GPS Overview Appendix B
MPC User Manual Rev 0D 87
B.5 Pseudorange Algorithms
Pseudorange algorithms correlate the pseudorandom code on the GPS signal received from a
particular satellite, with a version generated within the reference station receiver itself. The time delay
between the two versions, multiplied by the speed of light, yields the pseudorange (so called because
it contains several errors) between the reference station and that particular satellite. The availability of
four pseudoranges allows the reference station receiver to compute its position (in three dimensions)
and the offset required to synchronize its clock with GPS system time. The discrepancy between the
reference station receiver’s computed position and its known position is due to errors and biases on
each pseudorange. The reference station receiver sums these errors and biases for each pseudorange,
and then broadcasts these corrections to the remote station. The remote receiver applies the
corrections to its own measurements; its corrected pseudoranges are then processed in a least-squares
algorithm to obtain a position solution.
The “wide correlator” receiver design that predominates in the GPS industry yields accuracies of 3-5
m (SEP). NovAtel’s patented Narrow Correlator tracking technology reduces noise and multipath
interference errors, yielding accuracies of 1 m (SEP).
B.5.1 Pseudorange Differential Positioning
B.5.1.1 GPS System Errors
In general, GPS SPS C/A code single-point pseudorange positioning systems are capable of absolute
position accuracies of about 40 meters or less. This level of accuracy is really only an estimation, and
may vary widely depending on numerous GPS system biases, environmental conditions, as well as the
GPS receiver design and engineering quality.
There are numerous factors which influence the single-point position accuracies of any GPS C/A code
receiving system. As the following list will show, a receiver’s performance can vary widely when
under the influences of these combined system and environmental biases.
• Ionospheric Group Delays – The earth’s ionospheric layers cause varying degrees of GPS
signal propagation delay. Ionization levels tend to be highest during daylight hours causing
propagation delay errors of up to 30 meters, whereas night time levels are much lower and
may be as low as 6 meters.
• Tropospheric Refraction Delays – The earth’s tropospheric layer causes GPS signal
propagation delays. The amount of delay is at the minimum (about three metres) for satellite
signals arriving from 90 degrees above the horizon (overhead), and progressively increases
as the angle above the horizon is reduced to zero where delay errors may be as much as 50
metres at the horizon.
• Ephemeris Errors – Some degree of error always exists between the broadcast ephemeris’
predicted satellite position and the actual orbit position of the satellites. These errors will
directly affect the accuracy of the range measurement.
• Satellite Clock Errors – Some degree of error also exists between the actual satellite clock
time and the clock time predicted by the broadcast data. This broadcast time error will cause
some bias to the pseudorange measurements.
• Receiver Clock Errors – Receiver clock error is the time difference between GPS receiver
time and true GPS time. All GPS receivers have differing clock offsets from GPS time that
vary from receiver to receiver by an unknown amount depending on the oscillator type and
quality (TCXO vs. OCXO, etc.). However, because a receiver makes all of its single-point
pseudorange measurements using the same common clock oscillator, all measurements will
be equally offset, and this offset can generally be modeled or quite accurately estimated to
