Principles of Operation
P/N 7010-1004
1-5
number of common satellites in view at both the Base and the
Remote receivers.
– The quality of observed measurements can also affect
accuracy, and for this reason Topcon GNSS products use
sophisticated and patented techniques to produce highly
precise measurements. However, these measurements can
still be adversely affected by nearby natural and man-made
objects that block, interrupt, reflect, or partially obscure
satellite signals.
• Availability -
While a low number of satellites may adversely
affect accuracy, if very few satellites are visible this may result in
no position being available at all. A minimum of 4 GPS or 4
GLONASS, or 5 GPS+GLONASS (mixed) satellites must be
visible at all times.
• Integrity - Fault tolerance and redundancy allow a position
solution to have greater integrity, increasing its reliability for the
user. Several factors combine to provide fault tolerance,
including:
– Five or more visible satellites for only GPS or only
GLONASS; six or more satellites for a mixed scenario. As
the total number of satellites increases, so does the
measurement redundancy and the inherent reliability of the
position.
– Satellite Based Augmentation Systems (WAAS, EGNOS,
and so on) creates and transmit, along with DGPS
corrections, data integrity information (for example, satellite
health warnings).
– Current ephemerides and almanacs.
Calculating Differential Positions
DGPS, or Differential GPS, is a relative positioning technique where
the measurements from two or more remote receivers are combined
and processed using sophisticated algorithms to calculate the
receivers' relative coordinates with high accuracy. DGPS
accommodates various implementation techniques that can be
classified according to the following criteria:
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