State of the Art and Future Trends in Radionavigation

State of the Art and Future Trends in Radionavigation

Todd Humphreys, UT Austin Aerospace Dept.(with slide contributions from Mark Psiaki, Cornell MAE Dept.)

Briefing to USPTO| April 14, 2011

Outline of TopicsI. Overview of Radionavigation/GPSII. Advances in Weak-Signal GNSS Tracking

and Indoor Navigation + Network-aided Navigation

III.Vector Tracking for Improved Navigation Accuracy and Robustness

IV.Multipath Mitigation

Radionavigation

GPS GNSSRadionavigation

Systems GPS GNSSRadionavigation

Systems

The Three GPS Segments

(Courtesy of U.S. Air Force)

24 Satellites in 12-Hour Orbits Distributed in 6 Orbital Planes of 55 deg Inclination:

(Courtesy of B.W. Parkinson)

Spread Spectrum Radio Ranging

fL1 fL1 fL1

]2[cos)()()( 01 tπftDtACty LPRNtx

])/(2[cos)/()/()( 01 ctπfctDctCAty LPRNrcvdrx+1

+1 -1

-1

Transmitted CPRN(t)

Received CPRN(t-/c)

1-Chip Interval

PRN Chip Values (earliest to latest): +1, -1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1

TransmissionDelay = /c

t

t

PRNSpreading

RFTrans. Link

PRNDespreading

In Transmitter In ReceiverfL1

GPS Position & Time Determination

User ReceiverLocation

tcprrrr 1user1Tuser1 )()(

tcprrrr

3user3Tuser3

)()(

tcprrrr

2user2Tuser2

)()(

tcprrrr

4user4Tuser4

)()(

Data: r 1, r 2, r 3, r 4,p 1, p 2, p 3, p 4

Unknowns: ruser, t

Pseudorangemeasurement

equations

Pseudorange measurement: p = c(trcvd+trcvd-ttrans) Need >= 4 signals

GPS Errors & Accuracy Ephemeris errors in r i: 2 m Transmitter clock errors: 2 m Residual Ionospheric delay: 4 m*

Tropospheric delay: 0.5 m Multipath (reflected signals): 1 m#

Receiver noise: 0.5 m Multiplicative effect of geometry (GDOP) Typical accuracy: 10 m/axis, 30 nsec in time, 0.01

m/sec velocity* for single-frequency receiver w/model corrections, error > 15

m possible in unusual ionospheric conditions, low elevation# error > 15 m possible in strong multipath environments

Local Area Differential GPS12

12

BroadcastPosition & Time

ActualPosition & Time

Measured Scalar Correction

CORRECTION = expected pseudorange - measured pseudorange

User applies correctionto range measurementsKnown location

Reference Receiver

Xs

s

FAA LAAS version accuracy: 0.5 m within 45 km of ref. receiver at airport


Wide Area Differential GPS using SBAS

(Courtesy of B.W. Parkinson) FAA WAAS version accuracy: 1-2 m over North America Europe system: EGNOS; Planned Japanese system: QZSS Systems provide integrity signals

Carrier-Phase Differential GPS

iAB

iB

iABABA

iiB

iA NNttcrs )()()()( T

iB

iBBB

iB

iiB Ntcrrrr )()( T

Antenna A

Antenna B

GPSSatellite i

3.5

Two Possible Alternate Locations of Antenna B

BAr

isRMS precision ofrelative rangemeasurement:5 mm

Measurement Equations:iA

iAAA

iA

iiA Ntcrrrr )()( T

The End of Selective Availability

GPS–Equipped Cell Phones Motivations:

FCC Phase-II E911 requirement (50 m 67% of calls, 150 m 95% of calls)

Location based services (please tell me where is the nearest restaurant)

Money (projected $26B market in 2010, $104B in 2020)

Challenges: Weak signals/multipath in urban canyons

& indoors Solutions:

Assisted GPS (as in figure) MEMS Coarse position fix from communications

link (Fig. 3 of Ballantyne et al., "Achieving Low Energy-per-fix with A-GPS Cellular Phones," Proc. ION GNSS 2005)

GPS–Equipped Automobiles Motivations:

Never-lost/driving directions Location based services Money (projected $86B market

in 2020) Challenges:

Weak signals/multipath in urban canyons

Solutions: Aiding from inertial sensors Dead reckoning aids from odometer/steering data Maps & velocity/direction information included in fix solution Near-zenith augmentation system satellites, e.g., QZSS

(Fig. 1 of Normark & Ståhlberg "Hybrid GPS/Galileo Real Time Software Receiver," Proc. ION GNSS 2005)

L2 L1

L5

Block IIR-M (1st Launch 2005)

Block IIF(1st Launch

2010)

L2 Civil

Military M

C/AP(Y)

3rd Civil

LegacySignals

New GPS Signals


Galileo Signals (GIOVE-A Launched 12/2005)

(Fig. 1 of Wallner et al., "Interference Computations Between GPS and Galileo," Proc. ION GNSS 2005)

Opportunities Afforded by New Signals Dual-frequency ionospheric corrections in civil receivers GPS, Galileo, or a combination Flight-certifiable with 2 aviation-protected frequency bands

Better performance for indoor & high-altitude space applications Pilot signals (i.e., no data) allow better signal processing More power on some signals

Access to more spacecraft signals via interoperability GPS L1/L5 & Galileo L1/E5a bands are same 54 or more satellites available from combined constellation Improved GDOP, availability in urban canyons, RAIM

Non-Standard Applications of GNSS (Courtesy of B.W. Parkinson)

Note 4 antennas for attitude


Blind Landing, 0.30 m accuracy Robotic Farming, 0.08 m accuracy

Ionospheric Remote Sensing

(Fig. 4.f from Mitchell et al. Proc. ION GNSS 2004)

GPS Solves a Murder Mystery Headline:

“Jury will hear how GPS tracked murder suspect” – 9 April 2005, Citizens Voice, Wilkes Barre, PA

Facts:CSI-wireless Asset-Link GPS tracker in suspect’s rented Lincoln navigator placed him at crime scene minutes before firefighters discovered victim

GNSS Economics

2003 2004 2005 2006 2007 2008 2009 20100

2

4

6

8

10

Year

GP

S E

quip

men

t Sal

es in

U.S

. (B

illio

ns o

f $)

Over 10M civil sets in use, > 200,000/month sold at costs >= $100 World sales for cell-phones & automobiles alone projected at $190B in 2020

Outline of TopicsI. Overview of Radionavigation/GPSII. Advances in Weak-Signal GNSS

Tracking and Indoor Navigation + Network-aided Navigation



The Problem

Approaches to Indoor and Weak-Signal Nav. (1/3) Non-GNSS Solutions

Wi-Fi access points received signal strength + large database = ~10 m accuracy

Pseudolites: GPS-like signals from terrestrial transmitters Center frequency is usually offset from GPS, but same signal structure; as good as ~5 cm accuracy

Navigate off of cell phone towers Coarse but robust fallback: Cell tower ID Better: Advanced forward link trilateration in CDMA systems

Use sensors to dead reckon during short GNSS unavailability Inertial measurement units (accelerometers, gyros) Cameras Magnetometers Altimeters

IMES: Indoor measurement system Extremely weak (0.1 to 0.4 nano-watt) GPS-like signals used only for data transmission “If you’re near enough to detect my signal, you must be within 10 meters of my location, which is ...” No need for synchronization with GNSS signals since they are not used for ranging Scalable? (Would have to be densely deployed.)

Approaches to Indoor and Weak-Signal Nav. (2/3) Increasing Receiver Sensitivity

Narrow the search space: obtain rough user position and time and GNSS spacecraft ephemeris from the network (aided GPS (AGPS))

Massively parallel processing: search thousands of hypothesis cells simultaneously (currently implemented in high-performance chips by CSR, Broadcom, others)

Extend the coherent integration time Track pilot channels in new GNSS signals (no navigation data bits) Integrate across navigation data bit boundaries by “wiping off” the data bits with data

provided over network (e.g., via AGPS) Stabilize or compensate for clock and receiver dynamics to extend the receiver’s

coherence time Use high-quality, stable clock or frequency stability transfer to reduce unpredictable

clock variations Use IMU to compensate for receiver dynamics

Approaches to Indoor and Weak-Signal Nav. (3/3) Cafeteria Navigation: Cobble together a solution based on a subset

of the following sensors and signals: GNSS Non-GNSS Sensors

IMU Magnetometer Altimeter Camera

Signals of opportunity Wi-Fi Cellular telephone signals HDTV Iridium

Non-GNSS radionavigation signals Pseudolites IMES Nav-enhanced Iridium (future) Nav-enhanced Wi-Fi (future)

“The most suitable technology for indoors is a combination of GNSS with accelerometers, gyros, and Wi-Fi.” -- Kanwar Chadha of CSR, Oct. 2010

Massively Parallel Correlation

Silicon- and FFT-based MPC techniques allow all code offsets to be searched simultaneously, reducing TTFF and indirectly

improving sensitivity

Figure: Frank Van Diggelen

State-of-the-Art AGPS: CSR’s EGPS

26

Loose coupling between GPS & CDMA is practical and cheap, but prevents

nanosecond-level time aiding and further improvement in sensitivity

Future: Tightly-Coupled Opportunistic Navigation

Enabling configuration: (1) Same clock: Downmix and sample GPS

and SOP with same oscillator(2) Same silicon: Sample GPS and SOP in

same A/D converter

Details on Improving Sensitivity by Extending Coherence Time (1/2)

Example: For C/N0 = 7 dB-Hz, T must be > 7 dB-sec (about 5 seconds)

Details on Improving Sensitivity by Extending Coherence Time (2/2)

TCXO: Temperature-compensated crystal oscillator OCXO: oven-controlled crystal oscillator

Stable signals from CDMA cell towers can be used to discipline local clock





Traditional Receiver Architecture

(Fig. 1 of Lashley, 2009)

Vector Tracking Loop Architecture

(Fig. 2 of Lashley, 2009)

Vector Tracking Improves cross correlation immunity, helping to solve

the near/far problem In independent channel tracking, a tracking loop can get fooled into

tracking a cross-correlation peak instead of the autocorrelation peak In vector tracking, the centralized tracking loop is not fooled by cross-

correlation peaks because these do not follow the predicted trajectory Improves robustness

Navigation solution less sensitive to loss of individual channels. A solution is still possible with fewer than 4 satellites visible (degrades gracefully).

Faltering channels are “helped along” by the combined information contributed by the other channels

Amenable to “cafeteria navigation” “Hungry” estimator can take in signals of opportunity and data from a

diverse sensor suite





Multipath: A Dominant Error Source

(Van Diggelen, InsideGNSS, 2011)

Long Coherent Integration Time Provides Some Protection Against Multipath

(Fig. 8 of Pany, 2009)

Optimal Approaches To Multipath Mitigation: Maximum Likelihood

(See Sahmoudi, 2008)

Multipath model:

Likelihood function:

ML approach: Choose A_i, tau_i, and ph_i to maximize the likelihood function

More Information

http://radionavlab.ae.utexas.edu

Backup Slides

Emerging Threat: Civil GPS Spoofing

Civil Anti-Spoofing Techniques

Data bit latency defense (weak but easy to implement) Multi-antenna defense (patented in 1996; strong against

single spoofer; fails against multiple spoofers; requires additional hardware)

Vestigial signal defense (work in progress; appears strong)

Navigation message authentication (strong, practical, requires cooperation of control segment)

Cross-correlation using P(Y) code (pioneered by Lo, refined by Psiaki, very strong but not so practical)

Software-Defined GNSS Receivers: The GRID Receiver (2006)

V1

GRID Receiver Evolution (2006-2010)

V2V3

V4

GRID Receiver (2011)

V5

Documents

State of the Art and Future Trends in Radionavigation