GPS & Galileo Satellite Navigation Paul Lammertsma Universiteit Utrecht GPS & Galileo Satellite Navigation

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

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

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

Navy Navigation Satellite System TRANSIT System Navy Navigation Satellite 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

TRANSIT System ? ? ? Paul Lammertsma Universiteit Utrecht

TRANSIT System 150 MHz 200 MHz Paul Lammertsma Universiteit Utrecht

TRANSIT System 150 MHz 150 MHz 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 Paul Lammertsma Universiteit Utrecht

TRANSIT System single frequency dual frequency Paul Lammertsma Universiteit Utrecht

TRANSIT System Pros Cons Up and running 2 years after concept Only need 1 satellite per measurement Cons 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 Paul Lammertsma Universiteit Utrecht

Navigation Satellite Timing and Ranging NAVSTAR GPS Navigation Satellite Timing and Ranging Concept 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

NAVSTAR GPS History First three prototype satellites “Timation” from 1967-74 First prototype configuration “Block I” of 10 satellites from 1978-85 Current configuration is the “Block II” from 1989-94 Delayed partially because of the ’86 Challenger disaster Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS Configuration Paul Lammertsma Universiteit Utrecht

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

NAVSTAR GPS Configuration 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

NAVSTAR GPS Coverage Paul Lammertsma Universiteit Utrecht

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

NAVSTAR GPS How it works 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

NAVSTAR GPS How it works Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS How it works (cont.) 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

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

NAVSTAR GPS Satellite requirements 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 Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS Why is this important? 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? Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS The receiver 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 Paul Lammertsma Universiteit Utrecht

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

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

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

NAVSTAR GPS Pseudo range (cont.) 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 Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS The Broadcast Satellites broadcast over two reserved frequencies L1 frequency, at 1575.42 MHz L2 frequency, at 1227.6 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 Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS Content of the Broadcast frame header sub-frame headers Block of bits 1500 bits 1200 bits + 60 × 5 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 Paul Lammertsma Universiteit Utrecht

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

NAVSTAR GPS Content of the Broadcast (cont.) The Navigation Message is transmitted over the L1 frequency Although the frequency is 1575.42 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 Paul Lammertsma Universiteit Utrecht

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

NAVSTAR GPS Content of the Broadcast (cont.) Sub-frame 30 bits 30 bits 240 bits TLM HOW Data Telemetry Word (TLM) 10001011 Preamble (reserved) Parity Telemetry Word states the beginning of the sub-frame Contains reserved information Paul Lammertsma Universiteit Utrecht

NAVSTAR GPS Content of the Broadcast (cont.) Sub-frame 30 bits 30 bits 240 bits TLM HOW Data Handover Word (HOW) Sub-Frame ID Alert & AS-flag 17 bits: 100799 × 6 = 7 days 00 Time of week Data Parity Handover Word states the time of week Also states the current sub-frame Tells receiver if of possible inaccuracy Paul Lammertsma Universiteit Utrecht

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

NAVSTAR GPS Data in the Broadcast Header words Data words 240 bits sub-frame 1 sub-frame 2 sub-frame 3 Satellite clock & health data Satellite ephemeris (position) data Every sub-frame is split up in 10 words (word = block of 30-bits) The data is in words 3-10 7 × 30 = 240 bits sub-frame 4 sub-frame 5 Support data to be sent to Monitoring Station over 25 looping pages 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! 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 Paul Lammertsma Universiteit Utrecht

Galileo Concept 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 Paul Lammertsma Universiteit Utrecht

Galileo Services Open Service Safety of Life 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 Paul Lammertsma Universiteit Utrecht

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

Competition GPS against Galileo 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 Paul Lammertsma Universiteit Utrecht

Competition Galileo against GPS 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 Paul Lammertsma Universiteit Utrecht

Cooperation GPS & Galileo 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 Paul Lammertsma Universiteit Utrecht

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

Prospective 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 Paul Lammertsma Universiteit Utrecht

Questions, etc. Paul Lammertsma Universiteit Utrecht