GPS & Galileo Satellite Navigation Paul Lammertsma Universiteit Utrecht

2 Introduction Why by satellite? History –TRANSIT System GPS Galileo Competition Cooperation Prospective Paul Lammertsma Universiteit Utrecht

3 Why by satellite? Signal reception is better than by land Signals can pass through clouds and rain As long as a satellite is in the receiver’s horizon, a signal is always perceivable Worldwide High accuracy Paul Lammertsma Universiteit Utrecht

4 History The Russians kept the Doppler-effect in mind with the launch of Sputnik I in 1957 To keep radio contact with a moving object, you have to keep changing your frequency The Americans discovered how to invert this in 1959 with the start of the TRANSIT navigation system If you know the position of the satellite, you can determine your relative position to it Paul Lammertsma Universiteit Utrecht

5 TRANSIT System Satellite sends its exact position and time over a fixed frequency Receiver monitors the difference between the received frequency and the expected frequency When these frequencies are equal, the satellite is directly above the receiver Paul Lammertsma Universiteit Utrecht Navy Navigation Satellite System

6 Paul Lammertsma Universiteit Utrecht TRANSIT System ? ? ?

7 Paul Lammertsma Universiteit Utrecht TRANSIT System 150 MHz 200 MHz

8 Paul Lammertsma Universiteit Utrecht TRANSIT System 150 MHz

9 Paul Lammertsma Universiteit Utrecht TRANSIT System The receiver only knows that the satellite is neither approaching or departing So the ship must be on a line perpendicular to the orbit of the satellite However, farther from the orbit, the frequency transition is less A calculation will tell the receiver how far, but not which side

10 Paul Lammertsma Universiteit Utrecht dual frequency TRANSIT System single frequency

11 TRANSIT System Up and running 2 years after concept Only need 1 satellite per measurement Paul Lammertsma Universiteit Utrecht Low orbit + few satellites = bad coverage Receiver needs a continuous signal Receiver has to wait for satellite to pass overhead Only up to 500/25 meter accuracy Assumes sea level altitude Cons Pros

12 NAVSTAR GPS The American Department of Defense started development in 1973 Six orbital planes (plane = orbit containing multiple satellites) 21 active satellites, plus 3 spares Four per plane Paul Lammertsma Universiteit Utrecht Navigation Satellite Timing and Ranging Concept

13 NAVSTAR GPS First three prototype satellites “Timation” from First prototype configuration “Block I” of 10 satellites from Current configuration is the “Block II” from –Delayed partially because of the ’86 Challenger disaster Paul Lammertsma Universiteit Utrecht History

14 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Configuration

15 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht equator orbital plane 20,200 km 55° ×6 planes Configuration

16 NAVSTAR GPS Why not geostationary at 36,000 km? –Stronger transmitter required –More powerful launcher required –Poor coverage of polar regions Compromise: 20,200 km so period is 12h However, many satellites needed –At least 17 satellites required –Today: 27 satellites = five to twelve in range! Paul Lammertsma Universiteit Utrecht Configuration

17 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Coverage

18 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Satellite position Time Other parameters –Satellite status –Possible inaccuracies –Information about other satellites –etc. Satellite Broadcast

19 NAVSTAR GPS A receiver receives a signal from a GPS satellite It calculates the difference from the current time and the time sent by the satellite It now knows how far away the satellite is Because we know that radio signals travel at the speed of light, we can calculate this Paul Lammertsma Universiteit Utrecht How it works

20 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht How it works

21 NAVSTAR GPS There are two possible locations One is practically impossible, so it can be ruled out –Too far away from Earth (too high) –Velocity is not realistic Still, we need a fourth satellite –Confirm this location –Improve accuracy Paul Lammertsma Universiteit Utrecht How it works (cont.)

22 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht How it works (cont.)

23 NAVSTAR GPS Satellite requirements Paul Lammertsma Universiteit Utrecht Each satellite must be uniquely identified Satellites must know their exact position Satellites must know the exact time –2 rubidium & 2 cesium atomic clocks –At least once every 4 hours it synchronizes position and time with a Monitoring Station

24 NAVSTAR GPS Why is this important? Paul Lammertsma Universiteit Utrecht It only takes a signal about 63 milliseconds to reach the receiver Inaccuracy of 1 millisecond puts you off by 300 kilometers! So the satellites are equipped with four atomic clocks But what about the receiver?

25 NAVSTAR GPS The receiver Paul Lammertsma Universiteit Utrecht The receiver has a simple digital clock It doesn’t have to be spot-on It just has to get the travel time of each satellite’s signal relative to each other But this means we do need a fourth satellite

26 In two dimensions, this is the ideal situation Note that in 2D, we need 3 measurements! NAVSTAR GPS Pseudo range Paul Lammertsma Universiteit Utrecht We’re exactly here

27 In two dimensions, this would be the reality NAVSTAR GPS Pseudo range (cont.) Paul Lammertsma Universiteit Utrecht

28 With a calculation, we can make the circles intersect again NAVSTAR GPS Pseudo range (cont.) Paul Lammertsma Universiteit Utrecht

29 NAVSTAR GPS Pseudo range (cont.) Paul Lammertsma Universiteit Utrecht We can adjust the local time until the spheres more or less intersect The effect is twofold –We can more precisely determine our position –We can update the receiver’s clock

30 NAVSTAR GPS The Broadcast Paul Lammertsma Universiteit Utrecht Satellites broadcast over two reserved frequencies –L1 frequency, at MHz –L2 frequency, at MHz L1 carries a C/A code, which can be identified by civil receivers L1 & L2 carry a P code, which can only be identified by the U.S. military

31 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Content of the Broadcast Block of bits We need to send, say, 1200 bits of data The beginning of each frame must be identifiable A receiver shouldn’t have to wait until the next broadcast to join 1200 bits sub-frame headersframe header + 60× bits

32 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Content of the Broadcast (cont.) Frame 1500 bits Sub-frames 300 bits

33 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht The Navigation Message is transmitted over the L1 frequency Although the frequency is MHz, the message is carried at exactly 50 Hz That’s 50 bits per second To send a sub-frame of 300 bits, it takes precisely 6 seconds So a frame is repeated every 30 seconds Content of the Broadcast (cont.)

34 Sub-frames NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Content of the Broadcast (cont.) 6 seconds 30 s

35 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Content of the Broadcast (cont.) Sub-frame TLMHOWData Preamble(reserved)Parity Telemetry Word states the beginning of the sub-frame Contains reserved information Telemetry Word (TLM) 30 bits 240 bits

36 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Content of the Broadcast (cont.) Sub-frame TLMHOWData Time of weekDataParity Handover Word states the time of week Also states the current sub-frame Tells receiver if of possible inaccuracy Handover Word (HOW) 30 bits240 bits30 bits bits: × 6 = 7 days Sub-Frame ID Alert & AS-flag

37 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht 1.Acquire lock on frequency 2.Search for the preamble 3.Collect the following 16 bits of reserved data from TLM & check it with the parity 4.Gather all the data from the HOW and check the parity again 5.Identify the current sub-frame and start gathering data after HOW’s two 0-bits Reception of the Broadcast

38 NAVSTAR GPS Paul Lammertsma Universiteit Utrecht Data in the Broadcast Header wordsData words240 bits sub-frame 1 sub-frame 2 sub-frame 3 sub-frame 4 sub-frame 5 Satellite clock & health data Satellite ephemeris (position) data Support data to be sent to Monitoring Station over 25 looping pages Every sub-frame is split up in 10 words (word = block of 30-bits) The data is in words × 30 = 240 bits

39 Paul Lammertsma Universiteit Utrecht NAVSTAR GPS Data in the Broadcast (cont.) Receiver can use this data to pinpoint his relative location –Time elapsed to send signal –Position of that satellite Where the other satellites are Receiver now only needs to calculate the time from the other three satellites –This can happen at the same time!

40 Paul Lammertsma Universiteit Utrecht NAVSTAR GPS Limitations The chosen microwave-frequencies are highly sensitive They can’t even pass through thin foliage! This means reduced service –Worse coverage –“Multipath”: Range errors by signal bounce During wartime, the U.S. reduces accuracy or even shuts down civil GPS

41 Galileo Paul Lammertsma Universiteit Utrecht Four navigation services and one Search and Rescue service Six different navigation signals Three carrier frequencies Better performance than other satellite navigation systems Compatibility and interoperability with other satellite navigation systems Concept

42 Galileo Paul Lammertsma Universiteit Utrecht Open Service –Free of user charge Safety of Life Service –OS with timely warnings of integrity problems Commercial Service –Two additional signals improve accuracy Public Regulated Service –Two additional signals for high continuity Services

43 Galileo Paul Lammertsma Universiteit Utrecht Search and Rescue Service –Finds a beacon broadcasting a distress signal –Broadcasts the distress signal and beacon location globally Services (cont.)

44 Competition Paul Lammertsma Universiteit Utrecht The U.S. disliked the upcoming competitor Galileo Such accuracy poses a threat to the U.S. military GPS III, currently being researched, will match or surpass Galileo’s accuracy GPS against Galileo

45 Competition Paul Lammertsma Universiteit Utrecht The EU wants to be more than the consumer and partner in the background The EU dislikes the U.S.’s reduced accuracy policy They want to improve the existing service They want fully civil satellite navigation They want to have a guarantee that the service is always available Galileo against GPS

46 Cooperation Paul Lammertsma Universiteit Utrecht Political issues put aside, GPS and Galileo will cooperate Galileo will complement the existing GPS in accuracy and availablility However, Galileo will also be able to run independently GPS & Galileo

47 Cooperation Paul Lammertsma Universiteit Utrecht All the satellites will be able to communicate with each other Existing GPS-receivers will be able to make use of Galileo GPS & Galileo

48 Galileo will be fully active in 2008 Improved signal strength –Global positioning within buildings –Major improvements within cities “Always functioning” guarantee –Aircraft might be allowed official usage Improved service –Improved performance for existing uses –New uses Prospective Paul Lammertsma Universiteit Utrecht

49 Paul Lammertsma Universiteit Utrecht Questions, etc.