User`s guide

Appendix
Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05
209
RDOP
Relative dilution of precision. See Dilution of Precision.
Reconstructed Carrier Phase
The difference between the phase of the incoming Doppler shifted GPS carrier and the
phase of a nominally constant reference frequency generated in the receiver. For static
positioning, the reconstructed carrier phase is sampled at epochs determined by a clock
in the receiver. The reconstructed carrier phase changes according to the continuously
integrated Doppler shift of the incoming signal, biased by the integral of
the frequency offset between the satellite and receiver reference oscillators. The
reconstructed carrier phase can be related to the satellite to receiver range, once the
initial range (or phase ambiguity) has been determined. A change in the satellite to
receiver range of one wavelength of the GPS carrier (19 cm for L1) will result in a one-
cycle change in the phase of the reconstructed carrier.
Relative Navigation
A technique similar to relative positioning except that one or both of the points may be
moving. The pilot of a ship or an aircraft may need to know the vehicle's position relative
to a harbor or runway. A data link is used to relay the error terms to the moving vessel to
allow real-time navigation.
Right Ascension
The angular distance measured from the vernal equinox, positive to the east, along the
celestial equator to the ascending node. Typically denoted by a capital omega (Ω). Used
to discriminate between orbital planes.
RTCM
Radio Technical Commission for Maritime Services. Commission set up to define a
differential data link to relay GPS correction messages from a monitor station to a field
user. RTCM SC-104 recommendations define the correction message format and 16
different correction message types.
SATNAV
A local term referring to use of the older TRANSIT system for satellite navigation. One
major difference between TRANSIT and GPS is that the TRANSIT satellites are in low-
altitude polar orbits with a 90-minute period.
Selective Availability (SA)
A DoD program to control the accuracy of pseudorange measurements, whereby civilian
users receives a false pseudorange which is in error by a controlled amount. Differential
GPS techniques can reduce these effects for local applications. Under SA, the DOD
guarantees unauthorized users an accuracy of 100m 2DRMS at a 95% confidence level.
SA was deactivated in May of 2000 resulting in much better accuracies using standard
commercial GPS receivers, but DoD has the capability to reactivate it at any time.
Semi-major Axis
One half of the major axis of an ellipse.