Global Positioning System

1
Global Positioning System
LUCID Summer Workshop July 29, 2004
2
Background

• In the past, humans had to go to pretty extreme
  measures to keep from getting lost.
• They erected monumental landmarks, laboriously
  drafted detailed maps and learned to read the
  stars in the night sky.

3
Background (Contd)

• Things are much, much easier today.
• For less than 100, you can get a pocket-sized
  gadget that will tell you exactly
• where you are on Earth at any
• moment. As long as you have
• a GPS receiver and a clear
• view of the sky, you'll never be
• lost again.

4
Outline for Today

• Today, we will review the basics of the GPS
  system its key components, its history, etc.
• To gain a full appreciation of the complexity of
  this system, we will also provide an introduction
  to satellite communications.

5
GPS The Basics
6
What is it?

• GPS Global Positioning System is a worldwide
  radio-navigation system formed from a
  constellation of 24 satellites and their ground
  stations.
• A simplistic explanation
• GPS uses these man-made
• stars as reference points to
• calculate positions accurate
• to a matter of meters.

7
What is it? (Contd)

• Advanced forms of GPS make measurements to better
  than a centimeter.
• Devised by the U.S. Department of Defense for
  fleet management, navigation, etc.
• Although the U.S. military developed and
  implemented this satellite network as a military
  navigation system, it soon opened it up to
  everybody else.

8
A Little Bit of History

• For centuries, only way to navigate was to look
  at position of sun and stars.
• Modern clocks made it possible to find one's
  longitude.
• Using this information and estimate of latitude ?
  most accurate instruments could yield positions
  accurate only to within a few miles.

9
History (Contd)

• Then, when the Soviet Union
• launched Sputnik on Oct. 4,
• 1957, it was immediately
• recognized that this
• "artificial star" could be used
• as a navigational tool.
• Very next evening, researchers at Lincoln Labs
  at MIT were able to determine satellite's orbit
  precisely by observing specific properties of its
  transmitted radio wave (Doppler Shift).

10
More History

• The proof that a satellite's orbit could be
  precisely determined from ground was first step
  in establishing that positions on ground could be
  determined by homing in on signals broadcast by
  satellites.
• Then U.S. Navy experimented with a series of
  satellite navigation systems to meet navigational
  needs of submarines carrying nuclear missiles.
• These submarines needed to remain hidden and
  submerged for months at a time.

11
More History (Contd)

• By analyzing the radio signals transmitted by the
  satellites--in essence, measuring the Doppler
  shifts of the signals--a submarine could
  accurately determine its location in 10 or 15
  minutes.
• In 1973, Department of Defense was looking for a
  foolproof method of satellite navigation.
• A brainstorming session at Pentagon over Labor
  Day weekend produced concept of GPS on basis of
  the department's experience with all its
  satellite predecessors.

12
More History (Contd)

• The essential components of GPS were the 24
  Navstar satellites built by Rockwell
  International, each the size of a large
  automobile and weighing some couple of thousand
  pounds.
• Each satellite orbits the earth every 12 hours
  in a formation that ensures that every point on
  the planet will always be in radio contact with
  at least four satellites.
• The first operational GPS satellite was launched
  in 1978, and the system reached full 24-satellite
  capability in 1993.

13
More Background

• Each satellite is expected to last approximately
  7.5 years and replacements are constantly being
  built and launched into orbit.
• Each satellite transmits on three frequencies.
• Civilian GPS uses the L1 frequency of 1575.42
  MHz.

14
More Background (Contd)

• Day-to-day running of GPS program and operation
  of system rests with the Department of Defense
  (DoD).
• Management is performed by US Air Force with
  guidance from DoD Positioning/Navigation
  executive Committee.
• This committee receives input from a similar
  committee within Department of Transportation
  (DoT) who act as civilian voice for GPS policy
  matters.

15
Background (Contd)

• Each of these 3,000- to 4,000-pound solar-powered
  satellites circles the globe at about 12,000
  miles (19,300 km), making two complete rotations
  every day.
• The orbits are arranged so that at
• any time, anywhere on Earth,
• there are at least four satellites
• "visible" in the sky.

16
Triangulation

• A GPS receiver's job is to locate four or more of
  these satellites, figure out the distance to
  each, and use this information to deduce its own
  location.
• This operation is based on a simple mathematical
  principle called triangulation or trilateration.
• Triangulation in three-dimensional space can be a
  little tricky, so we'll start with an explanation
  of simple two-dimensional trilateration.

17
An Example of 2D Triangulation

• Imagine you are somewhere in the United States
  and you are TOTALLY lost -- for whatever reason,
  you have absolutely no clue where you are.
• You find a friendly local and ask, "Where am I?"
  He says, "You are 625 miles from Boise, Idaho."
• This is a nice, hard fact, but it is not
  particularly useful by itself. You could be
  anywhere on a circle around Boise that has a
  radius of 625 miles

18
Where in the U.S. am I?

• To pinpoint your location better, you ask
  somebody else where you are.
• She says, "You are 690 miles from Minneapolis,
  Minnesota. If you combine this information with
  the Boise information, you have two circles that
  intersect.

19
Where in the U.S. am I? (Contd)

• If a third person tells you that you are 615
  miles from Tucson, Arizona, you can eliminate one
  of the possibilities, because the third circle
  will only intersect with one of these points. You
  now know exactly where you are

20
Where in the U.S. am I? (Contd)

• You are in Denver, CO!
• This same concept works in three-dimensional
  space, as well, but you're dealing with spheres
  instead of circles.

21
Another 2D Example

• Consider the case of a mariner at sea (receiver)
  determining his/her position using a foghorn
  (transmitter).
• Assume the ship keeps an accurate clock and
  mariner has approximate knowledge of ships
  location.

Fog
22
Foghorn Example

• Foghorn whistle is sounded precisely on the
  minute mark and ship clock is synchronized to
  foghorn clock.
• Mariner notes elapsed time from minute mark until
  foghorn whistle is heard.
• This propagation time multiplied by speed of
  sound is distance from foghorn to mariners ear.

23
Foghorn Example (Contd)

• Based on measurement from one such foghorn, we
  know mariners distance (D) to foghorn.
• With measurement from one foghorn, mariner can be
  located anywhere on the circle denoted below

D
Foghorn 1
24
Foghorn Example (Contd)

• If mariner simultaneously measured time range
  from 2nd foghorn in same way.
• Assuming, transmissions synchronized to a common
  time base and mariner knows the transmission
  times. Then

A
D
Foghorn 1
B
Possible Location of Mariner
25
Foghorn Example (Contd)

• Since mariner has approximate knowledge of ships
  location, he/she can resolve the ambiguity
  between location A and B.
• If not, then measurement from a third foghorn is
  needed.

D
Foghorn 1
B
D3
Foghorn 3
26
How Foghorn Relates to GPS

• The foghorn examples operates in 2D space. GPS
  performs similar location but in 3D.
• The foghorn examples shows how time-of-arrival of
  signal (whistle) can be used to locate a ship in
  a fog. In this time-of-arrival of signal, we
  assumed we knew when the signal was transmitted.
• We measured the arrival time of the signal to
  determine distance. Multiple distance
  measurements from other signals were used to
  locate the ship exactly.

27
Foghorn Example Consider Effect of Errors

• Foghorn/mariner discussion assumed ships clock
  was precisely synchronized to foghorns time
  base.
• This may not be the case ? errors in TOA
  measurements.
• If we make a fourth measurement, we can remove
  this uncertainty.

D2e2
De1
Foghorn 1
Foghorn 2
D3e3
Foghorn 3
Estimated Location Area of Ship
28
3D Triangulation

• Fundamentally, three-dimensional trilateration is
  not much different from two-dimensional
  trilateration, but it's a little trickier to
  visualize.
• Imagine the radii from the examples in the last
  section going off in all directions. So instead
  of a series of circles, you get a series of
  spheres.

29
GPS Triangulation

• If you know you are 10 miles from satellite A in
  the sky, you could be anywhere on the surface of
  a huge, imaginary sphere with a 10-mile radius.

10 miles
Earth
30
GPS Triangulation (Contd)

• If you also know you are 15 miles from satellite
  B, you can overlap the first sphere with another,
  larger sphere. The spheres intersect in a perfect
  circle.

15 miles
10 miles
31
GPS Triangulation (Contd)

• The circle intersection implies that the GPS
  receiver lies somewhere in a partial ring on the
  earth.

Perfect circle formed from locating two satellites
Possible Locations of GPS Receiver
32
GPS Triangulation (Contd)

• If you know the distance to a third satellite,
  you get a third sphere, which intersects with
  this circle at two points.

33
GPS Triangulation (Contd)

• The Earth itself can act as a fourth sphere --
  only one of the two possible points will actually
  be on the surface of the planet, so you can
  eliminate the one in space.
• Receivers generally look to four or more
  satellites, however, to improve accuracy and
  provide precise altitude information.

34
GPS Receivers

• In order to make this simple calculation, then,
  the GPS receiver has to know two things
• The location of at least three satellites above
  you
• The distance between you and each of those
  satellites
• The GPS receiver figures both of these things out
  by analyzing high-frequency, low-power radio
  signals from the GPS satellites.

35
GPS Receivers (Contd)

• Better units have multiple receivers, so they can
  pick up signals from several satellites
  simultaneously.
• Radio waves travel at the speed of light (about
  186,000 miles per second, 300,000 km per second
  in a vacuum).
• The receiver can figure out how far the signal
  has traveled by timing how long it took the
  signal to arrive. (Similar to foghorn example.)

36
Aside Introduction to Satellite Communications
37
Satellites

• The basic component of a communications satellite
  is a receiver-transmitter combination called a
  transponder.
• A satellite stays in orbit because the
  gravitational pull of the earth is balanced by
  the centripetal force of the revolving satellite.
• Satellite orbits about the earth are either
  circular or elliptical.

38
Satellite Orbits

• A circular satellite orbit can be described as
• Using basic principles from physics, we can
  determine the orbit, i.e., find r, the radius of
  the circular orbit.

Satellite, m mass
r
R
Circular satellite orbit
Earth
39
Satellite Orbits (Contd)

• Satellites orbit the earth from heights of 100 to
  22,300 mi and travel at speeds of 6800 to 17,500
  mi/h.
• A satellite that orbits directly over the
  equator 22,300 mi from earth is said to be in a
  geostationary orbit.
• Geostationary (GEO) satellite revolves in
  synchronism with the earths rotation, so it
  appears to be stationary when seen from points on
  the earth.

40
3D Orbit

• Satellite orbits are not just determined by
  radius. There is also an inclination of the
  orbit relative to the equatorial plane (plane
  formed by the earths equator).

Orbit of Satellite
Inclination
Equatorial Plane
Earth
Orbital Plane
41
Satellite Motion Description

• To describe this 3D motion of a satellite, three
  typical measurements are used roll, pitch, and
  yaw.

42
Orbit Shapes

• Only some of the satellites have circular orbits.
• Others have elliptical orbits. These orbits have
  further classifiers
• Apogee
• Perigee

43
Apogee Perigee

• Perigee point on orbit when satellite is
  closest to earth.
• Apogee point on orbit when satellite is
  farthest from earth.

44
Stabilizing Satellite Orbits

• A satellite is stabilized in orbit by spinning it
  on its axis or building in spinning flywheels for
  each major axis (roll, pitch, yaw).
• Attitude adjustments on a satellite are made by
  firing small jet thrusters to change the
  satellites position or speed.
• Satellites are launched into orbit by rockets
  that give them vertical as well as forward motion.

45
Putting Satellites into Orbit

• All satellites today get into orbit by riding on
  a rocket or by riding in the cargo bay of the
  Space Shuttle.
• Several countries and businesses have rocket
  launch capabilities, and satellites as large as
  several tons make it safely into orbit on a
  regular basis.
• For most satellite launches, the scheduled launch
  rocket is aimed straight up at first. This gets
  the rocket through the thickest part of the
  atmosphere most quickly and best minimizes fuel
  consumption.

46
Putting Satellites in Orbit (Contd)

• After a rocket launches straight up, the rocket
  control mechanism uses the inertial guidance
  system to calculate necessary adjustments to the
  rocket's nozzles to tilt the rocket to the course
  described in the flight plan.
• In most cases, the flight plan calls for the
  rocket to head east because Earth rotates to the
  east, giving the launch vehicle a free boost.

N
S
47
Initial Boost offered by Earth

• The strength of this boost given by earths
  rotation depends on the rotational velocity of
  Earth at the launch location.
• The boost is greatest at the equator, where the
  distance around Earth is greatest and so rotation
  is fastest.

48
Initial Boost (Contd)

• How big is the boost from an equatorial launch?
• To make a rough estimate, we can determine
  Earth's circumference by multiplying its diameter
  by pi (3.1416).
• The diameter of Earth is approximately 7,926
  miles. Multiplying by pi yields a circumference
  of something like 24,900 miles.
• To travel around that circumference in 24 hours,
  a point on Earth's surface has to move at 1,038
  mph.

49
Initial Boost (Contd)

• A launch from Cape Canaveral, Florida, doesn't
  get as big a boost from Earth's rotational speed.
  
• The Kennedy Space Center's Launch Complex 39-A,
  one of its launch facilities, is located at 28
  degrees 36 minutes 29.7014 seconds north
  latitude.
• The Earth's rotational speed there is about 894
  mph.
• The difference in Earth's surface speed between
  the equator and Kennedy Space Center is about 144
  mph.

50
Initial Boost (Contd)

• Considering that rockets can go thousands of
  miles per hour, what does a difference of only
  144 mph mean?
• Rockets, together with their fuel and their
  payloads, are very heavy.
• For example, February 11, 2000 lift-off of the
  Space Shuttle Endeavor with the Shuttle Radar
  Topography Mission required launching a total
  weight of 4,520,415 pounds.

51
Initial Boost (Contd)

• It takes a huge amount of energy to accelerate
  such a mass to 144 mph, and therefore a
  significant amount of fuel.
• Launching from the equator makes a real
  difference.

52
Putting Satellites into Orbit (Contd)

• Once the rocket reaches extremely thin air, at
  about 120 miles (193 km) up, the rocket's
  navigational system fires small rockets, just
  enough to turn the launch vehicle into a
  horizontal position.
• The satellite is then released. At that point,
  rockets are fired again to ensure some separation
  between the launch vehicle and the satellite
  itself.

53
Putting a Satellite in Orbit (Contd)

• A rocket must accelerate to at least 25,039 mph
  to completely escape Earth's gravity and fly off
  into space.
• Earth's escape velocity is much greater than
  what's required to place an Earth satellite in
  orbit.
• With satellites, the objective is not to escape
  Earth's gravity, but to balance it.

54
Orbit Velocity

• Orbital velocity is the velocity needed to
  achieve balance between
• gravity's pull on the satellite and
• the inertia of the satellite's motion -- the
  satellite's tendency to keep going.
• Without gravity, the satellite's inertia would
  carry it off into space.

55
Orbit Velocity (Contd)

• Even with gravity, if the intended satellite goes
  too fast, it will eventually fly away. On the
  other hand, if the satellite goes too slowly,
  gravity will pull it back to Earth.
• At the correct orbital velocity, gravity exactly
  balances satellite's inertia.
• The orbital velocity of the satellite depends on
  its altitude above Earth. The nearer Earth, the
  faster the required orbital velocity.

56
Drag

• In general, the higher the orbit, the longer the
  satellite can stay in orbit.
• At lower altitudes, a satellite runs into traces
  of Earth's atmosphere, which creates drag.
• Drag causes orbit to decay until the satellite
  falls back into the atmosphere and burns up.
• At higher altitudes, where the vacuum of space is
  nearly complete, there is almost no drag and a
  satellite can stay in orbit for centuries (take
  the moon as an example).

57
Different Roles for Satellites

• Weather satellites help meteorologists predict
  the weather or see what's happening at the
  moment. The satellites generally contain cameras
  that can return photos of Earth's weather.
• Communications satellites allow telephone and
  data conversations to be relayed through the
  satellite. The most important feature of a
  communications satellite is the transponder -- a
  radio that receives a conversation at one
  frequency and then amplifies it and retransmits
  it back to Earth on another frequency.

58
Different Satellites (Contd)

• Broadcast satellites broadcast television signals
  from one point to another (similar to
  communications satellites).
• Scientific satellites perform a variety of
  scientific missions. The Hubble Space Telescope
  is the most famous scientific satellite, but
  there are many others looking at everything from
  sun spots to gamma rays.
• Navigational satellites help ships and planes
  navigate, e.g., GPS.

59
Different Satellites (Contd)

• Rescue satellites respond to radio distress
  signals.
• Earth observation satellites observe the planet
  for changes in everything from temperature to
  forestation to ice-sheet coverage.
• Military satellites are up there, but much of the
  actual application information remains secret.

60
Some Possible Military Applicaitons

• Relaying encrypted communications
• Nuclear monitoring
• Observing enemy movements
• Early warning of missile launches
• Eavesdropping on terrestrial radio links
• Radar imaging
• Photography (using what are essentially large
  telescopes that take pictures of militarily
  interesting areas)

61
Similarities between Satellites

• All satellites have a metal or composite frame
  and body, usually known as the bus. The bus holds
  everything together in space and provides enough
  strength to survive the launch.
• They have a source of power (usually solar cells)
  and batteries for storage.

62
Satellites Similarities

• They have an onboard computer to control and
  monitor the different systems.
• They have a radio system and antenna.
• All satellites have an attitude control system.
  The ACS keeps the satellite pointed in the right
  direction.

63
Transponder

• Some satellites have (hundreds of) transponders
  for communication purposes.
• A transponder
• receives transmissions from earth (uplink)
• changes signal frequency
• amplifies the signal and
• transmits the signal to earth (downlink).

64
Satellite Subsystems

• The main subsystems in a satellite are
• communications
• power
• telemetry tracking, and control (TTC)
• propulsion
• attitude stabilization and
• antenna subsystems.
• Power subsystem consists of solar panels,
  batteries, dc-to-dc converters, and regulators.
  Solar panels convert sunlight into power to
  operate all satellite electronics and to charge
  batteries (used when sunlight is blocked).

65
Satellite Subsystems (Contd)

• The TTC subsystem contains a receiver that picks
  up commands from a ground station and translates
  them into control signals that initiate some
  action on board.
• The telemetry system monitors physical conditions
  within the satellites and converts them into
  electrical signals that are transmitted back to
  earth.

66
Ground Stations The Other End

• Satellites in space communicate (transmit/receive
  radio waves) with ground stations.
• Ground stations consist of subsystems
• transmit/receive
• Power
• Antenna
• TTC and
• ground control equipment (GCE).

67
Satellite Dish

• Ground stations feature large parabolic dish
  antennas with high gain and directivity for
  receiving the weak satellite signal.

Satellite signals
The larger the dish is the higher the
received signal power.
68
Important Satellite Classifications

• GEO (Geostationary Earth Orbit) satellites orbit
  about 36,000 km above Earths surface.
• LEO (Low Earth Orbit) satellites are about
  500-1500 km above earths surface.
• MEO (Medium EO) satellites are about 6000-20,000
  km above earths surface.
• There are also HEO (Highly Elliptical Orbit)
  satellites.

69
Orbits of Different Satellites
70
GEO Satellites

• The majority of communications satellites are
  GEOs. These support voice, data, and video
  services, most often providing fixed services to
  a particular region.
• For example, GEO satellites provide back-up voice
  capacity for majority of U.S. long distance
  telephone companies and carry bulk of nation-wide
  television broadcasts, which commonly are
  distributed via from a central point to affiliate
  stations throughout country.

71
GEOs (Contd)

• GEO systems are less complicated to maintain
  because fixed location requires relatively little
  tracking capability at ground.
• High orbital altitude allows GEOs to remain in
  orbit longer than systems operating closer to
  earth.

72
GEOs (Contd)

• These characteristics, along with their high
  bandwidth capacity, may provide a cost advantage
  over other system types.
• However, their more distant orbit also requires
  relatively large terrestrial antennae and
  high-powered equipment and are subject to delays.
  

73
Satellite Delay

• An important artifact of satellite communications
  is delay.
• The radio signal has to travel a large distance
  to reach satellite from ground station (or to
  reach ground station from satellite).

Variation of Delay as a Function of Elevation
Angle
Delay
0
90
Elevation Angle, q, in degrees
74
LEOs

• Typical LEO satellite takes less than 2 hours to
  orbit the Earth, which means that a single
  satellite is "in view" of ground equipment for a
  only a few minutes.
• If transmission takes more than few minutes that
  any one satellite is in view, a LEO system must
  "hand off" between satellites to complete the
  transmission.

75
LEOs (Contd)

• Handoffs can be accomplished by relaying signals
  between satellite and various ground stations, or
  by communicating between satellites themselves
  using "inter-satellite links."
• LEO systems designed to have more than 1
  satellite in view from any spot on earth at any
  given time.

76
LEOs (Contd)

• LEO systems must incorporate sophisticated
  tracking and switching equipment to maintain
  consistent service coverage.
• Advantages very little delay, operate using
  smaller equipment (because signals travel shorter
  distance), etc.
• Disadvantages highly complex and sophisticated
  control and switching systems, shorter life span
  (subject to greater gravitational pull and higher
  transmission rates lead to shorter battery life).

77
MEOs

• MEOs are in between a GEO and a LEO.
• Advantages/Disadvantages are also in between
• PRO MEO systems will require far fewer
  satellites than LEOs, reducing overall system
  complexity and cost, while still requiring fewer
  technological fixes to eliminate signal delay
  than GEOs.
• CON MEO satellites, like LEOs, have a much
  shorter life expectancy than GEOs, requiring more
  frequent launches to maintain system over time.

78
HEOs

• Elliptical orbit causes satellite to move around
  earth faster when it is traveling close to earth
  and slower the farther away it gets.
• Satellites beam covers more of earth from
  farther away.
• Orbits are designed to maximize amount of time
  each satellite spends in view of populated areas.
  

79
HEOs (Contd)

• Delay characteristics depend on where the
  satellite is in its orbit.
• Several of proposed global communications
  satellite systems actually are hybrids of the
  four varieties reviewed above.

80
Satellite Costs

• Satellite launches don't always go well there is
  a great deal at stake. The cost of satellites
  and launches to name one.
• For example, a recent hurricane-watch satellite
  mission cost 290 million. A missile-warning
  satellite cost 682 million.

81
Satellite Costs (Contd)

• A satellite launch can cost anywhere between 50
  million and 400 million. Russian launches are
  generally the cheapest and the French launches
  are the most expensive.
• A shuttle mission pushes toward half a billion
  dollars (a shuttle mission could easily carry
  several satellites into orbit).

82
Major U.S. Satellite Firms

• Hughes
• Ball Aerospace Technologies Corp.
• Boeing
• Lockheed Martin

83
How can I see an Overhead Satellite?

• This satellite tracking Web site
  (http//www.heavens-above.com/) shows how you can
  see a satellite overhead, thanks to the German
  Space Operations Center.
• You will then need your coordinates for longitude
  and latitude, available from the USGS Mapping
  Information Web site (http//geonames.usgs.gov/).

84
Locating an Overhead Satellite

• Satellite-tracking software is available for
  predicting orbit passes. The above websites will
  help with this. Note the exact times for the
  satellites.
• Use binoculars on a clear night when there is not
  a bright moon.
• Ensure that your watch is set to exactly match a
  known time standard.
• A north-south orbit often indicates a spy
  satellite!

85
Space Junk Another Type of Satellite

• Space junk objects large enough to track with
  radar that were inadvertently placed in orbit or
  have outlived their usefulness
• Approximately 23,000 items of space junk are
  floating above Earth. The actual number varies
  depending on which agency is counting.
• Payloads that go into the wrong orbit, satellites
  with run-down batteries, and leftover rocket
  boosters all contribute to the count.

86
Some Other Causes for Space Junk

• Exploding rockets - This leaves behind the most
  debris in space.
• Jettisoned items - Parts of launch canisters,
  camera lens caps, etc.
• The slip of an astronaut's hand.
• Items initially placed into high orbits stay in
  space the longest.

87
Slip of the Astronauts Hand

• Suppose an astronaut doing repair in space drops
  a wrench -- it's gone forever.
• The wrench then goes into orbit, probably at a
  speed of something like 6 miles per second.
• If the wrench hits any vehicle carrying a human
  crew, the results could be disastrous.
• Larger objects like a space station make a larger
  target for space junk, and so are at greater
  risk.

88
Next Time

• This concludes our introduction to satellite
  communications.
• Next time, we will study the GPS system in
  greater detail.
• In the second half, we will switch gears and look
  at the basics of the Internet.
• This will setup our discussion on WiFi, which
  starts next week.