GNSS Receivers

DANISH GPS CENTER
GNSS Receivers,
One Step Deeper
Kai Borre, Head of DGC
Darius Plaušinaitis
Danish GPS Center, Aalborg, Denmark
The Signal Reception Problem
• The GNSS signal can be received only when:
– The frequency of the local carrier replica matches
the frequency of the carrier in the received signal
– The PRN replica code is well aligned in time to the
PRN code in the received signal
• There are number of parameters, that
influence how precisely these signals must
match to obtain desired processing qualities
Incoming signal
Carrier wave replica
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Integrator
()2
Correlation
result
PRN code replica
Danish GPS Center
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How Carrier Correlation Works
Correlation
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1
2
3
4
5
6
7
8
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How Code Correlation Works
Incoming
code
Generated
code
Correlation
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0
1
2
3
4
5
6
7
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Receiver Channel States
• An example of basic receiver states and
transitions between states
• Examples of additional states (or state flags):
re-acquisition, PLL lock, bit synchronization,
frame synchronization, ephemerides
received, high dynamics, data wipe-off, …
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GNSS Signal Acquisition
• Purpose of acquisition
–
–
–
–
Find satellites (signals) visible to the receiver
Estimate coarse value for C/A code phase
Estimate coarse value for carrier frequency
Refine carrier search result if it is needed for the
chosen tracking (receiver) design
• Acquisition in high sensitivity receivers might
also find bit boundaries
• The search space can be reduced if the
receiver has some a priori knowledge about
visible GNSS signals
• Re-acquire signals if tracking was interrupted
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GNSS Signal Acquisition
• Correct values of the
code phase (signal
alignment in the time
domain) and the
carrier frequency will
yield a high
correlation between
the locally generated
signal and the
received GNSS
signal
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Weak Signal Acquisition
• The weak signal acquisition process is an
extension of the basic acquisition:
– Coherent integration period is increased
– Non-coherent integration period is increased
∑M
()2
∑K
∑M
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Danish GPS Center
Correlation
result
()2
8
Non-Coherent Acquisition
•
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Non-coherent acquisition snapshot was made by student group 1049 (2005)
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Weak Signal Acquisition Aids
• Additional Information that reduces search
space
–
–
–
–
Precise GNSS time
Approximate position
Ephemerides (or at least almanac data)
Present GNSS signal parameters (Doppler etc.)
• Hardware acceleration
– Multiple physical correlators – limited application
• A classical GPS channel is using 2-3 complex correlators
– Other algorithms (parallel processing, FFT etc.)
• SirfStarIV – 400000 correlators
• u-blox 6 – “over 2 million effective correlators”
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Carrier Tracking Loop
I = 12 D(n) cos(φ )
D(n) cos(ωif n)
cos(ωif n + φ )
sin(ωif n + φ )
φ = tan −1
I
Q
Q = 12 D(n) sin(φ )
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Code Tracking Idea
Received code
Early
Locally
generated
copies of the
code
Prompt
Late
Correlation
1
0.5
0
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-1
-0.5
0
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0.5
1
Delay in chips, time
12
Noncoherent DLL
I
E
Incoming
signal
Local
oscillator
P
IE
Integrate
& dump
IP
Integrate
& dump
IL
L
Inputs for the
discriminator
PRN code
generator
E
P
L
90°
Q
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Integrate
& dump
Danish GPS Center
Integrate
& dump
QE
Integrate
& dump
QP
Integrate
& dump
QL
13
Tracking Results
7
3
Inphase Code Correlators
x 10
I2L
2.5
Amplitude
• Output from the 6
correlators, when
the the tracking is
locked
I2P
2
I2E
1.5
1
0.5
0
Discrete-Time Scater Plot
7
3
4000
Amplitude
Q prompt
0
-2000
-4000
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200
300
Time (ms)
400
600
500
Quadrature Code Correlators
x 10
Q2L
2.5
2000
-6000 -4000 -2000
100
0
Q2P
2
Q2E
1.5
1
0.5
0
2000
I prompt
4000
6000
0
0
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100
200
300
Time (ms)
400
500
600
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Tracking Errors Due To Multipath
• The multipath signal is a delayed and
attenuated copy of the direct signal. There can
be several (M) multipath signals.
M
x(t ) = ∑ Ai (t ) D(t − τ i (t ))C (t − τ i (t )) cos(2π ( f 0 + vi (t )) + ϕi (t )) + n(t )
i =1
1
Correlation
Correlation
• The figures show the constructive and
destructive interference of just one multipath
signal
0.5
0
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Delay in chips
1
Delay in chips
0
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GNSS Signal Bandwidth and the
Measurement Precision Relation
Frequency
Frequency
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Receiver Tracking Channel
I
E
Incoming
signal
E
90°
Local
oscillator
P
IE
Integrate
& dump
IP
Integrate
& dump
IL
Output
(nav. data
bit stream)
L
Code loop
filter
PRN code
generator
Q
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P
Integrate
& dump
Code loop
discriminator
L
Integrate
& dump
QE
Integrate
& dump
QP
Integrate
& dump
QL
Carrier
loop filter
Danish GPS Center
Output
(code phase
and
count of
complete
codes)
Carrier loop
discriminator
Output
(carrier phase)
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Examples Of Raw Nav. Data
Discrete-Time Scatter Plot
6000
4000
4000
3000
2000
2000
Q prompt
An example
of a strong
signal. The
bit transitions
are clearly
visible.
Prompt I output (strong signal)
5000
1000
0
0
-1000
-2000
-2000
-3000
-4000
-4000
-5000
-5000
-6000
0
I prompt
5000
0
200
400
600
Discrete-Time Scatter Plot
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1000
Time (ms)
1200
1400
1600
1800
2000
1400
1600
1800
2000
Prompt I output (weak signal)
5000
1500
4000
1000
3000
2000
Q prompt
An example
of a weak
signal. The
bit transitions
are not so
clear.
800
500
1000
0
0
-500
-1000
-2000
-1000
-3000
-1500
-4000
-5000
-5000
-2000
0
I prompt
5000
0
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200
400
600
800
1000
Time (ms)
1200
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An Example Of a GPS Sub-frame
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First Words Of a Subframe
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GPS Navigation Data Contents
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Error Detection And Correction
• Three types of techniques that deal with
bit errors in transmitted/received
signals:
– Error detection: CRC, parity check
– Error detection an correction: parity check,
FEC
– Techniques to mitigate loss or corruption of
a series of bits (burst errors): block
interleaving
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An Example Of Interleaved
Data Corruption
010111011
101011010
000110100
011001011
101100101
… , 0 1 0 1 1 1 0 1 1, 1 0 1 0 1 1 0 1 0, 0 0 0 1 1 0
1 0 0, 0 1 1 0 0 1 0 1 1, 1 0 1 1 0 0 1 0 1, …
Deinterleaving
… , 0 1 0 0 1, 1 0 0 1 0, 0 1 0 1 1, 1 0 1 0 1, 1 1 1 0 0, 1 1 0
1 0, 0 0 1 0 1, 1 1 0 1 0, 1 0 0 1 1, …
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GNSS Software Defined Radios
(SDR) And Other Alternatives
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Basic Facts
• Radio communication today: multi-standard,
multi-frequency communication in a single,
low power, compact device
– E.g. today’s mobile phone use Bluetooth, GSM (3
bands), GPRS, EDGE, 3G, 3.5G, 4G, WLAN, GPS,
FM, DVB and more …
• Devices continue to become smaller
– A need for fewer hardware components
– This means that the hardware in the device must be
reused for several different purposes
– Today’s devices have powerful DSP capabilities
• Intelligent radios need to handle all this
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Basic Facts (GNSS Receivers)
• GNSS positioning is also becoming multi-standard
and multi-frequency
– GPS II, GPS modernizations: M code and L2C, L5 signals,
L1C(GPS III)
– Galileo
– GLONASS + modernized signals
– QZSS (Japan), IRNSS (India), and Beidou (China)
– SBAS systems: EGNOS, WAAS, MSAS, GAGAN
• Today GNSS receivers often are part of devices which
have other radios too (hardware reuse)
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GNSS SDR Partitioning
Hardware
Traditional
Receiver
Radio
front-end
Software
Defined
Receiver
Radio
front-end
”Ideal”
Software
Receiver
Correlators
(Channels)
ADC
(analogue)
(analogue)
ADC
ADC
Correlators
(Channels)
Correlators
(Channels)
Channel
loop closure,
Positioning
Position
Channel
loop closure,
Positioning
Position
Channel
loop closure,
Positioning
Position
Software
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Solutions
• To use general hardware per new signal or
ASIC (Application Specific Integrated Circuit)
• To use reconfigurable hardware – FPGA (Field
Programmable Gate Array), etc.
• To use DSP (Digital Signal Processor)
• To use a general purpose processor (CPU) –
x86, ARM, etc.
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Comparison of Solutions
Unit Price
Performance
ASIC
FPGA
GPP
FPGA
DSP
DSP
GPP
ASIC
Flexibility
Power consumption
• The figures show only a general, rough picture.
• Other issues to consider in platform choice: development time,
development cost, development tools, learning curve.
• Today DSPs are ”squished” from both sides by GPPs and
FPGAs
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SDR Practical Conclusions
• Very, very flexible
• Matlab enables to write and test algorithms very
quickly (real life example: 6-8 lines in Matlab vs. 2000
lines in VHDL)
• Can be very slow (e.g. pure Matlab version)
– Matlab version of a GPS receiver is a few hundred times
slower than real-time
– Matlab & C code is close to real time (4 channels on a fast PC)
• Real-time GNSS SDR implementations exist (written in
C) for embedded and for PC applications. A very fast
PC can process about 5 x 12 channels in real-time
(2011)
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SDR Advantages
• A very convenient educational tool
• Quick prototyping
• A demo acquisition for Galileo in less than an hour
• Students have converted the GPS SDR to EGNOS and Galileo
SDRs in ~6 month
• SDR enables alternative positioning methods (e.g.
non-real time)
• “Easy” exploration of particular
signal cases (anomalies) or
algorithms because the GNSS
signal record can be replayed
again and again…
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GNSS Snapshot Idea
Data Aiding
GNSS
antenna
DGPS
Compact, low
power snapshot
device (rover)
RF front-end
Amplifier
Mixer
Frequency
synthesizer
Receiver
clock
Signal
recording
A/D
Wireless
or other
type data
delivery
SBAS
Multi-frequency,
multisystem
GNSS receiver
Software that does GNSS
signal processing,
derives measurements
and does the actual
position computation
…
PVT
solution
Additional tasks can be
precision improvement or
GNSS signal validation
Memory
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GNSS Snapshot Technique
• The rover devices can be a low power type devices (on the
opposite – the ordinary GPS is a very power consuming device)
• The rower device is relatively GNSS system independent, and
GNSS modernization independent
• The server software implements nearly all GNSS system
dependent signal processing parts – one place to update system
capabilities
• The server software can have more time, power and also other
types of resources to do position estimation
• The server software can implement signal authentication,
validation checks using all available resources
• Usually – not for true real-time applications
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SDR Demo
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Current Receiver Development
2007
2008-2010
Future
Research and
development
•High sensitivity
•Multi-system & multifrequency receiver
•Multipath mitigation
•Further SDR
development
•GNSS integrity
•Integration of other
kinds of positioning
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The ML507 Setup
GNSS
front-end
Battery
adapter
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Simulink Model
The adaptor block inside
calls nearly unmodified C
code of the FPGA receiver
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Matlab SDR Plots
Acquisition results
15
Acquisition Metric
Not acquired signals
Acquired signals
10
5
0
0
25
20
15
10
5
PRN number (no bar - SV is not in the acquisition list)
30
Real correlation result from GNSS SDR
6000
Correlation
1.5
Correlation
4000
2000
Theoretical
correlation
1
0.5
0
0
-1
-2000
4
4.005
4.01
4.015
4.02
Samples (time)
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Danish GPS Center
4.025
0
1
Code Offset [chips]
4.03
2
4.035
4
x 10
38
Matlab SDR
postProcessing.m script
USB driver
(C)
probeSignal.m
tracking.m
acquisition.m
plotTracking.m
postNavigation.m
satPos.m
findPreambles.m
leastSquarePos.m
Recorder application
(C, C++)
ephemeris.m
Signal record file
(1 byte per sample)
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Coordinate
transformations
plotNavigation.m
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Commercial DGPS Performance
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SDR Modification For
Galileo
One
sub-frame
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Thank You For Your Attention
DANISH GPS CENTER
http://gps.aau.dk
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