Carrier Phase Tracking (Surveying): An
Application of GPS
Carrier-phase tracking of GPS signals has resulted in a revolution in
land surveying. A line of sight along the ground is no longer necessary for
precise positioning. Positions can be measured up to 30 km from reference point
without intermediate points. This use of GPS requires specially equipped
carrier tracking receivers.
The L1 and/or L2 carrier signals are used in carrier phase surveying. L1
carrier cycles have a wavelength of 19 centimetres. If tracked and measured
these carrier signals can provide ranging measurements with relative accuracies
of millimetres under special circumstances.
Tracking carrier phase signals provides no time for transmission
information. The carrier signals, while modulated with time tagged binary
codes, carry no time-tags that distinguish one cycle from another. The
measurements used in carrier phase tracking are differences in carrier phase
cycles and fractions of cycles over time. At least two receivers track carrier
signals at the same time. Ionospheric delay differences at the two receivers
must be small enough to insure that carrier phase cycles are properly accounted
for. This usually requires that the two receivers be within about 30 km of each
Carrier phase is tracked at both receivers and the changes in tracked
phase are recorded over time in both receivers. All carrier-phase tracking is
differential, requiring both a reference and remote receiver tracking carrier
phases at the same time. Unless the reference and remote receivers use L1-L2
differences to measure the ionospheric delay, they must be close enough to
insure that the ionospheric delay difference is less than a carrier wavelength.
Using L1-L2 ionospheric measurements and long measurement averaging periods,
relative positions of fixed sites can be determined over baselines of hundreds
Phase difference changes in the two receivers are reduced using software
to differences in three position dimensions between the reference station and
the remote receiver. High accuracy range difference measurements with
sub-centimetre accuracy are possible. Problems result from the difficulty of
tracking carrier signals in noise or while the receiver moves. Two receivers
and one SV over time result in single difference.
Automatic Vehicle Location, A Tracking
Automatic Vehicle Location (AVL) is a technology
used for track-ing vehicles, vessels, and mobile assets such as trailers,
containers, and equipment. Each mobile unit has a GPS receiver that reports its
posi-tion to the base station over a communications network. This allows the
base station to monitor the entire fleet and manage the mobile as-sets.
Mobile GPS Unit
In each vehicle you need a GPS receiver to track the satellites and
calculate your position. But actually Trimble's mobile GPS units do a lot more
than just that. Altogether they:
Receive GPS satellite signals.
Calculate your position, speed, heading and
Make adjustments for Differential GPS and/or
Communicate with the base station - using either
communications or interfacing
with an external radio.
Use the IQ Event Engine to decide when to report.
Receive the precise time (the satellites use atomic clocks).