A GPS Overview
56 MiLLennium Command Descriptions Manual
•Ionospheric Group Delays – The earth’s ionospheric layers cause varying degrees of GPS
signal propagation delay. Ionization levels tend to be highest during daylight h ours causing
propagation delay errors of up to 30 metres, whereas night time levels are much lower and
may be up to 6 metres.
•Tropospheric Refraction Delays – The earth’s tropospheric layer causes GPS signal
propagation delays which bias the range measurements. 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 modelled or quite accurately estimated to
effectively cancel the receiver clock offset bias. Thus, in single point positioning, receiver
clock offset is not a significant problem. However, in pseudorange differential operation,
between-receiver clock offset is a source of uncorrelated bias.
•Selective Availability (SA) – Selective availability is when the GPS Control Segment
intentionally corrupts satellite clock timing and broadcast orbit data to cause reduced
positioning accuracy for general purpose GPS SPS users (non-military). When SA is active,
range measurements may be biased by as much as 30 metres .
•Multipath Signal Reception – Multipath signal reception can potentially cause large
pseudorange and carrier phase measurement biases. Multipath conditions are very much a
function of specific antenna site location versus local geography and man-made structural
influences. Severe multipath conditions could skew range measurements by as mu ch as 100
metres or more. See Appendix B, Multipath Elimination Technology for more information.
The NovAtel GPSCard receivers are capable of absolute single point positioning accuracies of 15 metres CEP
(GDOP < 2; no multipath) when SA is off and 40 metres CEP while SA is on. (As the status of selective availability
is generally unknown by the real-time GPS user, the positioning accuracy should be considered to be that of when
SA is on).
The general level of accuracy available from single point operation may be suitable for many types of positioning
such as ocean going vessels, general aviation, and recreational vessels that do not require position accuracies of
better than 100 metres CEP. However, increasingly more and more applications desire and require a much higher
degree of accuracy and position confidence than is possible with single point pseudorange positioning. This is
where differential GPS (DGPS) plays a dominant role in higher accuracy real-time positioning systems.
SINGLE POINT AVERAGING WITH THE GPSCARD
By averaging many GPS measurement epochs over several hours, it is possible to achieve an absolute position
based on the WGS 84 datum to better than five meters. This section attempts to explain how the position averaging
function operates and to provide an indication of the level of accuracy that can be expected versus total averaging
time.
The POSAVE command implements position averaging for reference stations. Position averaging will continue for
a specified number of hours or until the averaged position is within specified accuracy limits. Averaging will stop
when the time limit or the horizontal standard deviation limit or the vertical standard deviation limit is achieved.