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Implementation and Statistical Analysis of a Differential GPS System

Implementation and Statistical Analysis of a Differential GPS System

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## 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**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.**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 FreeWave Radio NovAtel Receiver Laptop Computer for analysis Antenna GPS Antenna**Block Diagram**Binary data stream Reference Station RF modem RF modem Mobile Station Commands Data Computer Matlab**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**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.**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 • 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 ') • 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**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);**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**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**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**• 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