Implementation of a Software- based GPS Receiver Anthony J. Corbin Dr. In Soo Ahn Thursday, June 25, 2015

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

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.

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

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

Tracking

Delay-Locked Loop [1]

Position Calculation

Functional Software Diagram

Changes to Project Objectives  Finding the satellite positions requires an accurate time…requiring 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.

USB GPS Dongle  USB 2.0 Interface  Simple software interface

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.

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.

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.

Position Results m

Position Results m

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

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.

Intel Threading Building Blocks  Intel’s TBB is a library for creating threaded programs  Platform independent  Relatively easy to use

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.

Real-time Functionality

Updated Schedule

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.