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?