Principles of Operation
P/N 7010-0752
1-5
Essential Components for Quality Surveying
Achieving quality position results requires the following elements:
• Accuracy – The accuracy of a position primarily depends upon
the satellite geometry (Geometric Dilution of Precision, or
GDOP) and the measurement (ranging) errors.
– Differential positioning (DGPS and RTK) strongly mitigates
atmospheric and orbital errors, and counteracts Selective
Availability (SA) signals the US Department of Defense
transmits with GPS signals.
– The more satellites in view, the stronger the signal, the lower
the DOP number, the higher positioning accuracy.
• Availability – The availability of satellites affects the calculation
of valid positions. The more visible satellites available, the more
valid and accurate the position. Natural and man-made objects
can block, interrupt, and distort signals, lowering the number of
available satellites and adversely affecting signal reception.
• Integrity – Fault tolerance allows a position to have greater
integrity, increasing accuracy. Several factors combine to provide
fault tolerance, including:
– Receiver Autonomous Integrity Monitoring (RAIM) detects
faulty GPS and GLONASS satellites and removes them from
the position calculation.
– Five or more visible satellites for only GPS or only
GLONASS; six or more satellites for mixed scenarios.
– Wide Area Augmentation Systems (WAAS, EGNOS, etc.)
creates and transmit, along with DGPS corrections, data
integrity information (for example, satellite health warnings).
– Current ephemerides and almanacs.
Conclusion
This overview simply outlines the basics of satellite positioning. For
more detailed information, visit the TPS website.
GMS-2_OM_Book.book Page 5 Friday, December 28, 2007 1:47 PM
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