TT_C BWG by suchenfz


									                         MORE Team Meeting

   On-Board Calibration System
    for the Range Delay of the
         BepiColombo KaT
G.Boscagli (ESA-ESTEC)
M.Mascarello (AAS-I)
27th February 2007

 27th February 2007
                                             Page 1
       Introduction for BepiColombo
         KaT On-board Calibration
            (for Ranging Delay)

27th February 2007
                                      Page 2
                 Approach for On-Board Calibration

      The approach hereafter preliminary analysed is based on the idea
       of implementing this function directly inside the KaT unit:
            Target  Calibration as KaT Internal Unit Function

      The idea is to include inside the KaT unit both the SSPA and the
       Diplexer function.
       In general more compact solutions improve S/C design (mass,
       interfaces routing, etc); it is believed that also calibration
       performances should be improved following this approach.

        NOTE – In this way the calibration doesn’t take into account the wave-guides
        (from-to-antenna) and the antenna itself. They are outside the calibration loop.
        Which is the contribution of wave-guides and antenna in the overall end-to-end
        ranging budget error? At present it is understood that the main effect to be
        considered is related to the input/output mismatching variation due to temperature
        variation. This might cause multi-path effects and error in the end-to-end ranging

27th February 2007
                                                                                     Page 3
                 Approach for On-Board Calibration
    For this reason (Calibration as an internal KaT function) the KaT unit must
     be commanded (by the on-board computer) in two different modes:
         Nominal Mode: RF link via antenna, unit in coherent mode (down-link coherent
          with the uplink both for carrier and ranging signal) when RX in Tracking Mode.
         Calibration Mode: RX and TX in Loop-back Configuration, unit running with the
          internal oscillator, RX coherent (in tracking) with the loop-back signal from the TX

    The approach hereafter proposed is based on the use of PN regenerative
     ranging, the reasons are:

         The use of this ranging scheme simplifies the calibration scheme in particular for
          the ambiguity resolution (when compared with other approaches as the ESA STD
          or the NASA Tone Ranging).
                Note - The ambiguity must be solved since the KaT loop-back delay (TX-RX) is
                expected of the order of microseconds, while the WBRS band should be in the range 5-
                20 MHz.
                This comment might not be valid anymore in case the group delay variation (versus
                environmental conditions and including aging) is inside the WBRS ambiguity resolution.
                This is difficult to be predicted at this stage.
         The PN regenerative ranging is already implemented inside the BepiColombo
          X/X/Ka Deep Space Transponder, so it can be easily re-used for the KaT unit
          simplifying the multi-frequency operation as needed by BepiColombo for plasma

27th February 2007
                                                                                              Page 4
         Impact of Calibration on
        BepiColombo KaT Front-End

    NOTE: this section has been written without considering the
    current subsystem architecture

27th February 2007
                                                              Page 5
                  Impact of Calibration on KaT Front-End Architecture

                                                   Preliminary RF Power Budget (TX Filter neglected)
                                                                                         RF Budget

T2 is indicated as variable attenuator, it might       TX                 2   W           33.01    dBm
be commanded for selecting the proper RF               TX coupler        30   dB           3.01    dBm
power (at RX input) for calibration: minimum           Attenuator- T1    30   dB          -26.99   dBm
value around -146 dBm (nominal value around            Passive Mixer     10   dB          -36.99   dBm
-125 dBm, TBC) .
                                                       Attenuator- T2    60   dB          -96.99   dBm
                                                       RX coupler        50   dB         -146.99   dBm

                                     34 GHz
                                                     RX Filter                LNA
          RX coupler                                                                     Loop-Back Path
     To Ant                                                   LO @ 2 GHz
                                                                              Amplifier Gain Control (TBC)
         TX coupler
                                                     TX Filter           Amplifier
                            32 GHz                                      2 Watt SSPA
   27th February 2007
                                                                                                         Page 6
             Impact of Calibration on KaT Front-End Architecture

   According to the proposed approach, the new
   circuits/functions to be developed are:
        The functions in the light-blue boxes: 2 GHz LO, T1, T2,
        The functions in the light-green boxes: TX coupler, RX coupler
          NOTE - The functions indicated in the light-red boxes represent the Diplexer and
          they are present in any case. In the current subsystem baseline (see dedicated
          slides by AAS-I on this issue), the Diplexer is indicated as external to the KaT;
          the advantage of having it integrated inside the KaT unit is clear from a
          calibration point of view and for a more compact solution.

   According to the architecture as proposed in the
   previous slide, when in calibration mode the loop-back
   signal is routed back from the TX to the RX side and to
   the antenna as well.
             NOTE – It might be useful (TBC) to introduce the control of the TX Power (2
             Watt SSPA) to minimise the TX power via antenna when in calibration mode;
             question: is the SSPA delay dependant on the selected Gain/Output-Power?

27th February 2007
                                                                                     Page 7
              Impact of Calibration on KaT Frequency Plan

    The KaT frequency plan must be carefully studied in order to simplify
     the generation of the 2 GHz LO signal and to avoid internal RFI
           NOTE - For instance considering the Cassini KaT frequency plan (next slide) we
           observe that the 1st IF chain is almost at the same frequency of the LO signal for
    The Cassini KaT turn-around ratio (next slide) is not included in the
     current CCSDS/ECSS recommendations for TT&C applications. The
     values from the current ECSS-E-50-05B (Radio Frequency and
     Modulation, draft issue under public review) are:

     The Ka/Ka turn-around ratio values are under discussion also in
      the frame of CCSDS, at present the draft recommendation
      (January 2007) is to use 3599/3344 and 3599/3360

27th February 2007
                                                                                      Page 8
                                     Cassini KaT Frequency Plan

                                                                                                          AGC LOOP
                        Front-End                                                                                                         AGC
                                                                                                                    XT AL Filter        DETECTOR
                                                                 Bw = 80 MHz                       Bw =2 MHz        Bw = 15 KHz
                P reselector   L.N.A. & Filter
       input                                             21F                                   F
                                                     X                         A          X
                                            294F                       1st IF           20F                    2nd IF
                      output                       x3
                                                 & Filter                                HPLL                                      X              X
                          294F                                                            x5
                    to external amplifier             98F                                                                              Q-branch
                                                                                              4F                              Loop                    I-branch
                                                   x49                                    x4
                                                            2F                                 F                                             Detector
                                                                                                               0°    90°
                                                                                                          VCXO                              Lock Status
                                                            F  109 MHz

                                                            Turn-around ratio = 294/315 = 14/15

27th February 2007
                                                                                                                                                       Page 9
     (from BepiColombo SRR Data Package)

This section has been provided by AAS-I (Marco Mascarello)

   Question: Are there any difficulties (due to on-board baseline
   architecture) for implementing the above proposed approach for

27th February 2007
                                                             Page 10
              Communications Subsystem Current baseline

27th February 2007
                                                          Page 11
          Communications Subsystem (Option KaT Amplifier)

27th February 2007
                                                            Page 12
                           KaT on board calibration including Triplexer

   Including a Triplexer inside the KaT, it would be possible to calibrate all the paths till the antenna interface.

                                                                         From Ka-Band TWTA (X/X/Ka DST)

              Ka TRIPLEXER
                                                                                             KaT assembly

                                                       34 GHz
                                                                          RX Filter              LNA

To Ant
                                                     T2                            LO @ 2 GHz


                                                                           TX Filter          Amplifier
                                                       32 GHz                               2 Watt SSPA

    27th February 2007
                                                                                                          Page 13
      Triplexer (from current BepiColombo SRR Data Package)
 The Ka-Band Triplexer is a 4 port device in charge of splitting input
  and output signals. It will consist of a new development for
  BepiColombo based on existing technology.
 The splitting will be accomplished by an E-plane trifurcation. Each
  sub-band will be selected by an H plane filter. The foreseen useful
  bandwidth of the filters will be the following:
            Rx Filter : 50 MHz (TBC) within 34 200 to 34 700 MHz
            Tx1 Filter : 200 MHz (TBC) within 31 800 to 32 300 MHz
            Tx2 Filter : 50 MHz within (TBC) 31 800 to 32 300 MHz
 As for the X-Band diplexer, the mechanical concept will be two
  symmetrical pieces. Interfaces will be standard WR28 waveguide
  flanges. The estimated dimensions for the assembly are 75 x 45 x
  23.1 mm, while the estimated maximum mass should be less than
  65 g.

27th February 2007
                                                                      Page 14
    The above solution based on the Triplxer inside the KaT
     unit shows an important drawback:
     The Ka-band DST signal is applied to the Antenna
     through the KaT unit.
            This represents a blocking point !

    Other solution must be addressed, for instance:
     1. Keeping the Triplexer external to the KaT unit
        (calibration not anymore an internal KaT function).
     2. Analysing different mixing approach between DST
        and KaT signals at HGA input.

27th February 2007
                                                         Page 15
       BepiColombo X/X/Ka DST PN
          Regenerative Ranging
        1999 JPL

        Balanced Weighted-Voting Tausworthe (v=2 and 4)

27th February 2007
                                                     Page 16
        Introduction to Pseudo Noise (PN) Ranging Sequence

•   The term “Pseudo-Noise (PN) ranging” refers in a strict sense to the use of a
    ranging-sequence system in which the ranging sequence is a logical
    combination of the so-called range clock-sequence and several Pseudo-
    Noise (PN) sequences.
•   The range clock sequence is the alternating +1 and –1 sequence of period 2

                                             (a) ranging–sequence waveform

          Range Clock Frequency =
            f RC        Hz .
                      2TC                      (b) range–clock waveform

•   A Pseudo-Noise (PN) sequence is a binary 1 sequence of period L whose
    periodic autocorrelation function has peak value +L and all (L–1) off-peak
    values equal to –1.

27th February 2007
                                                                             Page 17
        Introduction to Pseudo Noise (PN) Ranging Sequence

Example for the introduction of the Titsworth/Tausworthe generation scheme

                                                                Component Sequences
  PN Sequence                                                     Probe Sequences

   As an example, considering the following component sequences of period 2, 3 and 5,
   respectively (the first period of each sequence is underlined):
       Seq. Gen. # 1

       Seq. Gen. # 2

       Seq. Gen. # 3

   Combined by majority logic give the following period-30 sequence:
        PN Sequence

27th February 2007
                                                                                  Page 18
        Introduction to Pseudo Noise (PN) Ranging Sequence

Example for the introduction of the Titsworth/Tausworthe generation scheme

     Note that the period T of the PN sequence obtained with the
      Tausworthe scheme is given by: T  LCM T1 ,T2 ,..., Tm 
      with LCM = Least Common Multiple                         30 in the above example

             Importance of having prime length component sequences

   The correlation of this sequence (considered as +/-1 sequence) with the
    component/probe sequences gives the following results:

   Note that 2 + 3 + 5 = 10 operations of correlation are required instead
    of the 30 operations needed in the “classical” approach. In fact, only 9
    decisions are required because of the antipodal result of the sequence
    of period-2 (the clock sequence). Only one of the two operation of
    correlation must be performed because the other correlation will be the
    negative of the other.
27th February 2007
                                                                                  Page 19
         Introduction to Pseudo Noise (PN) Ranging Sequence
                        More in general we can state that:

      The ranging sequence is acquired by the receiver as the result of correlations
       between the received sequence and certain ±1 periodic sequences (and their
       cyclic shifts) whose periods are divisors of the rangingsequence period and that
       we will refer to as probing sequences.
      The probing sequences are related in some manner to the ranging sequence, e.g.,
       the ranging sequence might be the sequence resulting from some sort of voting
       by the chips of all the probing sequences at the same chip time.
      The probing sequences must have the property that when all these “in-phase”
       decisions are correctly made, then these decisions determine the delay (modulo
       the ranging sequence period L) in chips of the received ranging sequence relative
       to the corresponding model of the ranging sequence. The (one-way) ambiguity
       (U) due to the period of the ranging sequence in meters is
                      1          cL                      1     f Chip _ Rate
                U  c  L  Tc               f RC                          = ranging clock frequency
                      2          4 f RC                  2Tc         2
                   c = speed of the light      f Chip _ Rate  chip rate

                     For example, with L = 1,009,470 chips and
                     f RC  10 6 Hz, U  75,710,000 m or about 75,710 km.

27th February 2007
                                                                                             Page 20
               The 1999 JPL PN Ranging scheme (Tausworthe scheme)
                              Titsworth/Tausworthe generation scheme

   The combining logic is based on the following rule: the ranging-sequence chip is a +1
    if and only if either C1 has a +1 at that position or all five of the sequences C2, C3, C4,
    C5 and C6 have a +1 at that position, or both.

In literature this
sequence can be
indicated also as
JPL 99 or Taus

                         where the combining logic is   C  C1 C2  C3  C4  C5  C6

      C1, C2, … C6 are the so called Probing Sequences.

  27th February 2007
                                                                                           Page 21
            The 1999 JPL PN Ranging scheme (Tausworthe scheme)

 It is obvious from this combinational rule that the range clock will be strongly
  correlated with the ranging sequence, which facilitates locking on to the range clock at
  the receiver.

 Since the component sequences C2, C3, C4, C5 and C6 are all PN sequences with
  relatively prime periods 7, 11, 15, 19 and 23, respectively, the period of the 1999 JPL
  ranging sequence is L =2x7x11x15x19x23 = 1,009,470 chips.

 The probing sequences in the 1999 JPL PN ranging-scheme are the range clock
  sequence together with the five component PN sequences.

 The total number of correlation operations required for the probing sequences,
  excluding the range clock, is thus 7 + 11 + 15 + 19 + 23 = 75.

 27th February 2007
                                                                                     Page 22
           The 1999 JPL PN Ranging scheme (Tausworthe scheme)
           Correlation characteristics and spectrally relevant properties of
                    the ranging sequence and probing sequences

                                                                              Residual carrier
                                                                              Mod index = 0.82 rad-pk
Clock Components
at ±fRC

                                                                               Chip Rate at
                                                                               ± fChip_Rate = 2.5 Mcps

  The spectrum shows a powerful clock component at half the chip rate and below a noisy
  floor originating from the combination process with the other probing sequences. The fact the
  range clock is strongly correlated with the ranging sequence will facilitate locking on to the
  range clock at the receiver. The chip is square-wave shaped.
    27th February 2007
                                                                                           Page 23
                Weighted-Voting Tausworthe PN Ranging-Sequence Scheme

The Weighted-Voting Tausworthe sequences are derived from the 1999 JPL PN
Ranging sequence with an apparently small modification on the vote logic.
 The selection of different value for the clock vote (v=2 or 4) provides:
     flexibility in the choice of the strength of the range-clock component in the ranging
     different level for the power allocated to the clock and the other ranging spectral
         Select chip value with most votes

                                             v   +1 1    C1

                                             1   +1 +1 +1 1 1 +1 1     C2

                                             1   +1 +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

                                             1   +1 +1 +1 +1 1 +1 1 +1 1 1 1 1 +1 +1 1 +1 +1 1 1   C5

                                             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

                                                      6
      where the combining logic is C  sign  2 C1   Ci 
                                                    i 2 
27th February 2007
                                                                                                                             Page 24
      Balanced Weighted-Voting Tausworthe PN Ranging-Sequence Scheme

The Balanced Weighted-Voting Tausworthe sequences are derived from the
Weighted-Voting Tausworthe sequences (scheme above) with an apparently
small modification on the polarity of some probe sequences.

As the 1999 JPL PN Ranging scheme (Tausworthe scheme) also the Weighted-Voting
Tausworthe PN Ranging-Sequence Schemes (both for v=2 and 4) present a DC component.
A simple way to reduce the imbalance in the ranging sequence (and to produce what we call
the Balanced Weighted-Voting Tausworthe ranging-sequence scheme) is choosing the PN
probing sequences with the following first periods:
          C1 = +1 1
          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
          Note -   The key to elimination of imbalance is the fact the negative of a real
                   sequence has the same autocorrelation function as the original sequence.

  27th February 2007
                                                                                    Page 25
                        BepiColombo X/X/Ka DST: Code Phase Acquisition

                                                                                                                           Code Correlators                     Lock
The current model of
                                                                                                                              CODE CORRELATOR #1      Max.
BepiColombo X/X/Ka DST is                                      IN-PHASE        SIGN
programmable and can handle the           From Carrier       INTEGRATOR                                                       C1
                                          Quadrature                                                                                                 Dismiss
different schemes: JPL99, BT2 and         branch
                                                Qc                                          +/-1                               CODE GENERATOR #1

BT4.                                                          MID-PHASE                                                                                         Lock
The Regenerative Ranging                                                                                                      CODE CORRELATOR #2      Max.
Channel is composed by:                                                                                                        C2
                                                                TIMING                NCO                          LOOP        CODE GENERATOR #2

 the Chip Tracking Loop (CTL)
                                                                LOGIC                                             FILTER

for ranging code clock component                                                                         Nominal Chip
                                                                                                                              CODE CORRELATOR #3       Max.
phase and frequency recovery                                                                                Rate
                                                                                               N        Carrier Loop          C3
 the In-phase Integrator output                                                                                               CODE GENERATOR #3

is provided to Code Correlators:                                                       Code Components
                                                          Chip Tracking Loop           Generators Clock                                                         Lock
Six Correlators running in parallel for                                                                                       CODE CORRELATOR #4       Max.
probe sequences (C1,…. C6)                                                                                                                            Search
position recovery                                                                                                  C1                                Dismiss
                                                          Lock Status                                                          CODE GENERATOR #4

                                            Down-Link                                                              C2
 the Down-link Code Generator             Ranging Code
                                                                                                                   C3          CODE CORRELATOR #5      Max.
(In this case only the JPL99 case is                                      OR
                                                                                                   AND             C4
represented)                                                                                                                   C5
                                                                                                                   C5                                 Dismiss
                                                                                                                                CODE GENERATOR #5

                                                          Down-Link Code Generator                                             CODE CORRELATOR #6      Max.
                                                              REGENERATIVE RANGING CHANNEL                                                            Dismiss
                                                                                                                                CODE GENERATOR #6

     27th February 2007
                                                                                                                                                    Page 26
                       BepiColombo X/X/Ka DST: Chip Tracking Loop (CTL)
      The mid-phase integrator output is multiplied by +/-1 in order to provide
      the right correction to the loop. In a certain way the multiplication by +/-1
      replaces the transition detector typical of a DDTL, considering that the        Filtered Loop Error
      PN sequence resembles a square-wave.



Scaled Carrier
     Loop Error

     27th February 2007
                                                                                                   Page 27
      BepiColombo KaT Calibration
               based on
       PN Regenerative Ranging

27th February 2007
                               Page 28
     Impact of Calibration on BepiColombo KaT Baseband Processing

    We need a separate PN code generator on the TX side clocked by the on-
     board oscillator
         In the current X/X/Ka DST design the TX PN code is generated coherently with
          the received up-link PN code (see previous slide).

    The TX PN NCO and the RX PN NCO must be clocked with the same
     oscillator, avoiding any timing error between the two signals.

    At the start of the calibration procedure (defined by a strobe signal common
     to RX and TX processing functions) the two PN code generators (RX and
     TX) must be identically initialised.

    The loop back ranging signal acquired by the RX provides the delay from TX
     to RX (Loop-Back Delay).
         The PN code phase acquisition (using the Probe Sequences) is used for
          ambiguity resolution
         The phase difference between TX and RX ranging clock provides the accurate
          delay measurement

27th February 2007
                                                                                Page 29
     Impact of Calibration on BepiColombo KaT Baseband Processing

    The phase difference between the RX and TX PN Ranging Clock can be
     measured using the filtered phase error loop term of the CTL
                               CTL Detector
                                                   X           1/S
     To RX PN Code Generator
                               RX NCO                              E

                                                              B (nominal chip rate)

                        On-Board Clock

                               TX NCO                     B (nominal chip rate)

       To TX PN Code Generator
                                              CTL second order loop

27th February 2007
                                                                                      Page 30
     Impact of Calibration on BepiColombo KaT Baseband Processing
                                                               K
      Open Loop CTL Transfer Function       G ( s )  K d     NCO
                                                                            s     s
      In the X/X/Ka DST the CTL is digitally implemented inside the RX
       Digital Section (Ts is the loop sampling time), using the Z transfer
       function we have                           T 2  K
                                      G( z )  K d   Ts 
                                                                     NCO1
                                                                 1 
                                                            1 z  1 z
  CTL second order loop:
   digital representation

                         Kd                                        Ts           +
                       CTL Detector

                       RX NCO                                               E
                        N-BIT                                  +

                                                                   B (nominal chip rate)

27th February 2007
                                                                                           Page 31
     Impact of Calibration on BepiColombo KaT Baseband Processing

      After the transient phase, the error term E provides the
       measurement of the delay between the TX and RX ranging clock
       signal. In radiant we can write:         F              1
                                            2 clk E  Ts  2 N E
                                                 2N           2
      While in time we have:            N
                                           E  Ts
      It is evident that for typical loop sampling time of the order of 40
       MHz and N=32 bit NCO the phase/time resolution is well below
       the required BepiColombo ranging delay accuracy. The calibration
       resolution in time due to the digital loop implementation is:
            Ts 2 N
      The Probe Sequence acquisition phase and the CTL error term
       (E) must be transmitted via telemetry down-link (using the X/X/Ka
       DST link). This information (after proper post-processing) can be
       used (on-ground) to evaluate the accurate Loop-Back delay.

27th February 2007
                                                                     Page 32
     Impact of Calibration on BepiColombo KaT Baseband Processing

       Notice that the KaT Ranging Delay (RX => TX) and the Loop
        Back Ranging Delay (TX => RX) might be different, this is due
           1. The TX/RX different paths (between nominal and calibration mode)
              in the Front-End (Attenuators, Mixer, Couplers)
           2. Different routing of the signal in the baseband digital processing
              (ASIC gates).

       Probably the first contribution could be kept small (and
        negligible also under variations of environmental conditions) in
        terms of overall error budget. This to avoid further complications
        in terms of calibration.

       Also the second contribution (inside the DSP) might be kept
        negligible; however if not negligible, the delta (between the KaT
        Ranging Delay and the Loop Back Ranging Delay) can be
        measured at unit level in the LAB. Notice that this contribution is
        almost independent from the temperature since it is related to
        the clock drift (Note - the X/X/Ka DST is embarking an OCXO).

27th February 2007
                                                                          Page 33

27th February 2007
                                   Page 34

     In order to improve the calibration performances and to
      minimise the on-board complexity it is suggested to integrate
      inside the KaT unit the SSPA, the Diplexer (*), the RF
      Calibration Front-End (Attenuators, Couplers, mixer, LO).
      (*) The possibility to integrate the Diplexer/Triplexer is not clear
      (under discussion).l

     The use of PN Ranging (as per X/X/Ka DST) simplifies the
      calibration function in particular for ambiguity resolution.

     Minor changes are foreseen for the base-band digital signal
      processing(*). The approach is to transmit the CTL error term
      via TLM link for post-processing at the G/S.

         (*) However these have an impact on the current X/X/Ka DST FPGA/ASIC

27th February 2007
                                                                                Page 35

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