Introduction
Principles of Operation
Essential Components for Quality Measuring
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Using this technique, the spatially correlated errors – such as satellite orbital errors, ionospheric
errors, and tropospheric errors – can be significantly reduced, thus improving the position solution
accuracy. A number of differential positioning implementations exist, including post-processing
measuring, real-time kinematic measuring, maritime radio beacons, geostationary satellites, and
satellite based augmentation systems (WAAS, EGNOS, MSAS). The real-time kinematic (RTK)
method is the most precise method of real-time measuring. RTK requires at least two receivers
collecting navigation data and communication data link between the receivers. One of the
receivers is usually at a known location (Base) and the other is at an unknown location (Rover).
The Base receiver collects carrier phase measurements, generates RTK corrections, and sends this
data to the Rover receiver. The Rover processes this transmitted data with its own carrier phase
observations to compute its relative position with high accuracy, achieving an RTK accuracy of up
to 1 cm horizontal and 1.5 cm vertical.
1.1.4. Essential Components for Quality Measuring
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 GNSS 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.
– Satellite Based Augmentation Systems (WAAS, EGNOS, etc.) creates and transmit,
along with DGPS corrections, data integrity information (for example, satellite health
warnings).
– Current ephemerides and almanacs.
