GNSS SIGNALSINTRODUCTION

Gabriella Povero
Navigation Technologies

GNSS Training
AIT, Bangkok
14 -18 January 2019©2019 LINKS

1GNSS IN ONE SLIDEA Global Navigation Satellite System (GNSS) consists of aconstellation of satellites with global coverage, whose payloads areespecially designed to provide positioning of objects
GNSSs implement thetrilateration method (spherical
positioning systems)

The satellites are at known
positions, as we know satellite
orbits and time.

zSatellite broadcast signals
toward users on Earthxy©2019 LINKS

All the information data necessary
to perform the trilateration arecontained in the Signal in Space(SIS) transmitted by the satellites

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THE NAVIGATION EQUATION( xk , yk , zk )

k

known

Satellite clocks aresynchronised

measuredz

br( xo , yo , zo )xy©2019 LINKS

unknown

The receiver has aclock bias vs satellite
clocks k  ( xk  x 0 ) 2  ( y k  y 0 ) 2  ( z k  z 0 ) 2  b r
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THE NAVIGATION SOLUTION
If we consider 4 satellites:

 
 1
 
 2

3 

  4 measured©2019 LINKS( xs1  x0 ) 2  ( ys1  y0 ) 2  ( z s1  z0 ) 2  br
( xs 2  x0 ) 2  ( ys 2  y0 ) 2  ( z s 2  z0 ) 2  br
( xs 3  x0 ) 2  ( ys 3  y0 ) 2  ( z s 3  z0 ) 2  br
( xs 4  x0 ) 2  ( ys 4  y0 ) 2  ( z s 4  z0 ) 2  br

Known
(written in thenavigation message)

x0 , y0 , z 0 , bu
4 unknowns

br  c  bu
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THE NAVIGATION EQUATIONREMARKS••••©2019 LINKS

In order to estimate its position a
receiver must have at least four
satellites in view
The satellites must be in Line-of-Sight
If a larger number of satellites is in viewa better estimation is possible. In thepast the combination of four satellites
giving the best performance was chosen
Modern receivers use several channels
in order to perform the position
estimation

z

x
y

5

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MULTI-GNSS©2019 LINKS

6MULTI-GNSS©2019 LINKS

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GNSS SIGNAL IN SPACE
The signals broadcast by the navigation satellites:• Allow the user to estimate the distance (pseudorange)user-satellite• Carry some useful data
• Be robust to the transmission through the atmosphere• Identify in a unique way the satellites

The SIS is characterised by:• Frequency BandxRF (t )  2 Pc c(t )d (t ) cos( 2f L t   L1 )• Carrier Frequency• Modulation Schemet• Multiplexing Format• Ranging Codet

• Navigation Data Format• Transmitted Power©2019 LINKS

8GNSS SIGNAL IN SPACExRF (t )  2 PR c(t ) sc (t )d (t ) sin(2f RF t   )CarrierRanging code: Pseudo-random noise(PRN) sequence of chips(typ. 1023 chips per ms)

zoom

Subcarrier (not always present)
Navigation data: sequence of bits

Note: in the graphs the signal periods are not realistic (only pictorial)©2019 LINKS

9GNSS SIGNAL IN SPACE
The signals broadcast by the navigation satellites:• Allow the user to estimate the distance (pseudorange)user-satellite• Carry some useful data
• Be robust to the transmission through the atmosphere• Identify in a unique way the satellites

The SIS is characterised by:• Frequency Band
• Carrier Frequency• Modulation Scheme• Multiplexing Format• Ranging CodexRF (t )  2 Pc c(t )d (t ) cos( 2f L t   L1 )tt

• Navigation Data Format©2019 LINKS• Transmitted Power10©2019 LINKS

1575.42 MHz ---1278.75 MHz ---1207.140 MHz ---1176.45 MHz ---GNSS SIS - FREQUENCIES AND BANDS

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GNSS SIGNAL IN SPACE
The signals broadcast by the navigation satellites:• Allow the user to estimate the distance (pseudorange)user-satellite• Carry some useful data
• Be robust to the transmission through the atmosphere• Identify in a unique way the satellites

The SIS is characterised by:• Frequency BandxRF (t )  2 Pc c(t )d (t ) cos( 2f L t   L1 )• Carrier Frequency• Modulation Schemet• Multiplexing Format• Ranging Codet

• Navigation Data Format• Transmitted Power©2019 LINKS

12GNSS SIS - MODULATION SCHEMES••In legacy navigation SISs, the used modulation schemes are:
– BPSK– QPSKIn new and modernized SISs, innovative modulation schemes havebeen proposed (BOC, MBOC, …)

BPSK is the simplest form of phase shift keying(PSK).
It uses two phases which are separated by 180°.
Low data rate (1 bit/symbol)
Best BER performance among PSK modulations
QPSK can be obtained as the combination of 2
BPSK signals: one in-phase and the other inquadrature (90° phase shift)
Data rate: 2 bits/symbolQ
I©2019 LINKS

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SPLIT SPECTRUM MODULATION• One way to reduce mutual interference between signals modulated
over the same carrier is to introduce a frequency shift,modulating one of the signals with a subcarrier• Split spectrum:
the energy is allocated around
BOC subcarrier frequency andnot at the central frequency

Power Spectral Density [dBW/Hz]• In the navigation field thistechnique has been named
Binary Offset Carrier (BOC)
modulation

20
15
10
5
0
-5
-10
-15
-20
-4

©2019 LINKSBPSK(1)
BOC(1,1)-2

02Frequency shift [MHz]

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•

Code Division Multiple Access(CDMA) is a multiple-accesstechnique for transmitterssharing the same band
The data-signal band ismultiplied by a code, which is
unique for each transmitter

Bit (data signal)•

Chip (code signal)

GNSS SIS - CDMAtt

The code signal is a binary sequence, generally referred toas pseudo noise (PN)

Band
©2019 LINKS

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GNSS SIS – CDMA AND SPREAD SPECTRUM
Data signal with a narrow
band Bx is combined witha PN code (wider band)

1 bit period 1 chip period
Data signal
PN-codeCoded signal

The bandwidth By of the resulting
signal is the sum of band Bx and thelarge band of the code (Fouriertransform property)

Bx©2019 LINKS

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GNSS SIS – CDMA AND SPREAD SPECTRUM

Data signal with a narrow
band Bx is combined witha PN code (wider band)

1 bit period 1 chip period
Data signal
PN-codeCoded signal• The total transmitted power does
not change• The bandwidth By of the resulting
signal is larger than Bx. The name“spread spectrum” indicates thatthe spectrum is spread• The level of the power spectral
density decreases©2019 LINKS

Bx
By

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GNSS SIS – CDMA AND INTERFERENCE

RFI-TX

CDMA-TX©2019 LINKS

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GNSS SIS – CDMA AND INTERFERENCE

RFI-TX
CDMA-RX

FilterNoiseCDMA-TX©2019 LINKS

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GNSS SIS – CDMA AND INTERFERENCE

RFI-TX
CDMA-RX

FilterNoiseCDMA-TX©2019 LINKS

20GNSS SIS – CDMA AND INTERFERENCE

RFI-TX
CDMA-RX

FilterNoiseCDMA-TX©2019 LINKS

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GNSS SIS - FDMA

t

t0

f

Frequency domain©2019 LINKStTime domain

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GALILEO SIS•

One of the major driver in the Galileo signal design has been
the interoperability with GPS•

Interoperability means that receivers have to be potentiallyable to deal with both the systems, thus with both the
Signals-In-Space•As a consequence, SIS must be in close bandwidths, without
interfering each-other•

The open access service (free and unencrypted) signal sharethe same carrier of GPS C/A code (L1)•

BOC modulation introduced to reduce inter-systeminterference©2019 LINKS

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GALILEO BASELINE SIGNAL AND FREQUENCY PLAN

E5a Signal:
Data+Pilot
BPSK mod.
Rc=10.23 Mcps
Rs=50 sps
OS
Services

E5b Signal:
Data+Pilot
BPSK mod.
Rc=10.23 Mcps
Rs=250 sps
OS/CS
Services

50x1.023 MHz

E6P Signal:
BOCcos(10,5) mod.
Rc=5.115 McpsPRS Service

E6 Signal:
Data + Pilot
BPSK mod.
Rc=5.115 Mcps
Rs=1000 sps
CS Service

40x1.023 MHz

E1P Signal:
BOCcos (15,2.5) mod.
PRS Service

E1 Signal:
Data + Pilot
CBOC(6,1,1/11)
Rc=1.023 Mcps
Rs=250 sps
OS/CS/SoL
Services

24x1.023 MHz

Frequency(MHz)

E5 Signal:
AltBOC(15,10) mod.

©2019 LINKS

E5

E6

E1
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GALILEO BASELINE SIGNAL AND FREQUENCY PLAN

G/Nav, C/Nav

G/Nav, I/Nav

1164 - 1215 MHz

1260 - 1300 MHz

1559 - 1594 MHz

E5

E6

E1
L1

1278,750 MHz

1575,420MHz

E5a

E5b

1191,795 MHz
1176,450

F/Nav
50 sps
25 bpsPilot©2019 LINKS

Alt-BOC(15,10)

I

Q
Pilot

1207,140I/Nav
250 sps (125 bps)

G/Nav
BOC(10,5)100 spsC/Nav
BPSK(5)1000 sps + Pilot

Q

I

G/Nav
BOCc(15,2.5)
100 spsI

I/Nav
CBOC(6,1,1/11)
250 sps (125 bps)+ Pilot

Q

25

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GALILEO BASELINE SIGNAL AND FREQUENCY PLAN
SignalCarrier Frequency(MHz)

Receiver Reference
Bandwidth (MHz)E1

1575.420

24.552E6

1278.750

40.920E51191.795

51.150E5a1176.450

20.460E5b1207.140

20.460

Data
Data channel

Code

Pilot channel

CodeModulation
Modulation
Data

Data channel©2019 LINKS

X-band signal
Multiplexing

Code

Modulation

Pilot channel Code(present or not)Modulation
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GALILEO E1 SIGNALS
Carrier Frequency (MHz)

1575.420• 2 Navigation Signals (3 channels) transmitted in E1 band:➢ E1 (E1-B, E1-C): open access signals with navigation data
➢ E1P: restricted access signal (PRS)• Coherent Adaptive Sub-Carrier Modulation (CASM) multiplexing
Signal Channels

E1

E1P

©2019 LINKS

Modulation

Rc
Rd(Mcps) (sps)

Power
level
(dBW)

E1-B
Data

CBOC(6,1,1/11)

1.023

E1-C
Pilot

CBOC(6,1,1/11)

1.023

--

Min: -157
Max: 154/-150

OS ,CS(I/NAV)

E1-A
Data

BOCcos(15,2.5)

2.5575

N/A

N/A

PRS

250

Services

Multiplex.
scheme

CASM

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E1 MULTIPLEXING TECHNIQUE• CASM : Coherent Adaptative Subcarrier Modulation









2
1
~
S E1 t  
eE1 B t   eE1C t   j 2eE1 P t   eE1,int t 
3
3
INTERMODULATION PRODUCT TO ASSURE CONSTANT ENVELOPE• Relative power levels:

Channels

Beforemultiplexing

Aftermultiplexing

E1-B data

25%

22%

Q

I
E1-C pilot

25%

22%

E1-P

50%

44%

IM

--

11%

©2019 LINKS

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GALILEO E6 SIGNALS
Carrier Frequency (MHz)1278.750
2 Navigation Signals (3 channels):➢E6 (E6-B data and E6-C pilot channels): commercial
access signal➢E6P (data channel): restricted access signal (PRS)
SignalChannels

Modulation

Rc(Mcps)

Rd Power level Services
(sps)
(dBW)

E6-B DataBPSK(5)

5.115

1000

E6-C Pilot

BPSK(5)

5.115

--

Data

BOCcos(10,5
)

5.115

E6

E6P
©2019 LINKS

Min: -155
Max: -152

Multiplex.
schemeCS(C/NAV)
CASM

Min: -155
Max: -152

PRS
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GALILEO E5 SIGNALS
Carrier Frequency(MHz)

2 Navigation Signals (4 channels ):➢ E5a: open access signal containing basic data
for navigation and timing

E5
E5a

E5b

➢ E5b: open access signal containing navigationand integrity data•

E5

1191.795

E5a1176.450

E5b1207.140

AltBOC multiplexing

Signal

E5aE5b©2019 LINKS

ChannelsMHz
Modulatio
Rc
Rd
Power1191.795Servicesn(Mcps) (sps) level (dBW)

E5a-I
Data

BPSK-like

10.23

50

E5a-Q
Pilot

BPSK-like

10.23

---

E5b-I
Data

BPSK-like

10.23

250

E5b-Q
Pilot

BPSK-like

10.23

---

Min:-155
Max: -152

Multiplex.
schemeOS(F/NAV)
AltBOC(15,10)

Min:-155
Max: -152

OS/CS(I/NAV)
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ALTBOC MODULATION••

AltBOC Modulation allows the use of E5 band in two separate
sidebands (E5a and E5b)
In each sideband: 2 I-Q BPSK = 1 QPSK signal• Galileo receivers can use one or both sidebands••

Multiple codes locally generated and correlated (challenging
implementation of RX)
AltBOC is equivalent to 8-PSK

From the receiver point of view:
BPSK Signals• One or both sidebands separately• Received in non-coherent mode
• Triangular correlation• GPS receivers like©2019 LINKS

AltBOC architecture• Entire bandwidth• Coherently received• Narrower correlation• More complex structure

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GALILEO SPREADING CODE LENGTHS
•

•
•

•

©2019 LINKS

Spreading codes are used to acquire and track a specific satellite.
Each channel and satellite has a different code (CDMA)
Galileo signals features depend on their code properties
Code carefully selected by considering:• length• relation to data rates• auto/cross-correlation properties of the codes
Code lengths:• Data channels: code period duration is equal to one symbol
duration.
• Pilot channels: long pilot code periods (100 ms) to improvecross-correlation and channel isolation, as well as noise andinterference suppression.
• Tiered code construction: short primary and long secondary
codes used to build the code

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GALILEO SPREADING CODE LENGTHS

ChannelCode rate(Mcps)

Data Rate(symbol/s)

Code period(ms)Code length(chips)E5a-I data

10.23050

20204600

E5a-Q
pilot

10.230Pilot

1001023000

E5b-I data

10.230

250

4

40920

E5b-Q
pilot

10.230Pilot

1001023000

E6-B data

5.115

1000

1

5115

E6-C pilot

5.115

Pilot

100

1023000

E1-B data

1.023

250

4

4092

E1-C pilot

1.023

Pilot

100

1023000

These information are valid for signals excluded PRS service©2019 LINKS

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GALILEO NAVIGATION MESSAGE• Galileo Message Data Stream : The navigation message is transmitted in
the data stream as a sequence of frames.
• Each frame consists of a certain number (depending on the signal band) ofsubframes which contain several pages.
Frame #1

Frame #2……. Frame #N-1Frame #N Frame #1 Frame #2

Subframe #1 Subframe #2……. Subframe #M-1 Subframe #M……. Page #P-1 Page #P

Page #1

Page #2

Signals

Data rate

Page
duration# Pages in a
sub-frame# Sub-frames ina frame

F/Nav

E5a

50 sps

10 s

5

12

I/Nav

E5b E1F

250 sps

1s

30

18

C/Nav

E6B

1000 sps

1s

15

8

G/Nav

E6P E1P©2019 LINKS

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35

PAGE FORMAT
Synchro

Data

CRC

Tail

FEC encoded and interleaved(convolutional code with rate 1/2)

A three levels error coding is applied to the GALILEO Message Data
Stream:▪

A Cyclic Redundancy Check (CRC) with error detection
capabilities after recovery of the received data▪

A one-half rate Forward Error Correction (FEC). Tail Bits(sequence of zeros) to allow Viterbi decoding.

▪

Block Interleaving on the resulting frames: provides
robustness to the FEC decoding algorithm by avoiding packets
of errors

FEC and CRC are defined according to BER and FER targets.
©2019 LINKS35

GALILEO REFERENCE DOCUMENTS
https://www.gsc-europa.eu/electronic-library/©2019 LINKS

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CONTACTGABRIELLA POVERO
gabriella.povero@linksfoundation.com
www.navsas.eu
www.ismb.it

LINKS FOUNDATION
Via Pier Carlo Boggio 61 | 10138 Torino+(039) 011 2276 150©2019 LINKS@linksfoundation

linksfoundation@linksfoundation

linksfoundation.com

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