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1

GNSS - SDR Receivers

A-The Signal in Space (SIS)1. The navigation bands

2. The GPS signal

3. The Galileo signal4. Frequency plan

B-The received SIS

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Information data All the information data necessary toperform the trilateration are containedin the Signal in Space (SIS)

transmitted by the satellites

The SISs have to be allocated in a

satellite band

1. The navigation bands

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GPS and GLONASS bands

Navigation signals have to be allocated in theRNSS (Radio Navigation Satellite Services)bands

GPS occupies L1 and L2 bands

GLONASS occupies the adjacent bands

MHz60.12272L f MHz42.15751L f

L1L2

GPS GLONASS

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New generation GNSS signals

The new generation GNSS signals have to beallocated in the free frequency intervals in the Lband

MHz60.12272L f MHz42.15751L f

L1L2

GPS GLONASS

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Galileo in L1

GPS and Galileo have decided to beinteroperable in the L1 band

MHz60.12272L f MHz42.15751L f

GPS L1L2

Galileo E1

GalileoGPS GLONASS

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New GNSS services

GNSS providers have decided to enlargethe GNSS offer with new improved services

New signals have to be transmitted

MHz60.12272L f MHz42.15751L f

L1L2

GPS GLONASS Galileo

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Interoperability and compatibility (1/2)

Interoperability: Two systems which want towork together

Compatibility: Two systems which want towork without interfering each other

MHz60.12272L f MHz42.15751L f

L1L2

Two concepts become important:

GPS GLONASS Galileo

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MHz60.12272L f MHz42.15751L f

L1L2

At the end of the story the new signals have tobe bothinteroperableandcompatible

Interoperability and compatibility (2/2)

GPS GLONASS Galileo

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We need to know (1/2)

MHz60.12272L

fMHz42.15751L

f

L1L2

Structure of the GPS SIS

GPS GLONASS Galileo

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We need to know (2/2)

How the new signals reach the goal ofinteroperability and compatibility

MHz60.12272L f MHz42.15751L f

L1L2

Introduction to BOC modulation

GPS GLONASS Galileo

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GPS Signal In Space (SIS)

SIS transmitted by the i-th satellite)(txRF

MHz)10.23772(GHz57542.11L f

MHz)10.23602(GHz22760.12L f

Each GPS satellite transmits continuously using two radiofrequencies in the L-band

The frequencies are derived coherently from one of the

atomic standards aboard the satellite. The frequency of theatomic standards aboard a satellite is

MHz10.23

2. The GPS signal

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Three signals

Two signals are transmitted on L1- One for civil users (Open service OS)- One for DoD-authorized users

One signal is transmitted on L2 for DoD-authorized users

MHz42.15751L fMHz60.12272L f

MHz46.20

MHz46.20

MHz046.2)(Frequencyf

Note: the spectra are only qualitative

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Carrier

The signal structure

)2cos()()(2)( 1LLcRF tftdtcPtx

Ranging code: a uniquesequence of -1s and +1sassigned to each satellite(CDMA)

Navigation data: a binary-coded (+1,-1) messageconsisting of data on the satellites health status,ephemeris (satellite position and velocity), clock biasparameters, and an almanac giving reduced-precisionephemeris data on all satellites in the constellation.

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The signal power

)2cos()()(2)( 1LLcRF tftdtcPtx

2/

2/

2)(

1lim

T

T

RFT

RF dttxT

xP

c

T

T

LcT

RF PdttfPT

xP

2/

2/

11L

2)2(cos2

1lim

Power

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The signal components

)2cos()()(2)( 1LLcRF tftdtcPtx

Carrier

Ranging code: Pseudo-randomnoise (PRN) sequence of chips

Navigation data: sequence of bits(50 bits per second in GPS OS)

Note: in the graphs the signal periods are not realistic (only pictorial)

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The GPS SIS of the i-th satellite

)2sin()()(2

)2sin()()(2)2cos()()(2)(

22L222

11L11111L

LYYY

LYYYLcRF

tftdtcP

tftdtcPtftdtcPtx

SIS transmitted by the i-th satellite)(txRF

GHz57542.11L f L1 carrier frequency for OS

GHz22760.12L f L2 carrier frequency for DoD-authorized users

Coarse/Acquisition (C/A) code(a unique sequence 1023 chips, whichis repeated each millisecond)

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C/A code

Code (1023 chips repeated

each millisecond)

Code period: 1 ms

Chip duration (Tc) is about 1sChipping rate is 1.023 MHz

Note: Also the code frequency is derived coherently from the atomicstandards aboard a satellite (10.23 MHz)

0for0)( mmTR ci

m m

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Galileo and GPS interoperability

One of the major driver in the Galileo signal design hasbeen the interoperability with GPS

Interoperability means that receivers have to be

potentially able to deal with both the systems and thenboth the Signal-In-Space

As a consequence, SIS must be in close bandwidths,without interfering each-other

The open access service (free and unencrypted) signalshare the same carrier of GPS C/A code (L1)

3. The Galileo signal

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Split spectrum modulation

One way to reduce mutual interference betweensignals modulated over the same carrier is tointroduce a frequency shift, modulating one thesignal with a subcarrier

In the navigation field this technique has beennamed split spectrum or Binary Offset Carrier(BOC)modulation

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BOC modulation

BOC modulation (Binary Offset Carrier modulation) consistsin applying a squared subcarrier to a BPSK signal

In GNSS the BOC parameters are defined with respect to aReference Frequency:

sf

T

f

R

R

R

11

MHz023.1

)(tsb

t

Reference Period

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BOC subcarrier

The squared subcarrier

is applied to the code

)(tsb

t

tnf(t)s Rb 2sinsign

)(tc

t

nTT R

sc

Subcarrier period

m

T

T

R

r Chip duration

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BOC(n.m)

BOC(n,m): n: subcarrier frequency in multiples of 1.023 MHz

m: chip rate in multiples of 1.023 Mcps

)(tsb

t

n

TT Rsc

)(tc

t

m

TT Rr

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The BOC-modulated SIS

i R

Rii t

T

n

m

TitrdcPts 2sinsign)(

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The BOC-modulated SIS

i R

Rii t

T

n

m

TitrdcPts 2sinsign)(

Sequence of1 (chips of the code)

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The BOC-modulated SIS

i R

Rii t

T

n

m

TitrdcPts 2sinsign)(

Sequence of1 (bits of the navigation message)

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The BOC-modulated SIS

i R

Rii t

T

n

m

TitrdcPts 2sinsign)(

)(tr

rT t

Chip

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The BOC-modulated SIS

i R

Rii t

T

n

m

TitrdcPts 2sinsign)(

)(sin ts

scT t

Subcarrier

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An example: the BOC(10,5) modulation

A BOC(10,5) uses a square wave with afundamental frequency of 10.23 MHz tomodulate a code with chipping rate of 5.115Mchip/s (chip duration about 0.2 s)

2.0rT s)(t

)(tr )(sin ts

1.0scT s)(t

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Example: BOC(1,1)

s1

1

s1

1

1MHz023.11

Rsc

Rr

R

R

R

TT

TT

sTT

f

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BOC Power Spectral Density

The maximum of the power spectrum is shiftedwith respect to the center frequency

It is possible to theoretically evaluate the spectrumas

2

m)BOC(n,

2

cos

sin2

sin

)(

sc

rsc

sc

f

ff

f

f

f

f

ffG

r

rT

f1

sc

scT

f1

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Power spectra (normalized)

GPS

BOC(2,2)

BOC(10,5)

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BOC vs BOCcos

))2(sin(sign)(sin tfts sc

))2(cos(sign)(cos tfts sc

By default a BOC signal is generated by a sine subcarrier, a BOCcos signal

uses a cosine subcarrier

It results in areduction of thesecondary lobesand improvesisolation with

signals in thesame band

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Correlation property of a BOC modulated signal

The autocorrelation of a GPS PN code has a triangularshape in the interval [-Tc,Tc].

The BOC modulated signals have a narrower correlationfunction around the origin but with side peaks

The positioning performances of a system are related tothe ability of identifying the main peak of the correlationfunction: the BOC signal can potentially give betteraccuracy, but due to the presence of the side peaks theimprovement is traded-off with the complexity of the

receiver

C l i i

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Correlation properties

B d f R di N i i lli i (RNSS)

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Bands of Radio Navigation satellite service (RNSS)

GALILEO Bands (Navigation) GPS Bands (Current & modernized)

L5

E5 E6 L1E2 E1

1164MH

z

1214MH

z

1260

MH

z

1300MH

z

1559MH

z

1587

MH

z

1591MH

z

1563MH

z

121

5MH

z

1237

MH

z

L2

RNSS Bands RNSS Bands

ARNS Bands ARNS Bands

GLONASS Bands (Current & modernized)

161

0MH

z

1575.4

2MH

z

1278.7

5MH

z

1191

.795

MH

z

E2-L1-E1 and E5a/L5 are common to GPS Frequency bands for interoperability

Three Frequency Bands partof the RNSS allocated bands

Frequency plan

G lil Si l B li O i ( 2006)

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Galileo Signals Baseline Overview (~2006)

1278

.75MHz

40x1.023 MHz

E6P Signal:

BOCcos(10,5) mod.

Rc=5.115 Mcps

PRS Service

E6C Signal:

Data + Pilot

BPSK mod.

Rc=5.115 Mcps

Rs=1000 sps

CS Service

1575

.42MHz

40x1.023 MHz

L1P Signal:

BOCcos (15,2.5) mod.

PRS Service

L1F Signal:

Data + PilotBOC(1,1) mod.

Rc=1.023 Mcps

Rs=250 sps

OS/CS/SOL

Services

1191

.795M

Hz

E5A Signal:

Data+PilotBPSK mod.

Rc=10.23 Mcps

Rs=50 sps

OS/CS

Services

E5B Signal:

Data+PilotBPSK mod.

Rc=10.23 Mcps

Rs=250 sps

OS/CS/SOL

Services

Frequency

(MHz)

90x1.023 MHz

E5 Signal:

AltBOC(15,10) mod.

Not updated slide

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L1 band already crowded!!! Interoperability and

compatibility with GPS desired.

L1 modulations: design drivers and constraints (1/3)

Not updated slide

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The solution has to:

Make a good use of the spectrum

Keep the same carrier frequency than GPS C/A to assureinteroperability

Limit the overlap with other signals.

Galileo L1 baseline: L1F BOC(1,1)+L1P BOCcos(15,2.5)

L1 modulations: design drivers and constraints (2/3)

L1F open signal: relative small bandwidth desired.L1P restricted signal (PRS-Public RegulatedService): higher performances, larger bandwidthand spectrally separated from any open signal.

Not updated slide

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L1 modulations: design drivers and constraints (2/3)

Not updated slide

N t th h i f L1P d l ti

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Note on the choice of L1P modulation

Galileo Phase B baseline modulation for L1P was BOC(14,2) however not

enough isolation from the GPS M-code. The final choice depends on National Security Compatibility criteria: Spectral

Separation Coefficients Used to quantify interference with GPS M-code

M-code

SSC PSSC (5-12) PSSC (5-20)

BOC(14,2) -85.2 -85.4 -82.6

BOC(15,2.5) -85.6 -85.8 -84.7

BOCcos(15,2.5) -90.4 -90.4 -88.4

Better spectral isolation thanks to the 2ary lobesreduction of the BOC cosine subcarrier

Not updated slide

L1 m lti l i t h i

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L1 multiplexing technique

Three channels to be multiplexed:

L1F data channel:

L1F pilot channel:

L1P data channel:

Constraints:

Amplifier to be used in saturation: constant envelope

Power sharing: 50% for L1P and 50% for L1F

Optimise satellite implementation

Easy to separate the two signals at reception

)()5.2,15cos(111 tsctctdts BOCPLPLPL

)()1,1(111 tsctctdts BOCdFLFLdFL

)()1,1(11 tsctcts BOCpFLpFL

L1 multiplexing technique

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L1 multiplexing technique

CASM : Coherent Adaptative Subcarrier Modulation

Relative power levels:

Channels Beforemultiplexing

Aftermultiplexing

L1F data 25% 22%L1F pilot 25% 22%

L1P 50% 44%

IM -- 11%

Constellation

tstsjtststS LPLpFLdFLL int,11111 23

1

3

2

INTERMODULATION PRODUCT TO ASSURE CONSTANT ENVELOPE

I

Q

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The receiver chain

Antenna RF

Front-endADC

GNSSDigitalreceiver

SIS (Signal in Space)

)(tyIF

)(tyRF

Let us consider the SIS of a single SV

SV= Space Vehicle

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Received C/A SIS

C/A SIS received at the antenna input

ToA Doppler

Note: Doppler negligible at baseband

))(2cos()()(2)( L1 tfftdtcPty dRRF

Receivedpower

160dBW-watt1016

RP

ToA (Time of Arrival)

o5

RP

ElevationZenith

-164 dBW -156dBW

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The IF signal

Down conversion& filtering

)(tyRF )(tyIF

))(2cos()()(2)( )( tfftdtcPty dIFb

RIF

Filtered code with sub-carrier

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Adopted scheme

Antenna RF-IF

Front-endADC

GNSSDigitalreceiver

)(tyRF

)(tyIF

IF Sampling

BB Sampling

+ Noise

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SiS and Noise

AWGN

RF-IF

Front-endADC

GNSSDigitalreceiver

)(trRF)(trIF

fRFf

2/0N

f0

2/0N

IFfIFf

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Defintion of C/N0

(dBW/Hz)2/0N(dBW)RP

00 N

P

N

C R

f fRFf RFfSignal power

in the wholebandwidth

Hz)(dB/ 0NC

C/N0defined at the antenna and measured .

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Signal to Noise Ratio (SNR)

Down conversion& filtering

)(tyRF)()()( tNtytR IFIFIF

+

)(tN

(dBW)0BN

(dBW)RP

BNC

BNPR 1SNR

00

f

f

IFf IFfIFf

B

IFf

Signal andNoise power

in a band B

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C/N0 and SNR

BN

C

BN

PR 1SNR00

GPSElevation 5o zenith

SIS power at RX -164 dBW -156 dBW

Noise powerdensity (N0)

-201dBW/Hz -201dBW/Hz

C/N0 37 dBHz 45 dBHz

SNR (20MHz BW) -36 dB -28 dB

SNR (4MHz BW) -26 dB -18 dB