Title: TRANSIT
1Part 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
2Global 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.
3Global 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.
4TRANSIT 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
5GPS 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,
6GPS 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.
7Development 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
8Motto 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!
9Major Issues Identified in 1968 Studies
10Managed 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
11Convergence 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
12PHASE 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
13PHASE 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
14GPS 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
15Spread 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
16Spread 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.
17GPS 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.
18GPS 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.
19GPS 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)
20GPS 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
21GPS 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
22GPS 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
23GPS 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)
24GPS 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.
25Current 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
26(No Transcript)
27BLOCK I
BLOCK II/IIA
28BLOCK IIR
BLOCK IIF
29GPS 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)
30Who are GPS largest customers?
- Survey Mapping 54
- Navigation 20
- Tracking Comm 18
- Military 6
- Car Navigation 2
31GPS 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
32- GPS Satellite System
- 24 satellites
- altitude 20,000 km
- 12-hour period
- 6 orbital planes
- inclination 55o
33GPS 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)
34THE DEPLOYED CONSTELLATION
35GPS Antenna Coverage
Antenna has 28 field of view
36First GPS satellite Block I was launched in 1978
Air Force-launched Delta II carried the 18th GPS
satellite into orbit in February 1993.
36
37How 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).
37
38Why 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)
39- 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.
41(No Transcript)
42Source http//www.nasm.edu
43How 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.
43
44How 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.
44
45How 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.
45
46Finally 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
48Thus 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
49Measuring 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.
49
50Measuring 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.
50
51A 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".
51
52A 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.
52
53A 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.
53
54GPS Signal
54
55Getting 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.
55
56Atomic 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.
56
57Atomic 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.
58Rubidium 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
59Quartz 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!
60Getting 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.
60
61Getting 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.
61
62Getting 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.
62
63What 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.
63
64Getting 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.
65Getting 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)
65
66Getting 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...
66
67Getting 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.
67
68Atmospheric Errors on GPS Range
Boundary between iono and troposphere
Actual signal path
ionosphere
Geometric distance
troposphere
69Getting 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.
69
70Getting 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 -
70
71Getting 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.
72Summary 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