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## GPS Signal Structure

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GPS Signal Structure

- Sources:
- GPS Satellite Surveying, Leick
- Kristine Larson Lecture Notes

http://www.colorado.edu/engineering/ASEN/asen4519/asen4519.html

GPS Signal Requirements

- Method (code) to identify each satellite
- The location of the satellite or some information on how to determine it
- Information regarding the amount of time elapsed since the signal left the satellite
- Details on the satellite clock status

Important Issues to Consider

- Methods to encode information
- Signal power
- Frequency allocation
- Security
- Number and type of codes necessary to satisfy system requirements

Overview of Satellite Transmissions

- All transmissions derive from a fundamental frequency of 10.23 Mhz
- L1 = 154 • 10.23 = 1575.42 Mhz
- L2 = 120 • 10.23 = 1227.60 Mhz
- All codes initialized once per GPS week at midnight from Saturday to Sunday
- Chipping rate for C/A is 1.023 Mhz
- Chipping rate for P(Y) is 10.23 Mhz

Digital Modulation Methods

- Amplitude Modulation (AM) also known as amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal.
- Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1.
- Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa. This is the method used in GPS.

Modulo-2 recovery of GPS code

Modulo-2 arithmetic: 0 + 0 = 0; 0 + 1 = 1; 1 + 0 = 1; 1 + 1 = 0

Bit shifts aligned

MUST MOD-2 ADD RECEIVER-GENERATED CODE TO RECOVER

Superposition of codes - details

- Superposition of two codes is not unique because the bit transition occurs at the same epoch; remember that both codes and phases are multiples of the fundamental frequency
- Need to impose an additional constraint to arrive at a solution - quadri-phase-shift keying (QPSK), which puts the two codes 90° (p/2)

Phase and Quandrature - General

General Expression:

2

All spectral components of y1(t) are 90° out of phase

with those of y2(t). This allows this the two signals to

be separated in the receiver.

GPS signal strength - frequency domain

Note that C/A code is below noise

level; signal is multiplied in the

Receiver by the internally calculated

code to allow tracking.

C/A-code chip is 1.023 Mhz

P-code chip is 10.23 Mhz

Power = P(t) = y2(t)

The calculated power spectrum

derives from the Fourier

transform of a square wave

of width 2π and unit amplitude.

Common function in DSP

called the “sinc” function.

Digital Signal Processing Techniques

- Filtering: Allows one to remove some portion of the frequency spectrum that may contain unwanted signal.
- Low Pass Filter: lets all frequencies below a cutoff frequency through.
- High Pass Filter: lets all frequencies above a cutoff frequency through.
- Band Pass Filter: lets all frequencies within a specified window pass through. The window is called the passband

DSP Techniques, con’t.

- Frequency Translation and Multiplication: technique to shift frequency spectrum of some signal to another portion of the frequency domain.
- Up-conversion: translate signal to higher frequencies.
- Down-conversion: translate signal to lower frequencies. Commonly done in GPS receivers. Multiply signal by sine function in a “mixer.” Special case is signal squaring and may be used to recover the pure carrier phase from a bi-phase modulated ranging signal.

DSP Techniques, con’t.

- Spread Spectrum: broadly defined as a mechanism by which the bandwidth of the transmitted code is much greater than the baseband information signal (e.g. the navigation message in GPS)
- FDMA: Frequency Division Multiple Access. Requires different carriers. Used by GLONASS.
- TDMA: Time Division Multiple Access. Several channels share transmission link. Used by many cellular telephone providers and LORAN-C.
- CDMA: Code Division Multiple Access. Requires pseudorandom codes by transmitted and also generated for correlation within the receiver. Used by GPS.

DSP Techniques, con’t.

- Cross-correlation: Used by GPS receivers to determine what signal is coming from a specific satellite. Can be generalized to extracting information from any multiplexed digital signal.

PRN Cross-correlation

Correlation of receiver generated PRN code (A) with incoming data

stream consisting of multiple (e.g. four, A, B, C, and D) codes

Schematic of C/A-code acquisition

Since C/A-code is 1023 chips long and repeats every 1/1000 s, it is inherently ambiguous by 1 msec or ~300 km. Must modulo-2 add the transmitted and received codes after correlation to increase SNR and narrow bandwidth.

Methods to Cope with Anti-spoofing

- Anti-spoofing: Implemented in 1994 to make P-code unavailable to non-military users. Encrypted P-code is referred to as Y-code.
- Squaring: Yields half-wavelength carrier and greatly reduces SNR. Old technology.
- Code-aided squaring: Uses mathematical similarity of the Y-code to P-code. L1 carrier is down-converted and multiplied with a local replica of the P-code, then squared. Results in less reduction of SNR than simple squaring.

Anti-spoofing Methods, con’t.

- Cross-correlation: Takes advantage of the fact that both L1 and L2 are modulated with the same P(Y)-code, despite lack of knowledge of the actual P-code. Yields the difference in pseudoranges, P1(Y) - P2(Y), and the phase difference of L1 and L2. Again less SNR loss compared with squaring. Can be difficult to track at low elevation angles. Technique employed in Trimble 4000SSi/SSE.
- Z-tracking: Takes advantage of the fact that Y-code is the modulo-2 sum of the P-code with a lower encryption rate. Yields L1 and L2 Y-code pseudoranges and the full carrier phases of L1 & L2. This method yields the best SNR. Multipath performance is better than other methods. Technique employed in Ashtech Z-12 and micro-Z.

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