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Copyright 2001 Mani Srivastava
Mani SrivastavaUCLA - EE Departmentmbs@ee.ucla.edu
Location Sensing for Context-Aware Applications
Mani SrivastavaUCLA - EE Departmentmbs@ee.ucla.edu
EE206A (Spring 2001): Lecture #10
2
Required Reading for this Lecture
Ward, A.; Jones, A.; Hopper, A. A new location technique for the active office. IEEE Personal Communications, vol.4, (no.5), IEEE, Oct. 1997. p.42-7.
3
Location in Mobile Computing
Goal of mobile computing: User’s applications should be available wherever that user goes, in a suitably adapted form
User interfaces of the application follow the user These applications are called Follow-me applications
Special case of Follow-me applications: Context-aware Applications Ability to adapt behavior to a changing environment
e.g. Adapt to available proximate peripherals e.g. Adapt to location of the user
4
Context Awareness
What is context? Who What When Where How
Continuous vs. Registered context information Continuous: always available and appropriate to the
situation Registered: physical mapped to virtual
Context-aware applications need to know the location of users and equipment, and the capabilities of the equipment and networking infrastructure
5
Location-Aware Services
Multicasting selectively only to specific geographical regions defined by latitude and longitude e.g. sending an emergency message to everyone who
is currently in a specific area, such as a building or train station
Providing a given service only to clients who are within a certain geographic range from the server server may be mobile itself say within 2 miles
Advertising a given service in a range restricted way say, within 2 miles
Teleporting Ability by a user to access his desktop environment
from any networked machine
6
Location-Aware Services (contd.)
Providing contiguous information services for mobile users when information depends on the user's location location dependent bookmarks
provide user with any important information which happens to be local (within a certain range) possibly including other mobile servers.
Emergency 911 from cellular phones FCC’s E-911 mandate requires 125 m RMS accuracy
67% of the cases
Other location services in cellular systems location sensitive billing fraud detection resource management
Fleet management and intelligent transportation services [Stilp96]
7
Other Apps of Location Sensing
Monitoring large numbers of sensors dispersed over an area for nuclear, biological, or chemical threats
Synthesis of large aperture antennas for tight beam communication, using scattered transceivers that know their precise relative location and synchronization
Keeping track of mines, armaments, equipment, vehicles, etc.
Keeping track of personal items, such as one’s children, pets, car, purse, luggage, etc.
Inventory control in stores, warehouses, shipyards, railroad yards, etc.
Safety - finding fire fighters in a burning building, police officers in distress, or injured skiers on a ski slope.
Sports - arbitrating rules in a game, playback of motions for coaching, or viewing the re-creation of an event.
Home automation - keyless locks and rooms that adjust the light, temperature, and music sound level.
Motion pictures - automatically adjusting camera focus and motion-tracking for matching digital effects
8
What is Location?
Absolute position on geoid e.g. GPS
Location relative to fixed beacons e.g. LORAN
Location relative to a starting point e.g. inertial platforms
Most applications: location relative to other people or objects, whether
moving or stationary, or the location within a building or an area
Range and resolution of the position location needs to be proportionate to the scale of the objects being located
9 Self-positioning vs. Remote-Positioning Self-positioning
Mobile node formulates its own position e.g. by sensing signals received at the mobile from the
transmitters in the infrastructure
Remote-positioning Position of mobile node calculated at a remote
location e.g. by using signals received from the mobile by
sensors in the infrastructure
Indirect positioning Using a data link it is possible to send position
measurements from a self-positioning receiver to a remote site, or vice versa
A self-positioning system that sends data to a remote location is called indirect remote-positioning
A remote-positioning system transmitting an object’s position to the object is called indirect self-positioning
10
Techniques for Location Sensing
Measure proximity to “landmarks” e.g. near a basestation in a room example systems:
Olivetti’s Active Badge for indoor localization– infrared basestations in every room– localizes to a room as room walls act as barriers
Most commercial RF ID Tag systems– strategically located tag readers
improved localization if near more than one landmark Estrin’s system for outdoor sensor networks
– grid of outdoor beaconing nodes with know position– position = centroid of nodes that can be heard
• # of periodic beacon packets received in a time interval exceeds a theshold
a problem: not really location sensing it really is proximity sensing accuracy of location is a function of the density of landmarks
– Location accuracy = O(distance between landmarks)
11 Techniques for Location Sensing (contd.) Dead reckoning: position relative to an initialization point
work as supplement to a primary location sensing techniques
resynchronize when the primary location sensing technique works, and takes over if the primary fails
– e.g. supplement GPS during signal outages Use wheel and steering information in vehicles Integrating accelerometers mounted on gyroscopically
stabilized platforms Point Research’s Pointman Dead Reckoning Module
inertial measurement unit for personnel on foot– Latitude and longitude relative to the start point
magnetic compass + MEMS-based electronic pedometer + barometric altimeter + DSP
position error of 2-5% of total distance traveled since last resynchronization
no drift with time U. S. Patent No. 5,583,776. www.pointresearch.com
12
Pointman Dead Reckoning Module
Size: 1.9" x 2.9" x 0.6“
Weight: 1.5 oz.
Power: 0.5 Watts @ 3.3 V
(250 mW in new low-power DRM)
13
Trackman Personnel Locator
Combines a DRM with a GPS and a radio transmitter to provide continuous location tracking
Kalman filter is used to combine the dead reckoning data with GPS data when it is available
Specifications: Size: 3.2" x 7.5" x
2.3" Weight: 12 oz. Range: 0.25 miles
14 Techniques for Location Sensing (contd.) Measure direction of landmarks
Simple geometric relationships can be used to determine the location by finding the intersections of the lines-of-position
e.g. Radiolocation based on angle of arrival (AoA) measurements of beacon nodes (e.g. basestations)
can be done using directive antennas or antenna arrays need at least two measurements
BS
BS
BS
MS
1
2
3
15 Techniques for Location Sensing (contd.) Measure distance to landmarks, or Ranging
e.g. Radiolocation using signal-strength or time-of-flight also done with optical and acoustic signals
Distance via received signal strength use a mathematical model that describes the path loss attenuation
with distance– each measurement gives a circle on which the MS must lie
use pre-measured signal strength contours around fixed basestation (beacon) nodes
– can combat shadowing– location obtained by overlaying contours for each BS
Distance via Time-of-arrival (ToA) distance measured by the propagation time
– distance = time * c each measurement gives a circle on which the MS must lie active vs. passive
– active: receiver sends a signal that is bounced back so that the receiver know the round-trip time
– passive: receiver and transmitter are separate• time of signal transmission needs to be known
N+1 BSs give N+1 distance measurements to locate in N dimensions
16
Radiolocation via ToA and RSSI
x1
x2
x3
d1
d3
d2
MS
BS
BS
BS
17 Techniques for Location Sensing (contd.) Measure difference in distances to two landmarks
Time-difference-of-arrival (TDoA) Time of signal transmission need not be known Each TDoA measurement defines line-of-position as a
hyperbola hyperbola is a curve of constant difference in distance
from two fixed points (foci) Location of MS is at the intersection of the hyperbolas N+1 BSs give N TDoA measurements to locate in N
dimensions
18
Radiolocation via TDoA
19
Algorithms for Location
Depends on whether ToA (RSSI is similar) or TDoA is used
Straightforward approach: geometric interpretation Intersection of circles for ToA Intersection of hyperbolas for TDoA
But what if the circles or hyperbolas do not intersect at a point due to measurement errors?
20
Sources of Errors
Multipath Introduces error in RSSI, AoA, ToA, TDoA
RSSI– Multipath fading and shadowing causes up to 30-40 dB variation
over distances in the order of half a wavelength– Shadowing may be combated by using pre-measured signal strength
contours that are centered at BSs (assumes constant physical topography)
AoA– Scattering near and around the MS & BS will affect the measured
AoA– Problem even when there is a LoS component– In macrocells, basestations are elevated so that signals arrive in a
relatively narrow AoA spread– In microcells, signals arrive with a large AoA spread, and therefore
AoA may be impractical ToA and TDoA
– Conventional delay estimators based on correlation are influenced by the presence of multipath fading which results in a shift in the peak of the correlation
21
Sources of Errors
Non line-of-sight (NLoS) Signal takes a longer path or arrives at a different angle
Can be disaster for AoA if received AoA much different from true AoA
For time-based, the measured distance may be considerably greater than true distances
– in GSM system, ranging error due to NLoS propagation is 400-700 m
Multiple-access interference Most problem in CDMA where high power users may mask
the low power users due to near-far effect Power-control is used in CDMA But, MS is not power controlled to other BSs
So signal from MS may not be detectable at enough BSs to form a location estimate
A possibility is to temporarily power up MS to maximum, thus mitigating the near-far effect
22 Location Algorithms in Presence of Errors Geometrical algorithms fail
resort to estimation
2D scenario MS is located at BSs are located at vector of noisy measurements, , from a set of
BSs can be modeled by
where is an measurement noise vector, generally assumed to have zero mean and ancovariance matrix
The system measurement model depends on the location method used
Tss yx ],[sx
Tiii yx ],[x
1N r N
nxCr s )(
n 1NNN
Σ)( sxC
23 Location Algorithms in Presence of Errors (contd.) System measurement model
ToA TDoA AoA Note:
without loss of generality, TDoA are referenced to the first BS
if the time of transmission is needed to form the ToA estimate, it can be incorporated into as a parameter to be estimated along with and
– the unknown parameter vector can then be modified to while the system measurement model becomes
The AoAs are defined by
Although not shown, , , and are nonlinear functions of
TN ,,,)()( 21 ss xDxC
TN 1,1,31,2 ,,,)()( ss xRxC
TN ,,,)()( 21 ss xΦxC
ssx
sx sy
Tsss yx ],,[ sx
1DxC s sss yx ),()(
si
sii xx
yy
1tan
i 1,i isx
24 Location Algorithms in Presence of Errors (contd.) A well known approach for estimating from a noisy
set of measurements: method of least squares (LS) estimation
Weighted least squares (WLS) solution is formed as the vector that minimizes the cost function
LS methods can achieve the maximum likelihood (ML) estimate when the measurement noise vector is gaussian with and equal variances, i.e.
For unequal variances, WLS with gives the ML estimate assume from now on…
sx̂
sTss xCrWxCrx ˆˆˆ
0][ nE IΣ 2n
1ΣW
IW
25 Location Algorithms in Presence of Errors (contd.) is a nonlinear function of the unknown
parameter vector
The LS problem is a non linear one
One straightforward approach: iteratively search for the minimum of the function using a gradient descent method an initial guess is made of the MS location, and
successive estimates are updated according to
where the matrix is the step size,is the estimate at time , and denotes the gradient vector with respect to the vector
)( sxC
sx
)()()1( ˆˆˆ ks
ks
ks xxx υ
),( yxdiag υ)(ˆ k
sxk x /
x
26 Location Algorithms in Presence of Errors (contd.) In order to mold the problem into a linear LS
problem, the nonlinear function can be linearized by using a Taylor series expansion about some reference point so that
where is the Jacobian matrix of
Then the LS solution can be formed as
This approach can be performed iteratively, with each successive estimate being closer to the final estimate a drawback: an initial guess of MS position must be
made
)( sxC
0x
00 )()( xxHxCxC ss
H )( sxC
0
1
0ˆ xCrHHHxxs TT
0x
27 Location Algorithms in Presence of Errors (contd.) Problems with linearization: doesn’t work well if the
linearized function does not represent the nonlinear function well other approaches have been developed for TDoA that
avoid linearization
)( sxC
28
Measures of Location Accuracy
MSE and Cramer-Rao Lower Bound For location in M dimensions, the MSE (mean square
error) is given by
Calculated MSE can be compared with the theoretical minimum MSW given by CRLB which sets a lower bound on the variance of any unbiased estimator
Circular Error Probability Radius of the circle that has its center at the mean and
contains half the realizations of a random vector Measure of uncertainty in the location estimator
relative to its mean If the location estimator is unbiased, CEP is a measure
of the the estimator uncertainty relative to the true MS position
ssT
ssEMSE xxxx ˆˆ
sx̂ sxE ˆ
29 Measures of Location Accuracy (contd.) Geometric Dilution of Precision
GDOP provides a measure of the effect of the geometric configuration of the BSs on the location estimate
GDOP = ratio of the rms position error to thhe rms ranging error
where indicates the fundamental ranging error fo ToA and TDoA systems.
GDOP indicates the extent to which the fundamental ranging error is magnified by the geometric relation between the MS and the BSs
Furthermore:
ssT
ssEGDOP
μxμx ˆˆˆˆ
GDOPCEP 75.0
30
Basic Multilateration (simplified)
22 )()(),( aiaiiaaai yyxxDyxf
)( 2)0( OyxfeD yixiiiia ez
zAAA TT 1)(
Repeat until δ becomes 0
a
1
2
3
yaa
xaa
yy
xx
Linearize using Taylor Expansion
Residual of measured and estimated distance
Linear form
MMSE Solution
31 Cooperative Networked Ranging for Ad Hoc Networks Each node determines the range
to every other node and then shares the information with members of the network
With 4 nodes knowing all the ranges between them, a rigid tetrahedral structure is determined (assuming no 3 nodes are collinear) each node can then be in
one of two 3-dimensional locations with respect to the other 3 nodes
having a 5th node resolves this ambiguity
Advantage: no fixed infrastructure extensible, incremental,
mobile, and survivable Many open issues…
32 Some Other Technique/Systems for Location Sensing Celestial
complicated only works at night in good weather limited precision
OMEGA based on relatively few radio direction beacons accuracy limited and subject to radio interference
LORAN limited coverage (mostly coastal) accuracy variable, affected by geographic situation easy to jam or disturb
SatNav based on low-frequency doppler measurements so it's
sensitive to small movements at receiver few satellites so updates are infrequent.
33 Short-range Radio Proximity Sensing Technologies A variety of short-range radio-based technologies have been
employed to track items indoors identify objects with a sensor having a range of a few cms
to about 3 m, depending on the technology.
Electronic article surveillance (EAS) systems widely used in retail and library settings simple tags respond to a matched electronic field by
resonating when the resonance is detected restricted range, lack id codes, and limited reliability
Radio frequency identification (RFID) RFID tags are identified as they pass fixed sensors detectable up to about 3 meters away many applications
automated toll collection on highways hands-free access control replacement for bar codes in dirty or environmentally
challenging environments
34
RFID Tags
Broadly categorized as active or passive
Passive tags require no battery, so that they tend to cost less but have shorter range challenge of extracting operating power from the air as they pass within range of an interrogator, their
circuitry is charged either inductively (typically at 125 kHz) or electromagnetically (most commonly at 13.56 MHz)
once powered, passive RFID tags identify themselves to the interrogator using techniques such as frequency shifting, half-duplex operation, or delayed retransmission
range limited by the need for a nearby power source few centimeters to 2-3 meters
35
RFID Tags (contd.)
Active tags require battery, but have longer read ranges and more features more expensive operate at higher frequencies, typically 900MHz or 2.4GHz
ISM bands many use modulated backscatter to communicate
the tags modulate their radar cross-section in a pattern to identify themselves to the interrogator
modulated backscatter tags have limited range, around 3 meters for the most part,
– cannot be detected if blocked by a dense enough attenuator, such as a partition wall or a human body
– Backscatter reflections from the tag overwhelmed by reflections from file cabinets, white boards, fluorescent lights, and other objects
RFID tags are fundamentally tied to a nearby power source
36
IRID Tags
Infrared counterpart of RFID tags e.g. Olivetti/Xerox’s Active Badge
Tags periodically transmit their identification codes by emitting infrared light to readers installed throughout the facility
Problems: tag prices are relatively high installation is complicated by the large number of
readers required to ensure a line of sight to every possible tag
reliability: IRID systems do not work at all under various common lighting conditions
a scarf or tie in the wrong position (or a party with balloons) can disable an IRID personnel tag
37
GPS
History U.S. Department of Defense wanted the military to
have a super precise form of worldwide positioning Why?
Missiles can hit enemy missile silos… but you need to know where you are launching from
US missiles, unlike Soviet ones, were mostly sea-based US subs needed to know quickly where they were
After $12B, the result was the GPS system!
Approach “man-made stars" as reference points to calculate
positions accurate to a matter of meters with advanced forms of GPS you can make
measurements to better than a centimeter it's like giving every square meter on the planet a
unique address!
38
GPS System
Constellation of 24 NAVSTAR satellites made by Rockwell Altitude: 10,900 nautical miles Weight: 1900 lbs (in orbit) Size:17 ft with solar panels extended Orbital Period: 12 hours Orbital Plane: 55 degrees to equitorial plane Planned Lifespan: 7.5 years Current constellation: 24 Block II production satellites Future satellites: 21 Block IIrs developed by Martin Marietta
Ground Stations, aka “Control Segment” monitor the GPS satellites, checking both their operational health
and their exact position in space the master ground station transmits corrections for the satellite's
ephemeris constants and clock offsets back to the satellites themselves
the satellites can then incorporate these updates in the signals they send to GPS receivers.
Five monitor stations Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado
Springs.
39
How GPS Works
1. The basis of GPS is “trilateration" from satellites. (popularly but wrongly called “triangulation”)
2. To “trilaterate," a GPS receiver measures distance using the travel time of radio signals.
3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.
4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.
5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.
40 Earth-Centered Earth-Fixed X, Y, Z Coordinates
41 Geodetic Coordinates (Latitude, Longitude, Height)
42
Trilateration
GPS receiver measures distances from satellites
Distance from satellite #1 = 11000 miles we must be on the surface of a sphere of radius 11000
miles, centered at satellite #1
Distance from satellite #2 = 12000 miles we are also on the surface of a sphere of radius 12000
miles, centered at satellite #2 i.e. on the circle where the two spheres intersect
Distance from satellite #3 = 13000 miles we are also on the surface of a sphere of radius 13000
miles, centered at satellite #3 i.e. on the two points where this sphere and the circle intersect could use a fourth measurement, but usually one of the point
is ridiculous (far from earth, or moving with high velocity) and can be rejected
but fourth measurement useful for another reason!
43 Measuring Distances from Satellites By timing how long it takes for a signal sent from the satellite to
arrive at the receiver we already know the speed of light
Timing problem is tricky the times are going to be awfully short
if a satellite were right overhead the travel time would be something like 0.06 seconds
need some really precise clocks
– if timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error
– on satellite side, atomic clocks provide almost perfectly stable and accurate timing
– what about on the receiver side?
• atomic clocks too expensive!
Assuming precise clocks, how do we measure travel times?
44 Measuring Travel Times from Satellites Each satellite transmits a unique pseudo-random code, a copy of
which is created in real time in the user-set receiver by the internal electronics
The receiver then gradually time-shifts its internal code until it corresponds to the received code--an event called lock-on.
Once locked on to a satellite, the receiver can determine the exact timing of the received signal in reference to its own internal clock
If that clock were perfectly synchronized with the satellite's atomic clocks, the distance to each satellite could be determined by subtracting a known transmission time from the calculated receive time in real GPS receivers, the internal clock is not quite accurate
enough an inaccuracy of a mere microsecond corresponds to a 300-
meter error
The clock bias error can be determined by locking on to four satellites, and solving for X, Y, and Z coordinates, and the clock bias error
45 Extra Satellite Measurement to Eliminate Clock Errors Three perfect measurements can locate a point in 3D
Four imperfect measurements can do the same thing Pseudo-ranges: measurements that has not been corrected
for error If there is error in receiver clock, the fourth measurement
will not intersect with the first three
Receiver looks for a single correction factor that will result in all the four imperfect measurements to intersect at a single point
With the correction factor determined, the receiver can then apply the correction to all measurements from then on. and from then on its clock is synced to universal time. this correction process would have to be repeated
constantly to make sure the receiver's clocks stay synced
Any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously
46
Where are the Satellites?
For the trilateration to work we not only need to know distance, we also need to know exactly where the satellites are
Each GPS satellite has a very precise orbit, 11000 miles up in space, according to the GPS master Plan something that high is well clear of the atmosphere
it will orbit according to very simple mathematics
GPS Master Plan the launch of the 24th block II satellite in March of 1994 completed
the GPS constellation four additional satellites are in reserve to be launched "on need." spacings of the satellites are arranged so that a minimum of five
satellites are in view from every point on the globe GPS satellite orbits are constantly monitored by the DoD
check for "ephemeris errors" caused by gravitational pulls from the moon and sun and by the pressure of solar radiation on the satellites
satellite’s exact position is relayed back to it, and is then included in the timing signal broadcast by it
On the ground all GPS receivers have an almanac programmed into their computers that tells them where in the sky each satellite is, moment by moment
47
GPS Signals in Detail
Carriers the GPS satellites transmit signals on two carrier frequencies
the L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing
The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code
Pseudo-random Codes two types of pseudo-random code
the C/A (Coarse Acquisition) code – it modulates the L1 carrier– it repeats every 1023 bits and modulates at a 1MHz rate– each satellite has a unique pseudo-random code– the C/A code is the basis for civilian GPS use
the P (Precise) code – It repeats on a seven day cycle and modulates both the L1 and L2 carriers at a
10MHz rate– this code is intended for military users and can be encrypted
• when it's encrypted it's called "Y" code
Navigation message a low frequency signal added to the L1 codes that gives information
about the satellite's orbits, their clock corrections and other system status
48
GPS Signals
49
Encrypted GPS
Military maintains exclusive access to the more accurate "P-code" pseudo random code.
It has ten times the frequency of the civilian C/A code potentially much more accurate much harder to jam
When it's encrypted it's called "Y-code" and only military receivers with the encryption key can receive it
Because this code is modulated on two carriers, frequency diversity can be used to help eliminate errors caused by the atmosphere
50 Correcting Errors: Problems on the Way to the Earth Speed of light is only constant in a vacuum
As the GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down a bit this creates the same kind of error as bad clocks
Mathematical modeling can be used to predict what a typical delay might be on a typical day it helps but, atmospheric conditions are rarely typical
Dual frequency measurements can be used to handle these atmospheric effects low-frequency signals get "refracted" or slowed more than
high-frequency signals by comparing the delays of the two different carrier
frequencies of the GPS signal, L1 and L2, we can deduce what the medium (i.e. atmosphere) is, and we can correct for it
requires a sophisticated receiver since only the military has access to the signals on the L2 carrier
51 Correcting Errors: Problems on the Ground Multipath error: the signal may bounce off various
local obstructions before it gets to our receiver.
Sophisticated receivers use a variety of signal processing tricks to make sure that they only consider the earliest arriving signals (which are the direct ones) e.g. Rake Receivers
52 Correcting Errors: Problems at the Satellite Atomic clocks on the satellites are very, very precise
but they're not perfect minute discrepancies can occur, and these translate
into travel time measurement errors
Even though the satellites positions are constantly monitored, they can't be watched every second. slight position or "ephemeris" errors can sneak in
between monitoring times
53
Geometric Dilution of Precision
Basic geometry can magnify other errors with a principle called "Geometric Dilution of Precision" or GDOP
There are usually more satellites available than a receiver needs to fix a position, so the receiver picks a few and ignores the rest
Choice of satellites if it picks satellites that are close together in the sky the
intersecting circles that define a position will cross at very shallow angles
that increases the error margin around a position if it picks satellites that are widely separated the circles
intersect at almost right angles and that minimizes the error region.
Good receivers determine which satellites will give the lowest GDOP
54
GDOP
55 Intentional Errors: Selective Availability Until April 2000, DoD introduced some "noise" into
the satellite's clock data which, in turn, adds noise (or inaccuracy) into position calculations
The DoD may also send slightly erroneous orbital data to the satellites which they transmit back to receivers on the ground as part of a status message
Military receivers use a decryption key to remove the SA errors and so they're much more accurate
Why? to make sure that no hostile force or terrorist group
can use GPS to make accurate weapons
56
GPS Error Budget (in Meters)
Standard Differential
Satellite Clocks 1.5 0
Orbit Errors 2.5 0
Ionosphere 5.0 0.4
Troposphere 0.5 0.2
Receiver Noise 0.3 0.3
Multipath 0.6 0.6
SA 30 0
Typical Position Accuracy
Horizontal 50 1.3
Vertical 78 2.0
3-D 93 2.8
57 Quest for Greater Accuracy: Advanced Forms of GPS Differential GPS
involves the use of two ground-based receivers virtually same errors if receivers close to each other
one with know location monitors variations in the GPS signal and communicates those variations to the other receiver
solves equations in reverse to get error correction for each satellite the second receiver corrects its calculations for better accuracy Can eliminate all “common mode” errors, even SA
Carrier-phase GPS takes advantage of the GPS carrier signal to improve accuracy the carrier frequency is much higher than the GPS signal which
means it can be used for more precise timing measurements
Wide Area Augmentation System GPS developed by FAA and aviation industry uses a geostationary satellite as a relay station for the
transmission of differential corrections and GPS satellite status information
these corrections are necessary for instrument landings GEO satellite would provide corrections across an entire continent
58 Quest for Greater Accuracy: Advanced Forms of GPS Local Area Augmented GPS
work like the WAAS but on a smaller scale the reference receivers would be near the runways and so
would be able to give much more accurate correction data to the incoming planes
with a LAAS aircraft would be able to use GPS to make Category 3 landings. (zero visibility)
59 Code-Phase GPS vs. Carrier-Phase GPS
Using the GPS carrier frequency can significantly improve the accuracy of GPS
GPS receiver determines the travel time of a signal from a satellite by comparing the "pseudo random code" it's generating, with an identical code in the signal from the satellite the receiver slides its code later and later in time until
it syncs up with the satellite's code the amount it has to slide the code is equal to the
signal's travel time
60 Code-Phase GPS vs. Carrier-Phase GPS (contd.) Problem: bits (or cycles) of the pseudo random code
are so wide that even if you do get synced up there's still plenty of slop logical match below, but still slightly out of phase code-phase GPS compares pseudo random codes that
have a cycle width of almost a microsecond at the speed of light a microsecond is almost 300 meters
of error
61 Code-Phase GPS vs. Carrier-Phase GPS (contd.) Code-phase GPS isn't really that bad because receiver
designers have come up with ways to make sure that the signals are almost perfectly in phase. good machines get with in a percent or two but that's still at least 3-6 meters of error
Resorting to carrier frequency survey receivers beat the system by starting with the
pseudo random code and then move on to measurements based on the carrier frequency for that code
carrier frequency (1.57 GHz) is much higher so its pulses are much closer together and therefore more accurate
62 Code-Phase GPS vs. Carrier-Phase GPS (contd.) If one can get to within one percent of perfect phase like we do
with code-phase receivers one'd have 3 or 4 mm accuracy!
In essence we want to counting the exact number of carrier cycles between the satellite and the receiver the problem is that the carrier frequency is hard to count
because it's so uniform every cycle looks like every other the pseudo random code on the other hand is intentionally
complex to make it easier to know which cycle you're looking at
Trick: use code-phase techniques to get close if the code measurement can be made accurate to say, a
meter, then we only have a few wavelengths of carrier to consider as we try to determine which cycle really marks the edge of our timing pulse
resolving this "carrier phase ambiguity" for just a few cycles is a much more tractable problem computationally
63
GPS Technology Status
Standard Positioning Service (SPS): C/A code with SA Horizontal accuracy of ± 100 m (95%) [30m without SA] Vertical accuracy of ± 156 m (95%) UTC time transfer accuracy ± 340 ns (95 %)
Precise Positioning Service (PPS) : P code Horizontal accuracy of ± 22 m (95%) Vertical accuracy of ± 27.7 m (95%) UTC time transfer accuracy ± 200 ns (95 %)
Differential GPS Horizontal accuracy of ± 2 m Vertical accuracy of ± 3 m Requires a differential base station within 100 km
Real Time Kinematic GPS Horizontal accuracy of ± 2 cm Vertical accuracy of ± 3 cm Requires a differential base station within 10-20 km
64
GPS Technology Status (contd.)
The size and price of GPS receivers is shrinking World’s smallest commercial GPS receiver (www.u-
blox.ch)
Differential GPS receivers are inexpensive ($100-250)
Differential GPS available in all coastal areas
Real Time Kinematic GPS receivers are expensive
GPS needs line-of-sight to satellites does not work indoors, in urban canyons, forests etc.
65
AT&T Labs’ BAT System
Mobile units: Bats Use of ultrasound Consists of a radio transceiver, controlling logic, and an
ultrasound inducer
66
AT&T Labs’ BAT System (contd.)
Basestations: receivers Placed in ceilings Use of multilateration to location
67 Æther Wire & Lacation, Inc.’s Localizers http://www.aetherwire.com/
Low-Power, miniature, distributed position location and communication devices ultra-wideband, non-sinusoidal communication goes where GPS can’t: supplements GPS
works in buildings, urban areas, or forests inherently share position location information throughout the
network no separate communication channel
US Patent #5748891, 1998 and #6002708, 1999
Long-term goal: coin-sized devices that are capable of localization to centimeter
accuracy over kilometer distances with millions of localizers in a local area
Current DARPA project: pager-sized units powered by AAA-sized cells that are capable
of localization to submeter accuracy over kilometer distances in networks of up to a few hundred Localizers
Fourth generation prototype: 1 cm accuracy over 30-60 m
68
Ultra-wideband (UWB) Radio
Also known as Impulse Radio Non-sinusoidal Communications Baseband Pulse Technology
Communicates with baseband pulses of very short duration typically on the order of nanoseconds energy spread thinly from near DC to a few GHz pulse propagates with distortion when applied to a suitably
designed antenna
Must contend with interference from other signals, and not interfere with narrowband signals spread-spectrum via “time hopping” of the low duty-cycle
pulse train data modulation by pulse position modulation at the rate of
many pulses per data symbol
69
UWB Radio (contd.)
Benefits multipath resolution down to 1 ns eliminates significant
multipath fading, thus reducing fading margin in link budgets
carrierless transmission implies inexpensive manufacturing no inductors or off-chip filters
LPI LPJ
hard to jam GHz bandwidth! penetration ability from bandwidth at baseband
Challenges regulatory considerations over such a wide band will limit
radiated power ultra-fine time resolution increases sync acquisition times
and requires additional correlators to capture adequate signal energy
mobility exacerbates power control needs in multiple access networks
70
UWB Technology
Transceivers can be made very small, low power, low weight, and low cost because the electronics can be completely integrated in CMOS without any inductive components. MEMS can be used to integrate the resonator for the timebase on chip as well.
The antennas can be equally small, and can be driven directly by CMOS, because they are non-resonant, current-mode, and low voltage.
Ultra-wideband signals form a shadow spectrum which can coexist and does not interfere with the sinewave spectrum. The transmitted power is spread over such a large bandwidth that the amount of power in any narrow frequency band is very small.
The good features of spread spectrum are shared, including multipath immunity, tolerance of interference from other radio sources, and inherent privacy from eavesdropping (low probability of intercept).
Ultra-wideband signals have very good penetrating capabilities. Transceivers can operate within buildings, urban areas, and forests.
71
UWB for Localization
Accuracy of range determination is a function of the bandwidth of the exchanged signal with conventional sinewave technology, the
bandwidth of the signal relative to the carrier frequency is very small at most a few % using SS
Ultra-wideband signals consist of EM impulses have a relative bandwidth approaching 100%
Allows centimeter-level accuracy in determining range without using expensive microwave (GaAs MMIC) technology, because gigahertz bandwidth is obtained without a carrier in the 50 GHz range
72
Æther Localizer Prototype
Size: 2.0” x 3.6”
73 PinPoint 3D-iD Local Positioning System (LPS) Components
L3RF Tags (Long Range, Long Life, and Low Cost Radio Frequency)
networked Cell Controllers, each with up to sixteen (16) Antennas attached and covering an area as much as 5 acres (200,000 sq. ft.).
Features reads signals from distances of up to 200 feet to within a ± 10
feet resolution in indoor 3D space no line of sight is required
tags can be seen through walls, closets, desks and doors
Operation At a constant duty cycle, each L3RF Tag transponds and
reflects (or "transflects") a low-power, 2.4GHz radio signal that is transmitted by the system's Antennas
the emitted signal, which has a unique Tag Serial Number encoded in it, is a 5.8GHz radio signal
Cell Controllers continuously track hundreds of tags in real-time (a new location can be calculated every 0.5s)
74
Possibility: GPS-like Indoor LPS
Concept: tag designed to transmit a code for simultaneous arrival at three receivers installed in the facility If the Z (height) position is assumed to be fixed, three
receivers are enough to simultaneously solve for the tag's X-Y position and clock bias error
Drawbacks: need to solve for the tag's clock bias error adds to the
number of good readings required enough clear signal may not be there in cluttered
environments if a tag's clock is unknown, all the receivers need to
share a precisely calibrated time base e.g. by wiring them
need baseband rate of > 10 MHz for reasonable (3m) accuracy
Costly!
75
PinPoint’s LPS Approach
Transponding-tag 3D-iD readers emit codes that are received by the tags tags do not include sophisticated circuitry and software to
decode this signal instead, they simply change the signal's frequency and
transpond it back to the reader with tag ID information phase-modulated onto it
tags emit 1mw, giving range of 30 m range is limited by power (battery size)
The reader extracts the tag ID from this return signal, and also determines the tag's distance from the antenna by measuring the round trip time of flight since the reader generates the signal, there is no need to
calibrate the tag's clock since the distance to each reader is determined
independently, there is no need to synchronize the clocks on the various readers
since the tag is not generating the code, it is practical to send a baseband signal at 40 MHz, which makes for reasonably accurate location
76
PinPoint’s LPS Architecture
Cellular indoor antenna infra-structure each cell is handled by a cell controller, which is
attached to up to 16 antennas by means of coaxial cables
cell controller quickly cycles through the antennas
77
Determining LPS Tag Location
Conceptually a simple problem… but, in practice hard because of accuracy requirements
LPS simpler than GPS in some ways: a GPS receiver needs to synchronize its clock with the
satellite's clock LPS transmission time is a given because the cell controller
originates the signal a GPS receiver must first roughly frame its location--hence
the long synchronization time usual when the devices start up
in LPS the search can be limited to a relatively small window because the tag is known to be in the general vicinity of the cell controller
a GPS signal runs into atmospheric and relativistic distortions on its 13,500-km journey from satellite to receiver
none of these affects the relatively short-range LPS
78 Determining LPS Tag Location (contd.) Key challenge: how to extract
tag distance amid the clutter of the indoors multipath signals reflected from
objects such as steel beams, whiteboards, and fluorescent lights
Without multipath the time of arrival would be easily determined by finding the peak of the autocorrelation triangle.
Solution: processing gain! 40 MHz chipping rate in
PinPoint’s systems
79
Geospatial Addressing
How big an address space do we need? Earth’s radius is 6378 km Earth’s diameter is 40,075 km = 4.0075 x 109 cm 232 = 4,294,967,296
32 bits gives a resolution of .93 cm
In an Earth-centered frame of reference 96 bits is enough from the center of the earth to well beyond
geosynchronous orbit easily fits into an IPv6 address (128 bits)
80
Geospatial Addressing (contd.)
Any cubic centimeter in, on, or around the planet can be directly addressed as a triple
Higher-order geospatial addresses Sphere (<x, y, z>, r) 3D polygon Closed polygon <x0, y0, z0>, <x1, y1, z1>, …
<xn-1, yn-1, zn-1> <x0, y0>, <x1, y1>, … <xn-1, yn-1> is a general 2D polygon z0 = z1 … zn-1 height h
Set {g0, g1, … gk-1} each gi is itself a geospatial address Union of volume represented by g0, g1, … gk-1 gi’s not necessarily contiguous
81
Geospatial Domain Names
Introduce a new top-level domain “.geo”
Subdomains represent higher-level geospatial addresses dk-1. … .d1.d0.geo where di+1 is a subvolume of di ucla.westwood.la.ca.us.geo
However, there are many ways to organize geospace street address ZIP code long distance area codes Thomas Guide page numbers and grid (564, F6) relative addressing
200 yards north-east of the intersection of Westwood and Le Conte
82
Slicing Geospace
Subdividing interior space is even worse Floors and suites of office buildings Graduate student cubicles Equipment closets “Bill’s office” Relative descriptions
“Just down the hall from the water fountain” “Above and to the right of the bookshelf”
Need rich vocabulary for describing interior space
Many different kinds of maps
Any single geospatial namespace is inadequate
Need to rethink DNS service
83
Conclusion
Current: 2-3 m localization indoors 2-3 m, and even 1 cm, localization available outdoors
with GPS
Soon: centimeter resolution available ubiquitously coupled with wireless communication
What are the killer apps?
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