DATA MINING

Data Mining and Machine Learning

Data Mining and Machine Learning

Carrier Based Sliding Correlator

The tau-dither method just described is rarely used in the form outlined above. Rather it is used in carrier based system. Both the tau-dither and early late systems we have just covered are baseband correlators as no carrier (or IF) frequency was used. For the receiver described in this text a carrier based correlation method is used. In order to use the carrier based system requires that a sinewave be modulated with the C/A code so that it its phase is switched between 0 and 180 degrees. Such a device is the BPSK modulator, See Appendix C. The BPSK modulator is the basic building block of the carrier based sliding correlator. BPSK Modulators are reciprocal devices in that they can both modulate and demodulated binary phase shifted carriers. It is the “undoing” of the phase modulation function of BPSK modulators we seek to employ here.

Figure A8 shows a sliding carrier based correlator. Once again we have two C/A code generators with GEN#2 clock rate slightly offset from clock of GEN#1. The major difference from before is that GEN#1 C/A code is modulating sinewave carrier. This output models the GPS transmitter code modulation of the carrier with no 50hz data present. This signal is now fed into another multiplier where GEN#2 code is applied. The output of this second multiplier is now bandpass filtered with a filter that has a center frequency at the carrier frequency used. When the replica C/A code is within 2 chips of the reference C/A code the output of the bandpass contains a sinewave. The amplitude of this sinewave grows then decays as the two codes slide in and out of correlation. This is indicated in Fig. 10 by the Amp/Freq/Time plot and the carrier in the diamond shaped envelope.

The principle of operation is very simple. Once the first multiplier modulates the carrier, the second multiplier will completely remove the carrier modulation if the C/A code of GEN#2 is perfectly lined up with the C/A code of GEN#1. In this condition the output of the second multiplier is the original sinewave(at the input of first multiplier)along with some artifacts from the 2nd multiplier. The bandpass filter serves roughly the same function as the lowpass filter did in the baseband correlators, as an integrator and a to filter out unwanted multiplier artifacts. For those familiar with Fourier transform recall that multiplication by a sinewave results in a frequency shift in the frequency domain. So the baseband signal spectrums are shifted up by the carrier frequency as well as the lowpass filter operation. A frequency shifted lowpass filter is just a bandpass filter.

When the C/A codes from GEN#1 and GEN#2 are not aligned ( > two chip offset) the output of the bandpass filter would be the power spectrum a BPSK modulated carrier. But in a real world receiver the output of the bandpass filter is noise when the codes are not ...