Transcript No Slide Title

GPS
“The Next Utility”
Who What Where When Why How
GPS: Global Positioning System
US System:
NAVSTAR (NAVigation System
with Timing And Ranging)
Managed by: US Dept. of Defense
The Russian Federation system:
GLONASS (GLObal Navigation Satellite System)
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24 orbiting satellites (solar powered,
radio transmitting)
4 satellites in each of 6 orbiting planes
3 of the 24 are considered “spares”
Orbiting speed: 3.87 km/s
Orbiting angle: 55o angle from equator
Size: 9m width with solar panels extended
Control stations are positioned near the
equator, around the globe.
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A GPS system is comprised of several components:
• GPS satellites
• The satellite ground control system
• A GPS antenna and receiver
• Data loggers and/or computers
• Software for processing
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Geosynchronous
orbit (communications
satellites)
13,000 km
Zone of
20,200 km
space junk
35,420 km
GPS satellite
orbit
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Origin of problem: shipping in the 1800’s
Predecessor of GPS technology: WWII radar and later,
shipping radio transmitters.
1978: First GPS satellite launch
1983: GPS revealed (kept secret until now)
1994: All 24 satellites operational
1996: Investment so far - $12 billion
1999: “Washington, DC -- Vice President Gore announced today a
$400 million new initiative in the President's balanced budget that
will modernize the Global Positioning System (GPS) and will add two
new civil signals to future GPS satellites, significantly enhancing the
service provided to civil, commercial, and scientific users worldwide.”
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Originally develop by the military, for the military.
Now, civilian uses have far exceeded military uses, but
the DoD maintained strict control… leading to a long
political battle.
(GPS was used extensively in Desert Storm.)
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The Fundamental Principal:
Speed * Time = Distance
Radio waves are electromagnetic radiation, and travel
at a constant speed: 299,792,458 meters/sec
Thus, if we can measure how long it takes for a signal
to reach us, we know the distance to the satellite.
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x
(sphere)
If we know our distance from
one satellite, we know we lie
somewhere on a theoretical
sphere, with a radius equal to
that distance.
x = known distance
from satellite
= GPS satellite
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(spheres)
x
If we know our distance from
two satellites, we know we lie
somewhere on a theoretical
circle, that is the intersection
of the two spheres.
x = known distance
from satellite
= GPS satellite
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(spheres)
If we know our distance
from three satellites, we
know we lie on one of two
points, one of which is
impossible.
x
= one of two possible
positions
x = known distance
from satellite
= GPS satellite
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In theory, only three satellites are needed to acquire an
accurate position fix.
In practice, we need four satellites because of error that
arises from a variety of sources.
The core of a good understanding if GPS is understanding
these error sources.
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Sources of error:
Distance of error:
Satellite clock errors
Ephemeris errors (satellite position)
Receiver errors (fraction arithmetic)
Ionosphere (charged particles)
Troposphere (the dense part)
Multi-path errors
Selective Availability (when active)
< 1m
< 1m
< 2m
< 2m
< 2m
Variable
(< 33m)
Satellite Geometry (PDOP)
*4-6
PDOP = Position Dilution of Precision
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How does satellite geometry influence accuracy?
A telemetry example (2D):
90o
Shape of area
which may contain
transmitter
Receiver
30o
Shape of area
which may contain
transmitter
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Without correcting for these errors, we can achieve
about a 5m accuracy with parallel tracking units (good
ones), or 10m with serial tracking units (cheap ones).
Several sources of error are very difficult to correct for.
Fortunately, the largest error sources can be corrected
for using differential processing.
With differential processing, we can achieve accuracies
of < 1m, and even centimeter accuracy with the right
receiver.
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Averaging (no correction)
Actual location
+ Single GPS location
Averaged GPS
location
1-2 m
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Averaging increases
accuracy to around
2-4m. The more
points you average,
the better your
accuracy.
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Differential GPS
1. Using post-processing
Known location data
(base station)
Remote location data
collected simultaneously
Data from the known
location is used to
identify the error.
The post-processing
removes this error
from the remote
data.
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Differential GPS
2. Real-time differential
Known location data
(transmitting base station)
Data from the known
location is sent via radio
signals to the remote
receiver, which removes
the error using real-time
processing. No postprocessing is needed.
We can obtain sub-meter
locations in about 5
seconds.
Remote location data
collected simultaneously
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Differential GPS
US Coast Guard
transmitting station
coverage for realtime DGPS in
Wisconsin.
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Obstacles to GPS signals:
GPS signals are high frequency because low frequency
signals tend not to travel in a straight line through the
atmosphere.
The cost is that high frequency signals have very little
penetration through matter. They are also easily reflected.
The signals pass through: thin plastic, cloth, canvas, etc.
They do not pass through: anything metal, or anything
containing a high degree of water (flesh, deciduous leaves,
very heavy rain, etc).
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Obstacles to GPS signals (cont.):
Smooth surfaces act as mirrors to GPS signals.
“Smooth” to a high frequency radio wave means anything
as smooth or smoother than a coarse gravel road.
Open water is particularly reflective.
Reflectance leads to multipath errors...
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Multipath error:
Increases the length of time
taken for a signal to reach the
receiver.
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Types of GPS unit:
Low end (fishing boat GPS)
• Single channel
• Track in serial
• $100-$400
• Accuracy: 10m
High end (University / surveying / photogrammetry)
• 6-10 channels
• Track in parallel
• $2500 - $25000
• Accuracy: 5m, sub-meter real-time, centimeter
accuracy with post-processing.
Military: ? Real-time centimeter accuracy?
TEST
1. Why is this equatorial rainforest wildlife biologist sitting
on a horse in the middle of a river?
2. What problems is she likely to be having (with her GPS
unit)?
The Physics of GPS
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How is the signal sent?
The signal consists of two parts:
• the carrier - on all the time
• the modulation - carries the information
Signals are broadcast on 2 frequencies (only one of which
is for civilian use).
Coarse/Acquisition code:
Frequency: 1575.42 MHz (FM radio is around 100 MHz)
Wavelength: 20 cm (short, hence difficulty with obstacles).
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What is in the signal?
10011101101110001010101010
1023 bits repeated 1000 times/second
Called “pseudo-random noise”
Contains information about the satellite, the contents of the
code, the time the code was sent, etc.
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How is time measured?
Each satellite keeps accurate time using four atomic
clocks ($50,000 each).
The receiver has a much less accurate (and cheaper)
clock (this is one source of error).
The receiver and the satellite generate the same code
at the same time.
The receiver determines range by matching the
satellite code to its’ own code to calculate how long
the signal took to reach it, and therefore the distance
of the satellite (time x speed = distance).
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How does satellite geometry influence accuracy? (cont.)
GPS positioning involves 4 dimensions (3D space plus time)
The influence of geometry is measured with “Dilution of
Precision” (DOP).
A DOP value of 1 is perfect geometry.
DOP’s > 6 indicate poor geometry and readings are not
taken.
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DOP
North DOP (NDOP)
East DOP (EDOP)
Vertical DOP (VDOP)
Time DOP (TDOP)
The 4 dimensions
Horizontal DOP (HDOP) consists of NDOP and EDOP
Position DOP (PDOP) consists of HDOP and VDOP
Geometric DOP (GDOP) consists of PDOP and TDOP