GNSS - Receiver Archtecture

• Overall noise figure: 3-4dB, SNR = 22-23dB with 0dBic antenna

• Codes:

- C/A-code: L1 carrier, BPSK modulation, 2*1.023MHz bandwidth

- P(Y)-code: L1 and L2 carrier, BPSK modulation, 2*10.23MHz bandwidth

• De-spreading: multiplied by receiver generated spreading function (2*1.023MHz or 2*10.23MHz) -> the signal will be collapsed into a narrow bandwidth around the original carrier frequency band. Conceptually same as a narrow-band filtering. De-spreading gain.

• Tracking loop:

- Frequency bandwidth: 1Hz for code tracking, 20Hz for carrier tracking.

• Clock bias, receiver clock error

• Pseudo-range measurement

• Delta range

• Front-end

	LNA	SAW	Coax	RF	Mixer	IF	NF(total)
G(dB)	28	-1.5	-3.9	19	-6	19
NF(dB)	0.8	1.5	3.9	3.3	6	7	1

• Correlator:

- Number of correlators: directly affect TTFF and SNR, a few hundred correlators per channel are not adequate.

- Large number of correlators: fast fix with low signal level.

- NavSync CW25 chip: 12,288 correlators per channel. massively parallel search is possible. signal sensitivity as low as -155dBm can be tracked - tracking sensitivity (27dB down from normal level). Correlation gain 20-30dB.

- Code search resolution cell (code phase search): 1/2 PN chip. In the worst case, all the resolution cells in the entire uncertainty region must be tested before the signal is detected. Statistically half of the total number of resolutions cells must be tested. 511 resolution cells in time are tested in parallel. Code phase search is same as shifting the phase of the replica PRN code generated by the receiver until it correlates with the received satellite PRN code.

- Frequency search resolution cell (carrier phase search): reciprocal of the coherent integration period, use FFT to extend 511-time resolution cells to 64-frequency resolution cells. 511x64 = 32704 time-frequency resolution cells. Carrier phase search is same as changing the receiver frequency until it correlates with the received satellite carrier frequency plus Doppler.

The FFT search method is extremely efficient and effectively accomplishes massive parallel correlation comparable to the tens of thousands of correlators needed to acquire weak signals using serial search techniques. Since acquisition does not need to be a real-time continuous operation, it can be performed using a snap-shot technique to search for signals only when needed, saving both cost and circuit real-estate compared to a dedicated massive correlator chip.

The hardware search is a sequential serial search in the time domain using correlation techniques; whereas, the software search involves buffering samples to facilitate a Fourier transform. This operation is accomplished in the frequency domain by making use of the fundamental mathematical relationship that multiplication in the frequency domain is equivalent to convolution in the time domain (and vice versa). Acquisition techniques in software also complement assisted GPS (A-GPS) information as an SDR is easily tuned depending on the quality of the aiding information.

▪ Carrier racking loop

▪ Code tracking loop