Instruction manual

Page C-2 Z-12 Real-Time Sensor Operation and Reference Manual
Carrier Phase Differential (CPD)
In CPD mode, the pseudo-range and carrier phase measurement data, which are coded
in a DBEN message format, or RTCM Types 18 and 19, are transmitted from the base
station to the rover station. This allows the rover station to compute very accurate
differentially corrected positions.
The real-time Z system utilizes Ashtech’s PNAV data processing engine, which
processes the raw measurement data from both base receiver and the rover in double-
difference form. Double-difference processing, widely used in post-processing
geodetic surveying software, can achieve state-of-the-art differential GPS accuracy:
less than a centimeter.
On-the-Fly Ambiguity Resolution
The most important feature of the real-time Z system is its ability to resolve the cycle
integer ambiguities of the carrier phase measurements while the rover station is
moving, thus the term “on-the-fly”. With a PDOP less than two, and five or more
common satellites in view of the base and rover stations, real-time Z can achieve
centimeter-level accuracy very quickly, typically in less than a minute.
Fast Carrier Phase Differential
Another important feature of real-time Z is the fast CPD algorithm which reduces the
position solution latency, or delay.
Whether pseudo-range or carrier phase differential GPS, the most accurate solution is
obtained by processing the matched time-tag data from both the base station and the
rover station. This is not a problem in post-processing, since all of the raw data is
recorded in the files. For real-time applications using carrier phase differential (CPD),
the position solution could be delayed up to 2 seconds. This is due to the extra data
processing required and the latency of the radio data link. This slight delay is
acceptable for most applications such as static point surveying, but may not be
acceptable for certain vehicle navigation and kinematic surveying applications.
To reduce the position solution latency, the sensor incorporates a fast CPD algorithm
which utilizes differential correction, commonly used in pseudo-range differential
GPS, as well as an optimized data processing, reducing the CPD solution delay to
about 100 milliseconds. With fast CPD, a real-time rover can produce position
solutions up to 5 Hz, and each epoch solution is independent, not extrapolated or
interpolated.
Vector and Reverse Vector Processing
The most commonly used real-time differential GPS operation is to place one receiver
at a known position to serve as the base station. The base station transmits its raw