Implementation of a Software-based GPS Receiver

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Implementation of a Software-based GPS Receiver

• Anthony J. Corbin
• Dr. In Soo Ahn
• Thursday, June 20, 2013

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Overview

• Progress
• Flowcharts
• Acquisition
• Tracking
• Position Calculation
• Software Organization
• Changes to Project Objectives
• Results
• DLL/PLL Tracking
• Position
• Updated Schedule

3
Progress

• MATLAB GPS software 1 has been ported to C
• This includes
• Coordinate conversion
• Tracking loop
• Acquisition algorithms
• DSP design approach was abandoned due to
  technical issues which will be discussed later.
• C code can accurately find a position from
  stored sample data.

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Coarse Acquisition

• Coarse acquisition searches around the
  intermediate frequency in the range /- 10 KHz
  with a step of 500 Hz
• Frequency Domain Correlation

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Fine Acquisition

• Uses the frequency estimate from Coarse
  Acquisition to obtain a better estimate
• The overall functionality is very similar to
  Coarse Acquisition

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Tracking
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Delay-Locked Loop 1
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Position Calculation
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Functional Software Diagram
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Changes to Project Objectives

• Finding the satellite positions requires an
  accurate timerequiring collection of at least
  subframes 1-3 of the ephemeris data
• The equation below shows the number of
  multiplications per second required to track one
  satellite. This does not include C/A code
  generation, carrier demodulation, or the overhead
  involved with sampling.
• The DSP considered is clocked at 225 MHz which is
  simply not fast enough.

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USB GPS Dongle

• USB 2.0 Interface
• Simple software interface

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C/A Code Tracking

• The graphs to the right show the code error
  output from the delay-locked loop.
• The parameters have been selected in such a way
  that the loop converges very quickly.

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Carrier Tracking

• Carrier error is shown on the right with respect.
• In this example, the frequency of the carrier
  appears to be drifting further below the
  intermediate frequency.
• This is an illustration of the Doppler Effect.

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Navigation Data

• The figures to the right show resolved 50 Hz
  navigation data.
• The top graph shows 32s of data, while the bottom
  graph shows 3s.

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Position Results
51.81 m
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Position Results
104.4 m
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Current Display

• The display currently uses a console window.
• A GUI could be written in any language.

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Speed

• Currently the C code requires under a minute
  (per satellite) to read a full 36 s of satellite
  data.
• Compare this with the Matlab code which takes 6
  minutes per satellite.

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Intel Threading Building Blocks

• Intels TBB is a library for creating threaded
  programs
• Platform independent
• Relatively easy to use

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Real-time Functionality

• Taking a direct approach to implementing
  real-time functionality appears to be extremely
  difficult (possibly impossible) given current
  hardware limitations.
• However, a possibility exists, which may feasibly
  yield results.

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Real-time Functionality
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Updated Schedule
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References

• 1 Kai Borre, Dennis M. Akos, Nicolaj Bertelsen,
  Peter Rinder, and Soren Holdt Jensent,
  Software-Defined GPS and Galileo Receiver A
  Single-Frequency Approach. Birkhauser Boston,
  2007, pp. 29, 83, 105.
• 2 SiGe, SE4110L-EK1 Evaluation Board User
  Guide.
• 3 SiGe, SE4110L Datasheet.