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 Topics
I. Overview of Radionavigation/GPS
II. 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
GNSS
Radionavigation
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
RF
PRN
Trans. Link Despreading
PRN
Spreading
f
L1
fL1
In Transmitter
ytx (t )  ACPRN (t ) D(t ) cos[2πf L1t  0 ]
fL1
In Receiver
fL1
yrx (t )  Arcvd CPRN (t /c) D(t /c) cos[2πf L1 (t /c)  0 ]
1-Chip Interval
+1
Received CPRN(t-/c)
+1
-1
t
Transmission
Delay = /c
Transmitted CPRN(t)
t
-1
PRN Chip Values (earliest to latest): +1, -1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1
GPS Position & Time Determination
(r 1  ruser )T (r 1  ruser )  p1  ct
(r 2  ruser )T (r 2  ruser )
 p2  ct
1,
2,
3,
4,
Data: r r r r
p 1, p 2, p 3, p 4
Unknowns: ruser, t
(r 4  ruser )T (r 4  ruser )
User Receiver
4
 p  ct
Location
Pseudorange measurement: p = c(trcvd+trcvd-ttrans)
Pseudorange
measurement
equations
(r 3  ruser )T (r 3  ruser )
 p3  ct
Need >= 4 signals
GPS Errors & Accuracy
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Ephemeris errors in r i:
2m
Transmitter clock errors:
2m
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 GPS
Broadcast
Position & Time
12
FAA LAAS version
accuracy: 0.5 m
within 45 km of ref.
receiver at airport
Xs
s
12
Actual
Position & Time
Measured
Scalar Correction
Known location
User applies correction
to range measurements
Reference Receiver
CORRECTION =
expected pseudorange - measured pseudorange
(Courtesy of B.W. Parkinson)
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
Two Possible Alternate
Locations of Antenna B
GPS
Satellite i
ŝ
RMS precision of
relative range
measurement:
5 mm
i
Antenna B

rBA
Antenna A
Measurement Equations:
 Ai  (r i  rA )T (r i  rA )  ct A  N iA  i A
3.5
 Bi  (r i  rB )T (r i  rB )  ct B  N Bi  i B
 ( Ai  Bi )  ( ŝ i )T rBA  c(t At B )   ( N iAN Bi )  i AB
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:




(Fig. 1 of Normark & Ståhlberg "Hybrid GPS/Galileo Real Time
Software Receiver," Proc. ION GNSS 2005)
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
New GPS Signals
Legacy
Signals
P(Y)
L2 Civil
Block IIR-M
(1st Launch 2005)
Military M
L5
Block IIF
(1st Launch
2010)
3rd Civil
(Courtesy of B.W. Parkinson)
L2
L1
C/A
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)
(Courtesy of B.W. Parkinson)
Note 4 antennas
for attitude
Blind Landing, 0.30 m accuracy
Robotic Farming, 0.08 m accuracy
(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:
Ionospheric Remote Sensing
CSI-wireless Asset-Link GPS tracker in
suspect’s rented Lincoln navigator placed him at
crime scene minutes before firefighters
discovered victim
GPS Equipment Sales in U.S. (Billions of $)
GNSS Economics
10
8
6
4
2
0
2003 2004 2005 2006 2007 2008 2009 2010
Year
 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 Topics
I. Overview of Radionavigation/GPS
II. Advances in Weak-Signal GNSS
Tracking and Indoor Navigation +
Network-aided Navigation
III.Vector Tracking for Improved Navigation
Accuracy and Robustness
IV.Multipath Mitigation
The Problem
Approaches to Indoor and Weak-Signal Nav. (1/3)
 Non-GNSS Solutions
 Wi-Fi access points
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received signal strength + large database = ~10 m accuracy
 Pseudolites: GPS-like signals from terrestrial transmitters
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Center frequency is usually offset from GPS, but same signal structure; as good as ~5 cm accuracy
 Navigate off of cell phone towers
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Coarse but robust fallback: Cell tower ID
Better: Advanced forward link trilateration in CDMA systems
 Use sensors to dead reckon during short GNSS unavailability
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Inertial measurement units (accelerometers, gyros)
Cameras
Magnetometers
Altimeters
 IMES: Indoor measurement system
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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
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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
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Use high-quality, stable clock or frequency stability transfer to reduce unpredictable
clock variations
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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
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IMU
Magnetometer
Altimeter
Camera
 Signals of opportunity
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Wi-Fi
Cellular telephone signals
HDTV
Iridium
 Non-GNSS radionavigation signals
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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
Figure: Frank Van Diggelen
Silicon- and FFT-based MPC techniques
allow all code offsets to be searched
simultaneously, reducing TTFF and
indirectly improving sensitivity
State-of-the-Art AGPS: CSR’s EGPS
Loose coupling between GPS & CDMA is
practical and cheap, but prevents
nanosecond-level time aiding and further
improvement in sensitivity
26
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)
Stable signals from CDMA cell towers can be used to discipline local clock
TCXO: Temperature-compensated
crystal oscillator
OCXO: oven-controlled crystal
oscillator
Outline of Topics
I. Overview of Radionavigation/GPS
II. Advances in Weak-Signal GNSS Tracking
and Indoor Navigation + Network-aided
Navigation
III.Vector Tracking for Improved Navigation
Accuracy and Robustness
IV.Multipath Mitigation
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 crosscorrelation 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
Outline of Topics
I. Overview of Radionavigation/GPS
II. Advances in Weak-Signal GNSS Tracking
and Indoor Navigation + Network-aided
Navigation
III.Vector Tracking for Improved Navigation
Accuracy and Robustness
IV.Multipath Mitigation
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
Multipath model:
Likelihood function:
ML approach: Choose A_i, tau_i, and ph_i to maximize the likelihood function
(See Sahmoudi, 2008)
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
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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)
GRID Receiver Evolution (2006-2010)
GRID Receiver (2011)