Space Ops 2006

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Space Ops 2006
Delta-DOR and Regenerative Systems The New CCSDS
Frontier in Spacecraft Ranging
G. Boscagli, E. Vassallo (ESA) D. Lee, W.L.
Martin (NASA)
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Current ranging techniques

• Most commonly used ranging technique for
  interplanetary missions is the hybrid ranging
  system
• A ranging tone (or clock) is modulated by an
  agency specific sequential code
• The composite ranging signal phase modulates the
  residual carrier
• The transparent on-board transponder performs
  phase demodulation and re-modulation
• Ranging signal in the transponder is only
  wide-band filtered and level controlled (AGC)
• The downlink signal contains the ranging signal,
  the uplink noise and also the telecommand echo

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Current ranging techniques

• Therefore, a path loss proportional to 1/r4
  (where r is the distance) is incurred plus the
  sharing and intermodulation losses due to
  simultaneous command
• Very simple transponder ranging channels are
  needed
• Interoperability and cross support by other
  Agencies is made possible by imposing a limited
  number of transponder specs (bandwidth, phase
  linearity, group delay stability, etc.)

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The need for different techniques

• New interplanetary missions have more and more
  stringent requirements also from orbit
  determination point of view
• Gravity assists or orbit insertions demands very
  accurate orbital knowledge
• Radio science requirements performed over the TTC
  channel have more demanding ranging or Doppler
  accuracy requirements
• Missions to farther away planets suffer from
  higher path losses
• Ranging regeneration and delta-DOR (Differential
  One-way Ranging) can cope with new mission
  requirements
• Regeneration of the ranging signal on board
  eliminates uplink noise and telecommand echo

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Regenerative PN ranging

• The ranging SNR at the station is therefore
  proportional to 1/r2
• Substantial gains (up to 30 dB) can therefore be
  achieved to obtain better ranging accuracy or
  increase the telemetry return or both
• Regeneration of the current sequential ranging
  codes is impractical (sequences are transmitted
  for periods of time inversely proportional to the
  SNR)
• Solution is to use a set of pseudo-noise ranging
  codes which allows parallel acquisition to speed
  up process
• Regenerative transponders require complete
  ranging channel (ranging clock and codes) be
  standardized if interoperability is to be
  achieved
• CCSDS has tasked the Ranging WG to develop a
  regenerative PN ranging standard encompassing
  both stations and spacecraft specifications

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CCSDS regenerative PN ranging sequences

• True PN codes of the Tausworthe family
• A Pseudo-Noise (PN) sequence is a periodic binary
  (?1) sequence whose periodic autocorrelation
  function, over one period of length L, has peak
  value L and has all L 1 off-peak values
  equal to 1.
• The Tausworthe codes are Pseudo-Noise (PN)
  ranging codes whereby the ranging sequence is a
  logical combination of the so-called range
  clock-sequence and several component Pseudo-Noise
  (PN) sequences. The range clock sequence is a
  periodic binary (?1) sequence of period 2 (just
  the alternating 1 and 1 sequence.)
• PN-Like codes of the Stiffler family
• The term Pseudo-Noise-Like (PNL) ranging is
  used in a broader sense to denote any
  ranging-sequence system in which the ranging
  sequence is a logical combination of the
  range-clock sequence and several periodic binary
  (?1) sequences that are chosen to give desirable
  correlation properties to the ranging sequence
  but do not satisfy the criteria for Pseudo-Noise
  sequences

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CCSDS Tausworthe sequences

• Starting point Tausworthe (6-components code)
• C1 1 1 (the range clock)
• C2 1 1 1 1 1 1 1
• C3 1 1 1 1 1 1 1 1 1 1 1
• C4 1 1 1 1 1 1 1 1 1 1 1 1 1 1
  1
• C5 1 1 1 1 1 1 1 1 1 1 1 1 1 1
  1 1 1 1 1
• C6 1 1 1 1 1 1 1 1 1 1 1 1 1 1
  1 1 1 1 1 1 1 1 1
• Component sequences C2, C3, C4, C5 and C6 are all
  PN sequences with relatively prime periods 7, 11,
  15, 19 and 23, respectively.
• The ranging-sequence chip is a 1 if and only if
  C1 has a 1 at that position or if all five of
  the sequences C2, C3, C4, C5 and C6 have a 1 at
  that position, or both.
• The range clock is strongly correlated with the
  ranging sequence, which facilitates locking on to
  the range clock at the receiver.
• The period of the ranging sequence is L 2 ?
  7 ? 11 ? 15 ? 19 ? 23 1,009,470 chips.

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Basic Tausworthe code
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Tausworthe T2 and T4 codes

• The second option under investigation is the
  weighted-voting (?2) Tausworthe ranging code
  (T2)
• made up of the same six binary (?1) binary
  periodic component sequences as the first
  option
• with a different combination algorithm based on
  giving ?2 votes to the clock component C1
• The third option is the weighted-voting
  Tausworthe ranging code (T4) obtained from option
  2 by giving 4 votes instead of 2 to the ranging
  clock
• The number of 1s and 1s in these sequences is
  not balanced, resulting in a d.c. component
  useless for the code acquisition and that may
  affect the carrier tracking performances
• The key to elimination of imbalance is the fact
  that the negative of a real sequence has the same
  autocorrelation function as the original sequence
  
• Using the negatives of the PN sequences C3, C4
  and C6 balanced weighted-voting Tausworthe (T2B
  and T4B) are obtained

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Tausworthe T2 code
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CCSDS Stiffler sequences

• The first Stiffler sequence being considered for
  CCSDS standardization is the scrambled
  weighted-voting (v6) Stiffler ranging code (SS6)
  given by
• 
  
  
• where
• ?6 is the number of votes given to the ranging
  clock component s1(t)
• si(t) are the outputs of a 20-bit counter with
  increment d (221 1)/3 699051
• The second option is the scrambled
  weighted-voting (v8) Stiffler ranging code (SS8)
  generated as for the first option but by giving
  v8 votes to the ranging clock component s1(t)
• The resulting ranging sequences are periodic with
  length L 220 1,048,576 chips

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Stiffler sequence generation
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Ranging sequences performance comparison

• Tausworthe assumptions
• Six parallel correlators used for acquisition
• range-clock frequency fRC 1 MHz
• loop integration time of 10 s
• Pe2 0.999 and 2EC /N0 -33 dB .
• Stiffler assumptions
• Four parallel correlators used for acquisition
• range-clock frequency fRC 1 MHz
• loop integration time of 10 s
• Pe2 0.999 and 2EC /N0 -33 dB.

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Ranging sequences performance comparison

• Normalized acquisition time t (relative to the
  acquisition time Ka of antipodal sequences
  having unity in-phase percentage correlation)
• being Pe2 (0.999) the probability of
  successful acquisition at a chip signal-to-noise
  ratio 2EC /N0 (-33 dB) ? Ka ? 30000
• Standard deviation of measurement error due to
  thermal noise ?

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Tausworthe ranging performance
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Stiffler ranging performance
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PN ranging performance with TC/TM

• Mutual interference with simultaneous TC/TM
  simulated
• Conventional low-rate telecommand signal
  (PCM/NRZ-L/PSK with 16 kHz subcarrier)
• Low-rate (PCM/NRZ-L/PSK) and medium rate
  (Bi-Phase/PM) telemetry signals
• Bi-Phase/PM telemetry data rate selected close to
  PN range clock component (worst-case)
• Both semi-analytical and Monte-Carlo techniques
• Both ideal and real (filtered, saturated)
  transponder channels
• Both (half) sine-wave and square-wave formats for
  ranging clock
• 2 Mcps ranging code (1 MHz ranging clock)

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PN ranging interference to TC/TM

• Example medium-rate TM (ideal channel,
  sine-wave)

The table shows the good agreement in terms of
BER results between the MC (Montecarlo) and the
SA (Semi-Analitical) approach
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PN ranging interference to TC/TM(Ideal Channel)

• Negligible degradation to TC or low-rate TM
• In case of medium-rate TM
• As general rule the code with strongest clock
  component (for instance T4 and S8) induce lower
  degradation. Ranging codes T2 and T2B give rise
  to largest losses, followed by code SS6 the
  other codes are practically equivalent
• The loss of the telemetry system due to the
  interference of the ranging system becomes larger
  as the ranging modulation index increases
• There is not clear difference in terms of BER
  degradation between sine or square-wave ranging
  shaping for the same ranging power
• In general the loss due to the ranging signal
  lies in the range 00.7 dB both for sine or
  square-wave ranging shaping and for a peak
  modulation index lower than 0.7 rad
• If the chip rate is equal to or very close to the
  TM Bi-Phase rate or its multiple, there is a
  strong influence between the Stiffler ranging
  sequence and the telemetry signal and the TM BER
  depends on the transmitted telemetry bits.
  However, outside such situation this behavior
  disappears.

Note in case of real channel the contribution
of the non linearities masks the effects of the
interference due to the PN ranging, in particular
for the code with the strongest clock component.
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PN ranging interference to TC/TM(Ideal Channel)
Note JPL code correspond to T code in table at
page 18
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PN ranging interference to TC/TM(Ideal Channel)
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PN ranging interference to TC/TM(Ideal Channel)
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TM interference to PN ranging

• As general rule the code with strongest clock
  component (such as T, T4 and S8) give rise to
  faster clock acquisition but slower phase
  acquisition
• Easier clock acquisition is obtained for T,
  S8/SS8 and T4/T4B (very similar), S6/SS6 and
  T2/T2B
• The minimum ranging phase acquisition times are
  obtained for T2/T2B, S6/SS6, T4/T4B, S8/SS8 and
  finally T
• If the chip rate is equal to or very close to the
  TM Bi-Phase rate or its multiple, there is a
  strong influence between the Stiffler ranging
  sequence and the telemetry signal and the
  performance of the Stiffler codes highly depends
  on the telemetry bit sequence. However, outside
  such situation this behavior disappears.
• While all Tausworthe codes show practically the
  same loss (about 0.7 dB _at_ BER10-6), the Stiffer
  codes show a slightly smaller loss (about 0.4 dB
  _at_ BER10-6).

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TM interference to PN ranging Acquisition
probability
P(e)
Ec/No
Note Lo is the integration time and Lr is the
sequence length
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TM interference to PN rangingAcquisition
probability
P(e)
Ec/No
Note Lo is the integration time and Lr is the
sequence length
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PN ranging conclusions

• From these results, the best choice for the
  ranging code seems to be code T4 or T4B
  however, difference in performance (also with T)
  is generally modest
• Codes T, T4B, T4 and even T2/T2B can be generated
  by same hardware with simple re-programming
• Future work
• Complete missing simulations
• Perform hardware tests on ESAs BepiColombo
  breadboard
• Perform hardware tests on NASAs New Horizon
  spacecraft
• Select the code(s) for CCSDS standard and publish
  the new standard by Summer 2008

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Delta-DOR

• Delta-DOR (Delta Differential One-way Ranging)
  determines the angular position of a spacecraft
  by correlating the received signal at two ground
  stations
• The ?DOR measurements provide an independent
  navigation measurement of spacecraft position
  apart from Doppler and sequential ranging, and
  improve trajectory determination accuracy in many
  cases.
• When used in conjunction with ranging and
  Doppler, delta-DOR can reduce the amount of time
  needed to determine the spacecraft position after
  a trajectory maneuver.
• Missions such as Mars Odyssey, the Mars
  Exploration Rovers (MER), Mars Express, and Deep
  Impact have all utilized and benefited from
  Delta-DOR measurements.

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Delta-DOR principle
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Delta-DOR measurements

• The received signal (from either a spacecraft or
  a quasar nearby) is recorded simultaneously at
  two different antenna separated by a baseline B
• Measurements conducted on the spacecraft and the
  radio source in nearby angular position are then
  differenced to remove the common effects of
  station clock offsets, instrument group delay
  errors, and most media effects
• By correlating the signals recorded by the two
  antennas, the differential group delay ? between
  the two signals can be determined
• The delay difference ? is then used to calculate
  the path length difference ?R between the source
  and the two antennas
• Using precise knowledge of the baseline B and ?R,
  the angle ? can be calculated
• The spacecraft transmits widely separated
  sinusoidal tones (DOR tones) around the carrier.
  Using a large bandwidth is important because
  errors due to receiver noise and instrumental
  phase ripple are inversely proportional to the
  bandwidth.

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CCSDS delta-DOR

• CCSDS standardization would allow for ?DOR
  measurements to be made between ground stations
  of different space agencies
• It is envisioned that the CCSDS ?DOR standard
  will consist of the following sections
• 1.        Spacecraft DOR tone generation
  Specify DOR tone span bandwidths, number of DOR
  tones, frequency bands to be used, and waveform
  type.
• 2.        Radio star catalog Specify list of
  quasars to be used as reference sources and their
  angular positions.
• 3.        Raw Data File Transfer Specify the
  format of the file will containing the sampled
  received signal from a particular ground antenna.
  These files will be transferred to a common
  location for correlation and processing.
• 4.        ?DOR Observables Transfer Specify the
  format of the DOR measurement results which can
  be used by the navigation team
• NOTE Section 1 exists already.