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GPS FAQ: Global Positioning System Frequently Asked Questions.

1. What are the components that constitute a GPS?

a. Space segment

b. User segment

c. Control segment

 

a. Space segment: Space segment consist of several satellites orbiting the earth. The position and time signals transmitted by the satellites are used by User Segment to compute GPS information. Note that the information is passed one-way, and the GPS receivers will not transmit any signals to the satellites. Effectively, the GPS receiver uses the signals received from the GPS satellites to compute the location information.

b. User Segment: The user segment consists of GPS receivers. Since the GPS signals are broadcast in nature, any number of receivers can be used in a given area or GPS footprint.

c. Control Segment: Control segment consist of Satellite control earth stations. The location information is greatly depend on the accuracy of clock source used on the Space Segment (satellites). The Control segment corrects for any clock errors, and orbital positions of the satellites.

2. How a GPS receiver determines its position based on the satellite signals?

Position determination is based on determining the amount of time it takes for a GPS signal to reach a receiver (from a satellite). If both the satellites and the receiver are in time synchronization, thee satellites would be sufficient to determine the location of a GPS receiver accurately. However, it is very expensive to have a highly accurate clock source at the receiver (say 1 nano second inaccuracy). For this reason, a fourth satellite would be required to correct for timing error. Thus, it takes four satellites to accurately determine the location of an object on earth using GPS receiver. Note that the first three satellites determine the exact position (compare to X,Y, and Z axis) of the receiver, and fourth for timing correction.

3. Will the NAVSTAR GPS free?

Yes, GPS is free for all civil, commercial, and scientific purposes.

4. What is the accuracy with which the receiver position determined?

The NAVSTAR GPS (also known as the GPS) has two levels of positioning systems:

a. Standard positioning system, and

b. Precision positioning system

The Standard Positioning System is the one available for public use. It offers an accuracy of 100m horizontal, and 150 meters vertical (2 sigma) position.

2*sigma means that 5% of values are worse than 100 metres.

PPS is used by the Department of Defence (DoD), and not available for civilian use.

Effective from May 2000, the US Govt. has discontinued selective degradation of GPS signals available for Civilian use. As a result of this, the location accuracy has improved to better than 20 meters. If better accuracy is required, you need to use DGPS (Differential GPS). DGPS uses a fixed base station for computing the exact location of a GPS receiver within a meter.

5. What is Selective Availability (SA)of GPS?

Prior to May1, 2000, GPS signals were intentionally degraded. Effective from May 2000, the US has stopped intentional degrading of GPS signals. The intentional degradation of signals is called Selective Availability (SA).

6. Can the GPS be used for Military use by countries other than USA?

According to the official document, GPS is meant for Civilian use, and only DoD has access to the military use of GPS. It is possible that the US may prevent the GPS signals from reaching any hostile (or enemy) positions.

7. Are there any alternatives to the GPS?

The Russians have a GPS system in place by name GLONASS.

The European Union is putting together Galileo GPS and expected to be operational by 2008. It is claimed to be superior to the existing GPS systems with a service guarantee. Galileo is specifically designed for commercial use.

Sources:

gpsy.com/gpsinfo/gps-faq.txt
navcen.uscg.gov/faq/gpsfaq.htm
hunterpro.com/GPS/GPS_info.html

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