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C-Nav Hardware Reference Guide
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Post-processing is a method of differential data correction, which compares data logged from
a known reference point to data logged by a roving receiver over the same period of time.
Variations in the position reported by the reference station can be used to correct the positions
logged by the roving receiver. Post-processing is performed after the user collects the data and
returns to the office, rather than in real time as data is logged, so it can use complex,
calculations to achieve greater accuracy.
Precise code see P-code.
Precise Ephemeris i s the ephemeris computed after the transmission of the satellite signal and
based on satellite tracking information. It is used in post-processing of collected GPS data.
Precision is the degree of agreement or repeatability among a series of individual
measurements, values, or results. The precision of a numerical value can refer to the number of
significant digits used to express a quantity or that an instrument can measure to. Precision is
related to the quality of the operation through which the result is obtained.
PRN (Uppercase) typically indicates a GPS satellite number sequence from 1 – 32.
Projection is a mathematical formula that transforms feature locations between the earth’s
curved surface and a map’s flat surface. A projected coordinate system includes the information
needed to transform locations expressed as latitude values to x,y coordinates. Projections
cause distortion in one or more of these spatial properties-distance, area, shape and direction.
Protected code see P-code.
Pseudo-Random Noise (prn) is a sequence of data that appears to be randomly distributed
but can be exactly reproduced. Each GPS satellite transmits a unique PRN in its signals. GPS
receivers use PRNs to identify and lock onto satellites and to compute their pseudoranges.
Pseudorange is the apparent distance from the reference station’s antenna to a satellite,
calculated by multiplying the time the signal takes to reach the antenna by the speed of light
(radio waves travel at the speed of light). The actual distance, or range, is not exactly the same
because various factors cause errors in the measurement.
Radio Technical Commission for Maritime Services (RTCM) is a standard format for
Differential GPS corrections used to transmit corrections from a base station to rovers. RTCM
allows both real-time kinematic (RTK) data collection and post-processed differential data
collection. RTCM SC-104 (RTCM Special Committee 104) is the most commonly used version
of RTCM message.
Range is the distance between a satellite and a GPS receiver’s antenna. The range is
approximately equal to the pseudorange. However, errors can be introduced by atmospheric
conditions, which slow down the radio waves, clock errors, irregularities in the satellite’s orbit,
and other factors. A GPS receiver’s location can be determined if you know the ranges from the
receiver to at least four GPS satellites. Geometrically, there can only be one point in space,
which is the correct distance from each of four known points.
Real Time GIPSY (RTG) is a processing technique developed by NASA’s Jet Propulsion
Laboratory to provide a single set of real time global corrections for the GPS satellites.
Real-Time Kinematic (RTK) is a GPS system that yields very accurate 3D position fixes
immediately in real-time. The base station transmits its GPS position to roving receivers as the
receiver generates them, and the roving receivers use the base station readings to differentially
correct their own positions. Accuracies of a few centimeters in all three dimensions are possible.
RTK requires dual frequency GPS receivers and high speed radio modems.