GPS Worldwide Laboratory: a community of knowledge-seekers spanning the globe

Transcription

1 Laboratory B: (predicting and verifying satellite visibility) Lab Date: 1 November day depending on your time zone). YOU MUST DO THIS PARTICULAR LAB ON THE SPECIFIED DAY. Lab Goals: Predict when particular GPS satellites will be visible at your location, and then test your prediction. Be part of a community tracking the same satellite on the same day: Who saw it before you, and where were they? Who sees it next? Compare data collected from all students. Optional: Download a spreadsheet of all the data and do something with it (parse it, map it, analyze it, etc.) To Prepare: Install the free GPS Test App (AndroiTS GPS Test Free) on your Android device. To Submit Data: 1. Complete Lab Quiz B on the Coursera site (Due by Mon 3 Nov :59 PM PST). 2. Record your results in the GPS Worldwide Lab B (satellite visibility) Form. Introduction In this lab, you will predict when one of the GPS satellites PRN 4 and PRN 18 will be visible at your location, ideally close to the satellite s zenith. Then you will go outside at that time, and test your prediction: track the satellite with your Android 1 smartphone or tablet, and observe the satellite elevation angle (degrees above the horizon). This map predicts the satellite paths. And this App is what you will use to observe them. 1 At present (Oct 2014), iphone and ipad do not provide the necessary satellite data (such as which satellites are tracked, satellite ID, signal power, and elevation angle) that we need for these labs. 1

2 Prediction We use software known as mission planners to predict satellite orbits. These programs use the satellite almanac to predict where the satellites will be at any time in the future. This is the same almanac as broadcast by the satellites. It is available online from the US Coast Guard (Google: USCG GPS almanac ). You do not need to get the almanac for the lab; the online planning tool (see Appendix A) automatically updates the almanac that it uses. The date for performing this lab is Saturday 1 November 2014 ( 1 day, depending on your timezone). The two satellites we have chosen to follow are the GPS satellites with PRN codes 4 and 18. The following plot shows the ground paths directly beneath each satellite. You must convert the UTC time on the above plot to your local time to know when the satellite passes closest to you. This is when the satellite will have the highest elevation above your horizon. 2

3 Beware of Daylight Saving Time! Some of you will no longer be using daylight savings time (UK, for example), some of you will (USA, for example, where daylight savings ends on 2 November). If in doubt, use Google to check how your local time compares to UTC. Also, when you actually observe the satellite, watch for the maximum elevation angle: over a period of several minutes you should see the angle rising, reach a peak, and then reduce then you know you have observed the satellite zenith. For more sophisticated planning, see Appendix A. 3

4 Observation Install the app AndroiTS GPS Test Free on your Android smartphone or tablet (installation instructions in Lab A). Go outside several minutes before the predicted time of the satellite zenith. Start the AndroiTS app and check that you are tracking the expected satellite. 1. Touch the third icon along the bottom to get the display with satellite signal strength bars. 2. If there is a bar for one of the target satellites (4 or 18) then you are tracking it (the color of the bar does not matter for this experiment). 3. The satellite elevation (in degrees above the horizon) is shown in the last column. 4. Your position is shown. 5. The three dots on the bottom right take you to the menu where you can change app settings (such as GPS coordinates format) As time passes you should observe the elevation angle growing larger until the satellite is at its zenith, then the elevation angle will get smaller. Record the highest elevation angle you saw. 4

5 Next you will record your results on the GPS MOOC Lab website. NOTE: You must enter your Latitude and Longitude in degrees and decimal degrees. To set up AndroiTS in this way, tap the gear icon: Tap Settings, and select GPS coordinates format, DDD.DDDDDD 5

6 Results First record your results in the Lab B Quiz on the Coursera site this is necessary for you to get credit towards your certificate of completion - with distinction. Then go to this web page, and record your results in the Google Form: 6

7 Example of completed form: GPS Worldwide Laboratory: a community of knowledge-seekers spanning the globe 7

8 NOTES: 1) You must enter your Latitude and Longitude in degrees and decimal degrees. To set up AndroiTS in this way, go to the app settings, and select GPS coordinates format, DDD.DDDDDD (See page 5 for instructions on changing settings.) 2) The Google Form will make you enter time as AM/PM or 24 hour clock, depending on what Google has decided is normal for where you are. Please enter in whatever format it presents to you. For example, here s what you see in the USA and in Finland, respectively: Analysis of completed lab results This is your opportunity to be part of something unprecedented. There are approximately twenty thousand people registered for this course. As far as we know, no one has ever collected data from so many people tracking the same satellites around the world on the same day. If you each enter your data on the online form you may be part of the largest group ever to participate in this kind of experiment. In any case, we will collect all the data and make it available to all of you as Excel and CSV (comma-separated values) files. You are then free to do with it whatever you like analyze the statistics, plot a heatmap, write a conference paper, submit an entry to the Guinness Book of World Records, etc. Please Note: The data collected in this online form and in the Excel and CSV files is anonymous; your identity is neither collected nor available to anyone. 8

9 Appendix A: Mission Planning GPS Worldwide Laboratory: a community of knowledge-seekers spanning the globe An excellent online tool for mission planning is provided by Trimble Navigation Ltd. Go to Enter the settings, including Latitude, Longitude, Day, Time Zone. Set the visible interval to run for 24 hours (make sure the start time corresponds to 00:00, 1 November UTC). Then hit Apply. The tool can find your Lat,Lon for you if you select Pick and enter your city name. For example, here it finds Stanford, California: Click on Satellite Library and deselect all systems and satellites except for GPS 4 and 18. 9

10 Click on World View : and drag the time slider bar, you should be able to replicate the traces shown earlier in this document, but just for where the satellites are visible from Your Position: Notice that this is the same trace we saw earlier, but earlier we showed the complete ground paths over the whole world: 10

11 Now you can use the planning tool to see exactly when one of the satellites reaches its highest elevation. Click on Elevation, and you will see a plot showing the satellite elevation of the satellites as viewed from your location. Drag the time slider bar and you can see exactly when the highest elevation occurs, in this example GPS 18 reaches a zenith of 80 degrees elevation at 2:50 PM local time, 1 November 2014, when viewed from Stanford, California, USA. Installing AndroiTS GPS Test Free app See the detailed installation instructions in Lab A. For alternatives to this app, see the supplemental instructions posted on 11