Transcript No Slide Title

Pseudolite (Pseudo-Satellite) Localization
Shahram Rezaei , Yaser P. Fallah
March 19, 2009
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Topics
1.
Pseudolite Concept
2.
Pseudolite Literature
3.
How GPS Works? (GPS, D-GPS, A-GPS)
4.
Carrier-Phase GPS
5.
Pseudolite signals
6.
Pseudolite Clock Synchronization
7.
Pseudolite Initialization Issue
8.
Conclusion, Open Problems
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Pseudolite Concept
•
•
•
•
One receiver (user) with unknown location.
N stationary transmitters with known locations.
Pseudolite has been the testing method for GPS in early 70’s.
Pseudolite is the testing method for Galileo.
(xr , yr , zr)=?
RX
Y
TX1
TX2
TX3
(x2 , y2 , z2)
(x3 , y3 , z3)
(x1 , y1 , z1)
Z
X
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Literature
1)
Vlad Badea, Rikard Eriksson, “Indoor Navigation with Pseudolite (Fake GPS Satellite)”, MSc Dissertation,
University of Linköping, Sweden, 2005.
2)
J Barnes, C. Rizos, et. al., “High accuracy positioning using Locata’s next generation technology,”. 18th Int. Tech.
Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, pp.13-16, 2005.
3)
J. Barnes, C. Rizos, J. Wang , et. al., “High precision indoor and outdoor positioning using LocataNet”, Journal of
Global Positioning Systems, Vol. 2, No. 2, pp.73-82, 2004.
4)
T. Ford J. Neumann, N. Tos, et. al., “HAPPI-a High Accuracy Pseudolite/GPS Positioning Integration”, 9th Int.
Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation, Kansas City, Missouri, pp. 1719-1728, 1996.
5)
Anis Drira, “GPS Navigation for Outdoor and Indoor Environments”, MSc Dissertation, The University of
Tennessee, Knoxville, May 2006.
6)
J. Raquet, G. Lachapelle, et. al., “Development and Testing of a Mobile Pseudolite Concept for Precise Positioning”,
Proceedings of the Institute of Navigation GPS-95 Conference, Palm Springs, CA, pp. 149-165, 1995.
7)
Changdon Kee, Doohee Yun, Haeyoung Jun, “Precise Calibration Method of Pseudolite Positions in Indoor
Navigation Systems”, Computers and Mathematics with Applications, Vol. 46, No. 10, pp. 1711-1724, 2003.
8)
J. Barns, Joel Van Cranenbroeck, C. Rizos, et. al., “Long Term Performance Analysis of a New Ground-Transceiver
Positioning Network (LocataNet) for Structural Deformation Monitoring Applications”, FIG Working Iek, Strategic
Integration of Surveying Services, pp. 13-17, May 2007, Hong Kong.
9)
www.gmat.unsw.edu.au/snap/about/publications_year.html
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3. How GPS Works (GPS Signals)
There are 31 operational GPS satellites right now.
Entirely available to public
Most receivers can only track L1 frequency, so called single-frequency receivers.
Dual-frequency receivers can track both L1 and L2 frequencies. Most RTK receivers
are dual frequency receivers.
The L2 frequency is not entirely
available to the general public but
there are techniques to extract
code and carrier from the L2
signal.
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How GPS works? (GPS, AGPS, DGPS)
Distance Measurement (1/2)
Approximated or ignored.
Satellite sends models for this.
Unknown 1
Approximated.
In DGPS, the base station sends this info.
Approximated or ignored.
Approximated or ignored.
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How GPS works (GPS, AGPS, DGPS)
Distance Measurement (2/2)
P sr= c * dt
c [speed of light]
dt [time of flight of C/A signal]
Unknown 2
AGPS sends this info.
GPS can also decode from sky,
takes >30 sec and need high SNR.
Unknown 3
Unknown 4
Thus, there are 4 unknowns,
need measurements from 4
satellites to solve.
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Using PRN code for ranging
• A GPS receiver will generate the same PRN
code, then delays it to aligns it with the
received satellite signal,
• distance to satellite is = delay * C (assuming
no timing offset, that the satellite and receiver
were aligned initially)
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Carrier-Phase GPS (1/2)
•
Finding travel time using raw carrier signal is more accurate
than using C/A code.
o L1 carrier frequency = 1.575 GHz, λ = 19 cm
o C/A frequency= 1.024 MHz  λ = 293 m
•
This method is counting the exact number of carrier cycles
between the satellite and the receiver.
•
The problem is phase ambiguity. The trick with "carrier-phase
GPS" is to 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
[1].
[1] http://www.trimble.com/gps/dgps-advanced4.shtml
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Carrier Phase Tracking
Initial tracking of carrier phase
takes time (up to 10minutes)
Cycle slip is possible
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Carrier-Phase GPS (2/2)
-
+
The main challenge
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Pseudolites
• The same concepts as in GPS
– Ionospheric error does not
exist
– Tropospheric error is
negligible
– Multipath effects remain
unchanged
– No need for accurate clocks
• RTK concept is applicable too
• 10MHZ chipping rate -> better
accuracy using C/A
GPS
Error Source
Range
Error (m)
PL Range
Error (m)
Satellite Orbital (only
GPS) and Transmitter
1m
1m
Tropospheric
1m
~0
Ionospheric
10 m
0
Receiver Clock Error
1m
1m
Multipath
10 m
10 m
Clock Error
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Pseudolite Signals (no standard exists)
[1] J. Barnes, C. Rizos, et. al., “High accuracy positioning using Locata’s next generation technology,”. 18th Int.
Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, pp.13-16, 2005.
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Clock:
Precise Timing
•
Precise timing is critical due to speed of light:
1 nano-sec time error  0.3 m error
•
GPS satellites use atomic clocks: very accurate , but expensive ($100K or
more).
•
Pseudolite clocks (both transmitter and receiver) are low cost TCXO
(Temperature Compensated Cristal Oscillator).
•
Several methods have been proposed for clock synchronization of
pseudolites to achieve cm accuracy:
•
Locata uses its own technology: TimeLoc (next slide)
TimeLoc removes relative clock error between transmitters.
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Clock:
TimeLoc Technique
The Time-Loc procedure
for two LocataLites
(transmitters) [1]
Synchronization is
maintained, to if one
clock drifts, the others
can compensate
(assuming PLs are not
moving)
[1] J. Barnes, C. Rizos, J. Wang , et. al., “High precision indoor and outdoor positioning using LocataNet”,
Journal of Global Positioning Systems, Vol. 2, No. 2, pp.73-82, 2004.
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Initialization Issue (1/2):
What Locata does
LINE: Locata Integrated Navigation Engine
[1]
And I could not
find any thing
confirming that
it is available
now.
[1] J. Barnes, C. Rizos, et. al., “High accuracy positioning using Locata’s next generation technology,”. 18th Int.
Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, pp.13-16, 2005.
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Initialization Issue (2/2):
In case of vehicle?!
[1]
[1]
[1] J. Barnes, C. Rizos, et. al., “High accuracy positioning using Locata’s next generation technology,”. 18th Int.
Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Long Beach, California, pp.13-16, 2005.
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Multipath Fading
• Locata’s solution to Signal Multipath Fading
– Spatial: via multiple antenna elements physically
separated in space
– Multipath fading addressed at macro-level: if fading
results in no-signal, use a different antenna,
– Increasing the number of antennas increases the
chance of receiving the PL signal; assuming fading
effects are independent at different antennas
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Conclusion, Open Problems
1.
There are two companies that make Pseudolite:
•
Locata (www.locatacorp.com ), based in NSW, Austrlia
•
Novariant (www.novariant.com), based in Fremont, CA
Both solutions have been designed for local area applications, such as construction,
mining, and agricultural.
2.
To use Pseudolite for vehicular applications, the initialization issue must be first
addressed. Dual frequency receiver is a need for this, but not sure why Locata has
not been successful doing it.
3.
SNAP (Satellite Navigation and Positioning lab) which is research wing of Locata at
University of NSW in Australia has been very inactive in Pseudolite field; they don’t
have publication in this field after 2007. Recent publications are focused on low-cost
GPS receivers and their integration with INS and other sensors.
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Suggested Topics for Further
Investigation
• Dual Frequency Phase Ambiguity Resolution
• Multipath mitigation for Pseudolites
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Thank you!
Any question? Ask Yaser 
Thanks Yaser.
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Difference between DGPS & RTK
from Trimble website
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It can be difficult to distinguish between RTK and DGPS. Here's a quick
review the differences:
To get initialized, RTK needs a minimum of five satellites. After that it can
operate with four. DGPS needs a minimum of three, though at least four
are required for sub-meter accuracy. For RTK, you need a dual frequency
GPS receiver. Single frequency receivers are sufficient for DGPS. For
RTK, your GPS receiver must be capable of On-the-Fly initialization
(obtaining centimeter accuracy while moving). For DGPS, this isn't
necessary. With RTK, it takes one minute to initialize. DGPS receivers
initialize immediately. You can expect accuracy of a few centimeters in all
three dimensions using RTK. With DGPS, you can achieve submeter
accuracy in horizontal position only. To obtain GPS corrections for RTK,
you need your own base station that is no more than ten kilometers from
the field you are working in. For DGPS, you can use your own base
station, a correction service provider, or make use of the free radio
beacon broadcasts in many regions.
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GPS Augmentation methods
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RTK-GPS
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•
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DGPS : correction information is sent by a base station.
RTK is essentially a DGPS method, enhanced with carrier phase tracking
In general receivers are able to align the signals to about 1% of one bitwidth
– coarse-acquisition (C/A) code bit width ~1μs, receiver accurate to
0.01μs, or about 3 meters
– If signal carrier is used for ranging (1ns), and 1% precision is possible
(.01ns), receiver accuracy is 3mm. One wavelength accuracy: 19cm.
In practice,
– RTK systems use a base station receiver & a number of mobile units,
within a 10Km range.
– BS re-broadcasts the phase of the carrier that it measured; the mobile
units compare their own phase measurements with the ones received
from BS
– This allows the units to calculate their relative position to millimeters,
although their absolute position is accurate only to the same
accuracy as the position of the base station.
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