GPS uses a constellation of 24 satellites that continuously transmit radio signals. A GPS receiver uses these signals to calculate the user's position by measuring travel times and trilaterating distances from at least 3 satellites. The US DoD monitors and maintains the satellites, while users receive location data for navigation and other applications. GPS provides worldwide coverage and works anywhere without fees to users.

1. GPS
Global Positioning System

2. GPS Basics
• What is GPS?
• How does it work?

3. What is GPS
• GPS is a satellite-based system, operated
and maintained by the U.S. Department
of Defense (DoD), that provides accurate
location and timing information to people
worldwide.

4. • The system transmits radio signals that
can be used by GPS receivers to calculate
position, velocity and time anywhere on
earth, any time of day or night, in any
kind of weather

5. • The NAVSTAR GPS concept was developed in the
early 1970's to meet the U.S. military’s need for
improved navigation and positioning. The first
satellite was launched in 1978 and Full Operational
Capability (FOC) was achieved in April 1995.
• The Global Positioning System is a National resource
and an international utility for positioning, navigation
and timing.

6. • The NAVSTAR Global
Positioning System
(GPS) is a constellation
of orbiting satellites that
provides navigation data
to military and civilian
users all over the world.

7. What is GPS?
• NAVSTAR GPS (Navigation Satellite Timing
and Ranging system)
• 24 Satellites orbiting the earth
• Positioning, navigation and timing
• Operates 24 hrs/day, in all weather
• Can be used for any application that requires
location information

8. THREE SEGMENTS OF GPS
• Space
•Control
•User

9. SPACE SEGMENT
• The space segment is the satellite
constellation. The first Block I satellite was
launched in early 1978.
• The 1986 Challenger disaster slowed the
GPS constellation development. In February
1989 the first Delta 2 launch took place. The
constellation is now fully operational and
consists of 24 or more satellites.

10. CONTROL SEGMENT
• The control segment is operated by the
U.S. Department of Defense (DoD)
which tracks and maintains the satellites.
The Department of Transportation (DoT)
now has management responsibility,
along with DoD.

11. USER SEGMENT
• The user segment consists of both military
and civilian users. Military uses of GPS
include navigation, reconnaissance, and
missile guidance systems.
• Civilian use of GPS developed at the same
time as military use, and has expanded far
beyond anyone's original expectations

12. GPS Segments
User
Control
Space

13. Space Segment: GPS Satellites
• Power
– Sun-seeking solar panels
– Ni, Cd batteries
• Timing
– 4 atomic clocks

14. • The GPS satellites weigh about 900 kg and are about 5 meters
wide with the solar panels fully extended.
• They are built to last about 7.5 years, but many have
outlasted their original estimated life-span.
• The solar panels provide primary power; secondary
power is provided by Nicad batteries. On board each
satellite are four highly accurate atomic clocks.
• As of May 2003, there are 28 usable satellites in place.

15. Satellite Orbits
• 24 satellites in 6 orbital planes
• Orbit the earth at approx. 20,200 km (11,000
nautical miles)
• Satellites complete an orbit in approximately
12 hours
• Satellites rise (and set) approximately 4
minutes earlier each day

16. • There are four satellites in each orbit plane,
and each plane is inclined 55 degrees relative
to the equatorial plane (the satellite path
crosses the equator at a 55 degree angle).
• The high altitude ensures that satellite orbits
are stable, precise and predictable, and that the
satellites' motion through space is not affected
by atmospheric drag. It also ensures satellite
coverage over large areas.

17. Satellite Signals
• GPS satellites broadcast messages via radio
signals on 2 frequencies
– L1: 1575.42 MHz (C/A and P/Y code)
– L2: 1227.60 MHz (P/Y code)
• Two levels of service
– Standard Positioning Service (SPS)
– Precise Positioning Service (PPS)

18. • The radio signals travel at the speed of light:
300,000 km per second (186,000 miles per
second).
• It takes 6/100ths of a second for a GPS
satellite signal to reach earth. These signals are
transmitted at a very low wattage (about
300-350 watts in the microwave spectrum).

19. Coarse Acquisition Code
• C/A Code - Coarse/Acquisition Code available
for civilian use on L1 provides 300 m
resolution.
• C/A code (Coarse Acquisition code) is
available to civilians as the Standard
Positioning Service (SPS).

20. Precise Code
• P Code - Precise Code on L1 and L2 used by
the military provides 3m resolution
• The Precise Positioning Service (PPS),
available only to the military (and other
authorized users), provides higher accuracy
via the P code.

21. Satellite Signals, cont.
• Radio signals contain
– Unique pseudorandom code
– Ephemeris
– Clock behavior and clock corrections
– System time
– Status messages
– Almanac

22. Unique pseudorandom code
• Each satellite transmits a radio signal
containing its unique pseudorandom
(appears to be random but is not) code.
• This code identifies the satellite and
distinguishes it from other satellites

23. • Ephemeris data : The signal also contains the
precise location of the satellite (ephemeris
data).
• Status messages (usually referring to satellite
health).

24. Almanac Data
• An almanac is also provided which gives the
approximate location data for each active
satellite.
• The almanac is automatically downloaded
from the satellites to the GPS receiver when
the receiver is operating outside.

25. • It takes about 12 minutes to receive an
almanac. The almanac data can be transferred
to the office computer and used to display a
graphic showing the locations of all satellites.
This information can also be used to predict
satellite availability for a specific mapping
time and date.

26. Satellite Signals, cont.
• Satellite signals require a direct line to
GPS receivers
• Signals cannot penetrate water, soil, walls
or other obstacles

27. Satellite Almanac
• Sent along with position and timing messages
• Prediction of all satellite orbits
• Needed to run satellite availability software
• Valid for about 30 days

28. • The almanac has information about the orbits of all
24 satellites.
• A GPS receiver uses the almanac (for quick
acquisition of satellite positions), along with satellite
data messages, to precisely establish the position of
each satellite it is tracking.
• Satellite availability software uses the almanac to
make graphs of satellite locations overhead and to
calculate the best times to survey in a particular area.

29. • GPS receivers automatically collect a new almanac
each time they are turned on.
• It is important to use an up-to-date almanac when
viewing satellite availability during mission planning.
• Almanac data are valid for about 30 days, but a new
almanac should be transferred to satellite availability
software as frequently as possible.

30. GPS ORBITS

32. Control Segment: US DoD Monitoring
Colorado Springs
Hawaii
Ascension
Diego Garcia
Kwajalein

33. U.S. DoD Monitoring
• Orbits are precisely measured
• Discrepancies between predicted orbits
(almanac) and actual orbits are transmitted
back to the satellites

34. User Segment

35. How Does GPS Work?
Calculating a Position
• GPS receiver calculates
its position by
measuring the distance
to satellites (satellite
ranging)

36. Measuring Distance to Satellites
1. Measure time for signal to travel from
satellite to receiver
2. Speed of light x travel time = distance
Speed of light (300,000 km/sec)

37. Trilateration: 3 Distance Measurements
122 mi
80 mi
127 mi

38. • Here is an example of
trilateration in two
dimensions.
• Three ranges will locate a
point in two-dimensional
space.
• If we know that our location
is 127 miles from Great
Falls, then we are
somewhere on the red circle.

39. • If we also know that we are
122 miles from Billings,
then our position is
somewhere on the purple
circle, and, if we are 80
miles from Helena, we are
somewhere on the green
circle.
• Considering the three range
measurements together, our
position must be where the
three circles intersect, or,
Bozeman!

40. Measuring Travel Time of
Satellite Signals
• How do we find the exact time the signal left the
satellite?
• Synchronized codes
Time
difference

41. • In order to measure the travel time of the
satellite signal, we have to know when the
signal left the satellite AND when the signal
reached the receiver.
• Our receiver "knows" when it receives a
signal, but how does it know when the signal
left the satellite?

42. • GPS satellites generate a
complicated set of digital
codes (shown red)
• They are "pseudo-random"
sequences that actually
repeat every millisecond.
• The trick is that the GPS
satellites and our
receivers are
synchronized so they're
generating the same code
at exactly the same time.
Time
difference

43. • The time difference is
how long the signal
took to get from the
satellite to the receiver.
• In other words, the
receiver compares how
"late" the received
satellite code is,
compared to the code
generated by the
receiver itself.
Time
difference

44. One measurement narrows down our
position to the surface of a sphere

45. • 12,000 miles is the radius of a sphere centered
on the satellite.
• Our position could be anywhere on the surface
of that sphere.
• The intersection of two spheres is a circle. Now we
know that our position is somewhere on that circle

46. A second measurement narrows down our
position to the intersection of two spheres

47. A third measurement narrows down
our position to just two points

48. • The three spheres intersect at only two points.
Usually we can discard one of the two points because
one point might be nowhere near the earth or it might
be moving at a ridiculous speed.
• The computers in GPS receivers have various
techniques for distinguishing the correct point from
the incorrect one. But there is a reason we need a
fourth measurement...

49. Correcting for Timing Offset
• The first three measurements narrow down our
position
• A fourth measurement is needed to correct for
timing offset (difference in synchronization
between satellite and receiver clocks)
– Satellites use highly accurate atomic clocks
– Receivers use accurate quartz clocks

51. • Timing offset refers to the difference in
synchronization between the satellite clock and
the receiver clock.
• Atomic clocks are far too expensive to put in
GPS receivers, so a correction must be applied
to compensate for the difference between the
satellite and receiver clocks.

52. • In an ideal situation there would be no
timing error.
• Let's say we're 4 seconds from satellite A
and 6 seconds from satellite B.
• Our position is where the 2 circles
intersect.

53. 6 seconds
4 seconds
A
B

54. • If the receiver clock is one second fast (it's
ahead one second from the satellite clock).
• The receiver will "think" the distance from
satellite A is 5 seconds and the distance
from satellite B is 7 seconds.
• And it "thinks" our position is where the
two dotted circles intersect.

55. 6 seconds
4 seconds
5 seconds
(wrong time)
7 seconds
(wrong time)
A
B

56. • Back to the ideal situation with no timing
error.
• If we have accurate clocks, and we add a
third measurement, all three circles
intersect at the correct point, because the
circles represent the true ranges from the
three satellites

57. 6 seconds
4 seconds
8 seconds
A
B
C

58. • But with inaccurate clocks, the circles cannot
intersect: there is no point that can be 5 seconds from
A, 7 seconds from B and 9 seconds from C.
• When the receiver gets a series of measurements that
cannot intersect at a single point, it finds the
adjustment to all measurements that lets the ranges go
through one point. In this example, subtracting 1
second from all three measurements makes the circles
intersect at a point.

59. 5 seconds
(wrong time)
7 seconds
(wrong time)
9 seconds
(wrong time)
B
A
C

60. What’s wrong with this picture?

61. What’s wrong with this picture?

62. 5 Things to Take Away Today
1. 3 GPS segments
2. Satellites transmit radio signals containing
– Unique pseudorandom code
– Ephemeris
– Clock behavior and clock corrections
– System time
– Status messages
– Almanac
3. Formula for satellite ranging (D = t ∙ v)
4. 4 satellites to compute an accurate 3-D position (the 4th
measurement is needed to correct for timing offset)