An overview of GPS

Global Positioning System (GPS)

• The official name for GPS is NAVigation Satellite Timing And Ranging Global Positioning System or NAVSTAR GPS
• It was developed in 1973 to overcome the limitations of previous navigation Systems. It was created by the United States department of Defence (USDOD) and was originally run with 24 satellites. It became fully operational in 1995. Brad Parkinson, Roger L. Easton and Ivan A Getting invented it. It is maintained by the United States Government and is freely accessible by anyone with a GPS receiver.
• It consists of more than 30 satellites in medium Earth Orbit (2000 - 35000 km). Two dozen satellites working in harmony are known as a satellite constellation. It is basically used for 

1. -Navigation
2. -Map making and
3. -Surveying

• It consists of the following three segments:

1. -Space segment
2. -Control segment
3. -User segment

Space segment consists of:

1. -GPS satellites that fly in circular orbits at an altitude of 20,200 km and with a period of 12 hours
2. -They are powered by solar cells
3. -The satellites continuously orient themselves to point their solar panels toward the sun and their antenna toward the Earth
4. -The orbital planes of the satellites are centered toward the Earth
5. -Orbits are designed so that, atleast, six satellites are always within line of sight from any location on the plane.

Control segment consists of three units:

1. -Master Control System
2. -Moitoring stations and
3. -Ground Antennas

• -The master control station is located in Falcon Air Base in colorado springs
• -It is responsible for overall management of the remote monitoring and transmission sites
• -Here, a check-up is performed twice a day by each of the six stations as the satellites complete their journey around the Earth.
• -It can reposition satellites to maintain optimal GPS constellation

Monitor stations:

• -Check the exact altitude, position, speed and overall health of orbiting saellites.
• -The control segment ensures that the GPS satellite orbits and clocks remain within acceptable limits
• -A station can monitor upto 11 satellites at a time
• -This "check-up" is performed twice a day, by each station

Monitor stations are located at:

1. -Falco air base in colorado
2. -Cape canaveral
3. -Florida
4. -Hawaii
5. -Ascension island in Atlantic ocean
6. -Diego Garcia Atoll in the Indian Ocean and
7. -Kwajalein Island in the south Pacific ocean

Ground antennas:

1. -Ground antennas are used to monitor and track the satellites from horizon to horizon 
2. -They also transmit correct information to individual satellites
3. -They also communicate with GPS satellites for command and control

User segment consists of the GPS receiverwhich in-turn consists of:

1. -An antenna tuned to the frequencies transmitted by the satellite
2. -Receiver processors and
3. -A crystal oscillator as a highly stable clock

• -The GPS receiver may also include a display for showing location and speed information to the user
• -A receiver is often described by the number of channels signigying the number of satellites that it can monitor simultaneously
• -Receivers usually have anythin between twelve to twenty channels

WORKING PRINCIPLE

1. -GPS works on the principle of determination of any location if its distance from any two already known locations is available
2. -GPS satellites orbit the Earth at an altitude of 11000 miles.
3. -The orbits and locations of satellites are known in advance
4. -GPS receivers store the orbit information or all satellites in an ALMANAC which is a file containing positional information for ALL the GPS satellites
5. -A GPS receiver can tell its position by using its position data and comparing it with three or more GPS satellites
6. -The distance of each satellite is measured by the time taken by the radio signal to travel from the satellite to the receiver
7. -All electromagnetic radiation (Ex: Radio waves) travel at the speed of light
8. -The distance from satellite to receiver is computed using the satndard formula and the receiver's position is determined using trilateration.

-The position calculated by a GPS receiver depends on three accurate measurements:

1. -Current time
2. -Position of the satellite and
3. -Time delay for signal

Worst-case accuracy of a GPS signal is 7.8m at 95% confidence level

• Sources of GPS signal errors are due to:

1. -Satellite clock
2. -Receiver clock
3. -GPS jamming
4. -Atmospheric errors
5. -Multi-path error
6. Accuracy can be improved using
7. -Precision monitoring and
8. -Augmentation
9. Limitations

• -GPS can provide worldwide, 3 dimensional positions, 24 hours a day in any kind of weather. However, ther must be a clear line-of-sight between the GPS antenna and four or more satellites
• -The above condition may be a major problem in urban areas
• -GPS signal may bounce-off nearby objects causing a problem called "multi-path interference"

APPLICATIONS

1. -SURVEYING:
2. -TELEMATICS:
3. -Vehicle tracking
4. -Military applications
5. --GPS integrated into fighters, tanks, helicopters, ships, submarines, tanks, jeeps and soldier's equipment
6. --Target tracking
7. --Search and rescue operations