Lec05

one in the satellite and the other in the receiver The ranges are biased by satellite and receiver clock errors and, consequently, they are denoted as pseudoranges
主讲：张小红
1.3 Phase pseudoranges
Carrier Reconstruction
Code modulation is pseudorandom 180°phase shifts Decode (remove) the PRN signal ( from the modulated carrier phase signal Carrier Reconstruction is procedure that receiver picks up broadcast and decomposes the signal to recover
GPS Principle and Applications
主讲：张小红
1 Data acquisition
In concept, the GPS observables are ranges which are deduced from measured time or phase differences based on a comparison between received signals and receiver generated signals. “one way concept” –Satellite to receiver two clocks (active) are used
GPS Principle and Applications
主讲：张小红
1.1 Data types
Code Pseudoranges Carrier Phases Dopplers
GPS Principle and Applications
主讲：张小红
1.2 Code pseudoranges
discontinuous
Reconstructed carrier
Demodulation
Continuous signal
GPS Principle and Appltion techniques
The actual pseudorange measurements are performed in tracking loop circuits. The code ranges are determined in the delay lock loop (DLL) by using code correlation techniques. After removing the PRN code from the incoming signal and performing some filtering, the unmodulated carrier wave is obtained. This carrier ware is then passed to the phase lock loop (PLL) where the signal is compared with a carrier replica (generated in the receiver) to give the (fractional) phase offset between the two signals.
Δδ = δ R − δ S
R = cΔt = cΔt (GPS ) + cΔδ = ρ + cΔδ
GPS Principle and Applications
主讲：张小红
1.2 Code pseudoranges
to the distance between the position of the satellite at epoch ts(GPS) and the position of the antenna of the receiver at epoch tR (GPS) ρ = ρ (t S , t R ) = ρ (t s , (t s + Δt ))
c
ρ
ϕ R (t ) = f R t − ϕ 0 R
Pseudorange
Δt = t R − t S = [t R (GPS ) + δ R ] − [t S (GPS ) + δ S ] = Δt (GPS ) + Δδ
Where Δt (GPS ) = t R (GPS ) − t S (GPS ) C/A pseudorange P1 pseudorange P2 pseudorange
Observables and combinations
Lecture 5
Outline
Data acquisition
Code pseudoranges Carrier phases Doppler data Data combinations
Linear phase combinations Code pseudorange smoothing
Code Navigation message Carrier
GPS Principle and Applications
主讲：张小红
1.3.1 Phase Modulation and demodulation
PRN code
Carrier and data
Modulation
Resulting signal
GPS Principle and Applications
主讲：张小红
1.3.2 Reconstruction techniques
Code correlation Squaring Cross correlation Z-tracking
GPS Principle and Applications
Very accurate: 2~3mm noise 0.2~0.3mm is possible recently
Disadvantages
Ambiguities of carrier phase Cycle slips happen (lost cycle counts)
GPS Principle and Applications
GPS Principle and Applications
主讲：张小红
1.3.2 Reconstruction techniques
Squaring technique (codeless technique)
The procedure is based on autocorrelating the received signal The received signal is mixed (i.e., multiplied) with itself and, hence, all modulations are removed The result is the unmodulated carrier with twice the frequency, and thus, half the wavelength It is more difficult to resolve the ambiguities of the squared signals with halved wavelength. This technique has the advantage of being independent of PRN codes. The drawback of squaring is that the satellite clock and the satellite orbit information are lost in the process. signal-to-noise ratio (SNR) is substantially reduced in the squaring process
GPS Principle and Applications
主讲：张小红
1.3.2 Reconstruction techniques
Cross correlation techinque
The observable resulting from the correlation process are the range difference between the two signals obtained from the time delay of the Y-code on the two carriers, that is RL 2,Y − RL1,Y , and a phase difference Φ L 2 − Φ L1 obtained from the beat frequency carrier. The cross correlation outputs may be used to derive the L2 code range and phase by forming
3m
P1 pseudorange (Z-tracking)
0.3m
P2 pseudorange (Z-tracking, cross correlation)
0.3m
GPS Principle and Applications
主讲：张小红
1.3 Phase pseudoranges
Advantages
RL 2 = RL1,C / A + ( RL 2,Y − RL1,Y )
Φ L 2 = Φ L1,C / A + (Φ L 2 − Φ L1 )
GPS Principle and Applications
主讲：张小红
1.3.2 Reconstruction techniques
Z-tracking technique (please refer to Page 52 for details
Navigation message P1 and P2 L1 and L2 with full wavelength Good SNR
GPS Principle and Applications
主讲：张小红
1.3.3 carrier phase
Beat phase equation
ϕ = f (t − t ρ ) = f (t − ) c S S S ρ ϕ (t ) = f t − f − ϕ 0S
ρcorresponds
ts (GPS)
= ρ (t ) + ρ (t ) Δt
S S
i
t s + Δt
GPS Principle and Applications
主讲：张小红
1.2 Code pseudoranges
Pseudorange precision and types
Precision: about 1% of the chip length Recent a precision of about 0.1% of the chip length is possible C/A pseudorange (code correlation)
主讲：张小红
1.3.2 Reconstruction techniques
Code correlation (page 50)
The code correlation technique provides all components of the satellite signal: the satellite clock reading, the navigation message, and the unmodulated carrier. The drawback is that the procedure requires knowledge of one PRN code. Correlation steps refer to page 50