Lecture 4 – Examining GPS dataLecture 4 – Examining GPS data

Question #1 – How does GPS receiver know Question #1 – How does GPS receiver know where the satellites are?where the satellites are?

Issac Newton and Johannes Kepler will help Issac Newton and Johannes Kepler will help to estimate where the satellites are. to estimate where the satellites are. How?How?

- Issac Newton: gravityIssac Newton: gravity

- Johannes KeplerJohannes Kepler

http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

V02 = g RE

http://www.pd.astro.it/education/PlanetV/planetarium/L14_03S.html

P2 = k * a3

(Period 1)2 

----------------- =   (Period 2)2

( semimajor axis 1)3 

----------------------------   ( semimajor axis 2)3

Almanac and EphemerisAlmanac and Ephemeris

Almanac information is broadcast to provide Almanac information is broadcast to provide close satellite position information.close satellite position information.

The Almanac for all satellite is broadcast The Almanac for all satellite is broadcast from each satellite.from each satellite.

Ephemeris - Orbit prediction information Ephemeris - Orbit prediction information which is periodically transmitted by each which is periodically transmitted by each GPS satellite.  The Ephemeris provides GPS satellite.  The Ephemeris provides up-to-the-minute modifications to the up-to-the-minute modifications to the almanac.almanac.

How is the clock in the receiver How is the clock in the receiver kept accurately on GPS time?kept accurately on GPS time?

The receiver clock is reset to GPS time by The receiver clock is reset to GPS time by the satellites each time a position is found.the satellites each time a position is found.

- GPS receiver clock is not accurateGPS receiver clock is not accurate

- The clocks in the satellites keep time to The clocks in the satellites keep time to about a tenth a billionth of a second.about a tenth a billionth of a second.

- The 4The 4thth satellite can provide time fix. satellite can provide time fix.

How often does a satellite send a How often does a satellite send a signal? What is the signal like?signal? What is the signal like?

Each satellite sends a signal Each satellite sends a signal continuously.continuously.

Radio station signal consists two parts: Radio station signal consists two parts: A carrier, which is on all the time.A carrier, which is on all the time. Modulation of that carrier, which is the voice Modulation of that carrier, which is the voice

or music.or music.

Each satellite broadcasts on two Each satellite broadcasts on two frequencies (Actually, more than 2).frequencies (Actually, more than 2).

GPS signal VS FM signalGPS signal VS FM signal

The civilian carrier frequency is 1575.42 The civilian carrier frequency is 1575.42 Mhz, while FM is about 100 Mhz. Mhz, while FM is about 100 Mhz.

GPS signals are about 15 times as often GPS signals are about 15 times as often as FM signals. as FM signals.

GPS signals are about one-fifteenth as GPS signals are about one-fifteenth as long as FM signals. (about 20 centimeters long as FM signals. (about 20 centimeters from wave top to wave top.)from wave top to wave top.)

Modulation of the GPS signalsModulation of the GPS signals

A copy of the C/A code for a given satellite A copy of the C/A code for a given satellite might look like this:might look like this:

10001101001011110110001…10001101001011110110001…

- And on and on for a total of 1023 bits. And on and on for a total of 1023 bits. Then the sequence starts again.Then the sequence starts again.

- The C/A code is repeated every one-The C/A code is repeated every one-thousandth of a second.thousandth of a second.

How does the receiver use the 0s How does the receiver use the 0s and 1s to determine distance?and 1s to determine distance?

A satellite generates PRN code A satellite generates PRN code continuouslycontinuously

A GPS receiver generates PRN code A GPS receiver generates PRN code using the same computer programusing the same computer program

The GPS receiver compares the code The GPS receiver compares the code from the satellite and the code generated from the satellite and the code generated by itself to determine the time differenceby itself to determine the time difference

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

How does the receiver listen to How does the receiver listen to several satellites at once?several satellites at once?

Each satellite has its own distinctive PRN Each satellite has its own distinctive PRN code.code.

Old satellite can transfer its PRN to its Old satellite can transfer its PRN to its replacement.replacement.

Parallel channels on receiver can track Parallel channels on receiver can track multiple satellites at the same time.multiple satellites at the same time.

Expensive GPS receivers can dedicate Expensive GPS receivers can dedicate one channel for each satellite.one channel for each satellite.

What was selective availability?What was selective availability?

SA was the error deliberately introduced by SA was the error deliberately introduced by the GPS managers in the C/A code the GPS managers in the C/A code broadcast to diminish the accuracy of GPS broadcast to diminish the accuracy of GPS receivers.receivers.

- Sometimes the satellites lied about their Sometimes the satellites lied about their positions.positions.

- Sometimes they lied about when they sent Sometimes they lied about when they sent the code.the code.

How satellites can generate track How satellites can generate track that is almost due north-south in that is almost due north-south in

the vicinity of the equator?the vicinity of the equator?

A GPS satellite orbital plane cuts Earth’s A GPS satellite orbital plane cuts Earth’s meridians at about 55 degreemeridians at about 55 degree

It moves at 8600 mphIt moves at 8600 mph

Its track on Earth Surface is about 2100 mph.Its track on Earth Surface is about 2100 mph.

Equator moves about 1050 mphEquator moves about 1050 mph

Satellites moves 1700 mph at north/south Satellites moves 1700 mph at north/south component and 1200 mph at east/west component and 1200 mph at east/west component.component.