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 other.
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 of kilometres.
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 Technology Example
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:
1. Receive GPS satellite signals.
2. Calculate your position, speed, heading and altitude.
3. Make adjustments for Differential GPS and/or Dead Reckoning.
4. Communicate with the base station - using either built-in
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).
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