L2CS Technical Description

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L2CS Technical Description Tom Stansell
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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

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Signal Development Framework

• Objectives and Constraints

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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

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Code Spectra BOC (10,5) M C/A
C/A code spectrum
Effect on GPS noise floor of a strong M code
signal
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One Civil Component on L1
L1 Phase RelationshipsCivil is 3 dB stronger
than P/Y
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One Civil Component on L2
L2 Phase RelationshipsCivil is 0.4 dB weaker
than P/Y
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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• The outdated C/A should be replaced with a better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

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Technology Has Changed
1984
2001
Consumer 12 channel digital with color map
Consumer 12 channel digital for under 100
5 Channel Analog
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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

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Signals on IIR, IIR-M, IIF
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Civil Signal Availability
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L2 Civil Signal (L2CS) Description
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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

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IIF Signal Generation
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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)

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IIR-M Signal Generation
B1 is a potential software option to be uploaded
by the Control Segment
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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)

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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)

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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

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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

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L2CS Code Generator
Linear shift register generator with 27 stages
and 12 taps
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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

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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

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Narrow Correlator Tracking
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Narrow Correlator on L2CS
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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

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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

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Message Sequence Options
Type 4 message gives one satellite almanac per
sub-frame
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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

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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

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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

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Signal Performance Characteristics
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Relative Channel Power
Comparing L2CS with C/A on L2
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Data Tracking Thresholds
Comparing L2CS with C/A on L2
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Signal Acquisition
Modern, multiple correlator technology overcomes
the L2CS power deficit and permits rapid
acquisition of very weak signals
C/A code acquisition may be impossible for very
weak signals in the presence of a strong C/A
signal
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Power from IIR-M IIF
Comparing Three Civil Signals
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Relative Channel Power
Comparing Three Civil Signals
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Data Tracking Thresholds
Comparing Three Civil Signals
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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

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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

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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

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Design Decisions and Tradeoffs

• Why two codes?Why TDM? Why Chip by Chip? Why
  L5 type message? Why FEC?

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An Old Idea Revived

• Transit, the worlds 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
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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

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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

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Multi-Code Options

• Considered 3 ways to provide two codes
• Majority vote of 3 codes
• 0000, 0010, 0100, 1000, 0111, 1011, 1101,
  1111
• 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

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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

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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

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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)

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Three Individual Codes
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TDM of 409,200 Chips
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TDM of 1,534,500 Chips
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TDM of 613,800 Chips
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TDM with 1 msec Segments
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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

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Choosing Data FEC Rates
Theoretical requirements for data demodulation
with perfect carrier phase tracking
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Balance Tracking Demod.
For max acceleration 29.8 Hz/sec, maximum jerk
9.6 Hz/sec2, BL 8 Hz
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Higher G Tracking Demod.
Maximum acceleration 300 Hz/sec and maximum
jerk 100 Hz/sec2
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Eventual Civil Signal Options

• For each application,companies will choose the
  most appropriate signal to use

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Civil Signal Characteristics
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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