to determine a position. Because an aircraft can't sit still for several hours, TRANSIT was not sufficient for aircraft navigation. Both the NAVSTAR and GLONASS systems provide near-instantaneous position (close enough.)

Autonomous integrity checking
There must be some way for the user to determine the accuracy of the navigation solution in a timely fashion. It would be bad for an airplane to hit the ground in front of a runway because a GPS satellite is transmitting bad information. Generally this problem is solved by having the aircraft view a large enough number of satellites that it can calculate several sets of redundant positions, comparing them against each other. Bad solutions can be detected and thrown out. At this time, neither the NAVSTAR or GLONASS systems have enough satellites to do this. The only way to do it is to use both NAVSTAR and GLONASS satellites at the same time.

Accuracy sufficient for safe navigation.
The U.S. Department of Transportation has determined that the most stringent requirements for aircraft landing are 4.1 meters horizontal accuracy and 0.6 meters vertical accuracy. There is no way for either NAVSTAR or GLONASS to provide this accuracy without external aids. Therefore, a GNSS using NAVSTAR and GLONASS must include these aids.

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