текст по английскому (GPS) (559874), страница 2

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The typical configuration for differential positioning is shown below, two units receiving signals from the same constellation of satellites at the same time. The relative position of the two units can be determined to a very high accuracy, in many cases better than a centimeter. If one of these units was located over a point for which we had ground control coordinates it is then theoretically possible to obtain highly accurate coordinates for the other point. This is indeed the case, providing all the computations are performed in the one coordinate system.

This technique can be also applied if one of the receivers is on a moving platform, or is moved between points of interest while the base station remains fixed and continues to observe to the same satellites. This produces new procedures known as rapid-static positioning, kinematic positioning and pseudo-kinematic positioning. By the time these notes are copied and distributed there will no doubt be other new pieces of scientific jargon to describe new procedures. In general the units store the observations to the satellites and are down-loaded to PC type computers at the end of the project. The differential solution is then computed using the complete set of data from all the receivers. The latest hardware systems can transmit the corrections between the base station and the rover allowing the solution to be determined on-the-fly so that positioning accuracies of around 0.01m are available in real-time.

9. Coordinate Systems.

The type of coordinate system used in GPS surveying is an earth-centred cartesian system, of which a detailed discussion is beyond the scope of these reference notes in plane surveying. Generally coordinates produced by GPS units are geocentric coordinates which appear nothing like geographic coordinates. Geocentric coordinates are based on the centre of the earth and have an X, Y and Z component. The Z axis is from the centre of the earth through the north pole, the XZ plane passes through the Greenwich meridian and the XY plane passes through the equator. Geographic coordinates are in the form latitude and longitude and are located on a spheroid of 'best fit' over the surface of the planet. Transformations between the cartesian coordinates and the spheroidal coordinates can easily be performed in the receivers, as well as transformations between grid mapping systems like AMG. There is also a selection of spherical models that can be used as a coordinate datum, some of these can be seen during the set-up options on the navigational GPS units. If one wishes to output AMG coordinates in Australia one selects the Australian National Spheroid as the datum, UTM coordinates as the type and the appropriate zone if necessary.

There will be considerable more information provided on coordinate datums in the later years of the geomatics program. We have only looked at the matter superficially, the problem of coordinate frames of reference is part of on-going investigations and developments.

10. Conclusions

The Global Positioning system offers navigational precision positioning in all weather 24 hours per day. This facility offers people from a wide variety of non-technical backgrounds access to navigation facilities impossible even 10 years ago, ensuring increased safety and surety in travelling from one location to another. Anybody can stand in an open space, push a button and determine their location to 50m or so within minutes.

The full impact of this technology in the geomatics industries is still to be felt. Units like the Leica System 300 enable tasks like field-to-office topographic surveys to be performed using GPS instead of electronic theodolites and EDM, practically eliminating the need for control surveys and traverses. It is possible to achieve 0.1m positional accuracy at speeds of 100kmh-l, enabling the mapping of facilities and resources in a fraction of the time taken even only 4 years ago. If synchronised video cameras are coupled with kinematic GPS it is possible to map the position of road furniture simply by driving along the road. Ground control for mapping can be coordinated in a matter of days instead of months, and maps can be produced almost as fast as conditions change in the real world.

Further applications and implications of GPS positioning will be presented in the later years of the geomatics course. The technology and methodology is changing so rapidly that even the most current reference material is almost out of date by the time it is published.

References:
Elfick, M, J. Fryer, R. Brinkler and P. Wolf. 1995. Elementary Surveying, S.I. Edition. Harper Collins, pp 321-344.

Bannister, A., S. Raymond and R. Baker. Surveying, 6th Edition. Longman Scientific and Professional, pp 232-235.

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