GPS Signal Structure

1
GPS Signal Structure

• Sources
• GPS Satellite Surveying, Leick
• Kristine Larson Lecture Notes
• http//www.colorado.edu/engineering/ASEN/asen4519/
  asen4519.html

2
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

3
Important Issues to Consider

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

4
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

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Schematic of GPS codes and carrier phase
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GPS Signal Characteristics
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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.

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Modulation Schematics
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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
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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)

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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.
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Codes on L1 and L2
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Codes on L1 and L2 (cont.)
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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 2p
and unit amplitude. Common function in DSP
called the sinc function.
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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

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DSP Techniques, cont.

• 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.

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DSP Techniques, cont.

• 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.

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DSP Techniques, cont.

• 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.

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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
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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.
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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.

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Anti-spoofing Methods, cont.

• 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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AS Technologies Summary Table
Ashtech Z-12 µZ
Trimble 4000SSi
From Ashjaee Lorenz, 1992