TRANSIT

1
Part I WHAT IS GPS AND HOW IT WORKS PRIMARY GPS
ERROR SOURCES
GS609
This file can be found on the course web
page http//geodesy.eng.ohio-state.edu/course/gs6
09/ Where also GPS reference links are provided
2
Global Positioning System (GPS)

• The NAVSTAR Global Positioning System (GPS) is a
  satellite-based radio-positioning and
  time-transfer system, designed, financed,
  deployed and operated by the US Department of
  Defense.
• However, the system has currently significantly
  larger number of civilian users as compared to
  the military users.

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Global Positioning System (GPS)

• The NAVSTAR Global Positioning System (GPS)
  program was initiated in 1973 through the
  combined efforts of the US Army, the US Navy, and
  the US Air Force.
• The new system, designed as an all-weather,
  continuous, global radio-navigation system was
  developed to replace the old satellite navigation
  system, TRANSIT, which was not capable of
  providing continuous navigation data in real time
  on a global basis.

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TRANSIT System

• Researchers at Johns Hopkins observed Sputnik in
  1957.
• Noted that the Doppler shift provided closest
  approach to earth.
• Developed a satellite system that achieved
  accurate positioning
• Called TRANSIT and provided basic ideas behind GPS

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GPS Objectives 1/2

• Suitable for all classes of platform aircraft,
  ship, land-based and space (missiles and
  satellites),
• Able to handle a wide variety of dynamics,
• Real-time positioning, velocity and time
  determination capability to an appropriate
  accuracy,
• The positioning results were to be available on
  a single global geodetic datum,
• Highest accuracy to be restricted to a certain
  class of user,
• Resistant to jamming (intentional and
  unintentional),
• Redundancy provisions to ensure the
  survivability of the system,

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GPS Objectives 2/2

• Passive positioning system that does not require
  the transmission of signals from the user to the
  satellite(s),
• Able to provide the service to an unlimited
  number of users,
• World-wide coverage
• Low cost, low power, therefore as much
  complexity as possible should be built into the
  satellite segment, and
• Total replacement of the Transit 1 satellite and
  other terrestrial navaid systems.

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Development of Basic Navigation Satellite
Concept 1964-1967

• SYSTEMATIC STUDY OF EVERY WILD IDEA IMAGINABLE
• CONVERGED ON PSEUDORANGING IN 1967
• MAJOR STUDY CONTRACTS LET IN 1968 TO TUNE THE
  CONCEPT

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Motto Adopted by the Joint Program Office on GPS
Program
The mission of this Program is to 1. Drop 5
bombs in the same hole, and 2. Build a cheap set
that navigates (lt10,000), and dont you forget
it!
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Major Issues Identified in 1968 Studies
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Managed Concept Debates 1969-1972

• EXPANDED TRANSIT
• Insisted on worldwide overage
• 153 satellites in 400 mile polar orbits
• Transit carrier frequency

• EXPANDED TIMATION
• Initially only a Time Transfer System
• Insisted on worldwide coverage
• Expanded concept to intermediate altitude
  circular
• orbit constellation of 30 to 40 satellites

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Convergence on Final System 1973-1974

• SWITCHED CONCEPT TO 12-HOUR CIRCULAR ORBITS
• 3 planes, 8 satellites each
• i 63
• RETAINED DIRECT-SHIFT KEYED SPREAD SPECTRUM PN
  SEQUENCE
• DUAL FREQUENCY SIGNAL ON L-BAND
• PICKED INITIAL DEPLOYMENT OF 42 BLOCK I
  SATELLITES

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PHASE I DESIGN 1974-1980

• BLOCK I SATELLITE CONTRACTS WITH ROCKWELL
  INTERNATIONAL
• 6 satellites followed by 6 more
• All satellite performance projections achieved.
  3dB more transmitted power
• then required
• Exceptional (1x ) on-orbit Rubidium clock
  performance achieved.

• DETAILS OF SIGNAL STRUCTURE NAV MESSAGE DEFINED
• C/A code designed with civil sector in mind
• P-Code designed by Magnavox
• Navigation message identical on both signals

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PHASE II DESIGN 1981-1989

• BLOCK II SATELLITES
• Rockwell International
• Selective Availability and Anti-Spoof (Y-Code)
  Implemented
• Constellation downsized to 21 satellites (6
  planes)
• Nav message slightly modified

• OPERATIONAL CONTROL SEGMENT
• Monitors at Ascension, Diego Garcia, Guam,
  Hawaii, and Colorado Springs
• 24-satellite ephemeris (orbit) determination

• PHASE II/PHASE III USER EQUIPMENT
• Rockwell Collins, Magnavox and Teledyne Systems
• Rockwell Collins and Magnavox
• Rockwell Collins

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GPS Receiver Requirements
GPS user hardware must have the ability to track
and obtain any selected GPS satellite signal (a
receiver will be required to track a number of
satellites at the same time), in the presence of
considerable ambient noise This is now possible
using spread-spectrum and pseudo-random-noise
coding techniques
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Spread Spectrum Radio (SSR) Technique 1/2

• Spread Spectrum Radio (SSR) was almost
  exclusively used by military until 1985, when FCC
  allowed spread spectrums unlicensed commercial
  use in three frequency bands 902-928 MHz,
  2.4-2.4835 GHz and 5.725-5.850 GHz.
• SSR differs from other commercial radio
  technologies because it spreads, rather than
  concentrates, its signal over a wide frequency
  range within its assigned bands.
• A key characteristic of spread spectrum radios
  is that they increase the bandwidth of the
  transmitted signal by a significantly large ratio
  to the original signal bandwidth.
• The main signal-spreading techniques are direct
  sequencing and frequency-hopping

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Spread Spectrum Radio (SSR) Technique 2/2

• Direct sequencing continuously distributes the
  data signal across a broad portion of the
  frequency band it modulates a carrier by a
  digital code with a bit rate much higher than the
  information signal bandwidth (used by GPS).
• Alternatively, frequency-hopping radios move a
  radio signal from frequency to frequency in a
  fraction of a second.
• The spread spectrum receiver has to reconstruct
  the original modulating signal from the
  spread-bandwidth signal by a process called
  correlation (or de-spreading). The fact that the
  interference remains spread across a large
  bandwidth allows the receiver to filter out most
  of their signal energy, by selectively allowing
  through only the bandwidth needed for the
  de-spread wanted signal.
• Thus, the interference is reduced by SSR
  processing. Transmitting and receiving SSR radios
  must use the same spreading code, so only they
  can decode the true signal.

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GPS Policy Board

• Department of Agriculture
• Department of Commerce
• Department of Defense
• Department of Interior
• Department of State
• Department of Transportation
• NASA

created to give larger voice to civilian
applications of GPS.
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GPS Constellation

• Block I (not operational)
• Block II/IIA/IIR
• Currently
• - 28 satellites Block II/IIA/IIR
• AS1/SA capability (to limit the access to the
  system by unauthorized users)
• multiple clocks onboard

1 The process of encrypting the P-code by
modulo-2 addition of the P-code and a secret
encryption W-code. The resulting code is called
the Y-code. AS prevents an encryption-keyed GPS
receiver from being spoofed by a bogus,
enemy-generated GPS P-code signal. Y-code is not
available to the civilian users. 2 The
Department of Defense policy and procedure of
denying to most non-military GPS users the full
accuracy of the system. SA is achieved by
dithering the satellite clock and degrading the
navigation message ephemeris. Turned to zero on
May 2, 2000.
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GPS Constellation

• Block I
• vehicle numbers (SVN) 1 through 11
• launched between 1978 and 1985
• concept validation satellites
• developed by Rockwell International
• circular orbits
• inclination 63 deg
• one Cesium and two Rubidium clocks
• design life of 5 years (majority performed well
  beyond their life expectancy)

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GPS Constellation

• Block II
• vehicle numbers (SVN) 13 through 21
• launched between 1989 and 1990
• full scale operational satellites
• developed by Rockwell International
• nearly circular orbits
• inclination 55 deg
• two Cesium and two Rubidium clocks
• design life of 7.3 years
• AS/SA capabilities

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GPS Constellation

• Block IIA
• vehicle numbers (SVN) 22 through 40
• launched since 1990 (18 out of 19)
• second series of operational satellites
• developed by Rockwell International
• nearly circular orbits
• inclination 55 deg
• two Cesium and two Rubidium clocks
• design life of 7.3 years
• AS/SA capabilities

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GPS Constellation

• Block IIR
• vehicle numbers (SVN) 41 through 62
• total of 6 launched (1 unsuccessful)
• operational replenishment satellites
• developed by Lockheed Martin
• nearly circular orbits
• inclination 55 deg
• one Cesium and two Rubidium clocks
• design life of 7.8 years
• AS/SA capabilities

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GPS Constellation

• Block IIF
• will be launched between 2001 and 2010
• operational follow on satellites
• nearly circular orbits
• inclination 55 deg
• design life of 10 years
• will carry an inertial navigation system
• will have an augmented signal structure (third
  frequency)

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GPS Constellation
Block III In November 2000, Lockheed Martin and
Boeing were each awarded a 16-million, 12-month
study contract by the Air Force to conceptualize
the next generation GPS satellite, which will be
known as GPS Block-3.
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Current GPS Constellation
LAUNCH LAUNCH FREQ
ORDER PRN SVN DATE STD
PLANE -------------------------------------
-------------------------- II-1
14 14 FEB 89 Cs E1
II-2 02 13 10 JUN 89 Cs B3
II-3 16 16 18 AUG 89 Cs
E5 II-4 19 19 21
OCT 89 Cs A4 II-5 17
17 11 DEC 89 Cs D3 II-6
18 24 JAN 90 Cs F3
II-7 20 26 MAR 90
II-8 21 21 02
AUG 90 Cs E2 II-9 15
15 01 OCT 90 Cs D2
IIA-10 23 23 26 NOV 90 Cs E4
IIA-11 24 24 04 JUL 91 Rb
D1 IIA-12 25 25 23 FEB 92
Cs A2 IIA-13 28
10 APR 92 IIA-14
26 26 07 JUL 92 Rb F2
IIA-15 27 27 09 SEP 92 Cs A3

LAUNCH LAUNCH FREQ
ORDER PRN SVN DATE STD
PLANE -------------------------------------
-------------------------- IIA-16 01
32 22 NOV 92 Cs F1
IIA-17 29 29 18 DEC 92 Rb F4
IIA-18 22 22 03 FEB 93 Rb
B1 IIA-19 31 31 30 MAR 93
Cs C3 IIA-20 07 37 13
MAY 93 Rb C4 IIA-21 09
39 26 JUN 93 Cs A1
IIA-22 05 35 30 AUG 93 Cs B4
IIA-23 04 34 26 OCT 93 Rb
D4 IIA-24 06 36 10 MAR 94
Cs C1 IIA-25 03 33 28
MAR 96 Cs C2 IIA-26 10
40 16 JUL 96 Cs E3
IIA-27 30 30 12 SEP 96 Cs B2
IIA-28 08 38 06 NOV 97 Rb
A5 IIR-1 42 17 JAN
97 IIR-2 13 43 23 JUL 97 Rb
F5 IIR-3 11 46 07
OCT 99 Rb D2 IIR-4 20 51
11 MAY 00 Rb E1 IR-5 28
44 16 JUL 00 Rb B5
Satellite is no longer in service.
Unsuccessful launch. TOTAL 28 as of October 15,
2000  
ftp//tycho.usno.navy.mil/pub/gps/gpsb2.txt
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(No Transcript)
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BLOCK I
BLOCK II/IIA
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BLOCK IIR
BLOCK IIF
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GPS Receiver Manufacturers
NovAtel Inc. http//www.novatel.ca
Trimble http//www.trimble.com Topcon/Javad htt
p//www.topconps.com
Ashtech/Magellan http//www.ashtech.com Garmin ht
tp//www.garmin.com Leica http//www.leica-gps.co
m
Over 67 GPS manufacturers and over 467 types of
receivers, 106 antennas ! (GPS World, January
2000)
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Who are GPS largest customers?

• Survey Mapping 54
• Navigation 20
• Tracking Comm 18
• Military 6
• Car Navigation 2

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GPS Applications

• military
• civilian aircraft, land mobile, and marine
  vessel navigation
• time transfer between clocks
• spacecraft orbit determination
• geodesy (precise positioning)
• attitude determination with multiple antennas
• geophysics (ionosphere, crustal motion
  monitoring, etc.)
• surveying (static and kinematic, also real-time)
• Intelligent Transportation Systems
• GIS, Mobile Mapping Systems

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• GPS Satellite System
• 24 satellites
• altitude 20,000 km
• 12-hour period
• 6 orbital planes
• inclination 55o

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GPS Satellite System

• continuous signal transmit
• fundamental frequency 10.23 MHz
• almost circular orbit (e 0.02)
• at least 4 satellites visible at all times
• from any point on the Earths surface (5-7 most
• of the time)

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THE DEPLOYED CONSTELLATION
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GPS Antenna Coverage
Antenna has 28 field of view
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First GPS satellite Block I was launched in 1978
Air Force-launched Delta II carried the 18th GPS
satellite into orbit in February 1993.
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How accurate is GPS?

• Depending on the design of the GPS receiver and
  the measurement techniques employed, the accuracy
  is from 100 meters under Selective Availability
  (SA) policy (below 10 m with SA turned to zero)
  to better than 1 centimeter.
• In order to obtain better than 100 (10 with SA
  turned to zero) meter accuracy, differential GPS
  must be used (two simultaneously tracking
  receivers or differential services).

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Why is GPS so accurate ?

• The key to GPS accuracy is the fact that the
  signal is precisely controlled by the highly
  accurate atomic clock
• Atomic clocks stability is 10-13 10-14 per
  day (this means that the clock can loose 1 sec in
  3,000,000 years!)
• This highly accurate frequency standard produces
  the fundamental GPS frequency, 10.23 MHz, which
  is a basis for derived frequencies L1 (1575.42
  MHz 15410.23) and L2 (1227.60 MHz 12010.23)

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• The basis of GPS is
• "triangulation" from satellites.
• To "triangulate," a GPS receiver measures
  distance using the travel time of radio signals.
• To measure travel time, GPS needs very accurate
  timing, which it achieves with some tricks
• The primary unknowns are three coordinates of
  the receiver antenna (user)

40

• Mathematically we need four satellite ranges to
  determine exact position.
• Three ranges are enough if we reject ridiculous
  answers or use other tricks.

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(No Transcript)
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Source http//www.nasm.edu
43
How distance measurements from three satellites
can pinpoint you in space 1/3
Suppose we measure our distance from a satellite
and find it to be 11,000 miles. Knowing that
we're 11,000 miles from a particular satellite
narrows down all the possible locations we could
be in the whole universe to the surface of a
sphere that is centered on this satellite and has
a radius of 11,000 miles.
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How distance measurements from three satellites
can pinpoint you in space 2/3
Next, say we measure our distance to a second
satellite and find out that it's 12,000 miles
away. That tells us that we're not only on the
first sphere but we're also on a sphere that's
12,000 miles from the second satellite. Or in
other words, we're somewhere on the circle where
these two spheres intersect.
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How distance measurements from three satellites
can pinpoint you in space 3/3
If we then make a measurement from a third
satellite and find that we're 13,000 miles from
that one, that narrows our position down even
farther, to the two points where the 13,000
mile sphere cuts through the circle that's the
intersection of the first two spheres.
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Finally In order to find the correct location
(out of two points determined by the observation
of three ranges to three satellites) we may need
to make a fourth observation to the fourth
satellite this way we get the unique answer to
our positioning problem. But usually one of the
two points is a ridiculous answer (either too far
from Earth or moving at an impossible velocity)
and can be rejected without a measurement.
However, a fourth measurement becomes very handy
for another reason
47

• The dashed lines show the intersection point for
  ideal case (no observation errors), and the gray
  bands indicate the area of uncertainty
• Because of errors in the receiver's internal
  clock, the spheres do
  not intersect at one
  point (the time measurement is used to determine
  the distance to the satellite, as explained next)
  
• If three perfect measurements can locate a point
  in 3-dimensional space, then four imperfect
  measurements can do the same thing
• So, the fourth measurement is used to fix the
  time (receiver clock) problem, and find a unique
  3-D location in space

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Thus four range measurements to four GPS
satellites are needed for point positioning
But how do we measure the range to the satellite?
By precise measurement of the time that the radio
signal takes to travel from the satellite antenna
to the receiver antenna
49
Measuring distance from a satellite 1/2

• The timing problem is tricky. First, the signal
  travel times are going to be awfully short (about
  0.06 seconds), so we need some really precise
  clocks.
• But assuming we have precise clocks, how do we
  measure travel time?
• Suppose we start generating the same signal at
  the satellite and the receiver at the same time.
• The signal (Pseudo Random Code) coming from
  the satellite is delayed because it had to travel
  over 11,000 miles.

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Measuring distance from a satellite 2/2

• If we wanted to see just how delayed the
  satellite's signal was, we delay the receiver's
  version of signal until they fell into perfect
  synchronization.
• The amount we have to shift back the receiver's
  version is equal to the travel time of the
  satellite's version.
• So we just multiply that time times the speed of
  light and BINGO! we've got our distance to the
  satellite.

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51
A Random Code?
The Pseudo Random Code (PRC) or Pseudo Random
Noise code, PRN, is a fundamental part of GPS.
Physically it's just a very complicated digital
code, or in other words, a complicated sequence
of "on" and "off" pulses. The signal is so
complicated that it almost looks like random
electrical noise. Hence the name "Pseudo-Random".
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A Random Code?

• Since each satellite has its own unique
  Pseudo-Random Code, this complexity also
  guarantees that the receiver won't accidentally
  pick up another satellite's signal.
• So all the satellites can use the same frequency
  without jamming each other. And it makes it more
  difficult for a hostile force to jam the system.
• In fact the Pseudo Random Code gives the DoD a
  way to control access to the system.

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A Random Code?

• Another reason for the complexity of the Pseudo
  Random Code, is crucial to making GPS economical.
  
• The codes make it possible to use information
  theory to amplify the GPS signal. And that's
  why GPS receivers don't need big satellite dishes
  to receive the GPS signals.

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GPS Signal
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Getting Perfect Timing

• On the satellite side, timing is
• almost perfect because they have
• incredibly precise atomic clocks
• on board.
• But what about our receivers here on the ground?
  
• Remember that both the satellite and the
  receiver need to be able to precisely synchronize
  their pseudo-random codes to make the system
  work.

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Atomic Clocks

• Atomic clocks don't run on atomic energy. They
  get the name because they use the oscillations of
  a particular atom as their "metronome (device
  for marking time by means of a series of clicks
  at precise intervals).
• This form of timing is the most stable and
  accurate
• reference man has ever developed.
• With the development of atomic clocks a new era
  of precise time-keeping had commenced. However,
  before the GPS program was launched these precise
  clocks had never been tested in space.

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Atomic Clock Technology

• The development of reliable, stable, compact,
  space-qualified atomic frequency oscillators
  (rubidium, and then cesium) was therefore a
  significant technological breakthrough.
• The advanced clocks now being used on the GPS
  satellites routinely achieve long-term frequency
  stability in the range of a few parts in 1014 per
  day (about 1 sec in 3,000,000 years!).
• This long-term stability is one of the keys to
  GPS, as it allows for the autonomous,
  synchronized generation and transmission of
  accurate timing signals by each of the GPS
  satellites without continuous monitoring from the
  ground.

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Rubidium Atomic Clocks
Cesium clocks are the best time keeping devices
with a drift of 2-3 10-14/day Rubidium clocks
can drift by 2-3 10-13/day
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Quartz Crystal Oscillator Technology

• In order to keep the cost of user equipment down,
  quartz crystal oscillators were proposed (similar
  to those used in modern digital watches),
• Besides their low cost, quartz oscillators have
  excellent short-term stability.
• However, their long-term drift must be accounted
  for as part of the user position determination
  process this is where the fourth range
  measurement becomes handy!

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Getting Perfect Timing

• If our receivers needed atomic clocks (which
  cost upwards of 50K to 100K) GPS would be a
  lame duck technology. Nobody could afford it.
• Luckily the designers of GPS came up with a
  brilliant little trick that lets us get by with
  much less accurate clocks in our receivers.
• The secret to perfect timing is to make an extra
  satellite measurement (remember the fourth range
  observation that we need to get precise position
  in space?)
• By using an extra satellite range measurement
  and a little algebra a GPS receiver can eliminate
  any clock inaccuracies it might have.

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Getting Perfect Timing

• Since any offset from universal time (UTC, the
  civilian time system that we use) will affect all
  of our measurements, the receiver looks for a
  single correction factor that it can subtract
  from all its timing measurements to make them
  correct.
• That correction brings the receiver's clock
  back into sync with universal time, and BINGO! -
  you've got atomic accuracy time right in the
  palm of your hand (especially if you're using one
  of the hand-held receivers!)
• Once it has that correction it applies to all
  the rest of its measurements and now we've got
  precise positioning.

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Getting Perfect Timing

• One consequence of this principle is that any
  decent GPS receiver will need to have at least
  four channels so that it can make the four
  measurements simultaneously.
• But for the triangulation to work we not only
  need to know distance, we also need to know
  exactly where the satellites are.

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What else do we need to navigate (position) with
GPS?

• Along with distance, you need to know exactly
  where the satellites are in space. High orbits
  and careful monitoring are the secret.
• Finally you must correct for any delays the
  signal experiences as it travels through the
  atmosphere.

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Getting Satellite Position in Space 1/3

• Successful operation of GPS depends on the
  precise knowledge and prediction of a satellite's
  position with respect to an earth-fixed reference
  system.
• Tracking data collected by ground monitor
  stations are analyzed to determine the satellite
  orbit over the period of tracking (typically one
  week).
• This reference ephemeris is extrapolated into the
  future and the data is then up-loaded to the
  satellites.
• Prediction accuracies of the satellite
  coordinates, for one day, at the few meter level
  have been demonstrated.

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Getting Satellite Position in Space 2/3

• The Air Force has injected
• each GPS satellite into a
• very precise planned orbit.
• GPS satellites are so high up
• that their orbits are very
• predictable.
• On the ground all GPS receivers have an almanac
  programmed into their computers that tells them
  where in the sky each satellite is.
• Minor variations in satellite orbits are
  measured by the Department of Defense (data from
  permanently tracking stations allow determination
  of satellite position and speed)

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Getting Satellite Position in Space 3/3

• These errors (variations from the ideal orbit)
  are caused by gravitational pulls from the moon
  and sun and by the pressure of solar radiation on
  the satellites.
• That information is sent back up to the
  satellite itself. The satellite then includes
  this new corrected position information in the
  timing signals it's broadcasting.
• So a GPS signal is more than just pseudo-random
  code for timing purposes. It also contains a
  navigation message with ephemeris information as
  well.
• Now we are almost ready for perfect positioning,
  but there is one more trouble...

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Getting Errors Corrected

• A GPS signal doesnt travel
• in vacuum!
• We've been saying that you
• calculate distance to a satellite by
• multiplying a signal's travel time
• by the speed of light. But the speed of light
  is only constant in
• a vacuum.
• As a GPS signal passes through the charged
  particles of the
• ionosphere and then through the water vapor in
  the
• troposphere it gets slowed down, and this
  creates the same
• kind of error as bad clocks.

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Atmospheric Errors on GPS Range
Boundary between iono and troposphere
Actual signal path
ionosphere
Geometric distance
troposphere
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Getting Errors Corrected 1/3

• Some errors can be factored out using
  mathematics and
• modeling (tropospheric errors)
• One way to handle ionosphere-induced errors is
  to compare the relative speeds of two different
  signals. This "dual frequency" measurement is
  only provided by advanced GPS receivers.
• Problem on the ground -- is called multipath
  error and is similar to the ghosting you might
  see on a TV.
• Good receivers use sophisticated
• signal rejection techniques to
• minimize this problem.

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Getting Errors Corrected 2/3

• Other error sources satellite position.
• Random errors due to orbit perturbations
• Intentional errors - the policy is called
  "Selective
• Availability" or "SA" and the idea behind it is
  to make sure that no hostile force or terrorist
  group can use GPS to make accurate weapons.
• DoD introduces some "noise" into the satellite's
  clock
• data which, in turn, adds noise (or inaccuracy)
  into position
• calculations. DoD may also be sending slightly
  erroneous orbital data to the satellites

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Getting Errors Corrected 3/3

• Military receivers use a decryption key to
  remove the SA errors and so they're much more
  accurate.
• SA was turned down to zero on May 2, 2000
• Differential GPS can eliminate almost all error
  sources.

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Summary of GPS Error Sources m (Pseudoranging)
SA0 SA Differential GPS Satellite
Clocks 2.0 20.0
0 Orbit Errors 2.1
20.0 0 Ionosphere 5.0
5.0 0.4 Troposphere
0.5 (model) 0.5 (model) 0.2
Receiver Noise 0.3
0.3 0.3 Multipath
1.0 1.0 1.0
Typical Position Accuracy Horizontal
10.0 41.0 1.3
Vertical 13.0
51.0 2.0