NN 204 Advanced Navigation

1. NN 204 Advanced Navigation Global Positioning System

2. Objectives: 1. List the four major advantages of GPS. 2. Be able to list and describe the 3 components of GPS . 3. Describe how a GPS receiver determines a position from a satellite. 4. Be able to describe how many LOP’s are needed for a 2D & a 3D position.

3. Objectives: 5. Briefly describe the differences between the civilian SPS & the military PPS services. 6. Decribe the term “Figure of Merit”, the numerical value for the best and worst accuracy readings and some of the primary sources of error associated with GPS. 7. Briefly describe “selective availability”.

4. Objectives: 8. Briefly describe “anti-spoofing.” 9. Describe the four different modes in which GPS operates and normal operating mode. 10. Be able to list 5 military & civilian applications for GPS. 11. Be able to describe how DGPS works and some of the benefits/drawbacks to the system.

5. Advantages of GPS 1. Extremely accurate. 2. Worldwide 24 hour coverage. 3. Passive system. 4. All-weather operation.

6. Components of GPS 1. Satellites - A constellation of 24 satellites orbit the earth at very high altitude (10,900 NM) in six orbital planes - each satellite completing two revolutions per day. - broadcast continuously on 2 frequencies, with codes and navigation message modulated on the carrier signals.

7. GPS System components 1. Space Vehicles (Satellites): • Constellation of 24 orbiting at 10,900 NM (6 orbital planes). • 2 rev per day for each satellite. • 6 to 10 satellites always in view • Xmitting on 1572.42 and • 1227.6mhz

9. Components of GPS 2. Control Segment - Consists of the Master Control Station (MCS), 5 monitor stations and 4 ground transmitters. - Monitor stations passively track all satellites in view, accumulating range data. The MCS processes the data & updates each satellite’s navigation message. The ground stations transmit the new navigation message to the satellites.

10. GPS CONTROL STATIONS5 MONITOR STATIONS - HAWAII - KWAJALEIN - ASCENSION ISLAND - DIEGO GARCIA - COLORADO SPRINGS

11. GPS CONTROL STATIONS5 MONITOR STATIONS -Passively track satellites in view -MCS processes data & updates satellites nav message -Updated Info is Xmitted to satellites via 4 ground antennas

12. GPS CONTROL STATIONSMASTER CONTROL STATION The Master Control Station (MCS) is located at Schriever, AFB in Colorado Springs. Ascension Is, Diego Garcia, Cape Canaveral & Kwajalein Is have ground antennas

13. Correct orbit and clock frequency upload navigation message Observe ephemeris and clock Create new navigation message Compute errors Monitor Station Master Control Station Upload Station

14. Components of GPS 3. User Equipment - Receivers are lightweight, small, and relatively inexpensive. - Used by both conventional & SPECOPS forces. - Civilian community makes up 90% of current users.

15. GPS Operation • - GPS uses satellite triangulation to determine a position much like is done in cel nav with the stars. • - Positions are calculated from distance measurements to satellites. • - The distance is determined by measuring how long a radio signal takes to reach a receiver from a satellite. (D=SXT)

16. GPS Operation • - Pseudo random Noise Codes - a set of complicated digital codes that determines the time difference between receiver and satellite which will yield a single LOP. • - Data from a second satellite will produce a set of two distinct positions on the surface of the earth, of which only one is close to your DR (ie. a fix).

17. A B 4 seconds 6 seconds X

18. GPS Operation • - Each satellite has an atomic clock for time accuracy (to the nearest nanosecond), however, receiving units do not. • - The error in the receiving clocks, therefore, gives an error in position accuracy.

19. A B 4 seconds 6 seconds XX 5 seconds (wrong time) 7 seconds (wrong time)

20. GPS Operation • - Using a third satellite corrects for this error. Taking 3 LOP’s and assuming an equal error for each, the error can be calculated and an exact position obtained. • - Thus, three satellites are needed for a 2D position and four for a 3D position.

21. A B X 9 seconds (wrong time) XX 7 seconds (wrong time) 5 seconds (wrong time) C

22. GPS Services • Standard Positioning Service (SPS): a positioning service which is available to all GPS users on continuous, worldwide basis. • Precise Positioning Service (PPS): a service developed primarily for military use which is more accurate and “Protected.” P CODE

23. Figure of Merit (FOM or FG) • Modern technology cannot eliminate all sources of error to GPS ( depending on atmospheric or equipment condition), so receivers use a “figure of merit” for a general indication of fix accuracy. • FOM1 (FG1) is the most accurate and FOM9 (FG9) is the least accurate. A FOM5 or greater cannot be used for a fix.

24. System Error • The primary sources of error that are difficult to eliminate are: • 1. Ionospheric and atmospheric delays • 2. Small deviations with the atomic clocks • 3. Math errors with receiving units. • 4. Satellite signals taking a “circuitous” path to the receivers • 5. Geometry between the receiver and satellites.

25. Selective Availability and Anti-spoofing • Two distinct techniques are used to prevent unauthorized use of PPS: • Selective Availability: the intentional alteration and degradation of GPS accuracy • Anti-spoofing: designed to protect against spoofing (imitation) and unauthorized use by encrypting the P code

26. Selective Availability (S/A) • Key points regarding S/A: • 1. Allows control of the accuracy of the civilian/unencrypted GPS signal. • 2. Basically a rheostat control which can reduce GPS accuracy from 100m to 2500m. • 3. Controlled from the MCS • 4. Requires Presidential order to invoke.

27. Statement by the president regarding the US decision to stop degrading GPS accuracy …..I am pleased to announce that the US will stop the intentional degradation of GPS signals available to the public….We call this degradation feature Selective Availability . This will mean that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than they do now…….. …We have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened…..

28. Navy Utilization of GPS 1. Oceanic / coastal region navigation 2. Mine delivery and sweeping 3. NGFS 4. All-weather missions 5. Coordinated operations 6. Passive rendezvous

29. Navy Utilization of GPS 7. Search & Rescue 8. Command & Control 9. FBM accuracy improvement 10. Amphibious Assault 11. Submarine Warfare

30. Civil Applications of GPS 1. Space Shuttle 2. Air navigation 3. Static positioning / timing 4. Land navigation 5. Maritime navigation 6. Search & Rescue

31. Differential Global Positioning System (DGPS) • DGPS makes use of an additional correction signal added to the regular GPS signal. • this is the only “legal” means of correcting for S/A

32. DGPS • How does it work? • A GPS receiver on the ground in a known location receives positioning info from satellites and determines exactly what errors the satellite data contains. • The ground receiver can than transmit an error correction message to any GPS receivers that are in the local area and they can correct their position solutions.

33. GPS Satellites Differential Reference Station Broadcast Transmitter Control Center Integrity Monitor Data Communications DGPS User Equipment

34. DGPS Coverage Area

35. DGPS • Benefits: DGPS accuracy and integrity are better then GPS (bw/in 0.7 and 2.5 meters). • Drawbacks: DGPS requires additional receiver equipment and coverage area is limited.

36. Questions?