GPS “The Next Utility”. Who What Where When Why How GPS: Global Positioning System US System:...
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Transcript of GPS “The Next Utility”. Who What Where When Why How GPS: Global Positioning System US System:...
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)
Who What Where When Why How
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/sOrbiting angle: 55o angle from equatorSize: 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
13,000 km
20,200 kmZone of
space junk35,420 km
Geosynchronousorbit (communications
satellites)
GPS satelliteorbit
Who What Where When Why How
Who What Where When Why How
Origin of problem: shipping in the 1800’s
Predecessor of GPS technology: WWII radar and later, shipping radio transmitters.
1978: First GPS satellite launch1983: GPS revealed (kept secret until now)1994: All 24 satellites operational1996: Investment so far - $12 billion1999: “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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= GPS satellite(sphere)
x
x = known distancefrom satellite
If we know our distance fromone satellite, we know we liesomewhere on a theoreticalsphere, with a radius equal tothat distance.
Who What Where When Why How
(spheres)
= GPS satellite
x = known distancefrom satellite
If we know our distance fromtwo satellites, we know we liesomewhere on a theoreticalcircle, that is the intersectionof the two spheres.
x
Who What Where When Why HowIf we know our distance from three satellites, we know we lie on one of two points, one of which is impossible.(spheres)
x
= GPS satellite
x = known distancefrom satellite
= one of two possiblepositions
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In theory, only three satellites are needed to acquire anaccurate position fix.
In practice, we need four satellites because of error thatarises from a variety of sources.
The core of a good understanding if GPS is understandingthese error sources.
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Sources of error: Distance of error:
Satellite clock errors < 1mEphemeris errors (satellite position) < 1mReceiver errors (fraction arithmetic) < 2mIonosphere (charged particles) < 2mTroposphere (the dense part) < 2mMulti-path errors VariableSelective Availability (when active) (< 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 areawhich may containtransmitter
30o
Shape of areawhich may containtransmitter
Receiver
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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)
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1-2 m
Actual locationSingle GPS locationAveraged GPS location
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 datacollected simultaneously
Data from the knownlocation is used toidentify the error.The post-processingremoves this errorfrom the remotedata.
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Differential GPS
2. Real-time differential
Known location data(transmitting base station)
Remote location datacollected simultaneously
Data from the known location is sent via radio signals to the remote receiver, which removes the error using real-time processing. No post-processing is needed.
We can obtain sub-meter locations in about 5 seconds.
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Differential GPS
US Coast Guard transmitting station coverage for real-time 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 timetaken for a signal to reach thereceiver.
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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?
TEST1. 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).
Who What Where When Why How
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 EDOPPosition DOP (PDOP) consists of HDOP and VDOPGeometric DOP (GDOP) consists of PDOP and TDOP
