Electronic System of Position Fixing


Satellite Navigation Systems

The Global Positioning System (GPS)

The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. The satellites are all in polar orbits unlike the geo-stationary INMARSAT satellites.

Thus the GPS satellites do not have a fixed position with reference to a position on earth. They are all moving in their orbits.

The altitude is generally about 11000 km above the earth. GPS uses these ďman-made starsĒ as reference points to calculate positions accurate to a matter of meters.

The NAVigation Satellite Timing and Ranging (NAVSTAR) GPS is an all weather, radio based, satellite navigation system that enables users to accurately determine 3-dimensional position, velocity, and time worldwide.

The overall system consists of three major segments: the space segment, the ground control segment, and the user segment.

The space segment is a constellation of satellites operating in 12-hour orbits at an altitude of 20,183 km (10,898 NM).

The constellation is composed of 24 satellites in six orbital planes, each plane equally spaced about the equator and inclined at 55 degrees.

The ground control segment consists of a master control centre and a number of widely separated monitoring stations.

The ground control network tracks the satellites, precisely determines their orbits, and periodically uploads almanac ephemeris, and other system data to all satellites for retransmission to the user segment.

The user segment is the collection of all GPS user receivers and their support equipment.

More simply, the GPS Receiverís position is determined by the geometric intersection of several simultaneously observed ranges (satellite to receiver distances) from satellites with known co-ordinates in space.

The receiver measures the transmission time required for a satellite signal to reach the receiver.

Transit time is determined using code correlation techniques

The actual measurement is a unique time shift for which the code sequence transmitted by the satellite correlates with an identical code generated in the tracking receiver.

The receiver code is shifted until maximum correlation between the two codes is achieved.

This time shift multiplied by the speed of light is the receiverís measure of the range to the satellite.

This measurement includes various propagation delays, as well as satellite and receiver clock errors.

Since the measurement is not a true geometric range, it is known as a pseudo-range.

The receiver processes these pseudo-range measurements along with the received ephemeris data (satellite orbit data) to determine the userís three-dimensional position.

A minimum of four pseudo-range observations is required to mathematically solve for four unknown receiver parameters (i.e., latitude, longitude, altitude, and clock offset).

If one of these parameters is known (for example, altitude fixed) then only three satellite pseudo-range observations are required and thus only three satellites need to be tracked.

The level of accuracy is upto 100 metres. However it depends whether the chatty being used to plot the position is using WG84 datum or not. If not then there may be corrections to be applied prior such GPS derived positions are plotted on the charts.

Since the datum used is of so great a factor, it always prudent ot check on the chart whether the chart is to WGS84 datum or other. The correction is usually printed on the chart, but if it is not then due caution is to be used since the GPS position in extreme cases can be found to be on land.