GNSS Signals

Toulouse/Montauban, France, July 16 26, 2012

INTERNATIONAL SUMMER SCHOOL ON GNSS

GNSS Signals

Christopher J. Hegarty

The MITRE Corporation

ESA INTERNATIONAL SUMMER SCHOOL ON GNSS

MITRE 2

OVERVIEW

Modulation basics GPS signals GLONASS signals GALILEO signals COMPASS signals IRNSS signals QZSS signals


MITRE 3

OVERVIEW



MITRE 4

Binary Phase Shift Keying (BPSK)

carrier

T0

=

data bits, d(t)

(+1 or -1)

BPSK signal, s(t)

(180 deg phase

shift when data

bit changes)

Td

f0= 1/T0 = carrier frequency (Hz) Rd= 1/Td = data rate (bits/s)


MITRE 5

Direct Sequence Spread Spectrum

=

carrier

spread spectrum

waveform

Tc

Td

data waveform

modulated spread

spectrum signal

Rc= 1/Tc = chipping rate (chips/s)

T0

f0= 1/T0 = carrier frequency (Hz)

Rd= 1/Td = data rate (bits/s)


MITRE 6

Autocorrelation and Power Spectrum

Let s(t) be a BPSK signal created with random data waveform d(t).

Autocorrelation of data waveform:

else

TT

tdtdER

dd

,0

,1

)()()(

Power spectrum:

2

2

2

)(

)(sin

)()(

d

dd

fj

fT

fTT

deRfS

Power spectrum describes how total power in signal is distributed in frequency domain.

Note that ~90% of a BPSKs signal power is within +/-Rd Hz of carrier.

R()

0 Td -Td

S(f)

f Rd -Rd


MITRE 7

Why Spread?

Direct sequence spreading allows precise ranging

Use of different spreading waveforms for each satellite can provide a multiple access capability Multiple satellites can broadcast ranging signals at same

frequencies

Interference rejection


MITRE 8

DSSS Autocorrelation

Tc

Received signal:

Receiver replica:

Received signal

Receiver replica

Integrate & Dump

Out Out

-Tc Tc

Autocorrelation

+1

-1 -1


MITRE 9

DSSS Cross-correlation

Tc

Received signal (SV

j):

Receiver replica (SV

kj):

Received signal

Receiver replica

Integrate & Dump

Out

Out

Cross-correlation

+1

-1 -1

One code selection goal is to select codes for each satellite to minimize cross-correlation.


MITRE 10

Pseudorandom Sequences

Sequence of bits generated at chip rate to produce spread spectrum waveform Periodic for open (unencrypted) signals, aperiodic

for encrypted signals

Desired attributes: Good autocorrelation and cross-correlation

properties low amplitude sidelobes Balanced equal number of ones and zeros

Also known as pseudorandom noise (PRN) sequences, spread spectrum sequences

Often generated with linear feedback shift registers (LFSRs)


MITRE 11

LFSR Example

One Code Period

1 2 3 4

Code length = 24 - 1 = 15

G(x) = 1 + x1 + x4

Note: state (0,0,0,0) does not occur

State 1 2 3 4

1 1 1 1 1

2 0 1 1 1

3 1 0 1 1

4 0 1 0 1

5 1 0 1 0

6 1 1 0 1

7 0 1 1 0

8 0 0 1 1

9 1 0 0 1

10 0 1 0 0

11 0 0 1 0

12 0 0 0 1

13 1 0 0 0

14 1 1 0 0

15 1 1 1 0

1 1 1 1 1 Code repeats


MITRE 12

Some PRN Sequence Families

m is an arbitrary positive integer

Source: No and Kumar, IEEE Trans. Info. Theory, March 1989.


MITRE 13

Binary Offset Carrier Modulation

=

Carrier

Spreading code

Square wave

Data

BOC signal*

*Shown at baseband, i.e., without carrier.

Tsq

fsq= 1/Tsq = subcarrier frequency (Hz)

By convention, BOC(m,n) refers to a binary offset carrier modulation with m 1.023 MHz square wave subcarrier frequency and a n 1.023 MHz chipping rate.


MITRE

Sine-phased vs Cosine-Phased BOC

14

Spreading code

Square wave

Sine-phasing

Spreading code

Square wave

Cosine-phasing


MITRE

Multiplexing

Many GNSS satellites broadcast two or more signals on each carrier frequency

So that efficient switching-class amplifiers can be used on the spacecraft, multiplexing techniques that maintain constant envelope are preferred

Such techniques include: Phase quadrature for two signals on a carrier frequency, one

multiplies a sine the other a cosine

Interplexing for three signals (Butman and Timor, IEEE Trans Comm., 1972)

Majority vote for any odd number of signals

Time division multiplexing

Alternative BOC (ALTBOC)

15


MITRE

Constant Envelope

16

Constant envelope

Not constant envelope


MITRE

Dataless (Pilot) Components

Many modern GNSS signals include a component that is not modulated by navigation data Both signal components are still modulated by a PRN

Motivation allows carrier phase to be tracked using a phase locked loop (PLL) instead of a Costas loop A PLL can reliably track in 6 dB lower signal-to-noise ratio (SNR)

conditions

So, if one-half of the signal power is devoted to a dataless component, there is a net 3 dB SNR benefit

Data demodulation suffers a power loss, but this can be overcome by forward error correction

Data-modulated and dataless components are multiplexed on same carrier

17


MITRE

Secondary Codes

Secondary codes with lengths up to 1800 bits are used in many modern GNSS signal designs Each repetition of the spreading waveform is kept as is or

inverted following a deterministic pattern

Also referred to as synchronization code

Benefits: Reduces cross-correlation between signals

Helps receiver synchronize with data bits

Reduces impact of narrowband interference


MITRE 19

Polarization

Current and planned GNSS signals are right hand circularly polarized (RHCP)

User antenna should be also

+

+

+

-

-

-

+

+

+

-

-

-

Linear Polarization

1

0

RHCP

E-field


MITRE 20

Relativistic Effects

GNSS satellite clocks are set slow to appear at the desired

frequencies to an observer on the ground.

Source: Ashby, N., www.livingreviews.org


MITRE 21

OVERVIEW



MITRE 22

GPS Navigation Signals

Today - 2 navigation frequencies, 3 signals L1 = 1575.42 MHz (154 10.23 MHz)

Coarse Acquisition (C/A) code

Precision (P(Y)) code

L2 = 1227.6 MHz (120 10.23 MHz) P(Y) code

Future - 3 navigation frequencies, 8 signals L1 C/A, C, P(Y), and M-code

L2 C, P(Y), and M-code

L5 = 1176.45 MHz (115 10.23 MHz)

L2C, M-code, and L5 are being broadcast by a growing subset of the satellites in the

GPS constellation.


MITRE 23

GPS Signal Evolution

1227 MHz 1575 MHz 1176 MHz

L2 L1 L5

P(Y)

P(Y)

C/A

P(Y)

L2C M M

Present Signals

Signals After

Modernization

C/A

P(Y)

L1C


MITRE 24

GPS Spreading Codes

Signal Chipping Rate Carrier frequency Comments

(Mchip/s) (MHz)

C/A 1.023 1575.42 (L1) 1023 chip Gold codes repeat

every 1 ms

L2C 1.023 1227.6 (L2) 2 codes per SV each at 511.5

kHz, future

P(Y) 10.23 L1 and L2 Repeats 1/week. When P-

code is encrypted, referred to

as Y-code

L5 10.23 1176.45 (L5) 2 codes per SV, future

M 5.115 L1 and L2 BOC(10,5) modulation

future

L1C 1.023 L1 BOC(1,1)/BOC(6,1) - future


MITRE 25

Signal Power Spectra

Notes: (1) C/A codes actually have line spectra - continuous approximation shown.

(2) L5 signal spectrum resembles P(Y), except that L5 is also a line spectrum.

-15 -10 -5 0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1x 10

-6

Offset from Carrier Frequency (MHz)

Norm

aliz

ed P

ow

er S

pectrum

(W

/Hz)

C/A or L2C

L1C

P(Y)-code M-code


MITRE 26

C/A Code (PRN2) Spectrum

2000190018001700160015001400130012001100100090080070060050040030020010000-100

-90

-80

-70

-60

-50

-40

-30

-20

Frequency Offset from L1 (kHz)

Pow

er

Spe

ctr

al

Dens

ity (

dB

c/H

z)


MITRE 27

Spacecraft Signal Generation

Frequency Synthesizer

(10.23 MHz)

Navigation Data

Unit

C/A

P(Y)

L1 modulator/

Power Amplifiers/

Synthesizer

L2 modulator/

Power Amplifiers/

Synthesizer

Atomic clocks

Combiner

Phased

array

antenna

Nuclear detonation

detection

signal (L3)

Timing for all signals derived from 10.23 MHz atomic clock-based frequency

synthesizer. Note that C/A and P(Y) are in quadrature on L1.


MITRE 28

C/A Code Generation

SHIFT REGISTER

G GENERATOR 1

1 2 3 4 5 6 7 8 9 10

G GENERATOR 2

SET TO

"ALL ONES"

+

+

+

1 2 3 4 5 6 7 8 9 10

1.023

MBPS

CLOCK

SHIFT REGISTER

+

G = ---10101111111111 1

G = ---01001111111111 2

1023

DECODE

20 50 BPS DATA

CLOCK

G EPOCH

1kBPS

GOLD CODE XG (t)

C/A CODE i

XG

C/A CODE 2 2i G

S 1 S 2

PHASE

SELECTOR


MITRE 29

C/A Code Timing Relationships

1023 etc.

X1 Epoch @ 2/3 bps

0 1 2 18 19 0

1 ms 1023 BIT Gold Code @ 1023 kbps

1023 1023 1023 1023

Gold Code Epochs @ 1000/s

Data @ 50 cps

20 ms


MITRE 30

P-code Generation

Generator based on four

12-stage shift registers

10.23 Mchips per second

Reset once/week

For details, see Interface

Specification IS-GPS-200F


MITRE 31

Received Minimum Signal Levels

-155.5

-158.5

-161.5

-164.5

0 o 5 o 20 o 40 o 60 o 80 o 100 o 90 o

USER ELEVATION ANGLE (DEG)

RE

CE

IVE

D P

OW

ER

OU

T O

F 3

dB

il U

SE

R A

NT

EN

NA

(dB

W)

C/A - L 1

P - L 1

P - L 2 or

C/A - L 2


MITRE 32

Typical GPS L1 C/A Link Budget

Power in dBW = 10 log10 (Power in W)

Power in dBm = 10 log10 (Power in mW)

1 mW = 0.001 W = 0 dBm = -30 dBW

EARTH

Free space

path loss:

-184.7 dB

Transmit power:

40 W

= 16 dBW

Antenna gain:

12 dB

Received Signal:

2 10-16 W = -157 dBW

Received signal power is less than the thermal noise power in the receiver.

Thermal Noise

(2 MHz bandwidth):

1.4 10-14 W = -138.5 dBW


MITRE 33

C/A and P(Y) Navigation Data

Each subframe is 300 bits (6 s @ 50 bps). Entire message repeats every 12.5 min

(5 subframes 300 bits/subframe 25 pages = 37500 bits/message)


MITRE 34

L2C Characteristics Summary

L2 = 1227.6 MHz Minimum received power = -160 dBW PRN code chipping rate = 511.5 kHz for each

of two codes

Time Division Multiplexed (TDM) Signal Chip by chip multiplexing of two PRN sequences

Total chip rate: 1.023 MHz

Specification: IS-GPS-200F


MITRE 35

L2 Civil Signal Definitions

L2C the L2 civil signal CM the L2C moderate length code

10,230 chips, 20 milliseconds

CL the L2C long code 767,250 chips, 1.5 second

NAV the legacy navigation message provided by C/A and P(Y)

CNAV improved L2C and L5 navigation data message format and contents


MITRE 36

IIF L2C Signal Generation

C/A Code

Generator

10,230 Chip

Code Generator

767,250 Chip

Code Generator

L5-Like CNAV

Message

25 bits/sec

Chip by Chip

Multiplexer

1.023 MHz

Clock

Transmitted

Signal1/2

A1

A2B1

B2

Rate 1/2 FEC

Legacy NAV

Message

50 bits/sec

511.5 kHz Clock

CM

Code

CL

Code


MITRE 37

L2C Code Characteristics

Codes are disjoint segments of a long-period maximal length code

27-stage linear feedback shift register with multiple taps is short-cycled to get desired

period

Selected to have perfect balance

Separate shift registers for each of the two codes

1 cycle of CL & 75 cycles of CM every 1.5 s


MITRE 38

L2C Code Generator

DELAY

NUMBERS

SHIFT DIRECTION

OUTPUT

INITIAL CONDITIONS ARE A FUNCTION OF PRN AND CODE PERIOD (MODERATE/LONG)

1 3 1 1 3 3 2 3 3 2 2 3

Linear shift register generator with 27 stages and 12 taps


MITRE 39

L5 Characteristics Summary

L5 = 1176.45 MHz Minimum received power = -154.9 dBW Code chipping rate = 10.23 MHz QPSK Signal

In-Phase (I5) = Data Channel

Quadraphase (Q5) = Data-Free Channel

Equal Power in I5 and Q5 (-157.9 dBW)

Independent spreading codes on I5 and Q5

Specification: IS-GPS-705


MITRE 40

L5 Characteristics Summary (contd)

I and Q Modulation (1 kbps) Forward Error Correction (FEC) encoded 50 bps data on I5

(100 sps)

Further encoded with 10-bit Neuman-Hofman Code

Q5 encoded with 20-bit Neuman-Hofman Code


MITRE 41

L5 Codes

Codes with 2 - 13 stage shift registers Length of one (XA code) = 8190 chips

Length of second (XB code) = 8191 chips

Exclusive-ord together to generate longer code

Chipping rate of 10.23 MHz Reset with 1 ms epochs (10,230 chips)

Two codes per satellite (4096 available) One for I5, one for Q5


MITRE 42

L5 I and Q Code Generators

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

Exclusive OR

Initial XBI State

Exclusive OR

All 1's

1 ms Epoch

Code Clock

XA(t)

XBI(t+niT

c)

XIi(t)

XA Coder

XBI Coder

XBI State for SV i

ResetXQ

i(t)

XBQ(t+niT

c)

1 2 3 4 5 6 7 8 9 10 11 12 13

Initial XBQ State

Exclusive OR

XBQ Coder

XBQ State for SV i

Decode 1111111111101

Reset to all 1s on next clock


MITRE 43

L5 Neuman-Hofman Codes

Encoded symbols and carrier Modulate at PRN code epoch rate

Spreads PRN code 1 kHz spectral lines to 50 Hz spectral lines (including FEC)

Reduces effect of narrowband interference by 13 dB

Reduces SV cross-correlation most of the time

Provides more robust symbol/bit synchronization


MITRE 44

10-ms Neuman-Hofman Code on I5

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10

Code Delay - Milliseconds

Neu

man

-Ho

ffm

an

Co

de V

alu

e


MITRE 45

20-ms Neuman-Hofman Code on Q5

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Code Delay - Milliseconds

Neu

man

-Ho

ffm

an

Co

de V

alu

e


MITRE 46

L5 Data Content and Format

Six-Second 300-bit Messages Format with 24-bit cyclic redundancy code (CRC)

(same as satellite-based augmentation systems)

Convolutionally encoded: rate , length-7

Messages scheduled for optimum receiver performance

Lined up with L1 sub-frame epochs


MITRE 47

M-code

Unlike current GPS signals, M-code is generated with four components 10.23 MHz square wave, in addition to carrier, spreading

waveform, and data

Creates an effect similar to amplitude modulation - double sideband (AM-DSB) i.e., moves signal energy away from carrier to upper and lower

sidebands


MITRE 48

M-code Generation

=

Carrier (L1 or

L2)

5.115 Mchip/s

spreading code

10.23 MHz

square wave

Data

M-code signal*

*Shown at baseband, i.e., without carrier.


MITRE 49

M-code Autocorrelation

Note the presence of multiple peaks due to the square wave subcarrier.

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Auto

corr

ela

tion

Delay (microseconds)


MITRE 50

L5, L2C, and M-code Nav Data

Improvements made to clock and ephemeris representation

Clock resolution significantly enhanced Legacy message resolution ~ .5 ns

Ephemeris Resolution enhanced

Rate terms added for semi-major axis, mean motion, and inclination improve curve fit


MITRE 51

L1C

New L1 civil signal on GPS IIIA+ Interoperable with GALILEO L1 signal

Modulation is multiplexing of BOC(1,1) and BOC(6,1) symbols referred to as multiplexed BOC (MBOC)

Planned features: Dataless component, powerful forward error correction,

length-10230 PRN codes

Specified in IS-GPS-800


MITRE 52

L1C MBOC

-15 -10 -5 0 5 10 15-95

-90

-85

-80

-75

-70

-65

-60

-55

Frequency (MHz)

Pow

er S

pect

ral D

ensi

ty (d

BW

/Hz)

C/A Code

BOC(1,1)

TMBOC

-15 -10 -5 0 5 10 15-95

-90

-85

-80

-75

-70

-65

-60

-55

Frequency (MHz)

Pow

er S

pect

ral D

ensi

ty (d

BW

/Hz)

C/A Code

BOC(1,1)

TMBOC

(1,1) (6,1)29 4

33 33Pilot BOC BOCf f f

(1,1)Data BOCf f

(1,1) (6,1)

3 1

4 4

10 1

11 11

Signal Pilot Data

BOC BOC

f f f

f f

25% Power Data Component

75% Power Pilot Component

BOC(1,1) BOC(6,1)


MITRE


MITRE 54

OVERVIEW



MITRE 55

Current GLONASS Signals

DSSS modulation Frequency Division Multiple Access (FDMA)

scheme with multiple carriers in two sub-bands

Standard accuracy signal - 511 kHz chip rate, length-511 maximal-length codes

High accuracy signal 5.11 MHz chip rate, encrypted

Data at 50 bps, Manchester-encoded

1 1602 0.5625 MHzKf K

2 17 / 9K Kf f


MITRE 56

Standard Accuracy PRN Generation

See GLONASS Interface Control Document, version 5.1 for further signal details.


MITRE 57

Evolution of GLONASS Signals

GLONASS-K1 (launched Feb 2011) is broadcasting a test 10.23 MHz chip

rate CDMA signal at 1202.025 MHz. CDMA signals for L1, L2, L3 and

planned for future satellites.

Frequency (MHz)

L1 (~1593 1612) L2 (~1238 1593) L3

(1164 1215 MHz band)


MITRE 58

OVERVIEW



MITRE 59

Galileo Frequency Bands


MITRE 60

Frequencies and Power Levels

CS = Commercial Service

SoL = Safety of Life Service

Note that carrier frequencies were selected to be integer

multiples of 10.23 MHz for interoperability with GPS.


MITRE 61

PRN Codes

E5 primary codes may be generated with LFSRs or stored in memory

E1 primary codes are stored in memory E6 codes are not disclosed in Galileo OS ICD


MITRE 62

E5 Signal Characteristics

E5 is a wideband signal, centered at 1191.795 MHz Generated using alternative BOC (AltBOC) technique Similar in appearance to two coherently generated DSSS

signals with 10.23 MHz chip rates and centered at +/-

15.345 MHz from 1191.795 MHz

E5a at 1176.45 MHz E5b at 1207.14 MHz E5a and E5b each have data and pilot components


MITRE 63


E6 includes 3 components: A (for Public Regulated Service), B & C (for Commercial Service)

CS signal is DSSS modulated with 5.115 MHz chip rate High rate data: 500 bps/1000 sps Data and pilot components


MITRE 64


E1 includes 3 components: A (for Public Regulated Service), B & C (for Open, Safety of Life, and Commercial

Services)

B&C components represent Galileos implementation of MBOC (interoperable with GPS L1C)

Composite BOC (CBOC) technique used to achieve mixture (in power) of 10/11 BOC(1,1) and 1/11 BOC(6,1)

Data (125 bps/250 sps) and pilot components (50-50% power split)


MITRE 65

OVERVIEW



MITRE

COMPASS Signals

66


MITRE 67

OVERVIEW



MITRE

IRNSS Signal Characteristics

68


MITRE 69

OVERVIEW



MITRE

QZSS Signal Characteristics

70


MITRE

Summary of GNSS Signal Plans

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

Future CDMA signal

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)

SBAS

QZSS (Japan)

IRNSS (India)

COMPASS (China)

Galileo (Europe)

GLONASS (Russia)

GPS (US)

L1 L5 L2

Compass & IRNSS In S-band

1560 1570 1580 1590 1600 16101170 1180 1190 1200 1210 1220 1230 1240 1250 1260 1270 1280 1290 1300

Frequency (MHz)


MITRE 72

1. Kaplan, E., and C. Hegarty (Eds.), Understanding GPS: Principles and Applications, 2nd Edition, Artech House, 2006.

2. Misra, P., and P. Enge, Global Positioning System: Signals, Measurements, and Performance, 2nd Edition, Ganga-Jumana Press, 2006.

3. GPS Interface Specifications, available from www.gps.gov 4. GALILEO Open Service Signal in Space Interface Control

Document, available from www.gsa.europa.eu

5. GLONASS Interface Control Document, available from www.glonass-ianc.rsa.ru

6. COMPASS information from: www.unoosa.org/pdf/icg/2010/ICG5/18october/04.pdf

7. IRNSS information from: www.unoosa.org/pdf/icg/2010/ICG5/18october/05.pdf

8. QZSS ICD available from: qzss.jaxa.jp/is-qzss/index_e.html

References

Documents

GNSS Signals