Implementation and Statistical Analysis of a Differential GPS System Team Members: Jim Connor Jon...

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 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

AntennaFreeWave

RadioNovAtel Receiver

Laptop Computer

for analysis

GPS Antenna

Transmitter / Base Station

Receiver / Mobile Station

NovAtel Receiver

FreeWave Radio Antenna

GPS Antenna

Position Error

Corrections

Position Error

Corrections

Block Diagram

Reference Station

RF modem

RF modem

Mobile Station

Computer

Matlab

Data

Commands

Binary data stream

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® ReceiverBase Station

Serial Correction DataNovAtel® Receiver

Remote Station

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

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.


Latitude = 40 41 56.613512 N

Longitude = 89 37 1.613741 W

Height = 192.341 m

Hardware ImplementationConfigure 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 1log com1 rtcm59 ontime 1log 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)


Configure Mobile Station Accept differential corrections from

reference stationaccept com2 rt20

Log GPS datalog com1 p20a ontime 1

log com1 dopa ontime 1


Saving GPS Data Using Windows HyperTerminal,

save all data to a Notepad file.

Process data in Matlab.

Software ImplementationApproach

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 ChartOpen 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 ')

file = INPUTDLG('Enter the File name','Enter GPS log file to open')

[time lat long height] = textread(file, ' %*s %f %*[^\n]', 'delimiter',',')


Coordinate conversion Local (North, East, Down) Uses a reference point to find the

change in direction Converts to meters


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);


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


Calculating CEP

-0.12 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08CEP

START

END


Plotting graphs Scatter plotplot(east,north,'x'),title('CEP')

axis equal

Subplotssubplot(311),plot(east),title('East Coordinates'),

subplot(312),plot(north),title('North Coordinates'),

subplot(313),plot(height),title('Height')


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 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

CEP = 112 m

Procedure

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

Initial Data GatheringStand alone mode:

CEP = 112 m

Initial Data GatheringDifferential GPS mode:

CEP = 94 m

Initial Data GatheringAshtech:

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 GatheringCorrected Matlab Code:

CEP = 2.07 m

Data Gathering

CEP = 10.7 cm

Differential:

Data GatheringDifferential Steady State:

CEP = 4.7 cm

Statistical Analysis 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


CEP= 12.7cm

16 min

Steady State Response

CEP= 4.7cm


DGPS system operating Fix base station position with less

accuracy

What are the effects?


CEP = 40cm

30 min


Good DOP values are between 1 and 3.

Higher values mean poor position accuracy due to spacing of satellites.

Statistical AnalysisDOP:


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

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


50min

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

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

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