88 MPC User Manual Rev 0D
Appendix B GPS Overview
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 caus e 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.
NovAtel’s OEM4 GPSCard is a civilian dual frequency GPS receiver that normally tracks:
• L1 C/A Code
•L1 Carrier Phase
• L2 P or Y Code
• L2 (Codeless) Carri er Phase
P Code is never tracked on L1 even if AS is disabled.
•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 condit ions cou ld skew range measu rements by as much as 100
meters or more.
The NovAtel GPSCard receivers are capable of absolute single-point positioning accuracies of 15
meters CEP (GDOP < 2; no multipath) when SA is off and 40 meters CE P while SA is on. (As the
status of selective availabil ity is gener ally 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 accu racy available from single-po int operatio n may be suitable for many types of
positioning such as ocean going vessels, general aviation, and recreation a l vessels that do not require
position accuracies of better than 40 meters 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 pseud orange positioning . This is where dif ferentia l GPS (DGPS) plays a domi nant role in
higher accuracy real-time positioning systems.