Implementation and Statistical Analysis of a Differential GPS System

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Implementation and Statistical Analysis of a
Differential GPS System
Team Members Jim Connor Jon Kerr
Advisor Dr. In Soo Ahn
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Abstract

• Normal GPS (Global Positioning System) is not
  accurate enough for the applications here at
  Bradley University. For greater accuracy, a
  Differential GPS system will be implemented. To
  do this, two GPS units are required. A base
  station, with a known position, sends error
  correction data to a mobile unit. The error
  correction data is sent wirelessly through a
  radio link. The data can then be viewed on a
  laptop computer for statistical analysis.

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Project Purpose

• Previous project
• Autonomous Vehicle with GPS Navigation
• - Jason Seelye and Bryan Everett
• Problem
• GPS not accurate enough to control vehicle
• Our focus
• Create GPS system with the greatest accuracy
  possible for the control of autonomous vehicles
  (sidewalk)

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Topics of Discussion

• Explanations and Terminology
• Equipment used
• Project description
• Hardware implementation process
• Software implementation process
• Problems encountered and solutions
• Data gathering
• Statistical Analysis of data
• Conclusions and Recommendations

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Explanations and Terminology

• Global Positioning System (GPS) A satellite
  navigation system capable of providing highly
  accurate position, velocity, and timing
  information.
• Differential Global Positioning System (DGPS) A
  GPS system that is capable of being more accurate
  by taking into account position correction
  information.
• Circular Error Probability (CEP) Radius of the
  circle, centered at the known antenna position,
  that contains 50 of the data points in a
  horizontal scatter plot.
• Dilution of Precision (DOP) Accuracy of
  position due to satellite geometric positions.

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Specifications
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Equipment Used

• Two NovAtel RT-20 Receivers
• Operate at 1575.42 MHz
• 12 Channel Receivers
• Two FreeWave Radios
• Operate at 928 MHz
• 20 mile line of sight range
• NEC Laptop Computer

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Block Diagram
Transmitter / Base Station
Position Error Corrections
GPS Antenna
NovAtel Receiver
FreeWave Radio
Antenna
Receiver / Mobile Station
Position Error Corrections
FreeWave Radio
NovAtel Receiver
Laptop Computer for analysis
Antenna
GPS Antenna
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Block Diagram
Binary data stream
Reference Station
RF modem
RF modem
Mobile Station
Commands
Data
Computer
Matlab
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Functional Description

• An exact geographical position is determined.
• Reference station placed at this point.
• Since at a known position, able to calculate
  errors from GPS satellites.
• Sends error corrections across a wireless radio
  link to remote station.
• Remote station receives error corrections and
  also position information from same satellite
  constellation that reference station sees.
• Remote station uses both satellite data and error
  corrections to calculate position.

Serial Correction Data
NovAtel Receiver Base Station
NovAtel Receiver Remote Station
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Errors Removed by Differential GPS

• Ionosphere 0-30 meters Mostly Removed
• Troposphere 0-30 meters All Removed
• Signal Noise 0-10 meters All Removed
• Ephemeris Data 1-5 meters All Removed
• Clock Drift 0-1.5 meters All Removed
• Multipath 0-1 meters Not Removed
• SA 0-70 meters All Removed

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Design Approach

• Correct operation of NovAtel receivers borrowed
  from CAT
• Correct operation of FreeWave Radio communication
  link borrowed from Dr. Sennott (TISI)
• Successful GPS receiver radio link integration

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Hardware Implementation

• Reference Point
• Placed NovAtel GPS Receiver on Jobst Hall and
  collected position information (scatter plot) for
  about two hours.
• Used the average Latitude and Longitude of this
  plot as our reference point.

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Hardware Implementation
Latitude 40 41 56.613512 N Longitude 89 37
1.613741 W Height 192.341 m
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Hardware Implementation

• Configure Reference Station
• Data rate 9600 bps
• Minimum rate 2400 bps
• Fix position of NovAtel reference station.
• fix position 40.69903722, -89.61712110, 192.3415
• Log differential corrections.
• log com1 rtcm3 ontime 1
• log com1 rtcm59 ontime 1
• log com1 rtcm1 ontime 1

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RTCM Corrections

• Radio Technical Commission for Maritime Services
  (RTCM) set up a team composed of representatives
  of US federal authorities, GPS manufacturers and
  users.
• In early 1990, they adopted a first standard for
  the transmission format and contents for DGPS
  applications
• Special Committee 104 (SC104)

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Types of RTCM Log Commands
(Access to carrier phase)
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Hardware Implementation

• Configure Mobile Station
• Accept differential corrections from reference
  station
• accept com2 rt20
• Log GPS data
• log com1 p20a ontime 1
• log com1 dopa ontime 1

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Hardware Implementation

• Saving GPS Data
• Using Windows HyperTerminal, save all data to a
  Notepad file.
• Process data in Matlab.

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Software Implementation

• Approach
• Use Matlab to read a log file and process data
• Plot data points in a scatter plot
• Calculate CEP
• Plot drifting of position accuracy
• Plot position accuracy vs. number of satellites
  available

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Software Flow Chart
Open and read log file
Convert latitude and longitude to local
coordinates (meters)
Calculate CEP
Plot graphs
Calculate and display mean values
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Software Implementation

• Opening and reading log file
• R input('What type of log file is it? 1POSA
  2P20A 3P20A
• and DOPA ')
• file INPUTDLG('Enter the File name','Enter GPS
  log file to open')
• time lat long height textread(file, ' s f
  \n', 'delimiter',',')

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Software Implementation

• Coordinate conversion
• Local (North, East, Down)
• Uses a reference point to find the change in
  direction
• Converts to meters

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Software Implementation

• Coordinate conversion
• lat_refmean(lat)
• long_refmean(long)
• height_refmean(height)

a earth_shape north (a(2) (lat -
lat_ref))pi/180 d a(2) sin(lat) c a(1)
cos(lat) lat_angle atan2(d,c) east -(a(1)
cos(lat_angle).(long
long_ref))pi/180 down
-(height-height_ref)
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Software Implementation

• Calculating CEP
• Find the radius of a circle where half of the
  points lie
• Finds distances for all the points
• Compares to a incrementing radius
• Radius increments in millimeters starting at 1 mm

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Software Implementation

• Calculating CEP

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Software Implementation

• Plotting graphs
• Scatter plot
• plot(east,north,'x'),title('CEP')
• axis equal
• Subplots
• subplot(311),plot(east),title('East
  Coordinates'),
• subplot(312),plot(north),title('North
  Coordinates'),
• subplot(313),plot(height),title('Height')

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Software Implementation

• Displaying mean values
• 40.69896654 -89.61670040
• to
• 40 41 56.28N 89 37 0.11 W
• if(lat_refgt0) dirLat'N'else dirLat'S'end
• lat_refabs(lat_ref)
• degfloor(lat_ref)
• min(lat_ref-deg)60
• sec(min -floor(min))60
• Latsprintf(' d d f c',deg,floor(min),sec,d
  irLat)

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• Matlab screen output

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Problems Encountered

• CAT NovAtel receiver missing software
• Radio transmitter link doesnt work or transmit
  data when connected to GPS receiver
• Cant find geographic benchmark data

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Problems Encountered

• Transmitter doesnt transmit data when connected
  to GPS receiver
• Solution
• Null-modem/ Straight cable hardware conflict
• Bought Null Modem adapter from Radio Shack

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Initial Data Gathering

• Procedure
• Set up base station
• Set up remote station far away
• Start sending corrections
• Use laptop to capture remote station data
• Process in Matlab

CEP 112 m
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Initial Data Gathering
Stand alone mode
CEP 112 m
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Initial Data Gathering
Differential GPS mode
CEP 94 m
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Initial Data Gathering
Ashtech
CEP 2.6 m
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Code Problem

• We were converting Latitude and Longitude to
  meters without first converting to radians
• All our conversions were off by a factor of about
  57
• 1 radian 57.3 degrees

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Data Gathering
Corrected Matlab Code
CEP 2.07 m
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Data Gathering
Differential
CEP 10.7 cm
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Data Gathering
Differential Steady State
CEP 4.7 cm
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Statistical Analysis

• Scatter Plots - CEP
• Satellite Switching
• Steady State response
• DOP
• Warm and Cold Start
• DGPS GPS comparison

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Statistical Analysis

• DGPS system operating
• CEP
• Satellite effects
• Time to steady state

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Statistical Analysis
CEP 12.7cm
16 min
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Steady State Response
CEP 4.7cm
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Statistical Analysis

• DGPS system operating
• Fix base station position with less accuracy
• What are the effects?

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Statistical Analysis
CEP 40cm
30 min
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Statistical Analysis
Good DOP values are between 1 and 3. Higher
values mean poor position accuracy due to spacing
of satellites.
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Statistical Analysis
DOP
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Statistical Analysis

• DGPS system operating
• What are the effects of taking GPS data at warm
  and cold starts?
• Cold start Initial startup
• Warm start Been running for a while

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Statistical Analysis (cold)
CEP 1.83m
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Statistical Analysis (warm)
CEP 1.05m
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GPS/DGPS Comparisons

• Take DGPS data
• Turn off corrections after 50min
• What are the effects?

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GPS/DGPS Comparisons
50min
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GPS/DGPS Comparisons
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GPS/DGPS Comparisons
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Conclusions

• Solved accuracy problem, able to achieve greater
  position accuracy using the DGPS method
• NovAtel RT20 receivers performed better than the
  Ashtech G8 in stand alone mode
• NovAtel receivers are easier to integrate a DGPS
  system

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Conclusions

• The number of satellites the receiver uses in the
  position calculation effects the position
  accuracy and the DOP
• Once the receiver reaches steady state, position
  accuracy is less effected by errors or satellite
  switching

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GPS/DGPS Comparisons
GPS DGPS
CEP 1-3 m 4 - 40 cm
Avg. DOP 1.71 2.41
Avg. Satellites used in position 9 7
Sensitivity to satellite switching High Low
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Recommendations

• Purchase another NovAtel antenna instead of the
  Magnavox currently used (retail 595)
• Easy access to a permanent reference station on
  campus
• Power considerations
• Always transmitting

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Recommendations

• Investigate effects of transmitting corrections
  at different time intervals
• Investigate new correction standard, RTCA
• NovAtel has preliminary support
• Better error detection

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Special Thanks

• Dr. In Soo Ahn
• Bill Allen of Caterpillar
• Dr. James Sennott of TISI

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Questions