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Global Positioning System GPS and its application in Forestry

Maitreyi Mandal
Junior Research Fellow, Centre for Development and Environment Policy, Indian Institute of Management, Calcutta
mmandal@iimcal.ac.in, mmandal@hotmail.com

1. Introduction: What is Global Positioning System (GPS)?

Global Positioning System or GPS is a constellation of 27 satellites orbiting the earth at about 12000 miles. These satellites are continuously transmitting a signal and anyone with a GPS receiver on earth can receive these transmissions at no charge. By measuring the travel time of signals transmitted from each satellite, a GPS receiver can calculate its distance from the satellite. Satellite positions are used by receivers as precise reference points to determine the location of the GPS receiver. If a receiver can receive signals from at least 4 satellites, it can determine latitude, longitude, altitude and time. If it can receive signals from 3 satellites, it can determine latitude, longitude and time. The satellites are in orbits such that at any time anywhere on the planet one should be able to receive signals from at least 4 satellites. The basic GPS service provides commercial users with an accuracy of 100 meters, 95% of the time anywhere on the earth. Since May of 2000, this has improved to about 10 to 15 meters due to the removal of selective availability.

2. GPS Master Plan
The launch of the 24th block II satellite in March of 1994 completed the GPS constellation. Four additional satellites are in reserve to be launched "on need." The spacing of the satellites are arranged so that a minimum of five satellites are in view from every point on the globe. The basic orbits are quite exact but just to make things perfect the GPS satellites are constantly monitored by the Department of Defense. They use very precise radar to check each satellite's exact altitude, position and speed. The errors they're checking for are called "ephemeris errors" because they affect the satellite's orbit or "ephemeris." These errors are caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites.

GPS Satellites
Name: NAVSTAR Manufacturer: Rockwell International
Altitude: 10,900 nautical miles Weight: 1900 lbs. (in orbit)
Size: 17 ft with solar panels extended Orbital Period: 12 hours
Orbital Plane: 55 degrees to equatorial plane Planned Lifespan: 7.5 years
Current constellation: 24 Block II production satellites
Future satellites: 21 Block IIrs developed by Martin Marietta

3. Ground Stations or Control Segment
These stations monitor the GPS satellites, checking both their operational health and their exact position in space. The master ground station transmits corrections for the satellite's ephemeris constants and clock offsets back to the satellites themselves. The satellites can then incorporate these updates in the signals they send to GPS receivers. There are five monitor stations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs.

4. How GPS works
  1. The basis of GPS is "triangulation" from satellites.
  2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
  3. To measure travel time, GPS needs very accurate timing, which it achieves with some tricks.
  4. Along with distance, it is needed need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
  5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.
Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth. That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth. Forget for a moment how our receiver measures this distance. First let us consider how distance measurements from three satellites can pinpoint you in space.

The Big Idea Geometrically
Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centred on this satellite and has a radius of 11,000 miles. Next, say we measure our distance to a second satellite and find out that it's 12,000 miles away. That tells us that we're not only on the first sphere but we're also on a sphere that's 12,000 miles from the second satellite. Or in other words, we're somewhere on the circle where these two spheres intersect. If we then make a measurement from a third satellite and find that we're 13,000 miles from that one, that narrows our position down even further, to the two points where the 13,000 mile sphere cuts through the circle that's the intersection of the first two spheres. So by ranging from three satellites we can narrow our position to just two points in space. To decide which one is our true location we could make a fourth measurement. But usually one of the two points is a ridiculous answer (either too far from Earth or moving at an impossible velocity) and can be rejected without a measurement. A fourth measurement does come in very handy for another reason however, but we'll tell you about that later. Next we'll see how the system measures distances to satellites.

Triangulating: At a Glance
  • Position is calculated from distance measurements (ranges) to satellites.
  • Mathematically we need four satellite ranges to determine exact position.
  • Three ranges are enough if we reject ridiculous answers or use other tricks.
  • Another range is required for technical reasons to be discussed later.
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