THE USER SEGMENT |
GPS user equipment has undergone an extensive program of development, both in the military and civilian area. In this context, GPS "equipment" refers to the combination of:
There is a wide variety of GPS applications, which is matched by a similar diversity of user equipment. Nevertheless, the most basic classification system is provided by the type of observable that is tracked: (a) civilian navigation receivers using the satellite ranging code on the L1 frequency, (b) military navigation receivers using the satellite ranging codes on both L-band frequencies, (c) single frequency (L1) carrier phase tracking receivers, and (d) dual-frequency carrier phase tracking receivers. GPS instrumentation is discussed in sections 4.1.1, 4.1.2 and 4.1.3. Section 6.2.1 describes the biases and errors that affect all GPS measurements.
While military R&D programs have concentrated on achieving a high degree of miniaturisation, modularisation and reliability, the civilian user equipment manufacturers have, in addition, sought to bring down costs and to develop features that can enhance the capabilities of a system that was not optimised for many groups of civilian users, principally in terms of accuracy and reliability. This is particularly true of the survey user seeking levels of accuracy several orders of magnitude higher than that of the navigation user. It is fair to say that GPS user technology is being driven by the precise positioning market -- in much the same way that automotive technology benefits from car racing.
Yet another major influence on the development of GPS equipment has been the increasing variety of civilian applications. For although there may exist a similar positioning accuracy requirement across many user applications, to address a particular application in the most satisfactory manner, a specific combination of hardware and software features is often required. As the system matures it is likely that several trends in instrument development will emerge, some designed for high precision, while others will emphasise other attributes. There are at present over 100 manufacturers of GPS instruments of varying kinds.
It is important to emphasise that GPS was designed for navigation,
and not for surveying. Navigation requires real-time point positioning
at the several dekametre (tens of metres) level, while surveying aims to
achieve a high relative accuracy at the centimetre level, over distances
of a few tens of kilometres. Special observation, measurement and data
reduction procedures have been developed to enable these surveying accuracies
to be achieved.
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© Chris Rizos, SNAP-UNSW, 1999