Measurement methods

The distance is derived primarily through measuring the time difference between the transmission from the satellite and reception at the receiver of a coded signal. This range is more properly known as the pseudorange since it is affected by a number of system unknowns including clock biases, propagation delays and systematic errors described above, which must be solved for or estimated.

GPS Carrier Frequencies

In addition to time, ID, status etc messages, a satellite also sends on L1 a signal called the C/A (Coarse Acquisition) Code.

C/A Code on L1 Carrier

The above diagram shows the shift in time measured by comparing the C/A signal received from the satellite with an identical signal emitted at the same time by the ground receiver. The time shift is the time taken for the satellite signal to reach the receiver.

Measurement accuracy

In normal standalone operation, GPS will give a three-dimensional position accuracy of around 5 to 15m, and also provides velocity to approximately 20 cm/s and time to within 1 microsecond. These accuracies are dependent on the quality of user equipment, (low cost Commercial Grade receivers will give lower accuracy than higher cost Professional Grade receivers), error sources present, and the geometric configuration of the satellites that are being tracked. If the satellites tracked are all in one portion of the sky for example, the available fix geometry is poor and attainable accuracy will be affected. This is known as Dilution of Precision. Different GPS applications require varying degrees of positional accuracy. In-car and personal navigation, for example, require only the standard GPS positioning accuracy, whereas more demanding applications require augmentation of the standard GPS data, be it in terms of integrity or correction information.

Range measurement accuracy is estimated to be approximately 1% of wavelength. Wavelength of L1 C/A code is approximately 300m, so accuracy of measurement is approximately 3 metres. Add this to fundamental system errors discussed earlier of 11 to 12 m and total fix accuracy is approximately 15m for general Commercial Grade GPS, using code based measurement via the L1 carrier.

If you want better accuracy than this you need to use some form of Differential GPS (DGPS).

Blackroc manufacture a range of professional grade GNSS solutions which enables satellite fix accuracy in real time from submetre to 1cm (RTK)

Procyon GNSS range


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