Introduction to GPS Instructor: Yun Du

GPS Full name is “Global Positioning System”. It is a Global Navigation Satellite System (GNSS) developed by United States Department of Defense. Only one fully functional GNSS in the world. Managed by the United States Air Force 50th Space Wing Handset is not GPS. It is GPS receiver.

The other similar systems Russian GLONASS – incomplete as of 2008. European Galileo Positioning System – upcoming. Chinese COMPASS navigation system – proposed. Indian IRNSS. French DORIS.

Incident brought wide use In 1983, Korean Air Lines Flight 007 was shot down after straying into USSR’s prohibited airspace. Ronald Reagan issues a directive to make GPS available for civilian use as a common good. Widely used in navigation worldwide. A tool for map-making, land surveying, commerce, scientific uses, etc.

GPS satellite system A constellation of between 24 and 32 Medium Earth Orbit satellites transmitting precise microwave signals. Originally designed for 24 SVs, 8 each in 3 circular orbital planes, but this was modified to 6 planes with 4 satellites each. The orbital planes are centered on the Earth. Enable GPS receiver to determine its current location, time, and velocity.

Trilateration A circle and sphere surface in most cases of practical interest intersect at two points. Surface of Sphere Intersecting a Circle (not disk) at Two Points, shows this intersection. The two intersections are marked with dots. The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. If the three satellites are not in the same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near- Earth intersection will be the correct position for the case of a near-Earth vehicle.

Why is model like that? Two sphere surfaces intersecting in a circle Surface of a sphere intersecting a circle (i.e., the edge of a disk) at two points

Position Calculation Introduction It seems three satellite is enough, since space has three dimensions. However a small clock error would cause a large positional error. The fourth satellite correct the receiver’s clock. Sometimes three is enough, like a plane or ship. We know its elevation.

Position Calculation Introduction Imagining the GPS receiver received messages from a minimum of four satellites. It is able to determine the distance and time sent by [ , , , ]. Subscript i is the satellite number. The distance traveled is ( - ) c. is the time GPS receiver received

Position Calculation Introduction The time is so important even a very tiny error might make the GPS receiver get an imprecise position. There is a way to correct it: b = /c, = - is valid estimate distance from GPS receiver to 4th satellite. is pseudorange of 4th satellite. The GPS receiver’s clock is advanced if b is positive and delayed if b is negative. (correct time – time indicated by the receiver’s on-board clock)

The real model of satellite constellation http://upload.wikimedia.org/wikipedia/commons/9/9c/ConstellationGPS.gif

System details Three segments: space segment(SS), control segment(CS), and user segment(US). SS: each plane has inclination (relative to earth equator). right ascension to ascending node (angle along the equator from a reference point to the orbit’s intersection). As of March 2008, 31 satellites actively broadcast well. Some reports indicate 32nd satellite make difficulties on some GPS receivers.

Control Segment Flight paths are tracked by US Air Force monitoring in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs and Colorado. These stations are operated by National Geospatial-Intelligence Agency. Tracking info is sent to Air Force Space Command’s master control station operated by 2nd space operations squadron of USAF. 2 SOPS can update time and adjust the satellites. If satellite is marked unhealthy, GPS receiver would not use it any more.

User Segment In general, it is composed of an antenna (tuned to the frequency transmitted by the satellite), receiver- processor and a highly stable clock(a crystal oscillator). Usually it can monitor simultaneously several channels. As of 2007, 12 to 20 channels.

GPS application on mobile It uses hand-set based approaches which is different from above. It uses existing cell towers/APS and infrastructure to triangulate user’s location. Not real satellites.

GPS application on mobile It uses very accurate clocks to determine the difference in time in which uplink radio signal from user reaches different cell sites. Measures the direction of signal received at multiple towers with respect to antennas of known position to determine mobile position. At the cost of running radio, it is accurate to a couple hundred feet. It is working only at where it can be covered by cell towers. GPS here is only the locator. It can calculate where it is according to the cell towers’ numbers, and then find the position on the map which contains many attributes.

GPS application on mobile On the map, we can calculate the shortest path between two points according the attributes stored in the database. It calculates the position many times per second in order to get new position information because of moving. Another new app is that it can show the street views and satellite views, like Google G1. You can zoom in to see the real environment around you. This can help people who cannot really understand the map.

Google map on G1 Satellite views on G1 Street views on G1

Navigation Signals Each satellite continuously broadcasts a Navigation Signal at 50bit/s giving the time-of- week, satellite health info, an ephemeris and an almanac. All satellites broadcast at the same two frequencies, 1.57542GHz(L1) and 1.2276GHz(L2).

Signals L1 (1575.42 MHz): Mix of Navigation Message, coarse- acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites. L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites. L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties. L4 (1379.913 MHz): Being studied for additional ionospheric correction. L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life signal.

Other problems Interference: natural sources and artificial sources. Multipath effects. Accuracy enhancement. Civilian use. etc.

Project Multimedia data stream model of parking lot. Define data stream: Si, data rate, type(Micon), Ci weapon(frame-number, time-stamp, one-frame- of-video-data, type-of-weapon) Three tool to detect: camera, infrared, radar. Model the concrete examples with reasonable cost. Compare and combine them.