gnss receivers, one step deeper

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

2013 Danish GPS Center 2

Carrier wave replica

Incoming signal Correlation result

PRN code replica

Integrator ()2

How Carrier Correlation Works


1 2 3 4 5 6 7 8 Correlation

How Code Correlation Works


0 1 2 3 4 5 6 7

Incoming code

Generated code

Correlation

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, …


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 2013 Danish GPS Center 6

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


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

∑M ()2

∑K Correlation

result

Non-Coherent Acquisition • Non-coherent acquisition snapshot was made by student group 1049 (2005)


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”


Carrier Tracking Loop


)cos()(21 φnDI =

)cos( φω +nif)cos()( nnD ifω

)sin( φω +nif

)sin()(21 φnDQ =

QI1tan−=φ

Early

Prompt

Late

Received code

-1 -0.5 0 0.5 1 0

0.5

1

Corr

elat

ion

Delay in chips, time

Locally generated

copies of the code

Code Tracking Idea

2013 12 Danish GPS Center

Noncoherent DLL

Local oscillator

PRN code generator

Integrate & dump IE

IP

IL

Incoming signal

E P L

I Integrate & dump

Integrate & dump

90° QE

QP

QL

E P L

Q

Integrate & dump

Integrate & dump

Integrate & dump

Inputs for the discriminator


Tracking Results

• Output from the 6 correlators, when the the tracking is locked


-6000 -4000 -2000 0 2000 4000 6000

-4000

-2000

0

2000

4000

Discrete-Time Scater Plot

I prompt

Q p

rom

pt

0 100 200 300 400 500 6000

0.5

1

1.5

2

2.5

3x 10

7 Inphase Code Correlators

Time (ms)

Am

plitu

de

0 100 200 300 400 500 6000

0.5

1

1.5

2

2.5

3x 10

7 Quadrature Code Correlators

Time (ms)

Am

plitu

de

IL2

IP2

IE2

QL2

QP2

QE2

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.

• The figures show the constructive and destructive interference of just one multipath signal

Corr

elat

ion

Delay in chips 0

0.5

1

Delay in chips

0

1 Co

rrel

atio

n

)())())((2cos())(())(()()( 01

tnttvfttCttDtAtx iiii

M

ii +++−−=∑

=

ϕπττ


GNSS Signal Bandwidth and the Measurement Precision Relation


Frequency

Frequency

Receiver Tracking Channel


Output (nav. data bit stream)

Output (code phase

and count of complete

codes)

Output (carrier phase)

Incoming signal

Local oscillator

PRN code generator

Integrate & dump

IE

IP

IL

E P L

I Integrate & dump

Integrate & dump

90°

QE

QP

QL

E P L

Q

Code loop filter

Code loop discriminator

Integrate & dump

Integrate & dump

Integrate & dump

Carrier loop filter

Carrier loop discriminator

Examples Of Raw Nav. Data


-5000 0 5000-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000Discrete-Time Scatter Plot

I prompt

Q p

rom

pt

0 200 400 600 800 1000 1200 1400 1600 1800 2000-6000

-4000

-2000

0

2000

4000

6000

Time (ms)

Prompt I output (strong signal)

-5000 0 5000-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000Discrete-Time Scatter Plot

I prompt

Q p

rom

pt

0 200 400 600 800 1000 1200 1400 1600 1800 2000-2000

-1500

-1000

-500

0

500

1000

1500

Time (ms)

Prompt I output (weak signal)

An example of a strong signal. The

bit transitions are clearly

visible.

An example of a weak

signal. The bit transitions

are not so clear.

An Example Of a GPS Sub-frame


First Words Of a Subframe


GPS Navigation Data Contents


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


An Example Of Interleaved Data Corruption

… , 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, …


… , 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, …

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

GNSS Software Defined Radios (SDR) And Other Alternatives


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 2013 Danish GPS Center 25

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)


GNSS SDR Partitioning


ADC Correlators (Channels)

Channel loop closure, Positioning

Radio front-end (analogue)

Position



Radio front-end (analogue)

Position



Position

Hardware

Software

Traditional Receiver

Software Defined Receiver

”Ideal” Software Receiver

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.


Comparison of Solutions

• 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 2013 Danish GPS Center 29

Flexibility

Performance

ASIC FPGA

Power consumption

Unit Price

ASIC

FPGA GPP DSP GPP DSP

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)


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…


Data Aiding

GNSS Snapshot Idea


Compact, low power snapshot

device (rover) RF front-end

Amplifier

GNSS antenna

A/D

Signal recording

Memory

Software that does GNSS signal processing,

derives measurements and does the actual position computation

Additional tasks can be

precision improvement or GNSS signal validation

Multi-frequency, multisystem

GNSS receiver

Receiver clock

Frequency synthesizer

Mixer

SBAS DGPS

PVT solution

…

Wireless or other

type data delivery

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


SDR Demo


Current Receiver Development


2007 Future Research and development

2008-2010

•High sensitivity •Multi-system & multi-frequency receiver •Multipath mitigation •Further SDR development •GNSS integrity •Integration of other kinds of positioning

The ML507 Setup


Battery adapter

GNSS front-end

Simulink Model


The adaptor block inside calls nearly unmodified C code of the FPGA receiver

Matlab SDR Plots


4 4.005 4.01 4.015 4.02 4.025 4.03 4.035

x 104

-2000

0

2000

4000

6000

Samples (time)

Cor

rela

tion

Real correlation result from GNSS SDR

-1 0 1 2 0

0.5

1

1.5

Code Offset [chips]

Cor

rela

tion

Theoretical correlation

0 5 10 15 20 25 300

5

10

15Acquisition results

PRN number (no bar - SV is not in the acquisition list)

Acqu

isitio

n Metr

ic

Not acquired signalsAcquired signals

Matlab SDR


postProcessing.m script

postNavigation.m USB driver (C)

Recorder application (C, C++)

Signal record file (1 byte per sample)

probeSignal.m

acquisition.m

tracking.m

satPos.m

plotTracking.m

plotNavigation.m

Coordinate transformations

findPreambles.m

ephemeris.m

leastSquarePos.m

http://www.sparkfun.com/commerce/categories.php

Commercial DGPS Performance


SDR Modification For Galileo


One sub-frame

Thank You For Your Attention

http://gps.aau.dk


DANISH GPS CENTER

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gnss receivers, one step deeper

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