GPS stands for Global Positioning System and is a network that is made up of three main segments:
- Space segment – 24 satellites orbit the earth twice a day, traveling at over 7,000 mph. They are solar powered but have battery backup for when they are in the earth’s shadow. They are positioned so that at any given time there are at least 4 satellites ‘visible’ from any point on earth.
- Control segment – A master control station (located in Colorado Springs), unmanned monitor stations and ground antennas work together to make sure the satellites are working correctly and the information they beam down to earth is accurate.
- User segment – This is where you, the user comes into the picture. The user segment is made up of GPS receivers, which is any device built to receive signals from a GPS satellite. This can include mobile phones, laptops, in-car navigation devices and hand-held tracking units.
- GPS is a system that uses radio frequencies to find your exact location
- Developed by the military to know the location and movement of planes, ships and soldiers
- Accurate to meters, with newer satellites offering even greater accuracy
- Now used in cars, planes, boats, laptop computers and construction equipment
- When a GPS receiver is switched on it ‘listens’ (by receiving radio waves sent from space) to find the four nearest satellites to help calculate its current location
- The quartz clock in your GPS receiver continually updates to stay in sync with the very accurate atomic clocks used by the GPS satellites
- A GPS system can tell you how far you’ve traveled, your current direction, your speed and ETA
- The ways in which GPS is being used are constantly growing as people think of new ways to apply GPS technology. For example, seismologists use GPS units to detect plate movement.
Navigation Solution
Assume that the orbital positions of the satellites can be accurately computed with respect to the earth at any time. Further assume that a GPS receiver on the ground can measure the distance between a receiver and a satellite for at least three satellites at the same time. By defining the receiver location with three coordinates, such as latitude, longitude and height, one can readily write three equations that relate the three distance observations to the known coordinates of the satellites and the unknown coordinates of the receiver. These three equations can be solved for the three unknowns.
The distance to the satellites is measured by timing signals transmitted by the satellites that travel with the speed of light toward a receiver on the ground. Because of the high speed of light, it is necessary that the instant of signal transmission at the satellite and the instant of signal reception at the receivers antenna be accurately registered in order for the distance and, consequently, the position calculation to be accurate. Satellites in fact carry atomic clocks. Receivers, in contrast, contain inexpensive and therefore less accurate clocks. As a result, we must allow for a timing error to occur as the arriving satellite signal is timed at the receiver. Because the signals arriving at a receiver from all satellites are measured at the same time, the distance measurements are all falsified by the same receiver clock error, which must be calculated in order to determine an accurate position. The complete position determination of the receiver consequently requires four unknowns: the receiver clock error and the three receiver coordinates. Measuring distances to at least four satellites allows one to set up four equations that can be solved for these four unknowns. This leads to the fundamental requirement for a truly global positioning system that at least four satellites be visible at any time from any location on the earth. In practice, receivers observe all visible satellites to determine the best estimates of both the receiver clock and location.