How GPS Works Part 2

1. How GPS WorksPart 2 LUCID Summer Workshop July 30, 2004

2. Outline for Today • We spend a little more time reviewing the GPS system today. • Once we finish up GPS, we will switch gears and gain a basic understanding of how the Internet works. • This will be helpful for our WiFi discussion next week.

3. Recall: What it is • GPS: Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. • Uses the principle of triangulation and time- of-arrival of signals to determine the location of a GPS receiver.

4. Typical GPS Applications • Location - determining a basic position • Navigation - getting from one location to another • Tracking - monitoring the movement of people and things. • Mapping - creating maps of the world • Timing - bringing precise timing to the world

5. GPS Triangulation Procedure

6. Triangulation Requirements • To triangulate, a GPS receiver measures distance using the travel time of radio signals. • To measure travel time, GPS receiver needs very accurate timing. • Along with distance, receiver need accurate data on where satellites are in space. • System will also need to correct for any delays the signal experiences as it travels through atmosphere.

7. Components of GPS System • Control Segment: five ground stations located on earth. • Space Segment: satellite constellation (24 active satellites in space). • User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.

8. Ground Monitor Stations Falcon AFB Colorado Springs, CO Master Control Monitor Station Kwajalein Monitor Station Hawaii Monitor Station Diego Garcia Monitor Station Ascension Island Monitor Station

9. Basic Functions of Monitor Stations • These stations are the eyes and ears of GPS, monitoring satellites as they pass overhead by measuring distances to them every 1.5 seconds • This data is then smoothed using ionospheric and meteorological information and sent to Master Control Station at Colorado Springs. • The ionospheric and meteorological data is needed to get more accurate delay measurements, which in turn improve location estimation.

10. Functions of Monitor Stations (Cont’d) • Master control station estimates parameters describing satellites' orbit and clock performance,. It also assesses health status of the satellites and determines if any re-positioning may be required. • This information is then returned to three uplink stations (collocated at the Ascension Island, Diego Garcia and Kwajalein monitor stations) which transmit the information to satellites.

11. Space Segment • Space segment is the satellite constellation. • 24 satellites with a minimum of 21 operating 98% of the time • 6 Orbital planes • Circular orbits • 20-200 km above the Earth's surface • 11 hours 58 minute orbital period • Visible for approximately 5 hours above the horizon

12. GPS Satellite Orbits • We can obtain updates of GPS satellites at http://www.ngs.noaa.gov/GPS/GPS.html

13. GPS Satellite Orbits (Cont’d) • Orbits of GPS satellites need to be updated every once in a while because orbit does not stay circular without adjustments. • Adjustments needed because: • Other objects exert gravitational force on each satellite (e.g. sun, moon) • Effect of gravity is non-uniform during orbit. • Radiation pressure (due to solar radiation). • Atmospheric drag • Other effects

14. Interesting Aside on GPS Orbits • When GPS satellites are decommissioned, they are placed on a disposal orbit (outside the operating GPS orbit). • Some studies show that satellites in disposal orbits can eventually, perhaps over 20-40 years, encroach into operating constellation.

15. Aside (Cont’d) • This is because disposal orbits, while circular initially, become increasingly elliptical, mostly as result of sun-moon gravitational perturbations. • Besides intersecting GPS constellation, these satellites eventually could pose a threat to operational satellites in low Earth and geosynchronous orbits

16. Aside (Cont’d)

17. Aside (Cont’d) • Similar threats posed by other satellite systems. • The Russian Glonass constellation, a navigation system similar to GPS, will also experience orbit eccentricity growth and may pose a collision risk to itself and GPS. • Glonass, which has about 100 failed satellites within its constellation, is located about 1,000 kilometers (621 miles) lower than GPS and could pose a collision problem in 40 years, the studies show.

18. Aside (Cont’d) • Galileo satellites also may pose a threat to GPS. • Galileo is Europe’s own global navigation satellite system. • First experimental satellite will be launched in second half of 2005. • Galileo will be under civilian control.

19. Third Component of GPS: User Segment • User segment comprises receivers that have been designed to decode signals transmitted from satellites for purposes of determining position, velocity or time. • Receiver must perform the following tasks: • select one or more satellites in view • acquire GPS signals • measure and track signal • recover navigational data

20. Important Terminology • Satellite transmits Ephemeris and Almanac Data to GPS receivers. • Ephemeris data contains important information about status of satellite (healthy or unhealthy), current date and time. This part of signal is essential for determining a position. • Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.

21. Measuring Time Of Arrival (TOA) in GPS

22. TOA Concept • GPS uses concept of time of arrival (TOA) of signals to determine user position. • This involves measuring time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver. • Time interval is basically signal propagation time.

23. TOA Concept (Cont’d) • Time interval (signal propagation time) is multiplied by speed of signal (speed of light) to obtain satellite to receiver distance. • By measuring propagation time of signals broadcast from multiple satellites at known locations, receiver can determine its position. • Assuming we have precise clocks, how do we measure signal travel time?

24. Measuring Distance using a PRC Signal • At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code (PRC). • The receiver begins running the same digital pattern also exactly at midnight. • When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.

25. Measuring Distance • The length of the delay is equal to the signal's travel time. • The receiver multiplies this time by the speed of light to determine how far the signal traveled. • Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.

26. Synchronizing Clocks • In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. • Accurate time measurements are required. If we are off by a thousandth of a second, at the speed of light, that translates into almost 200 miles of error.

27. Synchronizing Clocks (Cont’d) • To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. • But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use. • The Global Positioning System has a clever, solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets.

28. Synchronizing Clocks (Cont’d) • The Global Positioning System has a clever, effective solution to this problem. • Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. • In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy.

29. Synchronizing Clocks (Cont’d) • When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. • Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. • Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

30. Synchronizing Clocks (Cont’d) • The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. • Based on this, it resets its clock to be in sync with the satellite's atomic clock. • The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

31. Synchronizing Clocks (Cont’d) • The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. • Based on this, it resets its clock to be in sync with the satellite's atomic clock. • The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

32. Knowing Satellite Locations • In order to properly synchronize clocks and figure out which PRC signal to listen to, the receiver has to know where the satellites actually are. • This isn't particularly difficult because the satellites travel in very high and predictable orbits.

33. Using Almanac Information • The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. • Things like the pull of the moon and the sun do change the satellites' orbits very slightly. • However, the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

34. 2 Types of Errors • Errors can be categorized as intentional and unintentional. • Intentional errors: government can and does degrade accuracy of GPS measurements. This is done to prevent hostile forces from using GPS to full accuracy. • Policy of inserting inaccuracies in GPS signals is called Selective Ability (SA). SA was single biggest source of inaccuracy in GPS. SA was deactivated in 2000.

35. Sources of Unintentional Timing Errors

36. Typical Errors Source of Error Typical Error in Meters (per satellite) Satellite Clocks 1.5 Orbit Errors 2.5 Ionosphere 5.0 Troposphere 0.5 Receiver Noise 0.3 Multipath 0.6 SA 30

37. Differential GPS • Technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment. • Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point. • Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites.

38. Improved Offered by Differential GPS Source Uncorrected With Differential Ionosphere 0-30 meters Mostly Removed Troposphere 0-30 meters All Removed Signal Noise 0-10 meters All Removed Orbit Data 1-5 meters All Removed Clock Drift 0-1.5 meters All Removed Multipath 0-1 meters Not Removed Receiver Noise ~1 meter Not Removed SA 0-70 meters All Removed

39. Using GPS Data • A GPS receiver essentially determines the receiver's position on Earth. • Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude of its current position. To make the navigation more user- friendly, most receivers plug this raw data into map files stored in memory.

40. Using GPS Data (Cont’d) • You can • use maps stored in the receiver's memory, • connect the receiver to a computer that can hold more detailed maps in its memory, or • simply buy a detailed map of your area and find your way using the receiver's latitude and longitude readouts. • Some receivers let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.

41. Using GPS Data (Cont’d) • A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. • If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. • This is what happens in cars equipped with GPS.

42. Using GPS Data With this information and its built-in clock, the receiver can give you several pieces of valuable information: • How far you've traveled (odometer) • How long you've been traveling • Your current speed (speedometer) • Your average speed • A "bread crumb" trail showing you exactly where you have traveled on the map • The estimated time of arrival at your destination if you maintain your current speed

43. Internet

44. Background • One of the greatest things about the Internet is that nobody really owns it. • It is a global collection of networks, both big and small. • These networks connect together in many different ways to form the single entity that we know as the Internet. In fact, the very name comes from this idea of interconnected networks.

45. The Internet Concept

46. The Internet Concept (Cont’d)

47. Background (Cont’d) • Since its beginning in 1969, the Internet has grown from four host computer systems to tens of millions. • However, just because nobody owns the Internet, it doesn't mean it is not monitored and maintained in different ways. • The Internet Society, a non-profit group established in 1992, oversees the formation of the policies and protocols that define how we use and interact with the Internet.

48. Outline for Remainder of Slides • In the next few slides, we will review basic underlying structure of the Internet. • We will learn about domain name servers, network access points and backbones. • First, we review how your computer connects to others.

49. Network of Networks • Every computer that is connected to the Internet is part of a network, even the one in your home. • For example, you may use a modem and dial a local number to connect to an Internet Service Provider (ISP). • At school/work, you may be part of a local area network (LAN), but you most likely still connect to the Internet using an ISP that your school/company has contracted with.

50. Network of Networks (Cont’d) • When you connect to your ISP, you become part of their network. • The ISP may then connect to a larger network and become part of their network. • The Internet is simply a network of networks.