l2cs technical description tom stansell. technical agenda signal development framework –objectives...

L2CS Technical Description

Tom Stansell

Technical Agenda

• Signal Development Framework– Objectives and Constraints

• The L2 Civil Signal (L2CS) Description• Signal Performance Characteristics• Design Decisions and Tradeoffs• Eventual Civil Signal Options

Objectives and Constraints

Signal Development Framework

Technical Framework (1 of 3)

• Civil L2 signal power ~2.3 dB less than L1 C/A • Code chip rate must remain at 1.023 MHz

– To separate the M Code and Civil Code spectra

• Only one bi-phase signal component available– L5-type quad-phase not possible – L2CS shares L2 with military signals

• Definition needed by the first of March– Technical meetings began in mid-January– Definition complete by mid-February– Coordinated with Lockheed-Martin and Boeing– First draft of ICD-GPS-200 PIRN completed

Code Spectra: BOC (10,5) M & C/A

Effect on GPS noise floor of a strong M code signal

C/A code spectrum

One Civil Component on L1

P /Y = 0

C ivil = 0

0,0 = 0 deg

C ivil = 1

P /Y = 1

0,1 = 109.5 deg

1,1 = 180 deg

1,0 = -70.5 deg

L1 Phase RelationshipsCivil is 3 dB stronger than P/Y

One Civil Component on L2

0,0 = 0 deg

P /Y = 0

C ivil = 0

C ivil = 1

P /Y = 1

0,1 = 84.7 deg

1,1 = 180 deg

1,0 = -95.3 deg

L2 Phase RelationshipsCivil is 0.4 dB weaker than P/Y


• Serve the current large and valuable dual frequency survey, science, and machine control applications – Approximately 50,000 in service– Primary need is for robust carrier phase measurements– Typically use semi-codeless L2 access, but many also are

equipped with an L2 C/A capability

• Improve cross-correlation for single frequency applications (e.g., wooded areas or indoor navigation)– A strong C/A code signal can interfere with weak signals

• Receiver technology has advanced enormously compared with the 1970s when C/A was developed – The outdated C/A should be replaced with a better code

Technology Has Changed

5 Channel Analog

2001

Consumer 12 channel digital with color map

Consumer 12 channel digital for under $100

1984


• New signals on IIR-M and IIF satellites• When will full coverage with the new signals

become available?– See estimated launch schedule chart

• Will the IIR-M be able to transmit an L5-type message on the L2CS? – Lockheed-Martin implementation study underway– Backup modes will be provided

Signals on IIR, IIR-M, & IIF

Signal\SV IIR IIR-M IIF

L1 C/A

L1 P/Y

L1 M

L2 Civil

L2 P/Y

L2 M

L5 Civil

Civil Signal Availability

0

5

10

15

20

25

30

35

Fe

b-0

3

Fe

b-0

4

Fe

b-0

5

Fe

b-0

6

Fe

b-0

7

Fe

b-0

8

Fe

b-0

9

Fe

b-1

0

Fe

b-1

1

Year

Nu

mb

er

of

Sat

elli

tes

wit

h N

ew

Sig

nal

s

L2CSL5

18

21

24

28

L2 Civil Signal (L2CS) Description

Definitions

• L2CS – the L2 Civil Signal• CM – the L2CS moderate length code

– 10,230 chips, 20 milliseconds

• CL – the L2CS long code– 767,250 chips, 1.5 second

• NAV – the legacy navigation message provided by the L1 C/A signal

• CNAV – a navigation message structure like that adopted for the L5 civil signal

IIF Signal Generation

C /A C odeG enerator

10,230 C hipC ode G enerator


L5-L ike C N A VM essage

25 b its/sec

C hip by C hipM ultip lexer

1 .023 M H zC lock

T ransm ittedS igna l1 /2

A 1

A 2B 1

B 2

R ate 1 /2 FE C

Legacy N A VM essage

50 b its/sec

511.5 kH z C lock

C MC ode

C LC ode

IIF L2CS Signal Options

• The ability to transmit any one of the following three signal structures upon command from the Ground Control Segment:– The C/A code with no data message (A2, B1)– The C/A code with the NAV message (A2, B2)– The chip by chip time multiplexed (TDM) combination

of the CM and CL codes with the CNAV message at 25 bits/sec plus FEC bi-phase modulated on the CM code (A1)

IIR-M Signal Generation

B1 is a potential software option to be uploaded by

the Control Segment

C /A C odeG enerator



L5-L ike C N A VM essage

25 b its/sec


25 B its/sec

C hip by C hipM ultip lexer

1 .023 M H zC lock

T ransm ittedS igna l1 /2

A 1

A 2B 1

B 2

R ate 1 /2 FE C


50 b its/sec

D 1

D 2 C 1

C 2

511.5 kH z C lock

C MC ode

C LC ode

IIR-M L2CS Preferred Mode

• The Preferred mode is the ability to transmit the following signal structure upon command from the Ground Control Segment:– The chip by chip time multiplexed (TDM)

combination of the CM and CL codes with the CNAV message at 25 bits/sec plus FEC bi-phase modulated on the CM code (A1, C1, D1)

IIR-M L2CS Backup Mode

• One backup mode is the ability to transmit the following signal structure upon command from the Ground Control Segment:– The chip by chip time multiplexed (TDM) combination

of the CM and CL codes with the NAV message at 25 bits/sec plus FEC bi-phase modulated on the CM code (A1, C1, D2)

IIR-M L2CS Optional Modes

• The ability to transmit any one of the following three signal structures upon command from the Control Segment:– The C/A code with no data message (A2, B1)– The C/A code with the NAV message (A2, B2)– The chip by chip time multiplexed (TDM) combination

of the CM and CL codes with the NAV message at 50 bits/sec bi-phase modulated on the CM code (A1, C2)

• Control Segment implementation is under evaluation for these & the previous options

L2CS Code Characteristics

• Codes are disjoint segments of a long-period maximal code– 27-stage linear shift register generator (LSRG) with multiple

taps is short-cycled to get desired period– Selected to have perfect balance

• A separate LSRG for each of the two codes• Code selection by initializing the LSRG to a fixed

state specified for the SV ID and resetting (short-cycling) after a specified count for the code period or at a specified final state

• 1 cycle of CL & 75 cycles of CM every 1.5 sec

L2CS 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

37 of the 100 Selected Codes

• Medium code = CM– 10,230 chips– 20 msec

• Long code = CL– 767,250 chips– 1.5 second

• Begin and end states• Perfectly balanced• 37 codes listed in the

ICD-GPS-200 PIRN• 100 codes defined

PERIOD=10230 PERIOD=767250 SHORT CODE STATES (OCTAL) LONG CODE STATES (OCTAL) PRN START END START END 1 742417664 552566002 624145772 267724236 2 756014035 034445034 506610362 167516066 3 002747144 723443711 220360016 771756405 4 066265724 511222013 710406104 047202624 5 601403471 463055213 001143345 052770433 6 703232733 667044524 053023326 761743665 7 124510070 652322653 652521276 133015726 8 617316361 505703344 206124777 610611511 9 047541621 520302775 015563374 352150323 10 733031046 244205506 561522076 051266046 11 713512145 236174002 023163525 305611373 12 024437606 654305531 117776450 504676773 13 021264003 435070571 606516355 272572634 14 230655351 630431251 003037343 731320771 15 001314400 234043417 046515565 631326563 16 222021506 535540745 671511621 231516360 17 540264026 043056734 605402220 030367366 18 205521705 731304103 002576207 713543613 19 064022144 412120105 525163451 232674654 20 120161274 365636111 266527765 641733155 21 044023533 143324657 006760703 730125345 22 724744327 110766462 501474556 000316074 23 045743577 602405203 743747443 171313614 24 741201660 177735650 615534726 001523662 25 700274134 630177560 763621420 023457250 26 010247261 653467107 720727474 330733254 27 713433445 406576630 700521043 625055726 28 737324162 221777100 222567263 476524061 29 311627434 773266673 132765304 602066031 30 710452007 100010710 746332245 012412526 31 722462133 431037132 102300466 705144501 32 050172213 624127475 255231716 615373171 33 500653703 154624012 437661701 041637664 34 755077436 275636742 717047302 100107264 35 136717361 644341556 222614207 634251723 36 756675453 514260662 561123307 257012032 37 435506112 133501670 240713073 703702423

Code Tracking

• Early minus late (E-L) code tracking loops try to center windows, e.g., narrow correlator windows, on code transitions

• For each of the two L2CS codes, there is a transition at every chip– Because the other code is perfectly balanced, the

alternate chips average to zero– Twice the transitions, half the amplitude, and double

the average noise power (time on) yields –3 dB S/N in a one-code loop

– Both codes can be tracked, but CL-only is OK

Narrow Correlator Tracking

Narrow Correlator on L2CS

The CNAV Message

• The CNAV message data rate is 25 bps• A rate-1/2 forward error correction (FEC),

without interleaving, (same as L5) is applied, resulting in 50 symbols per sec

• The data message is synchronized to X1 epochs, meaning that the first symbol containing information about the first bit of a message is synchronized to every 8th X1 epoch

CNAV Message Content

• The CNAV message content is the same as defined for the L5 signal with the following differences and notes: – Because of the reduced bit rate, the sub-frame

period will be 12 seconds rather than 6 seconds– The time parameter inserted into each data sub-

frame will properly represent the 12-second epoch defined by each sub-frame

– The terms provided by the Control Segment representing time bias between the P code and the civil codes for L1, L2, and L5 will be included

Message Sequence Options

Type 1

Type 2

Other Type

Type 1

Type 2

Other A

Other B

36 sec frame

48 sec frame

Message Types1 = Ephemeris A2 = Ephemeris B3 = Iono, Bias, Health4 = Almanac5 = Text

Type 4 message gives one satellite almanac per

sub-frame

CNAV Message Sequencing

• Message sequences will be determined by the Control Segment. One possible sequence is three sub-frames grouped into repeating frames of 36 seconds, each containing Ephemeris 1 and Ephemeris 2 messages plus another sub-frame

• The third sub-frame of each 36 second frame contains one almanac message or another message when and as needed

Another CNAV Sequence

• Another possible sequence is four sub-frames grouped into repeating frames of 48 seconds, each containing Ephemeris 1 and Ephemeris 2 messages plus two other sub-frames

• It also will be possible for different satellites to transmit different almanac messages at the same time, as defined or scheduled by the Ground Control Segment

Compact Almanac

• A new compact almanac message type is being developed to minimize the time required to collect a complete almanac

• Up to 7 satellite almanacs per sub-frame • The new message type will be described in a

following presentation

Signal Performance Characteristics

Relative Channel Power

Relative Data Channel

Power

Relative Data-Less

Channel Power

L2 C/A code 0.0 dB None (Costas)

L2 CS -3.0 dB -3.0 dB

Comparing L2CS with C/A on L2

Data & Tracking Thresholds

Relative Data Recovery Threshold

Relative Carrier Tracking

Threshold

L2 C/A code 0.0 dB 0.0 dB

L2 CS +5.0 dB (FEC = 5 dB)

(25 bps = 3 dB)

+3.0 dB(Phase locked

tracking = 6 db)

Comparing L2CS with C/A on L2

Signal Acquisition

Relative Acquisition Power

L2 C/A code 0.0 dB

L2 CS -3.0 dB (-1.0 dB using both codes)

C/A code acquisition may be impossible for very

weak signals in the presence of a strong

C/A signal

Modern, multiple correlator technology overcomes the L2CS power deficit and

permits rapid acquisition of very weak signals

Power from IIR-M & IIF

Received Power

Relative Total Power

L1 C/A code -157.7 dBW 0.0 dB

L2 CS -160.0 dBW -2.3 dB

L5 signal -154.0 dBW +3.7 dB

Comparing Three Civil Signals

Relative Channel Power

Relative Data Channel

Power

Relative Data-Less

Channel Power

L1 C/A code 0.0 dB None (Costas)

L2 CS -5.3 dB -5.3 dB

L5 signal +0.7 dB +0.7 dB


Data & Tracking Thresholds

Relative Data Recovery Threshold

Relative Carrier Tracking Threshold

L1 C/A code 0.0 dB 0.0 dB

L2 CS +2.7 dB +0.7 dB

L5 signal +5.7 dB +6.7 dB


Signal Acquisition

C/A code acquisition may be impossible for

very weak signals in the presence of a strong C/A

signal

Modern, multiple correlator technology overcomes the L2CS

power deficit and permits rapid

acquisition of very weak signals

Relative Acquisition Power

L1 C/A code 0.0 dB

L2 C/A code -2.3 dB

L2 CS -5.3 dB (-3.3 dB using both codes)

L5 signal +0.7 dB (+2.7 dB using both codes)

Tracking/Data Performance

• With 50% power split, 25 bps, and rate-½ FEC• Under moderate dynamic conditions (aviation)

– Max acceleration = 29.8 Hz/sec – Maximum jerk = 9.6 Hz/sec2 – BL = 8 Hz

• Balanced performance– 300 bit word error rate (WER) is 0.015

with total C/No = 22 dB-Hz – Phase slip probability within 60 seconds is 0.001

with total C/No = 23 dB-Hz

Tracking/Data Performance

• With 50% power split, 25 bps, and rate-½ FEC

• Under high dynamic conditions– Max acceleration = 300 Hz/sec – Maximum jerk = 100 Hz/sec2 – BL = 15 Hz

• Performance– 300 bit word error rate (WER) of 0.015 with total

C/No = 24.5 dB-Hz – Phase slip probability in 60 seconds of 0.001 with

total C/No = 25.5 dB-Hz

Why two codes?Why TDM?

Why Chip by Chip? Why L5 type message?

Why FEC?

Design Decisions and Tradeoffs

An Old Idea Revived

• Transit, the world’s first satellite navigation system, provided a coherent carrier

• But GPS used bi-phase data modulation, leaving no carrier

• Bi-phase modulation favors data over continuous lock and measurement accuracy

– But data is redundant, slowly changing, thus less important

• A carrier component makes signal tracking & navigation measurements more robust

TransitModulation

Why Two Codes?

• Carrier component first accepted for L5– Two equal power signal components in phase

quadrature, each with a separate code– One component with bi-phase data– The other component with carrier & no data– Forward error correction (FEC) raised bit error

probability to the level achieved with all the power in one bi-phase signal component

– The carrier component improves tracking threshold by 3 dB

– Win-win: better tracking, no data degradation

Two L2 Codes

• Quad phase was not available for L2• Two codes provided by time multiplexing one

bi-phase signal component• Data with forward error correction on

moderate length code, CM• No data on the long CL code, provides a

carrier component and a 3 dB better tracking threshold

• Longer CL code improves crosscorrelation

Multi-Code Options

• Considered 3 ways to provide two codes:– Majority vote of 3 codes

• 000=0, 001=0, 010=0, 100=0, 011=1, 101=1, 110=1, 111=1

• One with data, two without data• Tracking only one code loses 6 dB• Knowledge of all three regains 3 dB

– Time multiplexed, msec by msec– Time multiplexed, chip by chip

Chip by Chip TDM Chosen

• Majority vote eliminated because:– Requires 3 rather than 2 code generators– Requires synch to all 3 codes for best results– No other advantage found

• Msec by msec TDM eliminated because:– Requires care to avoid 500 Hz sidetone – No other advantage found

• Selected chip by chip TDM– Simple to implement with no disadvantages

Code Length Considerations

• The peak cross-correlation between existing C/A codes is ‑23.9 dB – The Gold bound for period 1023 chips – C/A codes are inadequate for indoor navigation

• Correlation sidelobe examples for TDM candidates– 20 msec period: 29 dB below full correlation– 200 msec period: 36 dB below full correlation– 1.5 sec period: 47 dB below full correlation

Code Correlation Studies

• Fig 1 – Three individual code lengths• Fig 2 – TDM 409,200 • Fig 3 – TDM 1,534,500 (10,230 & 767,250)

– This is the selected code pair– CM for faster acquisition– CL for better crosscorrelation

• Minimum crosscorrelation protection of 45 dB

• Fig 4 – TDM 613,800 (10,230 & 306,900)• Fig 5 – TDM 1,534,500 (1 msec segments)

Three Individual Codes

FIG 1. DISTRIBUTION OF CORRELATION FUNCTION OF TDMA CODE

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

25 30 35 40 45 50 55 60 65 70

CORRELATION LEVEL, DB BELOW FULL CORRELATION

PR

OB

TH

AT

CO

RR

EL

AT

ION

PE

AK

EX

CE

ED

S A

BS

CIS

SA

PERIOD OF TDMA CODE= 20460 CHIPS

40920 CHIPS

409200 CHIPS

TDM of 409,200 Chips

FIG 2. DISTRIBUTION OF CORRELATION FUNCTION OF TDMA CODE

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

40 45 50 55 60 65 70


PR

OB

TH

AT

CO

RR

EL

AT

ION

PE

AK

EX

CE

ED

S A

BS

CIS

SA

PERIOD OF TDMA CODE= 409200 CHIPS

AUTO

CROSS

TDM of 1,534,500 Chips

FIG 3. DISTRIBUTION OF CORRELATION FUNCTION OF TDMA CODE OF UNEQUAL PERIODS

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

45 50 55 60 65 70 75 80


PR

OB

TH

AT

CO

RR

EL

AT

ION

PE

AK

EX

CE

ED

S A

BS

CIS

SA

AUTO

CROSS

SHORT CODE PERIOD = 10230 CHIPS

LONG CODE PERIOD = 767250 CHIPS

PERIOD OF TDMA CODE = 1534500 CHIPS

TDM of 613,800 Chips


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

40 45 50 55 60 65 70 75 80


PR

OB

TH

AT

CO

RR

EL

AT

ION

PE

AK

EX

CE

ED

S A

BS

CIS

SA

AUTO

CROSS




TDM with 1 msec Segments


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

45 50 55 60 65 70 75 80


PR

OB

TH

AT

CO

RR

EL

AT

ION

PE

AK

EX

CE

ED

S A

BS

CIS

SA

AUTO

CROSS




INTERLEAVE 1 MSEC SEGMENTS

Data and FEC Rates

• Normally a signal can be tracked to a lower S/N than data can be demodulated reliably

• A team member suggested lowering the bit rate to 25 bps

• Using FEC with this change allows tracking and demodulation thresholds be be equivalent– Advantage in forest navigation

• The more compact and flexible L5-type message also makes this practical

• A bit rate of 25 BPS with a rate ½ FEC was chosen

Choosing Data & FEC Rates

Data rate&

FEC rate

For WER = 0.015, C/No in the data

component =

For 50% power split, C/No in the total

signal =

50 bps, uncoded 25.8 dB-Hz 28.8 dB-Hz

50 bps, rate-1/2 20.6 dB-Hz 23.6 dB-Hz

33.33 bps, rate-1/2 18.8 dB-Hz 21.8 dB-Hz



33.33 bps, rate-1/3 18.1 dB-Hz 21.1 dB-Hz


Theoretical requirements for data demodulation with perfect carrier phase tracking

Balance Tracking & Demod.

Data rate(bps) &

FEC rate

Carrier power percent

WER = 0.015 with total C/No

=

Phase slip = 0.001 with total

C/No =

50 & None Costas 26 dB-Hz 25.5 dB-Hz

50 & None 50 29 dB-Hz 23 dB-Hz

25 & None 50 26.5 dB-Hz 23 dB-Hz

50 & ½ 50 24 dB-Hz 23 dB-Hz

33.3 & ½ 50 22.5 dB-Hz 23 dB-Hz

25 & ½ 50 22 dB-Hz 23 dB-Hz

25 & ½ 25 24 dB-Hz 26 dB-Hz

25 & ½ 75 24 dB-Hz 21 dB-Hz

33.3 & 1/3 50 22 dB-Hz 23 dB-Hz

For max acceleration = 29.8 Hz/sec, maximum jerk = 9.6 Hz/sec2, BL = 8 Hz

Higher G Tracking & Demod.

Data &FEC rates

Carrier Power

percent

Optimum BL

WER = 0.015 with total C/No

=

Phase slip = 0.001 with total C/No =

50 & none Costas 15 Hz 27 dB-Hz 29 dB-Hz

50 & ½ 50% 15 Hz 25 dB-Hz 25.5 dB-Hz

25 & ½ 50% 15 Hz 24.5 dB-Hz 25.5 dB-Hz

25 & ½ 66.7% 15 Hz 24 dB-Hz 24.5 dB-Hz

25 & ½ 75% 13 Hz 24 dB-Hz 24 dB-Hz

33.3 & 1/3 50% 13 Hz 24.5 dB-Hz 25.5 dB-Hz

33.3 & 1/3 66.7% 15 Hz 24 dB-Hz 24.5 dB-Hz

33.3 & 1/3 75% 13 Hz 25 dB-Hz 24 dB-Hz

Maximum acceleration = 300 Hz/sec and maximum jerk = 100 Hz/sec2

For each application,companies will choose the most

appropriate signal to use

Eventual Civil Signal Options

Civil Signal Characteristics

Carrier Frequency

(MHz)

Code Length (Chips)

CodeClock (MHz) Phases Available

Correlation Protection

1,575.42 1,023 1.023 Bi-Phase

Now > 21 dB

1,227.60 10,230767,250

1.023 Bi-Phase

~ 2011 > 45 dB

1,176.45 10,23010,230

10.23 Quad Phase

~ 2015 > 30 dB

L2CS Features

• Best crosscorrelation protection – Aids navigation indoors and in forest areas– Provides headroom for increased SV power

• Lower chip rate: – Saves power and minimizes thermal rise – Allows use of narrowband RF/IF filters

• Lower cost • Protection against nearby interfering signals

• Available years sooner than L5

l2cs technical description tom stansell. technical agenda signal development framework –objectives...

Documents