Global Positioning Systems in Surveying (GPS)

Global Positioning Systems in Surveying

(GPS)

    Global Positioning Systems (GPS) is a system of determining position on the earth’s surface by the use of satellites orbiting above the earth. The first generation of satellite systems used in surveying was the US Navy's TRANSIT satellites, operating on the Doppler principle.

    In this system, receivers located at ground stations measured the changes in Frequencies of radio signals transmitted from satellites orbiting in polar orbits at an altitude of about 1075 km. In 1973, the US Department of Defence began a concept validation program to investigate the viability of a proposed new satellite system to replace TRANSIT and NAVSTAR GPS was born.

(Navigation Satellite Timing and Ranging Global Positioning System.)  The first satellite was launched in 1978.

   This system is an all-weather radio navigation system which provides 3 dimensional positional information anywhere on the globe, 24 hours per day, from any position.

    It provides this information faster, more accurately and at lower cost than TRANSIT and has superseded all previous long range navigation systems.

   Although this was designed as a navigation system, GPS is having a tremendous impact on surveying and is being used more and more as a means of determining position in surveying.

   GPS is a military navigation system and civilian users are tolerated. GPS is a major navigation technology that has been modified to provide surveying accuracy without downgrading its navigation capabilities, and without the need for elaborate data processing and long observation periods.

Description of the System

   There are currently 24 satellites in 6 orbital planes (4 in each), each at a distance of 20,200 km above the earth. The satellites have a 12 hr orbit period and a circular orbit at 55°E to equatorial plane, and spaced 60° apart.

   The system has worked continuously since 1991 and is capable of centimetre accuracy. There is a 30 second fix resolution which is updated every 6.5 seconds.

There are two types of receivers, single frequency receivers and dual frequency receivers (dual frequency is faster and more accurate).

The System Consists of Three Main Segments

·         The space segment:

This is essentially the 24 satellites in orbit.

·         The user segment:

It receives the satellite signals and calculates the positions or position

differences, obtaining real-time kinematic positions or more accurate static position fixes.

·         The control segment:

This has as its main function the control and synchronization of the

highly accurate clocks in the satellites, the determination of the satellite Orbits and the injection of such data into the satellites for re-transmission as Broadcast Ephemeris.

The Principle of GPS

   GPS procedures for determining precise point positions consist fundamentally in measuring distances from points of unknown location to satellites whose positions are known at the instant the distance is measured.

   From these distances, a precise position can be determined similar to conventional resections.

GPS is a uni-directional method of distance measurement that depends upon accurate time measurement, facilitated by precise synchronization of clocks in both the satellites and the receivers.

   The distance is determined by calculating the time taken for the carrier wave to travel from the satellite to the receiver. The time is determined by comparing the signal received by the receivers with a constant reference signal being generated in the receiver.

Relative Positioning or Differential Positioning

 

There are two basic methods of using GPS.

The first is to obtain a position by using just a single receiver. When this method is used, there is no check to determine any error in the machine.

The coordinates that are calculated must be accepted as being correct.

   Far more accurate and quicker results are obtained, however, if two

receivers are used. This is known as relative or differential positioning. When using this method,

one receiver remains on a known fixed survey control point, with known coordinates, whilst the other visits the unknown points.

   Since the errors due to clock errors and refraction errors are the same at each receiver, the error at the known control points can be applied to the coordinates obtained from the unknown points and thus virtually eliminated, producing more accurate results.

Advantages of GPS

1. Co-ordinates can be brought into remote areas where the cost of conventional methods would be prohibitive.

2. The system can be used 24 hours a day regardless of weather conditions.

3. Intervisibility between points is not necessary. Therefore no expensive and time-consuming line clearing is required.

4. The method is relatively quick and accurate and field techniques are easy to master.

Disadvantages of GPS

1. The full differential positioning equipment is still reasonably expensive, although prices are reducing all the time.

2. In heavily forested areas, GPS surveys are not as effective since satellite visibility is inhibited.

3. The GPS system is controlled and owned by the US Department of Defense and thus the ultimate control of the whole system lies with them.

    NOTE: The GPS receivers obtain coordinates based on an earth centered World Geodetic Datum and these coordinates must then be converted to the AMG, although World Geodetic Datum Coordinates (WGS81) are almost identical to GDA96 coordinates.

   Care must be taken when using a map for coordinates, to check which spheroid has been used to calculate the coordinates.