Implementation and Statistical
Analysis of a Differential GPS System
Team Members:
Jim Connor
Jon Kerr
Advisor: Dr. In Soo Ahn
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.
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)
Topics of Discussion

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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
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.
Specifications
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
Block Diagram
Transmitter / Base Station
Position
Error
Corrections
GPS
Antenna
NovAtel
Receiver
FreeWave
Radio
Antenna
Receiver / Mobile Station
Position
Error
Corrections
Antenna
FreeWave
Radio
NovAtel
Receiver
GPS
Antenna
Laptop
Computer
for analysis
Block Diagram
Commands
Reference
Station
RF
modem
Binary data
stream
RF
modem
Data
Computer
Matlab
Mobile
Station
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.
NovAtel® Receiver
Base Station
Serial Correction Data
NovAtel® Receiver
Remote Station
Errors Removed by
Differential GPS
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Ionosphere
Troposphere
Signal Noise
Ephemeris Data
Clock Drift
Multipath
SA
0-30 meters
0-30 meters
0-10 meters
1-5 meters
0-1.5 meters
0-1 meters
0-70 meters
Mostly Removed
All Removed
All Removed
All Removed
All Removed
Not Removed
All Removed
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
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.
Hardware Implementation
Latitude = 40 41 56.613512 N
Longitude = 89 37 1.613741 W
Height = 192.341 m
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
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)
Types of RTCM Log Commands
(Access to carrier phase)
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
Hardware Implementation
Saving GPS Data
 Using Windows HyperTerminal,
save all data to a Notepad file.

Process data in Matlab.
Software Implementation
Approach
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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
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
Software Implementation
Opening and reading log file

R = input('What type of log file is it? 1=POSA 2=P20A 3=P20A
and DOPA ')
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file = INPUTDLG('Enter the File name','Enter GPS log file to open')

[time lat long height] = textread(file, ' %*s %f %*[^\n]', 'delimiter',',')
Software Implementation
Coordinate conversion
 Local (North, East, Down)
 Uses a reference point to find the
change in direction
 Converts to meters
Software Implementation
Coordinate conversion

lat_ref=mean(lat)
long_ref=mean(long)
height_ref=mean(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);
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
Software Implementation
Calculating CEP
CEP
0.08
0.06
0.04
0.02
END
0
-0.02
-0.04
START
-0.06
-0.08
-0.12
-0.1
-0.08 -0.06 -0.04 -0.02
0
0.02
0.04
0.06
0.08
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')
Software Implementation
Displaying mean values
 40.69896654
-89.61670040
to
 40° 41’ 56.28”N
89° 37’ 0.11” W

if(lat_ref>0) dirLat='N';else dirLat='S';end
lat_ref=abs(lat_ref);
deg=floor(lat_ref);
min=(lat_ref-deg)*60;
sec=(min -floor(min))*60;
Lat=sprintf(' %d %d %f %c',deg,floor(min),sec,dirLat);
Matlab screen output
Problems Encountered


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CAT NovAtel receiver missing
software
Radio transmitter link doesn’t work
or transmit data when connected to
GPS receiver
Can’t find geographic benchmark
data
Problems Encountered


Transmitter doesn’t transmit data when
connected to GPS receiver
Solution
Null-modem/ Straight cable hardware conflict
 Bought Null Modem adapter from Radio Shack

Initial Data Gathering
Procedure
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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
Initial Data Gathering
Stand alone mode:
CEP = 112 m
Initial Data Gathering
Differential GPS mode:
CEP = 94 m
Initial Data Gathering
Ashtech:
CEP = 2.6 m
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
Data Gathering
Corrected Matlab Code:
CEP = 2.07 m
Data Gathering
Differential:
CEP = 10.7 cm
Data Gathering
Differential Steady State:
CEP = 4.7 cm
Statistical Analysis
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Scatter Plots - CEP
Satellite Switching
Steady State response
DOP
Warm and Cold Start
DGPS – GPS comparison
Statistical Analysis
DGPS system operating
 CEP
 Satellite effects
 Time to steady state
Statistical Analysis
CEP= 12.7cm
16 min
Steady State Response
CEP= 4.7cm
Statistical Analysis
DGPS system operating
 Fix base station position with less
accuracy

What are the effects?
Statistical Analysis
CEP = 40cm
30 min
Statistical Analysis
Good DOP values are between 1 and 3.
Higher values mean poor position accuracy due to spacing
of satellites.
Statistical Analysis
DOP:
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
Statistical Analysis (cold)
CEP= 1.83m
Statistical Analysis (warm)
CEP= 1.05m
GPS/DGPS Comparisons
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Take DGPS data
Turn off corrections after 50min
What are the effects?
GPS/DGPS Comparisons
50min
GPS/DGPS Comparisons
GPS/DGPS Comparisons
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
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
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
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

Recommendations

Investigate effects of transmitting
corrections at different time
intervals

Investigate new correction
standard, RTCA
NovAtel has preliminary support
 Better error detection

Special Thanks

Dr. In Soo Ahn

Bill Allen of Caterpillar

Dr. James Sennott of TISI
Questions