GPS Navigation Solution Laboratory: Analyzing Satellite Selection and Clock Offset | Lab Reports Electrical and Electronics Engineering

ECE 498AL Laboratory 4

The GPS Navigation Solution

Laboratory Goals

In this lab, we will exercise MATLAB code written to solve the Newton-Raphson

method utilizing raw pseudorange measurements from the OSGPS receiver. The code

can solve for the receiver location and clock offset using four or more satellites. You will

investigate the effect of satellite choice on the resultant navigation solution, the

improvement gained in using all available satellites, the effects of the various correction

terms in the pseudorange equation, and the behavior of the receiver clock offset as a

function of time.

Prelab

Before coming to lab, download the code from the course website at

http://airglow.csl.uiuc.edu/Teaching/ECE498JM/. You will need:

• GPS_LAB4.zip

The package contains all of the code that you should need for this lab. Some are

duplicates of code you have written before. You are encouraged to use the code you have

written, in these cases, but are also welcome to use the code provided.

Laboratory Exercises

Data Collection

You will need at least twenty minutes of data from an OSGPS receiver. Make sure that

the program will output RINEX data files by setting the “rinex_file” flag in the

“rcvr_par.dat” file before running the receiver software. You also need to make sure the

“N_obs” (2), “Obs_1” (C1) and “Obs_2” (D1) variables in the “rinexpar.dat” file are

correctly set (to the values in parenthesis). The data you collect should be with as many

satellites as possible successfully tracking (aim for twenty minutes with at least eight

satellites). Keep an eye of the receiver software to make sure that it continues tracking

and that the altitude error does not grow too large. If it does, this is a sign that the

receiver is having issues. Call over the lab instructor to fix the problem.

Dilution of Precision

In the earlier labs, you should have noticed that the receiver calculates several types of

Dilution of Precision terms (HDOP, VDOP, GDOP, TDOP). These are geometrical

terms that assist a user in choosing which satellites to use in performing a navigation

solution. Granted, it had greater utility in the days when receivers had limited channels,

but it is still a useful instructional tool. Essentially, the DOP tells you, all other things

being equal, how the geometry of the tracked satellites will affect the accuracy of the

navigation solution. Lower DOPs correspond to a better distribution of satellites in the

sky, and lower overall error.

To investigate this, later in the lab, we will be calculating the navigation solution using

two subsets of four satellites; one with high DOP and one with low DOP. We will now

figure out what these two subsets will be.

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ECE 498AL Laboratory 4 The GPS Navigation Solution Laboratory Goals In this lab, we will exercise MATLAB code written to solve the Newton-Raphson method utilizing raw pseudorange measurements from the OSGPS receiver. The code can solve for the receiver location and clock offset using four or more satellites. You will investigate the effect of satellite choice on the resultant navigation solution, the improvement gained in using all available satellites, the effects of the various correction terms in the pseudorange equation, and the behavior of the receiver clock offset as a function of time. Prelab Before coming to lab, download the code from the course website at http://airglow.csl.uiuc.edu/Teaching/ECE498JM/. You will need:

GPS_LAB4.zip The package contains all of the code that you should need for this lab. Some are duplicates of code you have written before. You are encouraged to use the code you have written, in these cases, but are also welcome to use the code provided. Laboratory Exercises Data Collection You will need at least twenty minutes of data from an OSGPS receiver. Make sure that the program will output RINEX data files by setting the “rinex_file” flag in the “rcvr_par.dat” file before running the receiver software. You also need to make sure the “N_obs” (2), “Obs_1” (C1) and “Obs_2” (D1) variables in the “rinexpar.dat” file are correctly set (to the values in parenthesis). The data you collect should be with as many satellites as possible successfully tracking (aim for twenty minutes with at least eight satellites). Keep an eye of the receiver software to make sure that it continues tracking and that the altitude error does not grow too large. If it does, this is a sign that the receiver is having issues. Call over the lab instructor to fix the problem. Dilution of Precision In the earlier labs, you should have noticed that the receiver calculates several types of Dilution of Precision terms (HDOP, VDOP, GDOP, TDOP). These are geometrical terms that assist a user in choosing which satellites to use in performing a navigation solution. Granted, it had greater utility in the days when receivers had limited channels, but it is still a useful instructional tool. Essentially, the DOP tells you, all other things being equal, how the geometry of the tracked satellites will affect the accuracy of the navigation solution. Lower DOPs correspond to a better distribution of satellites in the sky, and lower overall error. To investigate this, later in the lab, we will be calculating the navigation solution using two subsets of four satellites; one with high DOP and one with low DOP. We will now figure out what these two subsets will be.

Using the almanac data and the GPS_Plotsat.m routine, create an az/el plot corresponding to the time that you are collecting data. Remember that you can use an old almanac datafile. However, you must correctly account for the number of seconds that have elapsed since the time of the almanac (e.g., you must add on the correct number of seconds corresponding to the weeks that have elapsed since the almanac was collected). Q1 : Verify that the plot is correct by comparing the output with the satellites being tracked by the OSGPS receiver and the reported az/el values. Provide a copy of the plot.
Find four satellites that are evenly distributed about the sky. You would likely want one satellite at zenith, and three satellites separated by 120 degrees at an elevation angle of 30 degrees or so.
Find four satellites that are clustered in the same portion of the sky. Ideally, you would want four satellites that are at low elevation angles in the same quadrant of the sky. Q2 : List the two sets of four satellites you have determined in the previous two steps. Use the GPS_CalcDOP.m routine to calculate the DOP for each of the set of four satellites. As with the GPS_Plotsat.m routine, you must make sure to send in the GPS time referenced to the week that the alamanc data are from. You also only want to send in the almanac data for the four satellites being used in each case. How do the DOP values compare for the two sets? If the DOP gives a sense of the expected error in the resultant navigation solution, which set do you expect to be a better choice? Calculate the Navigation Solution By now, the receiver should have collected the data required. Make sure to save copies of the rinex.obs and rinex.nav files. More information on the file formats for these files can be found at http://www.ngs.noaa.gov/CORS/instructions2/. You will also want the ion_utc.dat, current.alm, and current.eph files from your data directory. The remainder of the lab is concerned with calculating the navigation solution using the data that you have just collected. A script file, Lab04.m is provided. There is some information that you will have to fill out, which should be fairly self-explanatory. You may want to generate multiple copies of Lab04.m to document your work below. It might also be useful to only work with a subset of the data (determined by the for loop) at first just to make sure everything is working (analyzing the entire dataset can take a couple of minutes).
Use Lab04.m to calculate the navigation solution using the high-DOP set of four satellites. Q3 : What is the average error from the true location of the antenna? What is the standard deviation? Are the errors biased or distributed about the true location? Provide a plot of

satellite clock error, and propagation delay terms. The difference between this corrected pseudorange and the true range is then the receiver clock offset. Plot the receiver clock offset calculated on each channel of the receiver (a channel should correspond to a single satellite, unless your receiver lost lock on the satellite) as a function of time on the same plot. What does this plot say about our assumption we had to make regarindg the receiver clock offset? PL2 : Investigate the effects of the initial guess in the convergence of the Newton- Raphson method. Add a line of code to the GPS_NavSolnIon.m function to print out the number of iterations needed for the solution to converge. Systematically change the initial guess and note how many iterations are needed (you may need to put in really bad initial guesses here to see a difference). Can you get the solution to fail to converge or converge to a wrong answer? What does this say about the robustness of the Newton- Raphson method? Lab Writeup Your lab report should include answers to each question asked in this lab handout as well as appropriate graphs and figures to make your point. Copies of the code written in the lab should also be included as appendices. Grammar, spelling, completeness, and brevity are all part of your overall grade.

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