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

2. Introduction Why by satellite? History GPS Galileo Competition
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. 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

6. TRANSIT System ? ? ?
Paul Lammertsma
Universiteit Utrecht

7. TRANSIT System
150 MHz
200 MHz
Paul Lammertsma
Universiteit Utrecht

8. TRANSIT System
150 MHz
150 MHz
Paul Lammertsma
Universiteit Utrecht

9. 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

10. TRANSIT System single frequency dual frequency Paul Lammertsma
Universiteit Utrecht

11. 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

12. 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

13. NAVSTAR GPS
History
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

14. NAVSTAR GPS
Configuration
Paul Lammertsma
Universiteit Utrecht

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

16. 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

17. NAVSTAR GPS
Coverage
Paul Lammertsma
Universiteit Utrecht

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

19. 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

20. NAVSTAR GPS
How it works
Paul Lammertsma
Universiteit Utrecht

21. 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

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

23. 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

24. 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

25. 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

26. 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

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

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

29. 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

30. NAVSTAR GPS The Broadcast
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
Paul Lammertsma
Universiteit Utrecht

31. 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

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

33. NAVSTAR GPS Content of the Broadcast (cont.)
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
Paul Lammertsma
Universiteit Utrecht

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

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

36. 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: × 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. Questions, etc.
Paul Lammertsma
Universiteit Utrecht