INTRODUCTION |
Conventional GPS surveying has the following characteristics (section 2.3.2):
The points being coordinated are not moving.
GPS data are collected over some "observation
session", typically ranging in length from less than one hour
to several hours, or perhaps days for very precise applications.
Relative positioning mode of operation
is the only mode employed.
The measurements used for data reduction are those made on the
transmitted L-band carrier wave, requiring
specialised hardware and software.
A variety of processing algorithms can be
employed, including triple-difference, double-difference ambiguity-free
and ambiguity-fixed solutions.
Mostly associated with the traditional surveying
and mapping functions.
During the late 1980's considerable attention was paid to the first two points, as they were considered to be unnecessarily restrictive for precise GPS technology. That is, if antennas could be moving during a GPS survey, then new applications for the GPS technology could be addressed. If the length of time required to collect phase data for a reliable solution could be shortened, then GPS survey productivity would improve and the technology would be attractive for many more surveying applications. It is possible now to distinguish between the two basic modes of GPS surveying: static GPS positioning and kinematic GPS surveying.
New GPS surveying methods have been developed with the two liberating characteristics of: (a) static antenna setups no longer having to be insisted upon, and (b) long observation sessions no longer essential in order to achieve survey level accuracies. These modern GPS surveying techniques are given a variety of names by the different instrument manufacturers, but the following generic terminology will be used in these notes:
Rapid static positioning techniques.
Reoccupation techniques.
Stop & go techniques.
Kinematic positioning techniques.
All require the use of specialised hardware and software, as well as
new field procedures. GPS receivers capable of executing these types
of surveys can also be used for conventional static GPS surveying. Although
the field procedures are different from conventional GPS surveying, the
principles of planning, quality control and network processing are the same
for both modern and conventional GPS surveying practices.
Each of the techniques represents a technological solution to the problem
of obtaining high productivity (measure as
many baselines in as short a period of time as possible) and/or versatility
(for example, the ability to obtain results even while the receiver is in
motion) without sacrificing very much in terms of accuracy and
reliability. None of these techniques is as accurate
or reliable as conventional static GPS surveying, and each of these techniques
has its special strengths and weaknesses. They represent the
state-of-the-art in precision GPS positioning, and are a direct outcome
of considerable innovation by instrument manufacturers addressing survey
applications. In many cases the most significant advances are in the software,
but nevertheless the receiver hardware is of the top-of-the-line variety.
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© Chris Rizos, SNAP-UNSW, 1999