# Presentation: 219048

- 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

- L1 Phase RelationshipsCivil is 3 dB stronger than P/Y

## One Civil Component on L2

- L2 Phase RelationshipsCivil 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- The outdated C/A should be replaced with a better code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 $100

- 1984

## 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\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

- 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 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 |

- Comparing Three Civil Signals

## 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 |

- Comparing Three Civil Signals

## Signal Acquisition 

**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-1⁄2 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-1⁄2 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

**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

## 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 1⁄2 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  |
| 25 bps, rate-1/2 | 17.6 dB-Hz  | 20.6 dB-Hz  |
| 50 bps, rate-1/3  | 19.9 dB-Hz  | 22.9 dB-Hz  |
| 33.33 bps, rate-1/3  | 18.1 dB-Hz  | 21.1 dB-Hz  |
| 25 bps, rate-1/3  | 16.9 dB-Hz  | 19.9 dB-Hz  |

- Theoretical requirements for data demodulation with perfect carrier phase tracking

## Balance Tracking & Demod.

| Data rate
(bps) &
FEC rate | Carrier power percent | WER = 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 & 1⁄2 | 50 | 24 dB-Hz  | 23 dB-Hz  |
| 33.3 & 1⁄2  | 50 | 22.5 dB-Hz  | 23 dB-Hz  |
| 25 & 1⁄2 | 50 | 22 dB-Hz  | 23 dB-Hz  |
| 25 & 1⁄2 | 25 | 24 dB-Hz  | 26 dB-Hz  |
| 25 & 1⁄2 | 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 & 1⁄2    | 50% | 15 Hz | 25 dB-Hz  | 25.5 dB-Hz  |
| 25 & 1⁄2  | 50% | 15 Hz | 24.5 dB-Hz  | 25.5 dB-Hz  |
| 25 & 1⁄2 | 66.7% | 15 Hz | 24 dB-Hz  | 24.5 dB-Hz  |
| 25 & 1⁄2   | 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) | CodeClock (MHz) | Phases | Available | Correlation Protection |
| --- | --- | --- | --- | --- | --- |
| 1,575.42 | 1,023 | 1.023 | Bi-Phase | Now | > 21 dB |
| 1,227.60 | 10,230767,250 | 1.023 | Bi-Phase | ~ 2011 | > 45 dB |
| 1,176.45 | 10,23010,230 | 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