POS MV V5 Installation and Operation Guide
Theory
of Operation
Copyright © Applanix Corporation, 2017
B-21
Each receiver can measure the fraction of a wavelength from each tracked satellite but cannot determine
the integer number of wavelengths between the transmitting satellite and the antenna.
By using satellite pseudo-range, the receiver can reduce the uncertainty in knowing the integer number of
wavelengths to less than 100. The remaining unknown integer number of wavelengths is the carrier
phase ambiguity.
GAMS uses an On-the-Fly (OTF) ambiguity resolution algorithm to resolve the carrier phase ambiguities.
On-the-fly implies that the algorithm resolves the phase ambiguities while the antennas are moving.
The OTF algorithm uses carrier phase measurements from five or more satellites to estimate and,
eventually, to identify a set of integer phase ambiguities for each satellite being tracked by both receivers.
The Carrier Phase Differential Position topic on page B-22 explains this process simply.
For the OTF algorithm to work, both receivers must track at least five common satellites to resolve the
ambiguities. Preferably, they should track six or more to resolve the ambiguities within a reasonable
length of time.
The GAMS OTF algorithm uses the roll, pitch and initial heading computed by POS MV (without GAMS
heading aiding) to accelerate the ambiguity resolution process. Practically, if POS MV uses the IMU
heading alignment process to compute the heading with an RMS accuracy of 1° or better, then ambiguity
resolution is almost instantaneous and can be completed in less than ten seconds. You can achieve this
degree of heading measurement accuracy without GAMS heading aiding by executing a few vessel turns
after you power-on POS MV.
This facility also allows GAMS to recover very quickly following a GNSS dropout caused by a signal
blockage when passing under a bridge for example.
Surveyed Antenna Baseline Vector
Figure 91 shows the geometry of the two GNSS antennas and the IMU. The antenna baseline vector is
shown as
∆
R
AB
.
During normal operation, GAMS measures this vector resolved in the geographic or North-East-Down
frame (the geographic antenna baseline vector) by using a carrier phase differential GNSS algorithm.
