software defined radio for processing gnss signals - …jmfriedt.free.fr/efts_gps.pdf · software...

Software definedGPS receiver

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Software Defined Radio for processing GNSSsignals

S. Martinez Gutierrez, J.-M Friedt, G. Cabodevila, P.Y Bourgeois,E. Rubiola

FEMTO-ST Time & Frequency, Besancon, France

www.femto-st.fr first-tf.com

Slides available at http://jmfriedt.free.fr/efts_gps.pdf

January 31, 20151 / 36

www.femto-st.fr

first-tf.com

http://jmfriedt.free.fr/efts_gps.pdf


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Introduction: GNSSGNSS:

• Spaceborne atomic clocks 1

• Aim: use of ultrastable time references beyond positioning ...• ... eg. tide gauge 2, moisture detection 3 ,

phase monitoring (TEC)• Requires at least phase recovery.• Educational purpose: detailed understanding

of the steps needed to complete a GPSreceiver low phase noise LO !

Use a low cost DVB-T receiver for acquisition:• 8 bit-resolution

• poor sensitivity ⇒ pre-amplified antenna

• 2.4 MHz measurement bandwidth limited byUSB bandwidth (I, Q components)

• gnss-sdr /w GNURadio processing blocks

1http://geodesie.ign.fr/journee-gnss-science/2

K.M. Larson: http://spot.colorado.edu/~kristine/Kristine_Larson/Home.html3

Bock et. al., West African Monsoon observed with ground-based GPS receivers ..., J. Geophysical Research 113, D21105 (2008),at http://onlinelibrary.wiley.com/doi/10.1029/2008JD010327/pdf 2 / 36

http://geodesie.ign.fr/journee-gnss-science/

http://spot.colorado.edu/~kristine/Kristine_Larson/Home.html

http://onlinelibrary.wiley.com/doi/10.1029/2008JD010327/pdf


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Introduction: GPS basics

Borre & al. p.18

3 / 36


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Objectives• a modulator generates the information, here encoded in the phase

of the carrier

• the information is carried on a signal whose frequency varies(Doppler, thermal drift of LO)

• recovering the transmitted information is a matter of eliminatingcarrier information

• two degrees of freedom (carrier frequency and CDMA for satelliteidentification) will require two feedback loops to recover theinformation

⇒ carrier recovery and code position (delay) recovery

PRN (1.023 Mbps)

NAV (50 bps)

PRN

NAV

feedback loop

feedback loop

1575.42 MHz+/−df

4 / 36


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SDR: user perspective

• DVB-T dongle application : L1 C/A at 1.023 MHz barely meets theNyquist criteria when sampling at 2 MHz (no need to: strongassumption on modulation, i.e. 2/1.023=1.95 symbol/period)

• Receiver sensitivity ?

Receiver 137 MHz 434 MHz 1090 MHz 1500 MHz

E4000 -105 -105 -96 -95R820T -112 -112 -109 -102FC0013 -112 -111 -76 -58

Receiver 137 MHz 434 MHz 1090 MHz 1500 MHz

E4000 -109 -110 -101 -101R820T -117 -119 -113 -110FC0013 -116 -115 -80 -62

Carrier power (dBm) needed for a demodulated signal SNR of 10 dB. All dongles set

to their max gain (RF=39 dB and IF=45 dB for E4000, RF=50 dB for R820T,

RF=20 dB for FC0013). Top: FM, bottom: AM.

5 / 36


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Link budget: is it possible ?• Emitted power: 25.6 W=14 dBW

• 13 dBi antenna gain

• FSPL (21000 km)=182 dB

• Received power=-155 dBW=-125dBm

• Themal noise in a 2 MHz BW:-174 dBm/Hz+10·log10(2 MHz)=-111 dBm

• Compression (coding) gain:BW×τ=2 MHz×1 ms=33 dB

• SNR=(−125 + 33)− (−111)=19 dB• Receiver antenna amplification: +27 dB

⇒ (-125+27)-(-102DVB−T ) '4 dB SNR

R820T+RTL2832U based DVB-T receiver, bias T, antenna (30$) 4

4http://www.adafruit.com/product/960: 13$ antenna andhttp://www.dx.com/p/

rtl2832u-r820t-mini-dvb-t-dab-fm-usb-digital-tv-dongle-black-170541:12$ dongle

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http://www.adafruit.com/product/960

http://www.dx.com/p/rtl2832u-r820t-mini-dvb-t-dab-fm-usb-digital-tv-dongle-black-170541

http://www.dx.com/p/rtl2832u-r820t-mini-dvb-t-dab-fm-usb-digital-tv-dongle-black-170541


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SDR: user perspective• Frequency offset budget: LO offset (±50 ppm) & drift

(temperature) + remote carrier dependence with position in space• Identify carrier frequency mismatch: carrier recovery (phase) issue• Temperature dependence : front-end-cal 5 over > a week

-62

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off

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per

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C)

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PSK signal is hidden by phase rotation ifincoming carrier and LO mismatch

[......7...........19.......27.....]

Total signal acquisition run time 11.703 [seconds]

Reference Time:

GPS Week: 790

GPS TOW: 209107 16728.560000

~ UTC: Tue Oct 14 12:05:08 2014

Current TOW obtained from SUPL assistance = 209107

Reference location (defined in config file):

Latitude=47.3 []

Longitude=6 []

Altitude=10 [m]

Doppler analysis results:

SV ID Measured [Hz] Predicted [Hz]

7 92437.50 1848.25

19 94125.00 3080.82

27 93000.00 2116.75

Parameters estimation for Elonics E4000 Front-End:

Sampling frequency =1999884.81 [Hz]

IF bias present in baseband=90736.35 [Hz]

Reference oscillator error =-57.60 [ppm]

Corrected Doppler vs. Predicted

SV ID Corrected [Hz] Predicted [Hz]

7 1701.15 1848.25

19 3388.65 3080.82

27 2263.65 2116.75

GNSS-SDR Front-end calibration program ended.

5gnss-sdr.org 7 / 36

gnss-sdr.org


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SDR: user perspectiveCalculation of Doppler shift as a function of orbital parameters

θ

90−θ

Earth

orbit

HH’

P

R

R+r

r

v

v

• Orbiting at 20000 km in 12 hours⇒ 2π(6400 + 20000)/12 =13800 km/h=3840 m/s

• Receiver at position P observes thesatellite from horizon to horizon HH′.

• At H, velocity vector ~v along thesatellite trajectory is projectedtowards P along ~v‖ with ϑ meeting

the condition sin(ϑ) = RR+r

and

hence |~v‖| = |~v | · cos(90− ϑ) =|~v | · sin(ϑ) = |~v | · R/(R + r).

• Here|~v‖| = 13800 ·6400/(6400+20000) =3345 km/h and the Doppler shift is∆f = f0 · |~v‖|/c = ±4800 Hz'3.1 ppm

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SDR: user perspectiveCalculation of Doppler shift as a function of orbital parameters

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00

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00 0

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00

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Doppler shift (Hz)

tim

e (h

)

• Orbiting at 20000 km in 12 hours⇒ 2π(6400 + 20000)/12 =13800 km/h=3840 m/s

• Receiver at position P observes thesatellite from horizon to horizon HH′.

• At H, velocity vector ~v along thesatellite trajectory is projectedtowards P along ~v‖ with ϑ meeting

the condition sin(ϑ) = RR+r

and

hence |~v‖| = |~v | · cos(90− ϑ) =|~v | · sin(ϑ) = |~v | · R/(R + r).

• Here|~v‖| = 13800 ·6400/(6400+20000) =3345 km/h and the Doppler shift is∆f = f0 · |~v‖|/c = ±4800 Hz'3.1 ppm

9 / 36


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SDR: developer perspective

• Problem: BPSK modulates the carrier as ϕ = [0;π]: sin(ωt + ϕ)

• Ideal recovery: mix with ω and cos(ϕ) is left

• Real life: LO offset introduces ∆ω and receiver mixer yieldscos(∆ω · t + ϕ): ϕ variation hidden in ∆ω · t

Mixer acts as BPSK modulator

cos(ϕ)2 ∝ cos(2ϕ)ϕ ∈ [0;π]⇒ 2ϕ = 0 [2π]

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po

rteu

se

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rteu

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ab

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Carrier recovery by squaring BPSK

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SDR: developer perspective• Problem: BPSK modulates the carrier as ϕ = [0;π]: sin(ωt + ϕ)

• Ideal recovery: mix with ω and cos(ϕ) is left

• Real life: LO offset introduces ∆ω and receiver mixer yieldscos(∆ω · t + ϕ): ϕ variation hidden in ∆ω · t

Mixer acts as BPSK modulator

cos(ϕ)2 ∝ cos(2ϕ)ϕ ∈ [0;π]⇒ 2ϕ = 0 [2π]

DVB−TBPSKmodulatedsignal

Source 1.21 GHz

Mixer createsthe BPSK

modulation of thecarrier at

1210 MHz

LO, −43 dBm

100 kHz1Vpp

RFIF

receptionemission

Practical BPSK signala, here←− squared by oscilloscope software

aE. Rubiola, Tutorial on the double balanced mixer,

http://arxiv.org/pdf/physics/0608211.pdf

11 / 36



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Software based carrier recovery (feedback loop not allowed inGNURadio): use the ready made Costas loop block

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Software based carrier recovery (feedback loop not allowed inGNURadio): use the ready made Costas loop block

modulation

30

dB

(1)

(3)

modulation

carrier

squared signal = removes

the modulation

20

dB

phase of signal shiftedby the carrier frequency

(2)

(4)

f∆

frequency offset

= locked loopcentered on 0 Hz

received signal: carrier

shifted wrt 0 Hz

carrier

(1)

(3)

(2)

(4)

phase of the signal hiddennot centered on 0 Hz= unlocked loop by phase rotations du to freq.

offset

excessive carrier offset

received signal:

carrier

Locked Costas loop Unocked Costas loop

13 / 36


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• Single carrier demodulation is not applicable as is to GPS: CDMA

• All GPS satellites send over the same frequency (shifted byDoppler): need for correction by the BPSK modulation beforecarrier recovery (but PRN requires carrier ?!)

• Mockup system with a 7-bit FLSR code generated by C. Fluhr(EFTS labs)

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code 1, after Costas

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code 1, shifted by 1050

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code 2, shifted by 1600 (’perfect’)

Before and after Costas With frequency compensation14 / 36


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From mock-up to actual GPSsignals

We have demonstrated that cross-correlating with the PRN will yieldnon-zero signal even if carrier offsets exist.

→ get familiar with GPS PRN code: a bit more complex (combinationof two LFSR)

→ how resistant is GPS PRN code (10 bits) to carrier frequencyoffset ?

→ applicability to recorded DVB-T data ?

DVB-T |xcorr(s × LO),PRN)|Q

I

NCOf ∈ C PRN∈ R

exp(jωt)× exp(jω′t)

cos(ωt)× exp(jω′t)

exp(−jωt)× exp(jω′t)

when handling complex functions, −f 6= f since exp(jft) 6= exp(−jft)

+ no need for band pass filter (mixer output is a single frequency)

15 / 36


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10-bit PRN code resistance tocarrier offet

0

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xcorr

(a.u

.)

sample offset (no unit)

PRN1,PRN1PRN2,PRN1

PRN1,1.003*PRN1PRN1,1.01*PRN1

0

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1000 1020 1040 1060 1080 1100

xcorr

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.)

sample offset (no unit)

PRN1,PRN1PRN2,PRN1

PRN1,1.003*PRN1PRN1,1.01*PRN1

1 more o f f ; c l o s e a l l ; c l e a r a l ls e t ( 0 , ” d e f a u l t a x e s f o n t n a m e ” , ” H e l v e t i c a ” )

3a=cacode ( [ 1 : 3 1 ] , 1 ) ;

5 a=a ’ ;a=a−mean ( a ) ;

7 p l o t ( x c o r r ( a ( : , 1 ) , a ( : , 1 ) ) , ’ r ’ )h o l d on

9 p l o t ( x c o r r ( a ( : , 1 ) , a ( : , 2 ) ) )x l a b e l ( ’ sample o f f s e t ( no u n i t ) ’ )

11 y l a b e l ( ’ x c o r r ( a . u . ) ’ )

13 h o l d ona2=cacode ( 1 , 1 . 0 0 3 ) ; a1=cacode ( 1 , 1 ) ; p l o t ( x c o r r

( a1−mean ( a1 ) , a2−mean ( a2 ) ) , ’ c ’ )

15 a2=cacode ( 1 , 1 . 0 1 ) ; a1=cacode ( 1 , 1 ) ; p l o t ( x c o r r (a1−mean ( a1 ) , a2−mean ( a2 ) ) , ’ g ’ )

% 10 kHz /1023 kHz=0.01

cacode.m available at MatlabCentrala

aK.S. Raju & al., Digital GPS Signal Generator for L1

Band, Signal & Image Processing: An International Journal(SIPIJ) 3 (6), Dec. 2012, available athttp://airccse.org/journal/sipij/papers/3612sipij07.pdf

16 / 36

http://airccse.org/journal/sipij/papers/3612sipij07.pdf


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Application to real GPS signals

• gnss-sdr provides a calibration software that stores the recordedsamples (2 MS/s) in a temporary file

• when calibrating the dongle, we have saved the result of thegnss-sdr calibration process and the raw sample

• we now process these datasets and compare our analysis with thoseof gnss-sdr

x=r e a d c o m p l e x b i n a r y ( ’ 1413968929 c a p t u r e . dat ’ ) ;

2 t ime =[0:1/2 e6 : l e n g t h ( x ) /2 e6 ] ’ ; t ime=t ime ( 1 : end−1) ;

4 f o r m=1:37a=cacode (m, 2 / 1 . 0 2 3 ) ; a=a−mean ( a ) ;

6 l =1; m % d i s p l a y s a t PRNf o r f r e q =8e4 : 2 0 0 : 1 e5 % run through p o s s i b l e f r e q u e n c y o f f s e t s : c f f r o n t−end−c a l

8 mysine=exp ( j∗2∗p i∗(− f r e q )∗t ime ) ; % LOxx=x .∗ mysine ; % f r e q u e n c y s h i f t t h e s i g n a l

10 [ u ( l ,m) , v ( l ,m) ]=max ( abs ( x c o r r ( a , xx ) ) ) ; % check f o r c r o s s c o r r e l a t i o n max .l=l +1;

12 endend

14 f i g u r eimagesc ( abs ( u ) )

Notice the extended frequency offset range: hi accuracy LO will speedup acquisition time

17 / 36


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2D plot with PRN number as X-axis and frequency offset in Y-axis

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4005 10 15 20 25 30 35

fre

q o

ffse

t

PRN

Borre & al. p.23: sat ID = PRN, here 4, 7, 11, 15,

19, 27, 30

Latitude=47.3 []

Longitude=6 []

Altitude=10 [m]



4 93875.00 3547.36

7 88625.00 -1439.45

11 93625.00 3455.35

15 89437.50 -694.02

19 92000.00 1783.79

27 90250.00 96.13

30 90375.00 139.85







4 3792.04 3547.36

7 -1457.96 -1439.45

11 3542.04 3455.35

15 -645.46 -694.02

19 1917.04 1783.79

27 167.04 96.13

30 292.04 139.85


18 / 36


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0

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4005 10 15 20 25 30 35

fre

qu

en

cy s

hift

(20

0 H

z/p

oin

t)

PRN

sat ID = PRN, here 19, 27

[......7...........19.......27.....]

Total signal acquisition run time 11.4224 [seconds]

Reference Time:

GPS Week: 791

GPS TOW: 299342 23947.360000

~ UTC: Wed Oct 22 13:09:03 2014



Latitude=47.3 []

Longitude=6 []

Altitude=10 [m]



7 88687.50 -1596.95

19 92000.00 1664.98

27 90250.00 -55.40







7 -1518.25 -1596.95

19 1794.25 1664.98

27 44.25 -55.40


19 / 36


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0

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4005 10 15 20 25 30 35

fre

q o

ffse

t

PRN sat

ID = PRN, here 7, 11, 19, 27, 30

GPS TOW: 299662 23972.960000

~ UTC: Wed Oct 22 13:14:23 2014



Latitude=47.3 []

Longitude=6 []

Altitude=10 [m]



7 89250.00 -1745.40

11 93875.00 3382.16

19 91625.00 1541.05

27 90000.00 -209.33

30 90000.00 -244.43







7 -1052.65 -1745.40

11 3572.35 3382.16

19 1322.35 1541.05

27 -302.65 -209.33

30 -302.65 -244.43


20 / 36


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Did we get this right ? Nav.messages

• Navigation message is XORed with C/A

• 50 bps or 20 ms ⇒ longer record than front-end-cal

• C/A PRN repeats every 1023 cycles at 1.023 MHz ⇒ probecorrelation phase every 1 ms

80000

85000

90000

95000

100000

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t (H

z)

PRN number (no unit)

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90000 92000 94000 96000 98000 100000

cro

ss c

orr

ela

tion

frequency offset

PRN 31

PRN 31: visibility chart & cross-correlation for carrier freq. identification

21 / 36


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Did we get this right ? Nav.messages

• Navigation message is XORed with C/A

• 50 bps or 20 ms ⇒ longer record than front-end-cal

• C/A PRN repeats every 1023 cycles at 1.023 MHz ⇒ probecorrelation phase every 1 ms

-60

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ase

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d)

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FFT freq estimatemanual correction: +100 Hz

unwrapped manual corr.

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ph

ase

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d)

time (ms)

FFT freq estimatemanual correction: +100 Hz

unwrapped manual corr.

Manual frequency correction to get a flat phase in each bit (+100 Hz)Nav signal: bit recovery requires phase tracking (here lin. fit removal) !

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Feedback loop: carrier frequencytracking

• Need for frequency tracking to compensate for Doppler shift andtemperature drift

• Need for PRN phase positioning (?)

PRN

PRN

A/DC atan(Q/I)

PRN position

frequency tracking

8 bits2 MS/s

DVB−T

Quick reminder on implementing a feedback loop control:

C (z) = O(z)I (z) = a·z+b

c·z+d = a+b·z−1

c+d·z−1 ⇒ c ·Ok + d ·Ok−1 = a · Ik + b · Ik−1 or

Ok = a/c · Ik + b/c · Ik−1 − d/c · Ok−1

23 / 36


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Phase discriminator

• The phase is needed for carrier frequency tracking yet encodes theBPSK information ⇒ discriminator insensitive to 180o phase shifts

• Keep the phase close to 0 means arctan(Q/I ) ' 0⇒ Q ' 0

• arctan(Q/I ) is the exact phase estimate insensitive to 180o rotations

• atan2(Q, I ) provides the full phase including 180o rotations ⇒needed to exctract the navigation message

• Q × sign(I ) is easier to compute and yields the samefunctionality6.

Following graphs: from top to bottom thephase of the cross correlation maximum, theNCO frequency, and the extractednavigation bits.

Q

I

(−1,1)=(1,−1)

(1,1)=(−1,−1)

6C. O’Driscoll, M.G. Petovello & G. Lachapelle, Choosing the coherent integrationtime for Kalman filter-based carrier-phase tracking of GNSS signals, GPS Solut 15345–356 (2011)

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Te=1e-3;A=150;Cz=tf((1/A)*[2-a -1],[1 -2 1],Te);

1 [ num , den ]= t f d a t a ( Cz , ’ v ’ ) ;mys ine=exp ( j ∗(2∗ p i∗mod( ( ( f r e q 0+f r ( l ) )∗t ime ( kk : kk+f s ∗1000−1) ) , 1 ) ) ) ;

3 xx=x ( kk : kk+f s ∗1000−1).∗ mysine ;zp=x c o r r ( ap , xx ,MAXLAG) ; % we have d e f i n e d MAXLAG=2000

5 yp ( l )=atan ( imag ( zp ( k ( l ) ) ) . / r e a l ( zp ( k ( l ) ) ) ) ; % d i s c r i m i n a t o rf r e q=−den ( 2 )∗freqm1−den ( 3 )∗f reqm2+num ( 1 )∗yp ( l )+num ( 2 )∗ym ; % f e e d b a c k l o o p

Discriminator: arctan(Q/I )finit = 92860 Hz

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

−1

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ph

ase

[p

i],

rad

)

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fre

q.

(Hz)

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ase

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psk)

time (ms)

sampling rate: 2.0 MS/sfinit = 92960 Hz

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

−1

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phase [pi], ra

d)

500 1000 1500 2000 2500 3000 3500 4000 4500

−20

−10

0

10

freq. (H

z)

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

0

2

4

6

phase (

bpsk)

time (ms)

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Friedt & al.

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What next ...


• Difficulty in carrier tracking due to low sampling rate ?

• Attempt at 2.4 MS/s

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

−1

0

1

2

phase [pi], ra

d)

500 1000 1500 2000 2500 3000 3500 4000 4500

−6

−4

−2

0

freq. (H

z)

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

0

2

4

6

phase (

bpsk)

time (ms)

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

−1

0

1

2

phase [pi], ra

d)

500 1000 1500 2000 2500 3000 3500 4000 4500

0

5

10

15

20

freq. (H

z)

0 500 1000 1500 2000 2500 3000 3500 4000 4500−2

0

2

4

6phase (

bpsk)

time (ms)

⇒ Much poorer results ! (sampling rate mismatch ?)

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9-second long decoding without divergence, after implementing PRNcode positioning (R&S SMA100)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000−2

−1

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1

2

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ata

n (

rad)

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0

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Hz)

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0

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6

time (ms)

angle

(ra

d)

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Friedt & al.

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(Temporary) conclusions –perspectives

Conclusion ...

• Rather good understanding of the modulation scheme

• Operational carrier recovery even in a CDMA environment

• The so-called GPS acquisition is functional

• Tracking loop functional but VERY sensitive

... and perspectives:

• if interested, could provide navigation information (to bedemonstrated – 50 Hz, output of the correlation scheme)

• postprocessing → real time processing with gnss-sdr ...

• ... whose code remains to be understood !

• Beyond DVB-T: L2 tracking ? L5 tracking ? 7

• Passive RADAR: use remote emitter for Doppler-distance mapping

7http://pmonta.com/blog/2014/07/08/gps-p-code-exploration/28 / 36

http://pmonta.com/blog/2014/07/08/gps-p-code-exploration/


Friedt & al.

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Software defined radio: secondarycorrelator peaks

• Access to raw correlator output ⇒ search for multipath signal

• Cross correlation of direct and reflected signal ⇒ even compatiblewith P-code

• Water height measurement performed by amplitude measurement orphase measurement 8

• SNR hints at surface structure

• opportunistic signal source: extension to passive RADAR analysis• Replace the known C/A pseudorandom code with any information

transmitted• PRN v.s Doppler is now replaced with time-delay v.s Doppler

(Doppler helping to get rid of clutter)• common clock on reference and measurement receivers to get rid of

relative oscillator drift

8trs-new.jpl.nasa.gov/dspace/bitstream/2014/14430/1/00-0867.pdf orhttp://arxiv.org/pdf/physics/0212055.pdf

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trs-new.jpl.nasa.gov/dspace/bitstream/2014/14430/1/00-0867.pdf



Friedt & al.

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Experimental setup

PRN v.s Doppler is now replaced with phase shift (delay=distance) v.sDoppler (speed)

• Common clock to bothdongles (either external orfrom one of the quartz)

• Simultaneous acquisition fromboth dongles

• No apparent effect of USB bus(load due to file transfer doesnot induce phase slip)

• Ability to detect phase butslow signal drift ?! dependenton how the dongles areinitialized ?

30 / 36


Friedt & al.

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Experimental setup






31 / 36


Friedt & al.

Intro & basics

User perspective

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Experimental setup






Example on commercial FM broadcast

32 / 36


Friedt & al.

Intro & basics

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What next ...

Perspectives• Stabilize understanding of carrier frequency feedback loop

• Demonstrate phase shift when antenna is moved (carrier phasetracking, λ = 300/1575 ' 19 cm

• Use carrier phase information in local oscillator stabilizationfeedback loop

Replace E4k with R820T

• so far, no GPS signal decoded from a R820T based receiver

From educational to research analysis

• replace DVB-T with high-bandwidth multichannel acquisition card

EGNOS (European WAAS 9) decoding ?

• L1-compatible signal sent by geosynchronous satellites

• extended PRN codes beyond 37:• 135, 138: over USA• 120, 124 (decomissioned), 126, 136: active over Europe

• new PRN code generator seems functional (≤31) but no EGNOSsignal detected

9Wide Area Augmented System33 / 36


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References1 K. Borre et al., A Software-Defined GPS and Galileo Receiver – A

Single-Frequency Approach, Birkhauser Boston, 2007

2 E.D Kaplan, Understanding GPS: Principles and Applications, 2ndEd., Artech House (2005)

3 gnss-sdr.org, significantly http://gnss-sdr.org/node/50

4 C. Fernandez-Prades et al., GNSS-SDR: an open source tool forresearchers and developers, Proc. ION GNSS Conference 2011 10

5 Michele’s GNSS blog at http://michelebavaro.blogspot.fr/

6 SoftGPS web page kom.aau.dk/project/softgps, now moved tothe rather useless gfix.dk/matlab-gnss-sdr-book

7 P. Boven, Hacking the GPS, OHM2013 program athttps://program.ohm2013.org/event/314.html, unfortunatelyno trace other than notes and memories

Archive of data and scripts:http://jmfriedt.free/fr/efts_archive.tar.gz in addition to

the slides at http://jmfriedt.free/fr/efts_slides.pdf

10http://www.cttc.es/publication/

gnss-sdr-an-open-source-tool-for-researchers-and-developers/34 / 36

http://gnss-sdr.org/node/50

http://michelebavaro.blogspot.fr/

kom.aau.dk/project/softgps

gfix.dk/matlab-gnss-sdr-book

https://program.ohm2013.org/event/314.html

http://jmfriedt.free/fr/efts_archive.tar.gz

http://jmfriedt.free/fr/efts_slides.pdf

http://www.cttc.es/publication/gnss-sdr-an-open-source-tool-for-researchers-and-developers/

http://www.cttc.es/publication/gnss-sdr-an-open-source-tool-for-researchers-and-developers/


Friedt & al.

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What about this squared carrierstory ?

P. Boven at OHM2013: track GPS by squaring the received signal toconcentrate the spread spectrum into a single frequency bin ...

35 / 36


Friedt & al.

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What next ...

What about this squared carrierstory ?

... but the de-spread GPS signal is narrowband (±5 kHz) and hidden inthe broadband noise + LO offset

Knowing what to look for, we found it !36 / 36

software defined radio for processing gnss signals - …jmfriedt.free.fr/efts_gps.pdf · software...

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