User's Manual Part 1

Positioning Modes of Operation Chapter 5
OEMV Family Installation and Operation User Manual Rev 5B 91
5.5 Carrier-Phase Differential
Carrier-phase algorithms monitor the actual carrier wave itself. These algorithms are the ones used in
real-time kinematic (RTK) positioning solutions - differential systems in which the rover station,
possibly in motion, requires base-station observation data in real-time. Compared to pseudorange
algorithms, much more accurate position solutions can be achieved: carrier-based algorithms can
achieve accuracies of 1-2 cm (RMS).
Kinematic GPS using carrier-phase observations is usually applied to areas where the relation
between physical elements and data collected in a moving vehicle is desired. For example, carrier-
phase kinematic GPS missions have been performed in aircraft to provide coordinates for aerial
photography, and in road vehicles to tag and have coordinates for highway features. This method can
achieve similar accuracy to that of static carrier-phase, if the ambiguities can be fixed. However,
satellite tracking is much more difficult, and loss of lock makes reliable ambiguity solutions difficult
to maintain.
A carrier-phase measurement is also referred to as an accumulated doppler range (ADR). At the L1
frequency, the wavelength is 19 cm; at L2, it is 24 cm. The instantaneous distance between a GPS
satellite and a receiver can be thought of in terms of a number of wavelengths through which the
signal has propagated. In general, this number has a fractional component and an integer component
(such as 124 567 967.330 cycles), and can be viewed as a pseudorange measurement (in cycles) with
an initially unknown constant integer offset. Tracking loops can compute the fractional component
and the change in the integer component with relative ease; however, the determination of the initial
integer portion is less straight-forward and, in fact, is termed the ambiguity.
In contrast to pseudorange algorithms where only corrections are broadcast by the base station,
carrier-phase algorithms typically “double difference” the actual observations of the base and rover
station receivers. Double-differenced observations are those formed by subtracting measurements
between identical satellite pairs on two receivers:
ADR
double difference
= (ADR
rx A,sat i
- ADR
rx A,sat j
) - (ADR
rx B,sat i
- ADR
rx B,sat j
)
An ambiguity value is estimated for each double-difference observation. One satellite is common to
every satellite pair; it is called the reference satellite, and it is generally the one with the highest
elevation. In this way, if there are n satellites in view by both receivers, then there are n-1 satellite
pairs. The difference between receivers A and B removes the correlated noise effects, and the
difference between the different satellites removes each receiver’s clock bias from the solution.
In the RTK system, a floating ambiguity solution is continuously generated from a Kalman filter.
When possible, fixed-integer ambiguity solutions are also computed because they are more accurate,
and produce more robust standard-deviation estimates. Each possible discrete ambiguity value for an
observation defines one lane. That is, each lane corresponds to a possible pseudorange value. There
are a large number of possible lane combinations, and a receiver has to analyze each one in order to
select the correct one. L2 measurements provide additional information making results faster and
more reliable. In summary, NovAtel’s RTK system permits L1/L2 receivers to choose integer lanes
while forcing L1-only receivers to rely exclusively on the floating ambiguity solution.
Once the ambiguities are known, it is possible to solve for the vector from the base station to the rover
station. This baseline vector, when added to the position of the base station, yields the position of the
rover station.