Lecture 4 – Examining GPS data

1. Lecture 4 – Examining GPS data Question #1 – How does GPS receiver know where the satellites are? Issac Newton and Johannes Kepler will help to estimate where the satellites are. How? • Issac Newton: gravity • Johannes Kepler

2. V02 = g RE http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

5. http://www.pd.astro.it/education/PlanetV/planetarium/L14_03S.htmlhttp://www.pd.astro.it/education/PlanetV/planetarium/L14_03S.html P2 = k * a3

7. Almanac and Ephemeris Almanac information is broadcast to provide close satellite position information. The Almanac for all satellite is broadcast from each satellite. Ephemeris - Orbit prediction information which is periodically transmitted by each GPS satellite.  The Ephemeris provides up-to-the-minute modifications to the almanac.

8. How is the clock in the receiver kept accurately on GPS time? The receiver clock is reset to GPS time by the satellites each time a position is found. • GPS receiver clock is not accurate • The clocks in the satellites keep time to about a tenth a billionth of a second. • The 4th satellite can provide time fix.

9. How often does a satellite send a signal? What is the signal like? • Each satellite sends a signal continuously. • Radio station signal consists two parts: • A carrier, which is on all the time. • Modulation of that carrier, which is the voice or music. • Each satellite broadcasts on two frequencies (Actually, more than 2).

10. GPS signal VS FM signal • The civilian carrier frequency is 1575.42 Mhz, while FM is about 100 Mhz. • GPS signals are about 15 times as often as FM signals. • GPS signals are about one-fifteenth as long as FM signals. (about 20 centimeters from wave top to wave top.)

11. Modulation of the GPS signals A copy of the C/A code for a given satellite might look like this: 10001101001011110110001… • And on and on for a total of 1023 bits. Then the sequence starts again. • The C/A code is repeated every one-thousandth of a second.

12. How does the receiver use the 0s and 1s to determine distance? • A satellite generates PRN code continuously • A GPS receiver generates PRN code using the same computer program • The GPS receiver compares the code from the satellite and the code generated by itself to determine the time difference

13. Based on above information, can you calculate the distance between the satellite and the receiver?

14. How does the receiver listen to several satellites at once? • Each satellite has its own distinctive PRN code. • Old satellite can transfer its PRN to its replacement. • Parallel channels on receiver can track multiple satellites at the same time. • Expensive GPS receivers can dedicate one channel for each satellite.

15. What was selective availability? SA was the error deliberately introduced by the GPS managers in the C/A code broadcast to diminish the accuracy of GPS receivers. • Sometimes the satellites lied about their positions. • Sometimes they lied about when they sent the code.

16. How satellites can generate track that is almost due north-south in the vicinity of the equator? • A GPS satellite orbital plane cuts Earth’s meridians at about 55 degree • It moves at 8600 mph • Its track on Earth Surface is about 2100 mph. • Equator moves about 1050 mph • Satellites moves 1700 mph at north/south component and 1200 mph at east/west component.