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 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 L1 Phase Relationships Civil is 3 dB stronger than P/Y

One Civil Component on L2 L2 Phase Relationships Civil is 0.4 dB weaker than P/Y

Technical Framework (2 of 3) 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with weak signals Receiver technology has advanced enormously compared with the 1970s when C/A was developed –The outdated C/A should be replaced with a better code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 $

Technical Framework (3 of 3) 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\SVIIRIIR-MIIF L1 C/A L1 P/Y L1 M L2 Civil L2 P/Y L2 M L5 Civil

Civil Signal Availability

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

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

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 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

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 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 code0.0 dBNone (Costas) L2 CS-3.0 dB Comparing L2CS with C/A on L2

Data & Tracking Thresholds Relative Data Recovery Threshold Relative Carrier Tracking Threshold L2 C/A code0.0 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 code0.0 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 dBW0.0 dB L2 CS dBW-2.3 dB L5 signal dBW+3.7 dB Comparing Three Civil Signals

Relative Channel Power Relative Data Channel Power Relative Data-Less Channel Power L1 C/A code0.0 dBNone (Costas) L2 CS-5.3 dB L5 signal+0.7 dB Comparing Three Civil Signals

Data & Tracking Thresholds Relative Data Recovery Threshold Relative Carrier Tracking Threshold L1 C/A code0.0 dB L2 CS+2.7 dB+0.7 dB L5 signal+5.7 dB+6.7 dB Comparing Three Civil Signals

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 code0.0 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/sec 2 –BL = 8 Hz Balanced performance –300 bit word error rate (WER) is with total C/No = 22 dB-Hz –Phase slip probability within 60 seconds is 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/sec 2 –BL = 15 Hz Performance –300 bit word error rate (WER) of with total C/No = 24.5 dB-Hz –Phase slip probability in 60 seconds of 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 Transit Modulation

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

TDM of 409,200 Chips

TDM of 1,534,500 Chips

TDM of 613,800 Chips

TDM with 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/N o in the data component = For 50% power split, C/N o in the total signal = 50 bps, uncoded25.8 dB-Hz28.8 dB-Hz 50 bps, rate-1/220.6 dB-Hz23.6 dB-Hz bps, rate-1/218.8 dB-Hz21.8 dB-Hz 25 bps, rate-1/217.6 dB-Hz20.6 dB-Hz 50 bps, rate-1/319.9 dB-Hz22.9 dB-Hz bps, rate-1/318.1 dB-Hz21.1 dB-Hz 25 bps, rate-1/316.9 dB-Hz19.9 dB-Hz Theoretical requirements for data demodulation with perfect carrier phase tracking

Balance Tracking & Demod. Data rate (bps) & FEC rate Carrier power percent WER = with total C/N o = Phase slip = with total C/N o = 50 & NoneCostas26 dB-Hz25.5 dB-Hz 50 & None dB-Hz23 dB-Hz 25 & None dB-Hz23 dB-Hz 50 & ½ 5024 dB-Hz23 dB-Hz 33.3 & ½ dB-Hz23 dB-Hz 25 & ½ 5022 dB-Hz23 dB-Hz 25 & ½ 2524 dB-Hz26 dB-Hz 25 & ½ 7524 dB-Hz21 dB-Hz 33.3 & 1/35022 dB-Hz23 dB-Hz For max acceleration = 29.8 Hz/sec, maximum jerk = 9.6 Hz/sec 2, B L = 8 Hz

Higher G Tracking & Demod. Data & FEC rates Carrier Power percent Optimum B L WER = with total C/N o = Phase slip = with total C/N o = 50 & noneCostas15 Hz27 dB-Hz29 dB-Hz 50 & ½50%15 Hz25 dB-Hz25.5 dB-Hz 25 & ½50%15 Hz24.5 dB-Hz25.5 dB-Hz 25 & ½66.7%15 Hz24 dB-Hz24.5 dB-Hz 25 & ½75%13 Hz24 dB-Hz 33.3 & 1/350%13 Hz24.5 dB-Hz25.5 dB-Hz 33.3 & 1/366.7%15 Hz24 dB-Hz24.5 dB-Hz 33.3 & 1/375%13 Hz25 dB-Hz24 dB-Hz Maximum acceleration = 300 Hz/sec and maximum jerk = 100 Hz/sec 2

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) Code Clock (MHz)PhasesAvailable Correlation Protection 1, , Bi- Phase Now> 21 dB 1, , , Bi- Phase ~ 2011> 45 dB 1, , 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