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Basics of GPS Satellite Surveying
In GPS Satellite Surveying, the points of observation are considered coordinated as stationary, and the collected GPS data pertains to a specific "observation session.” Before we move towards the details, let us start with the significant features of GPS Satellite Surveying:
• GPS data collection is done instantly, thereby rendering a real-time solution.
• The positioning modes of operation can be either relative or absolute, involving relatively lower degrees of accuracy.
• All the measurements are usually done primarily based on PRN codes; hence, pseudo-range data processing becomes essential.
• With the application of this software, safe passage of aircraft and ships is ensured to the maximum extent.
• The measurements made on the L-band carrier wave, require special instrumentation and software associated with the traditional surveying and mapping functions.
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Basic Instrumentation Set-uP Involved With a GPS Network Survey
The GPS network survey involves a number of fixed-height tripods, antennas, receiver sets, meteorological and minor scientific instruments.
Identical equipment is used whenever possible in order to nullify the equipment bias effects.
A validation survey is usually used to determine and demonstrate the compatibility of using a mixed bag of instrument models. Calibrations are done to all the equipments and possibilities of erroneous measurements are ruled out. Finally, validation survey serves as a starting point to start the proceedings of live GPS Survey.
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Survey Planning Considerations
Survey planning considerations are usually based on following parameters:
Nature and Aim of Project: It is done just as in case of conventional GPS surveys.
Determining Unique Characteristics of GPS: This brings about severe simplification in survey design, and there are no requirements for station inter-visibility in particular.
Deciding Upon Total Number of Points to be Surveyed: This has always been one of the biggest logistical problems involved with a GPS Survey as the resources are at the surveyor’s disposal. Depending upon the strategy to be used for propagating the survey, a decision is made on the count.
Prudent Survey Practice: This involves check measurements and increases the level of redundancy in the network design, but it usually helps in obtaining better results.
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Rapid Static GPS Surveying
Also referred to as quick-static or fast-static, the rapid static GPS survey has the following salient features, which tend to differ from the conventional ones.
Observation Time Requirements: The Observation times are significantly shorter in comparison to the conventional GPS surveying time. They are calculated as function of satellite geometry, number of satellites that are tracked, and the baseline length.
Baseline and Number of Satellites: In a typical GPS satellite surveying setup with four to six satellites and a baseline between five and 20 kms, receivers occupy baseline for five to 20 minutes, depending upon the value of baseline.
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Depending upon the GPS products involved with the survey, the hardware requirements may change to a considerable extent. In some cases, merely dual-frequency phase measurements may suffice, while surveys involving tedious measurements require accurate dual-frequency pseudo-range measurements as well.
Unlike conventional GPS surveys, the GPS Satellite Surveys requires lot of data processing, but the option of "mixing" of receivers has not been successfully devised to date.
The exact configuration of hardware to be used during the survey also greatly depends upon the tracking software being made use of. For instance, the rapid static GPS survey produces results with single frequency phase data. Hence, high-end hardware is not needed.
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The GPS Satellite Surveying heavily relies on the software to resolve the ambiguities (if any) and estimate their integer values, with the help of a relatively short observation period.
Varieties of software are available in market that can be used during different surveys involving variable levels of sophistication and peculiar characteristics. However, fast ambiguity resolution capability remains the primary requirement in all the cases.
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For more information on using GPS in surveying, read the Bright Hub articles Handheld GPS Surveying Unit – Fast and Easy Survey, GPS Survey - Benefits of Using a GPS in Surveying and GPS Systems Used in Construction Surveying.