GNSS Principles

1) Signal acquisition

▪ GPS signal from satellite is very weak. To boost the signal, the receiver should know the exact frequency and the synchronization timing of the special code of each GPS signal. The code of each satellite is known.

▪ The process of boosting a weak signal by matching frequency and timing is called 'acquisition'.

(More details)

▪ Satellite search: 32 possible PRN codes, 1023 C/A code chips, ±5kHz Doppler frequency range (due to 500m/s movement of SV relative to stationary receiver), 50Hz typ. Doppler bin (0.7T, T: dwell time)

▪ Acquisition procedures:

- Correlation (de-spreading): integration (multiply and add) for pre-integration interval (PDI) -> shift the code chip and do the integration again until the maximal signal power is detected.

- Processing gain = (signal bandwidth)/(message bandwidth)

- Find code peak, find frequency peak, close code loop, close frequency loop, code and frequency loops aid each other.

- Code tracking loop(DLL), carrier tracking loop(PLL), decode navigation data (20m boundary bit edge detection, decode ephemeris taking 15-28s)

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- Simultaneous (2D) search Doppler frequency shift and code offset (delay)

- Correlation: multiplying GPS signal with a locally generated version of the satellites CDMA code with a given delay.

- Integration: search of the exact delay producing the maximum correlator output. 1ms(=C/A code repetition time) for strong signal. For weak signals, the integ. time increased to enhance S/N.

- Time for Doppler freq. fix: proportionally increased as integration time increased.

- Acquisition time: proportional to the square of integration time. N²

- World record: -183dBW using 265ms integration period

- Cross correlation: correlation between SV codes. less than autocorrelation by 24dB.

- Doppler processing: sequential method (slower, overall acq. time N²), non-sequential FFT method (faster, overall acq. time, N^1.5)

2) Position fix

▪ SV(satellite vehicle): transmits the exact time and location of data transmission

▪ Data from 3 SVs are enough to obtain the receivers location (triangulation, or fix) if the receiver's clock is same as the SV's clock. At any given time 6 SVs are visible.

▪ Without an exact clock at the receiver, we need 4 SVs to solve for the receiver's location.

▪ When only 3 SVs are available, the fix is obtained by assuming the receiver is on the mean sea level, with resulting position less accurate.

▪ TTFF(time-to-first-fix): time from the receiver power on to the first position fix.

(More details)

▪ More than 4 pseudoranges need: 3D position + clock error

▪ Pseudorange error will affect positioning accuracy

▪ Apply least-square solutions to find the range.

▪ Final data available: position, velocity, receiver clock offset from GPS time.

3) Tracking

▪ SVs are constantly moving and the receiver may be moving as well. Some SV might disappear from view and other SVs may come into view.

▪ The receiver keeps track of SVs in view and acquires new SV appearing into view constantly calculating the position data. This process is called 'tracking'

▪ Initial acquisition is followed by tacking mode. The receiver's revised estimate of pseudo-range and carrier frequency derived from the initial acquisition are used to track the signal.

4) Reacquisition

▪ Re-acquiring SVs and re-fixing the receiver's location after a temporary loss of SVs' signals due to signal blockage, power-off etc.

▪ Usually a very fast TTFF is possible since data stored in a moment ago for acquisition and location can be used.