Lab B supplementary instructions (answering questions raised in the forum) 1 November 2014. We will update this document as necessary. Q. (a) I can enter 24 hour time on Google forms, or AM/PM (b) I can only enter 24 hour time, not AM/PM (c) I can only enter AM/PM A. We've discovered that depending on where you are, or your computer settings, or some combination thereof, Google Forms may allow 24 hour time or it may force you to use AM/PM. If you can enter your time in 24 hour format in Lab B in Google Forms, please do (that's the proper Navigator way to tell time anyway) Q. What if both satellites pass overhead where I am? A. You can pick either one. Ideally the one that gets highest. For the "Quiz" answer (on the Coursera page), just fill in one - that will get you credit for the lab. For the community answer (on the Google Form) please do both - and thank you, you will be contributing double to the community experiment. Q. I am getting a pop up telling me that I must install "Microsoft Silverlight" before I can view the contents of the "GNSS planning online" page. A. Yes, you must install Microsoft Silverlight. Q. What if I can t run the Trimble Planning app on my computer? A. You can estimate "by hand" when the satellite will be closest to you by looking at the worldwide plot provided in the lab instructions. Q. I m confused about converting UTC to local time. What can I do? A. Lab B is all about getting the time right (by the way, Navigation and GPS is all about getting time right so you d better work hard at this!). The Trimble Planning website does the time zones for you. Also, if you are consulting the map shown in the lab and want to convert times by hand, then there are good tools online to help. One option is www.worldtimebuddy.com, where you can enter a city name and calculate the difference between current time and UTC. 1
Time Examples: Example 1: The planning map shows that satellite 18 passes close to Detroit USA at 23:00 UTC, on 1 November. The time app shows you that this is 7:00 PM, 1 November in Detroit. (by the way, this app takes care of daylight savings for you if you set the right date so you don t have to worry about that). Example 2: The planning map shows satellite 18 passing Bangalore at about 09:00 UTC on 1 November. The www.worldtimebuddy.com site shows this is 2:30 PM, 1 November, in Bangalore. 2
For certain places (like India), it s hard to see, just from the above global map, when the satellite will be closest (and highest) for you. Use the Trimble Planner for more precise planning see the Lab B instructions, and read Appendix A: Mission Planning carefully. I tried it for Bangalore and got the following elevation plot...... which shows that 2:30 PM (local time) actually is when satellite 18 is highest in the sky. NOTE: when you do this be sure you set the following right: your location, the date, the time zone, and click Apply whenever you can. 3
Q. The AndroiTS GPS Test app doesn t work on my phone, can you recommend other apps. A. Any app that shows you which satellites are tracked, their elevation above the horizon and their C/No (sometimes called SNR), will do. Here are some options, comments, and known issues: App AndroiTS GPS Test GPS Test barbeaudev GPS Test Chartcross GPS NMEA Comments Known version Issues Recommended app. Shows all the required information clearly. And correctly shows the different constellations (like GPS, GLONASS and BeiDou in the picture above) Didn t work on a Jellybean (Android release 4.1) phone. Works with later versions. Does the job. Slightly amateurish and gets some things wrong (like puts a US flag next to a BeiDou satellite). But will work for all 3 labs. Worked on Jellybean phone where AndroiTS did not. Doesn t show numerical elevation, you have to estimate it from Az-El plot shown above. This will work, but for Lab B it s not as good as seeing the actual elevation. Worked on Jellybean phone where AndroiTS did not. Allows you to log NMEA data files. Not userfriendly and not recommended for beginners. 4
Q. Is it possible to complete the lab exercises by using a GPS receiver and reading data from the NMEA data strings? A. For those who don t know, NMEA stands for National Marine Electronics Association, and the NMEA 0183 standard is one of the oldest (and most enduring) standards for GPS data. Unless you are already familiar with NMEA it is not recommended for these Labs. However, it is possible to do the labs using NMEA data. For Lab A go to the lab page, and read the part entitled: What if I can't obtain or borrow an Android device with GPS? This will explain how to do Lab A by looking up lat/lon. Just be sure you are aware of the units used for lat/lon in NMEA (degrees and minutes), and use compatible units when you look up your true position. For Lab B you will need satellite ID (PRN) and elevation angle, both available in the GSV message. For Lab C you will need C/No, also available in the GSV message. 5