gnss signals & satellite position -...

2010‐05‐27

1

Global Navigation Satellite System‐ Signals & Satellite Orbit‐

Dr Falin WuSchool of Instrument Science and Opto‐electronics Engineering,

Beijing University of Aeronautics and Astronautics

[email protected]

GNSS Signals & Satellite Orbit

• GNSS Signals

– Global Positioning System

– Galileo & Other Navigation Satellite System

• GNSS Satellite Orbit

2

Global Satellite System

• Modulations for Satellite Navigation

• Legacy GPS Signals

• Navigation Message Format

• Modernized GPS Signals

3

Modulation Types ‐ BPSK ‐

• BPSK: Binary Phase Shift Keying– Change in code state causes a 180 degree phase shift in carrier.

– GPS uses BPSK to modulate the codes on to the carrier.

4

Modulation Types ‐ DSSS ‐

• DSSS: Direct Sequence Spread Spectrum

– An extension of BPSK or other phase shift keyed modulation used by GPS and other satellite navigation systemsg y

5

Why DSSS?

• The frequency phase inversion in the signal introduced by the PRN waveform enable precise ranging by the receiver

• The use of different PRN sequences from a well‐designed set enables multiple satellites to transmit signals simultaneously and at the same frequency (CDMA)

• DSSS provides significant rejection of narrowband interference

• BPSK‐R: DSSS signals generated using BPSK signaling with rectangular chips– BOC (Binary Offset Carrier) signals: Generated using DSSS techniques

but employ portions of a square wave for the spreading symbols

6

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Multiplexing Techniques

• FDMA/FDM: Frequency Division Multiple Access / Frequency Division Multiplexing

• TDMA/TDM: Time Division Multiple Access / Time Division Multiplexing

• CDMA/CDM: Code Division Multiple Access / Code Division Multiplexing– Two binary DSSS signals: Quadrature Phase Shift Keying (QPSK)

– Three binary DSSS Signals:

– More than two binary DSSS signals while retaining constant envelope• Majority vote

• Intervoting

7

Signal Models & Characteristics

• General quadrature signal Complex‐envelope

• Autocorrelation function for a lowpass signal with• Autocorrelation function for a lowpass signal with constant power

• Power spectral density: Fourier transform of the autocorrelation function

8

Signal Models & Characteristics – Random binary code ‐

• A random binary code producing

• Autocorrelation function

9

Signal Models & Characteristics– Random binary code ‐

• Power spectral density

• Pseudo random codes (PRN)?

– GNSS employing rectangular chips • similar autocorrelation and power spectrum properties to those described for the random binary code case

– PRN codes are perfectly predictable and reproducible.

10

• A DSSS signal generated from a maximum‐length PRN sequence

– A DSSS signal without data employing a PRN sequence that repeats every N bits ( )

Signal Models & Characteristics– DSSS Signal ‐


11

Signal Models & Characteristics– DSSS Signal ‐

• Line spectrum

12

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Signal Models & Characteristics– General baseband DSSS Signal ‐

• General baseband DSSS signal

– Autocorrelation function

• BPSK R signal• BPSK‐R signal

– Autocorrelation function

– Power spectrum

• BPSK‐R(n) – Pseudorandom Code Rate of n1.023 MHz

13

Signal Models & Characteristics– BOC ‐

• BOC (Binary Offset Carrier)


14

Signal Models & Characteristics– BOC ‐


– Sine‐phased BOC

– Cosine‐phased BOC

15

Signal Models & Characteristics– BCS ‐

• BCS: Binary Coded Symbol



16

Signal Models & Characteristics– BOC & BCS

• Why BOC & BCS?

– BPSK‐R: Only allow the signal designer to select carrier frequency and chip rate

– BOC & BCS: Provide additional design parametersBOC & BCS: Provide additional design parameters for wave designers to use

• Enhanced performance when bandwidth is limited

• Better share limited frequency bands available for use by multiple GNSS

• Shape the spectra to limit interference

17

BPSK/BOC Modulation BiBi--Phase Shift Keying (BPSK) ModulationPhase Shift Keying (BPSK) Modulation

Binary Offset Carrier (BOC) ModulationBinary Offset Carrier (BOC) ModulationEach Pseudorandom Chip Multiplied by Binary CarrierEach Pseudorandom Chip Multiplied by Binary Carrier

Pseudorandom Code

BPSK(5)BPSK-R(k) – Pseudorandom Code Rate of k1.023 MHz

p p y yp p y yBOC(BOC(k,jk,j) ) –– Binary Carrier Frequency of jBinary Carrier Frequency of j1.023 MHz1.023 MHz

BOCcos(15,2.5) BOC(1,1)BOC(10,5)

Bin

ary

Car

rier

Sine Carrier BOC(m,n)

Cosine Carrier BOCcos(m,n)

Note: data signals additionally multiplied by binary data stream

18

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BCS Modulation 19 20BCS Modulation






21

Legacy GPS Signals

• GPS satellites send very weak radio signals on two L – band frequencies (L1 and L2)

• L1 and L2 are carrier frequencies.L1 and L2 are carrier frequencies.

These are sinusoidal signals

22

C/AC/A

P(Y)P(Y)

Legacy GPS Signals• All GPS satellites use the same frequency carriers (L1 and L2)– L1: 1575.42 MHz

• Modulated by C/A‐code & P‐code• Signal Power: ‐160 dBW

– L2: 1227.6 MHz• Modulated by P code only• Modulated by P‐code only• Signal Power : ‐166 dBW

• But each satellite has its own identification code• These are two types of codes modulating the L1 and L2 carriers.– C/A – Code– P – Code

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L1 1575 MHz

L2

P(Y)P(Y)

1227 MHz

Legacy GPS Satellite Signal Generator

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Legacy GPS Satellite Signal Generator

2, 2 2sini i iL t B P t D t t

25

1, 1 1

1

cos

2 sin

i i i

i i

L t A P t D t t

A G t D t t

GPS Signal Structure for L1

• P(Y) code signal = long secure code with 50‐bps data

• C/A code signal = 1023 chip Gold / g pcode with 50‐bps data

26

1 1

1

cos

2 sin

i i i

i i

L t A P t D t t

A G t D t t

GPS Code Mixing with Data

10.23 Mbps

27

1.023 Mbps

GPS L1 Carrier Modulationa) L1 carrier (0° phase)

b) L1 carrier (90 ° phase)

c) P(Y) code ⊕ data

d) C/A code ⊕ datad) C/A code ⊕ data

e) P(Y) code ⊕ data BPSK modulated on L1 carrier (0 ° phase) with 3-dB attenuation

f) C/A code ⊕ data BPSK modulated on L1 carrier (90 ° phase)

g) Composite modulated L1 carrier signal

28

GPS Code Generators

29

C/A Code Generator

• Two 10‐bit shift registers designated G1 & G2

30

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P Code Generator

• Four 12‐bit shift registers designated X1A, g ,X1B, X2A, & X2B

31

GPS Code Generator Polynomials and Initial States

32

Code Phase Assignments & Initial Code

Sequences for C/A Code and P Code

33

GPS SignalsCarriers (L1/L2)

Binary Phase Shift Keying (BPSK) Modulation

C/A ‐ Code (L1)

P ‐ Code (L1/L2)

34

Phase Quadrature

SA Degredation

Nav Data (L1/L2)

A‐S Encryption P – P(Y)

Power Levels‐User Received Minimum Signal Power Levels‐

• Minimum Received GPS Signal Power

• User received minimum signal power levels

35

Power Levels(Block II)

36

‐158.5‐3.0+184.4+0.5+3.4=26.8dBW

1lg 26.8 13.4 /10 21.9W

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Autocorrelation Functions ‐ C/A ‐

37

• Spectrum

Power Spectral Densities‐C/A ‐

• Power ratio of a typical C/A code

38

Autocorrelation Function‐ P(Y) ‐

39

Comparisons Between C/A Code & P(Y) Code Autocorrelation

40

C/AC/A

P(Y)P(Y)

Power Spectral Densities

Power spectrum of L1 P(Y) & C/A codes

P(Y)P(Y)

41

Power spectrum of L2 P(Y) code

Power spectrum of L1 C/A code showing the line spectrumof the C/A code

Cross‐Correlation Function

• Cross‐correlation function

• Each SV PRN code used in the CDMA system must be minimally cross‐correlated with another SV’s PRN code for any phase or Doppler shift combination

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PRN Code Auto/Cross Correlation

43

GPS Signals

• GPS receivers generate the equivalent of these codes internally and compares with the ones coming from the satellites.

• GPS receiver shifts the internally generated code until it matches with the received one (cross‐correlation)

44






45

GPS Signals

• Another Message on the L1 & L2 carrier frequency is the “Navigation Message”

• Navigation Message

50 H Cl k t– 50 Hz Clock rate

– Has information specific for each satellite

– Has the satellite position and time delay information

46

Navigation Message Format

• 50 Hz navigation message

– 6 s for one subframesubframe

– 30 s for one frame

– 12,5 min for the whole set

47

GPS Navigation Data Structure

48

• Each subframe containing 300 bits lasts 6 s. • Subframes 1, 2, and 3 repeat every 30 s while subframes 4 & 5

have 25 versions before repeating. • The entire navigation message repeats after 12.5minutes.

Courtesy: Frank van Diggelen.

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49

GPS Modernization Program

• Need for upgrades recognized as GPS entered Full Operational Capability– Anti‐jam military needs

– Better, more reliable civilian service

– Recognized growing importance of GPS to both sectors

d l l d / d l• 1996 Presidential policy and 1998/1999 Vice Presidential announcements committed U.S. to modernization and improvement path– New signals, better service ( no direct user fees)

– Selective Availability (SA) discontinued

– Over $1 billion added to future U.S. GPS investment

50

Basic GPS

L2C on L2M-Code (Earth)

NAVWAR CapableFull Civil Rqmts

Add’l CapabilitiesNew Civil Signal – L5

GPS Modernization at a GlanceIncreasing

System CapabilitiesIncreasing

Civil/Defense Benefit

SA Setto 0

GPS IIA/IIR

GPS III

GPS IIR-M, IIF

IIR-M: Improved on all IIA capabilities and added

• 2nd Civil Signal on L2• New L1 & L2 M-CodeIIF: IIR-M capability and:• Add 3rd Civil Signal on L5

• Standard Service (~100 m)• Precise Service (~16 m)• Two Nav frequencies

L1: Civil (C/A) &Precise (P) CodeL2: P-Code

GPS-III:• Improved Anti-jam (+20dB)• Increased Accuracy• Greater Availability• Controlled Integrity• Greater Survivability Other Transformational needs • Blue Force Tracking• Nav-related Messaging• Responsive Ops

51

The End of Selective AvailabilityMay 2, 2000

0

20

4060

80

100120

140

160

or

(met

ers)

Horizontal Error (meters)

Vertical Error (meters)

2 May 2000Colorado Springs, Colorado

SPS CEP AFTER TRANSITION: 2.8 metersSPS SEP AFTER TRANSITION: 4.6 meters

52

-200-180

-160

-140-120

-100

-80-60

-40

-200

0 1 2 3 4 5 6 7 8 9 10

Time of Day (Hours UTC)

Inst

anta

neo

us

Err

o

ANALYSIS NOTES

- Data taken from Overlook PAN Monitor Station, equipped with Trimble SVeeSix Receiver- Single Frequency Civil Receiver- Four Satellite Position Solution at Surveyed Benchmark- Data presented is raw, no smoothing or editing

P(Y)P(Y)

C/AC/A

Modernized Signal Evolution

C/AC/A

P(Y)P(Y)

P(Y)P(Y)

P(Y)P(Y)

MML2CL2CMM

Legacy Legacy SignalSignal(Block II/IIA/IIR)(Block II/IIA/IIR)

22ndnd Civil; MCivil; M--CodeCodeBlock IIRBlock IIR--MM

33rdrd CivilCivilBlock IIFBlock IIF

C/AC/A

P(Y)P(Y)

MM

P(Y)P(Y)

L2CL2CMM

1176 MHz1176 MHz(L5)(L5)

1227 MHz1227 MHz(L2)(L2)

1575 MHz1575 MHz(L1)(L1)

53

Modernized GPS Signals

• L1

• L2

• L5

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New Civil Signal Rollout

• Second Civil Signal (L2C) ‐ Block IIR‐M Satellites– First launch in 2003, then every satellite thereafter– Provides a redundant signal for civil users

• Improved continuity in case L1 signal reception is lost• Improved accuracy via dual‐frequency ionosphere correction• Wide‐lane for extremely‐precise local area differential GPSWide lane for extremely precise local area differential GPS

• Third Civil Signal (L5) ‐ Block IIF Satellites– First launch in 2005, then subsequent satellites thereafter– Provides redundant dual‐frequency capability for civil users

• Improved continuity in case L1 or L2 signal reception is lost• Improved accuracy via triple‐frequency ionosphere correction• Tri‐lane for ultra‐precise local area differential GPS

– Provides an interference‐resistant signal for civil users

55

New Civil Code on L2

• Advantages of a New Signal

– Improved cross‐correlation properties

– Improved tracking capability ‐‐ 3dB higher power than Coarse Acquisition (C/A) on L2

• Signal Characteristics– Two codes: one with data (medium code); one without (long code)

– Codes longer than C/A code to minimize cross correlation

– Separated by time ‐‐ Time Division Multiplexed (TDM)

– Overcome some limitations of current C/A coded signals

56

Baseband L2C Signal Generator

• Two different PRN codes per satellite

– CM (civil moderate) code: 10,230 chips

– CL (civil long) code: 767,250 chips

• L2C overall chip rate: 2x511.5‐kchip/s rate = 1.023 Mchip/s

• L2C signal has a similar power spectrum to the C/A code (2.046 MHz null‐to‐null bandwidth)

57

Baseband L2C Signal Generator

• CM and CL PRN code generation

– 27‐stage linear feedback shift register

• L2C data convolution encoder

– A rate one‐half constraint‐length 7 FEC code

– 25‐bps ‐> 50 sps

58

L5 Signal Generation• QPSK: an in‐phase signal component (I5) & a quadraphase

signal component (Q5)

• I5 & Q5: Different length‐10,230 PRN codes

• I5: Modulated by 50‐bps navigation data

• I5 & Q5: 10.23‐MHz chipping rate, a 1‐ms code repetition period

59

I5 & Q5 PRN Code Generation

• Three 13‐bit linear feedback shift registers

• Every 1 ms, the XA coder is initialized tocoder is initialized to all 1s

• Simultaneously, the XBI and XBQ coders are initialized to different values to yield the I5 & Q5 PRN codes

60

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M Code Signal Generation

• M code is designed exclusively for military use & is intended to eventually replace the P(Y) code

• Improved security + support anti‐jam resistance

• Enhanced tracking and data demodulation performance, robust acquisition, and compatibility with C/A code and P(Y) code

( )• BOCs(10,5): – the frequency of an underlying squarewave subcarrier = 10x1.023 MHz,

– the underlying M code generator code chipping rate = 5x1.023 Mchip/s

61

GPS Modernization

Block IIA/IIR Block IIR-M IIF Block III

Increasing System Capabilities Increasing Defense / Civil Benefit

62

Block IIA/IIR Block IIR M, IIF Block III

Basic GPS• Std Service (16-24m SEP)– Single frequency (L1)– Coarse acquisition (C/A)

code navigation• Precise Service (16m SEP)– Y-Code (L1Y & L2Y)– Y-Code navigation

IIR-M: IIA/IIR capabilities plus• 2nd civil signal (L2C)• M-Code (L1M & L2M)

IIF: IIR-M capability plus• 3rd civil signal (L5)• Anti-jam flex power

Block IIIA:• Increased anti-jam power• Increased security• Increased accuracy• Navigation surety• Backward compatibility• Assured availability• Controlled integrity• System survivability• 4th civil signal (L1C)

Source: US National Space Based Coordination Office (www.pnt.gov)

Summary of GPS Signals

63


• GNSS Signals




64

Galileo Frequencies

• Each satellite transmits 6 navigational signals over 4 carrier frequencies

• The Carriers are:– E5a (1176.450 Mhz)E5a (1176.450 Mhz)

– E5b (1207.140 Mhz)

– E6 (1278.75 Mhz)

– E2‐L1‐E1 (1575.42 Mhz) (same frequency as GPS L1)

65

Galileo Signals in Space

1164.00

1215.00

E5

1260.00

1300.00

E6

1563.00

1587.00

L1

1559.00

E2

1591.00

E1

1544.10

L6

66

SAR downlink

f [MHz]

ARNS960 MHz 1214 MHz

1151 MHz 1300 MHz

RNSS

1559 MHz 5250 MHz

1559 MHz 5030 MHz

RNSS

ARNS

ARNS – Aeronautical Radio Navigation ServiceRNSS – Radio Navigation Satellite Service

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Navigation Signal Properties

SignalRanging

Code Rate (Mcps)

Sub-Carrier Frequency

Primary Code

Length

Secondary Code

Length

Data Rate (SPS)

Encrypted

L1F-d 1.023 1.023 4092 None 250 No

L1F-p 1.023 1.023 4092 25 - No

L1P 2.51.023 151.023 - - - Yes

E5a-d 101.023 - 10230 20 50 No

E5a-p 101.023 - 10230 100 - No

E5b d 10 1 023 10230 4 250 N

67

E5b-d 101.023 - 10230 4 250 No

E5b-p 101.023 - 10230 100 - No

E6C-d 51.023 - - - 1000 Yes

E6C-p 51.023 - - - - Yes

E6P 51.023 101.023 - - - YesNotes: L1 signals multiplexed using Coherent Adaptive Sub-carrier Modulation (also known as Interplex or

Modified Tricode Hexaphase) giving a constant signal envelope prior to satellite’s high power amplifier.

Low rate (one chip per primary sequence) Secondary Codes multiply Primary Codes. This improves signal cross-correlation and can aid data bit edge detection.

Data rate expressed in Symbols Per Second (i.e. after Forward Error Correction)

BPSK/BOC Modulation Bi-Phase Shift Keying (BPSK) Modulation

Binary Offset Carrier (BOC) ModulationEach Pseudorandom Chip Multiplied by Binary Carrier

Pseudorandom Code

BPSK(5)BPSK(k) – Pseudorandom Code Rate of k1.023 MHz

68

p p y yBOC(k,j) – Binary Carrier Frequency of j1.023 MHz

BOCcos(15,2.5) BOC(1,1)BOC(10,5)

Bin

ary

Car

rier

Sine Carrier BOC(m,m)

Cosine Carrier BOCcos(m,m)

Note: data signals additionally multiplied by binary data stream

GALILEO ‐ BOC Correlation Function

0.5

1

BOC(1,1)

69

-8 -6 -4 -2 0 2 4 6 8

x 10-6

-0.5

0

m = 1 – subcarrier frequency is 1.023 MHzn = 1 – range code chip frequency is 1.023 MHz

GALILEO ‐ BOC Spectrum

-20

-15

-10

-5

0

BOC(1,1)BPSK(1)

70

-5 -4 -3 -2 -1 0 1 2 3 4 5

x 106

-40

-35

-30

-25

m = 1 – subcarrier frequency is 1.023 MHzn = 1 – range code chip frequency is 1.023 MHz

GALILEO ‐ BOC modulation

0

0.5

1

-0 8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

BOC(1,1) BOC(5,1) BOC(5,2)

correlation

function

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

x 107

-40

-35

-30

-25

-20

-15

-10

-5

0

-5 -4 -3 -2 -1 0 1 2 3 4 5

x 106

-40

-35

-30

-25

-20

-15

-10

-5

0

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

x 107

-40

-35

-30

-25

-20

-15

-10

-5

0

-8 -6 -4 -2 0 2 4 6 8

x 10-6

-0.5

-1.5 -1 -0.5 0 0.5 1 1.5

x 10-6

-1

0.8

-1.5 -1 -0.5 0 0.5 1 1.5

x 10-6

-0.8

-0.6c fspectrum

71

GALILEO ‐ Signals, Services and Spectra

service modulationcode

encryptiondata rate

[symbol/s]data

encryption

PRS BOC(15,2.5) yes 100 yes

OS/SoL/CS

BOC(1,1) none 250some(CS)

OS/SoL/CS

BOC(1,1) noneno data(“pilot”)

- 1559.00

1591.00

L1

1563.00

1587.00

E2 E1

I

Q

IN PHASE

sin()

1575.420

modulationsubcarrierfrequency

MHz

code rateMchips/s

BOC(15,2.5) 15.345 2.5575

BOC(1,1) 1.023 1.023

SoL uses the same signals as OS with integrity message

72

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Optimised CBCS for L1 Option Within the EU-US agreement there is scope to optimise the L1

BOC(1,1) signal.

CBCS = (1 - )BOC(1,1) + BCS(1,20)

Signal can be optimised by adding a smaller Binary Coded Symbol (BCS) signal to a Binary Offset Carrier (BOC) signal to produce a Composite Binary Coded Symbol (CBCS)Can be tracked as BOC(1,1)

73

+B

inar

y C

oded

Sy

mbo

l

-Bin

ary

Cod

ed

Sym

bol

For balance sign of BCS is alternated

Like BOC, but subwaveform can take any binary sequence

Increased bandwidth gives CBCS receiver much better tracking/multipath performance – even exceeds BOC(2,2) for 12MHz BW

5MHz Lobe in GPS M-Code NullL1 CBCS

GALILEO ‐ Signals, Services and Spectra


encryptiondata rate

[symbol/s]data

encryption

OS/SoL AltBOC(15,10) none 50 none

OS/SoL AltBOC(15,10) none no data (“pilot”) -

OS/SoL/CS AltBOC(15,10) none 250 some (CS)

OS/SoL/CS AltBOC(15,10) none no data („pilot“) -

1176.450

1207.140

1191.795

E5a E5b

1164.00

1215.00

E5

I

Q

IN PHASE

sin()

Differerent signals are broadcast• on I and Q channels• in upper (E5b) and lower(E5a) part of the band

E5a and E5b may be used asa single ultra wide channel 74

E5 Alt‐BOC Modulation

AltAlt--BOC can be represented as an 8BOC can be represented as an 8--PSK SignalPSK SignalPhase Angle (1 to 8) determined from binary values of the 4 coherent E5 codedata streams:

--11 --11 --11 --11 --11 --11 --11 --11 11 11 11 11 11 11 11 11

--11 --11 --11 --11 11 11 11 11 --11 --11 --11 --11 11 11 11 11

--11 --11 11 11 --11 --11 11 11 --11 --11 11 11 --11 --11 11 11

--11 11 --11 11 --11 11 --11 11 --11 11 --11 11 --11 11 --11 11

E5aE5a‐‐ddE5bE5b‐‐ddE5aE5a‐‐pp

E5bE5b‐‐pp

1

I

Q

23

4

56

7

8

Receiver can Correlate:Receiver can Correlate:1 Entire Alt BOC

55 44 44 33 66 33 11 22 66 55 77 22 77 88 88 11

55 44 88 33 22 33 11 22 66 55 77 66 77 44 88 11

11 44 88 77 22 33 11 22 66 55 77 66 33 44 88 55

11 88 88 77 22 33 11 66 22 55 77 66 33 44 44 55

11 88 88 77 22 77 55 66 22 11 33 66 33 44 44 55

11 88 44 77 66 77 55 66 22 11 33 22 33 88 44 55

55 88 44 33 66 77 55 66 22 11 33 22 77 88 44 11

55 44 44 33 66 77 55 22 66 11 33 22 77 88 88 11

151

.023

8 M

Pha

ses/

s . . .

.

. . .

.

Circ

ular

Buf

fer

1176

.45

1191

.795

1207

.14

1. Entire Alt-BOC2. As if each lobe (E5a/E5b) were

QPSK like GPS L5. (This gives small correlation loss)

GALILEO ‐ Spectrum, Services and Spectra


encryptiondata rate

[symbol/s]data

encryption

PRSBOC(10,5)

TDMAgovernment 100 yes

CS PSK(5) commercial 1000 yes

CS PSK(5) commercial no data („pilot“) -

1278.750

1268.520

1288.980

1260.00

1300.00

E6

I

Q

IN PHASE

sin()

CS PSK(5) commercial no data („pilot )

76

GALILEO ‐ Signal, Services and Spectra

50

40

50

20

1544.10

95

20

80

1164.00

1215.00

1260.00

1300.00

1559.00

1591.00

E5 E6 L1

1563.00

1587.00

E2 E1

E5a E5b

L6

I

Q

1176.45

1207.14

1278.75

1575.42

1191.79

1268.52

1288.98

IN PHASE

sin()

SAR downlink

77

Galileo Signals in Space10 Navigation Signals ‐ Right Hand Circularly Polarised

E5a E5b

OS/SOL Alt-BOC(15,10)Data + Pilot

E6 L1

OS/SOL BOC(1,1)* Data + Pilot

CS BPSK(5) Data + Pilot

PRS BOCcos(10,5) PRS BOCcos(15,2.5)

E2 E1

-155 dBW

-155 dBW

-152 dBW

-152 dBW

78

Commercial Service (CS) Public Regulated Service (PRS)

Open Service (OS) Safety Of Life Service (SOL) * BOC(1,1) or Optimised CBCS

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14

Navigation Data

• Ephemeris data

• Time parameters

• Almanacs

• Using this data, positioning for any user on earth can be derived.

79

Message Structure

• FEC

– Forward Error Correction, Rate ½ ConvolutionalE d d S b l R t i

Frame (1) Frame (2) Frame (N)

Superframe (1)

Frame (1)

Subframe (1) Subframe (M)Subframe (2)

Frame (i)

Subframe (j)

. . . . . .. . . . . .

. . . . . . . . . .

. . . . . .

Encoded – Symbol Rate is twice Data Rate

• UW

– Unique Word to Synchronisewith Data Fields

• CRC

– Cyclic Redundancy Check –checks parity for data errors

• Block Interleaving

– After convolutional encoding excluding UW

80

Subframe (1) Subframe (M)Subframe (2) Subframe (j)

CRCData Field Tail BitsUW

FEC Encoded & Block Interleaved

. . . . . . . . . .

Almanacs

• Used to identify the position of all of the satellites that are in orbit.

• Will identify:–Mean of Semi‐Major Accessj–Eccentricity– Inclination–Right Ascension of the Ascending Node–Argument of Perigree–Mean Anomaly

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Ephemeris Message StructureSimilar, but not identical, to GPS

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BEIDOU/COMPASS ‐ Overview

• China decided to build an independent satellite navigation system in 1980’s.

• In 2003, the Compass Demonstration Navigation System was built and the system has been used inSystem was built and the system has been used in many areas nowadays in China.

• Now the COMPASS global navigation system is under construction.

• China Satellite Navigation Project Center is in charge of the construction.

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BEIDOU/COMPASS ‐ Space Segment

• 5 GSO satellites and not more than 30 MEO satellites

GSO Satellite

MEO SatelliteConstellation

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BEIDOU/COMPASS ‐ The 1st Step

• COMPASS Navigation Demonstration System.

– Three GEO satellites have been launched since 2000, the demonstration system can provide some basic services including positioning, timing, and short‐message communication.

• October 2000: Launch of Beidou 1A (slot 140°E)

• December 2000: Launch of Beidou 1B (slot 80°E)

• May 2003: Launch of Beidou 2A (slot 110.5°E), currently only used as backup

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BEIDOU/COMPASS ‐ The 2nd Step

• COMPASS Navigation Satellite System.

– As a global system COMPASS will cover Asia‐Pacific area firstly in about 2010.

2010 The near future

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Launches ‐ $1.46 Billion First Phase

• 10 rockets of Long‐March 3 will be launched in recent two years, more than 10 satellites will be put into use, after that the system can offer services regionally.

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Beidou Signal Characteristics

• Frequencies

– B1: 1559.052~1591.788MHz

– B2: 1166.22~1217.37MHz

B3: 1250 618~1286 423MHz

Till the year Constellation Signals (actual emission)

2012 5GEO+5IGSO+4MEO（Regional Service）

mainly COMPASS Phase(CP) II signals

2020 5GEO+3IGSO+27MEO（Global Service）

mainly CP III signals

– B3: 1250.618~1286.423MHz

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ComponentCarrier Frequency

(MHz)Chip Rate

(cps)Bandwidth

(MHz)Modulation

TypeService

Type

Beidou Signal Characteristics

• CP II: B1, B2, and B3 as below

B1(I)1561.098

2.0464.092 QPSK

Open

B1(Q) 2.046 Authorized

B2(I)1207.14

2.04624 QPSK

Open

B2(Q) 10.23 Authorized

B3 1268.52 10.23 24 QPSK Authorized

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Beidou Frequencies & Signals Updated

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Compass‐M1 – Frequency & Signals

• April 2007: Launch of the 1. MEO

• Spring‐Summer 2007: University of StanfordUniversity of Stanford tracked B1, B2 and B3 Signal

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Compass‐M1 – Frequency & Signals• Spectra observed on 24/04/2007 – Figure centre frequency : L1 (Includes partial RF equalization)

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Estimated Signal Properties

B1 ‐ 1561.1 MHz• BPSK(2)


• Spectra observed on 24/04/2007 – Figure centre frequency : E6 (Includes partial RF equalization)

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B3 ‐ 1268.52 MHz• BPSK(10)


• Spectra observed on 24/04/2007 – Figure centre frequency : E5b (Includes partial RF equalization)

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B2 ‐ 1207.14 MHz• BPSK(2)• BPSK(10)


• GNSS Signals




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Satellite Orbits

• Treat the basic description and dynamics of satellite orbits

• Major perturbations on GPS satellite orbits

S f bi i f i• Sources of orbit information:

– SP3 format from the International GPS service

– Broadcast ephemeris message

• Accuracy of orbits and health of satellites

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Dynamics of satellite orbits

• Basic dynamics is described by F=Ma where the force, F, is composed of gravitational forces, radiation pressure (drag is negligible for GPS) and thruster firings (not directlyfor GPS), and thruster firings (not directly modeled).

• Basic orbit behavior is given by

3eGM

r r r

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Simple dynamics

• GMe = = 3986006x108 m3s‐2

• The analytical solution to the central force model is a Keplerian orbit. For GPS these are elliptical orbits.

f d• Mean motion, n, in terms of period P is given by

• For GPS semimajor axis a ~ 26400km

n 2P

a3

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Solution for central force model

• This class of force model generates orbits that are conic sections. We will deal only with closed elliptical orbits.

• The orbit plane stays fixed in space• The orbit plane stays fixed in space

• One of the foci of the ellipse is the center of mass of the body

• These orbits are described Keplerian elements

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Keplerain elements: Orbit plane

Z

Satelliteperigee

Node

i

0

Greenwich

Vernalequinox

equator

i Inclination Right Ascension of ascending node Argument of perigee True anomaly

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Keplerian elements in plane

Satellite

P iA

b r

a

FocusCenter of Mass

ae PerigeeApogee E

a semimajor axisb semiminor axise eccentricity

True anomalyE Eccentric anomalyM Mean anomaly

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Satellite motion

• The motion of the satellite in its orbit is given by

M (t) n(t T0 )

• To is time of perigee

( ) ( 0 )

E(t) M (t) esin E(t)

(t) tan1 1 e2 sin E(t) /(1 ecosE(t))(cos E(t) e) /(1 ecosE(t))

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True anomaly

0.05

0.1

0.15

0.2

0.25 Difference between true anomaly and Mean anomaly for e 0.001‐0.100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

x 104

-0.25

-0.2

-0.15

-0.1

-0.05

0

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Eccentric anomaly

0.05

0.1

0.15

0.2

0.25Difference between eccentric anomaly and Mean anomaly for e 0.001‐0.100

0 0.5 1 1.5 2 2.5 3 3.5 4

x 104

-0.25

-0.2

-0.15

-0.1

-0.05

0

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Vector to satellite

• At a specific time past perigee; compute Mean anomaly; solve Kepler’s equation to get Eccentric anomaly and then compute true anomalyanomaly.

• Vector r in orbit frame is

2

2

cos cos

sin1 sin

(1 )(1 cos )

1 cos

E ea r

e E

a er a e E

e

r

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Final conversion to Earth Fixed XYZ

• Vector r is in satellite orbit frame

• To bring to inertial space coordinates or Earth fixed coordinates, use

• This basically the method used to compute positions from the broadcast ephemeris

ri R3()R1(i)R3()r

re R3()R1(i)R3()r

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Perturbed motions

• The central force is the main force acting on the GPS satellites, but there are other significant perturbations.

• Historically, there was a great deal of work on analytic expressions for these perturbations e.g. Lagrange planetary equations which gave expressions for rates of change ofequations which gave expressions for rates of change of orbital elements as function of disturbing potential

• Today: Orbits are numerically integrated although some analytic work on form of disturbing forces.

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Perturbation from Flattening J2

• The J2 perturbation can be computed from the Lagrange planetary equations

222 2 2

3 cos

2 (1 )e

ina J

22 2 2

22

22 2 2

22

22 2 3

2 (1 )

3 5cos 1

4 (1 )

3 3cos 1

4 (1 )

e

e

e

a e

ina J

a e

iM n na J

a e

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J2 Perturbations

• Notice that only and n are effected and so this perturbation results in a secular perturbation

• The node of the orbit precesses the argument• The node of the orbit precesses, the argument of perigee rotates around the orbit plane, and the satellite moves with a slightly different mean motion

• For the Earth, J2 = 1.08284x10‐3

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Gravitational perturbation styles

Parameter Secular Long period Short period

a No No Yes

e No Yes Yese No Yes Yes

i No Yes Yes

Yes Yes Yes

Yes Yes Yes

M Yes Yes Yes

Other perturbation on orbits and approximate size

Term Acceleration (m/sec2)

Distance in 1/2 orbit (21600 sec)

Central 0.6

J2 5x10-5 12 km

Other gravity 3x10-7 70 mg y

Third body 5x10-6 1200 m

Earth tides 10-9 0.2 m

Ocean tides 10-10 0.02 m

Drag ~0 ~0

Solar radiation 10-7 23 m

Albedo radiation 10-9 0.2 m

GPS Orbits

• Orbit characteristics are

– Semimajor axis 26400 km (12 sidereal hour period)

– Inclination 55.5 degrees

– Eccentricity near 0 (largest 0.02)Eccentricity near 0 (largest 0.02)

– 6 orbital planes with 4‐5 satellites per plan

• Design lifetime is 6 years, average lifetime 10 years

• Generations: Block II/IIA 972.9 kg, Block IIR 1100 kg

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Basic Constellation

Orbits shown in inertial space and size relative t E th i tto Earth is correct

4-5 satellites in each plane

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Broadcast Ephemeris

• Satellites transmit as part of their data message the elements of the orbit

• These are Keplerian elements with periodic terms added to account for solar radiation and gravity perturbations

• Periodic terms are added for argument of perigee, geocentric distance and inclination

• The message and its use are described in the ICD‐GPS‐200 icd200cw1234.pdf(page 106‐121 in PDF)

• Selected part of document with ephemeris information icd200cw1234.Nav.pdf

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Distribution of Ephemerides

• The broadcast ephemeris is decoded by all GPS receivers and for geodetic receivers the software that converts the receiver binary to an exchange format outputs an ASCII version

• The exchange format: Receiver Independent Exchange format (RINEX) has a standard for the broadcast ephemeris.

• Form [4‐char][Day of year][Session].[yy]ne.g. brdc0120.02n

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RINEX standard

• Description of RINEX standard can be found at ftp://igscb.jpl.nasa.gov/igscb/data/format/rinex2.txt

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References

• Borre, K., Akos, D. M., Bertelsen, N., Rinder, P., and Jensen, S. H. (2007). A Software‐Defined GPS and Galileo Receiver ‐ A Single‐Frequency Approach. Boston • Basel • Berlin: Birkhäuser.

• Tsui, J. B.‐Y. (2005). Fundamentals of Global Positioning System Receivers A Software Approach (Second ed.): A John Wiley & Sons, Inc. Publication.

• Misra, P., and Enge, P. (2006). Global Positioning System ‐ Signals, Measurements, and Performance (Second ed.). Lincoln, Massachusetts, USA: Ganga‐Jamuna Press.

• Kaplan E D and Hegarty C J (2006) Understanding GPS Principles and• Kaplan, E. D., and Hegarty, C. J. (2006). Understanding GPS ‐ Principles and Applications (Second ed.). Boston | London: Artech House.

• Parkinson, B. W., Spilker, J. J., Axelrad, P., and Enge, P. (1996). Global Positioning System: Theory and Applications, Volume I (Vol. 163). Washington, DC, USA: American Institute of Aeronautics and Astronautics, Inc.

• Parkinson, B. W., Spilker, J. J., Axelrad, P., and Enge, P. (1996). Global Positioning System: Theory and Applications, Volume II (Vol. 164). Washington, DC, USA: American Institute of Aeronautics and Astronautics, Inc.

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Thank You!

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