OPS Forum Delta-DOR 22.04.2005
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OPS Forum Delta-DOR 22.04.2005

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D-DOR stands for Delta-Differential One Way Ranging. D-DOR is the measure of the difference in signal arrival time between two stations. The observable is an uncalibrated time-delay between the two ...

D-DOR stands for Delta-Differential One Way Ranging. D-DOR is the measure of the difference in signal arrival time between two stations. The observable is an uncalibrated time-delay between the two antennas.

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  • 1. ESA Delta DOR ESOC, 22 nd April 2004
  • 2.  DOR definitions (1)
    •  -DOR stands for Delta-Differential One Way Ranging
    • DOR is the measure of the difference in signal arrival time between two stations. The observable is an uncalibrated time-delay between the two antennas.
    • “ Delta” is respect to a simple DOR, and refers to quasar calibration of the S/C DOR.
    • Since the quasar signal is recorded on the same BW of the S/C channels, ideally any errors which are station or light path dependent will cancel.
  • 3.  DOR definitions (2)
    • The extent to which these error sources cancel depends on the angular separation of the two sources being observed. The maximum angular distance between S/C and Quasar is 15 deg.
    • Thus, one is able to evaluate a potentially error-free relative station delay, which leads to an accurate determination of the S/C position in the plane of the sky
    • The measurement accuracy is given by:
    Longer baselines, better accuracies
  • 4.  DOR generic signal structure
    • S/C signal
      • Set of TM harmonics (possibly unmodulated) or dedicated DOR tones
      • Low order TM harmonics (and DOR tones) are typically not embedded in noise, high-order harmonics normally are
      • Accuracy on S/C signal time-delay determination is:
    • Quasar signal
      • Random noise-like signal
      • Quasar signal is totally embedded in antenna noise
      • Accuracy on Quasar signal time-delay determination is:
      • “ f max –f min ” is the “total spanned bandwidth”, i.e. the frequency span between the most distant tones, used in the observation.
  • 5. Current station architecture
    • XDC output BW: 100 MHz
    • LDC output BW: 30 MHz
    • IFMS input BW: 28 MHz
    • IFMS can handle only 2 35 Mb/s sampled inputs
    • IFMS CPU and connectivity not suitable to handle the expected data rate generated by the DDOR process
    8400 – 8500 MHz X-band D/C L-band D/C 1 Switching Matrix IFMS1 C F E GDSP RHC LHC 540-640 MHz 70 MHz Each input BW: 28 MHz L-band D/C 2 GDSP Test Test UCPU Estrack LAN
  • 6. Driving requirements (1)
    • Current ESA Deep Space Missions :
      • MEX, Rosetta, VEX transponders do not generate dedicated DOR Tones
      • Use of low and high order even TM Harmonics.
      • Recommended maximum spanned BW of about 15 – 20 MHz. This means using up to the 40 th harmonic (around 44 dBc).
      • This is within the input BW of current IFMS
  • 7. Driving requirements (2)
    • JPL Missions :
      • Current in-flight JPL missions have transponder with dedicated DOR tones at  3 MHz,  19 MHz (X-band). Maximum spanned BW: 38MHz
      • This is outside the maximum input BW of the IFMS (28MHz)
    • Band Re-centring :
      • Different parts of the spectrum have to be tuned to the current inputs of the IFMS
      • The relevant parts of the spectrum can be then properly processed in chunks (Band re-centring)
    X-band F C 3MHz 19MHz 19MHz 3MHz 20MHz 20MHz Input 1 Input 2
  • 8. Station configuration for Band re-centring LDC1 LDC2 IFMS1 CFE LDC3 IFMS3 FMS2 CFE CFE Input1 Input1 Input1 Input2 Input3 Input2 Input2 Input3 Input3 A B C A..B.C A B C SWITCHING MATRIX X 2 1 Y 2 1  2 1 X 1 1 Y 1 1  1 1 X 3 1 Y 3 1  3 1 X 1 2 Y 1 2 X 1 1 Y 1 1  1 1 X 2 1 Y 2 1  2 1 X 3 1 Y 3 1  3 1 X 1 2 Y 1 2 X 1 2 Y 1 2
  • 9. Driving requirements (3)
    • Ka-band
    • Future ESA Deep Space Missions (BepiColombo) :
      • Transponders CCSDS compliant (DOR Tones at  4 MHz,  20 MHz in X-band, at  4 MHz,  20 MHz,  80 MHz in Ka-band, i.e. BepiColombo).
      • Maximum spanned BW 160 MHz (in Ka-band).
      • This is also outside current IFMS input BW (again, band re-centring has to be used but this will not cover the maximum spanned BW).
    80MHz F C 4MHz 20MHz 20MHz 80MHz 4MHz 28MHz 28MHz Input 2 Input 1
  • 10. Implementation approach
    • All Station, IFMS, Storage Device, Link modifications are highlighted in red
    CORRELATOR D/C IFMS (EOLP GDSP) ESU DSA1 Estrack Lan D/C IFMS (EOLP GDSP) ESU DSA2 Estrack Lan
  • 11.  DOR observation (1) Sequence
    • Observation Sequence: S/C – Quasar – S/C (or vice versa)
    DOR Tone or TM Harm. F C Ch. BW (50kHz) DOR Tone or TM Harm. DOR Tone or TM Harm . DOR Tone or TM Harm. Ch. BW (50kHz) Ch. BW (50kHz) Ch. BW (50kHz)
    • S/C Signal: Telemetry Harmonic (or DOR Tone)
    Ch. BW (2MHz) Ch. BW (2MHz) Ch. BW (2MHz) Ch. BW (2MHz)
    • Quasar Signal: White Noise
  • 12. (1) IFMS modifications
    • IFMS GDSP Modifications
      • 4 channels with BW from 1kHz to 2 MHz
      • Quantization 1,2,4,8,16 bits
      • Synchronization of GDSPs internal clock
      • 2 GDSPs will be modified, to allow reception of up to 8 channels
    • IFMS Internal LAN
      • Private VLAN for  DOR data routing (up to 36 Mbits/s)
    IFMS C F E CH4 CH3 CH2 CH1 36Mbits/s GDSP2 36Mbits/s GDSP1
  • 13. (1) Data rates and volumes
    • Data Rates & Volumes (each GDSP)
      • S/C Observation :
        • Data rate: 50kHz sampling I & Q, 8 bits quantization, 4 channels = 3.2 Mbits/s.
        • Data Volume: 10 minutes observation = 0.24 GB
      • Quasar Observation :
        • Data rate: 2MHz sampling I & Q, 2 bits quantization, 4 channels = 32 Mbits/s.
        • Data Volume: 10 minutes observation = 2.4 GB
    • Need for an External Storage Unit (ESU)
  • 14.  DOR observation (2) Data collection
    • Redundant ESUs
    • Each ESU collects and formats data coming from any IFMS
    • ESUs are physically located in the station MER
    • Redundant connectivity between IFMSs and ESUs is provided by station LAN
    ESU1 IFMS1 GDSP1 GDSP2 IFMS2 GDSP1 GDSP2 IFMS3 GDSP1 GDSP2 ESU2 LAN Switch AER AER LAN Switch MER MER Existing station LAN (100 Mb/s)
  • 15.  DOR observation (3) Data type
    • Data type is “open loop”
      • Time-tagged I and Q samples on a selected polarization collected in files (1 file per minute)
      • Observation configuration (fixed during observation) parameters in a Primary Header
      • Parameters that may vary during observation are stored in a Frame Header. The size of each frame is of 1526 bytes.
    Primary Header Frame Header and Data
  • 16.  DOR observation (4) Data transfer
    • Data collected at the two station are transferred to a centralised facility (ESOC) to perform the correlation process.
    • In the worst case sequence of observation (Quasar – Spacecraft – Quasar), the data volume will be of up to 5 GB (using 4 channels), and up to 10 GB (using 8 channels).
    • Data transfer from the stations to the correlator has to be completed within 8 hours (in critical phases).
    • Link speed requirement: 10 GB / 8 hours = 2.77 Mbits/s
  • 17. (4) Data transfer requirements
    • CEB and NNO Deep space stations will be equipped with 2Mbit/s lines
    • Assuming a maximum operational throughput of about 1Mbit/s, the remaining 1Mbit/s can be assigned to DDOR data transfer.
    • The extra capacity (about 2Mbit/s in the worst case), during DDOR data transfer, shall come from ad-hoc leased lines.
    • The feasibility of this solution has already been assessed with OPS-ONC
  • 18.  DOR observation (5) Correlator I/F
    • Correlator interfaces definition
    • Station1, Station2 : open loop data
    • FD (input): orbital data to help the correlation process
    • FD (output): the final product of the correlation
    • CP: configuration parameters
    • The correlator will be implemented on a Linux workstation placed in an operational area
    Inputs Outputs FD Station 1 CP S/W Correlator Station 2 FD
  • 19.  (5) Signal structure
    • S/C signal Correlation
      • Set of TM harmonics
      • Low order TM harmonics are typically not embedded in noise, high-order harmonics are.
      • Signal characteristic permits phase extraction
    • Quasar signal Correlation
      • Noise-like signal
      • Quasar signal is totally embedded in receiver noise
      • Signal characteristic forces to go for a direct correlation method
  • 20.  DOR observation (6) S/C signal correlation
    • S/C signal Correlation (1)
      • Phase extraction by means of a S/W frequency estimator on the lowest TM even harmonic (highest in S/N)
      • Update of the expected phase (produced after an input by FD), with the estimation obtained above.
      • Drive the phase extraction of all other tones with the model (properly Doppler-scaled) built on the lowest TM harmonic
      • At this point, we have the phase sequences of each TM harmonic at each Station, sufficiently corrected for phase uncertainties.
      • Correlation of the phase of each channel of Station 1 with the phase of the corresponding channel of Station 2
      • The correlation result as such is “modulo 2  ” ambiguous
  • 21.  DOR observation (7) Ambiguity resolution
    • S/C signal Correlation (2)
      • In order to solve such ambiguity, some operation is performed on each pair of observed channels.
      • The right delta-delay is the one identified by the slope (S) straight line.
      • The S/C DOR delay (  S/C ) is then calculated as follows:
      
  • 22.  DOR observation (8) Quasar signal correlation
      • Each data stream (channel) from each station is delay- and Doppler- compensated (using the model provided by FD). The delay is mostly given by Earth rotation
      • After delay- and Doppler- correction, each data stream of Station 1 will be correlated with the corresponding data stream of Station 2 for a range of delays (few  s) around the expected value (provided by FD)
      • The analysis of the observed data is split in observation periods (“accumulation periods”) of 1 s, in order to keep a tolerable level of error in Doppler compensation
      • Correlation is performed for a suitable integration time in order to maximise the signal-to-noise ratio
      • Delay resolution is improved by the use of available multi-band recordings (enlarging the total spanned bandwidth)
      • The result of the correlation is then added to the value given by FD
      • As a result, one obtains a Quasar DOR (  Q )
  • 23.  DOR observation (9) Correlator final output
    • The final output of the correlator is a file containing three DOR measurements (in the Table only the most important parameters are reported).
     S/C  S/C  Q
  • 24.  DOR observation (10) Orbit determination
    • To obtain a single  DOR observation the two S/C observations are linearly interpolated to the time of the single Quasar observation. Direct differencing of the observations can then be made.
    • The obtained time is used to correct FD estimation of the delay we are looking for.
    • The corrected delay is then used to calculate the angle between the orthogonal to the baseline and the S/C direction
    • Results are then calibrated for media effects
    • Overall accuracies are mainly driven by:
      • Maximum spanned BW
      • SNR of S/C and Quasar signals
  • 25. Summary of the work to be done
    • Use of 3 rd IFMS in ESA Deep Space facilities as primary  DOR processor
    • Modification of the IFMS internal DSPs (GDSP)
    • Availability of all IFMSs in Deep Space facilities for redundant  DOR measurements
    • Private VLAN for  DOR data routing (IFMS-ESU up to 36 Mbps)
    • Use of a PC (ESU) with a fast Hard Disk for  DOR data storage (up to 8 GB per  DOR observation)
    • Enhancement of link capacity (station-correlator) for the availability of data in near real time
    • S/W Correlator development limited to MEX, VEX, Ros support only
  • 26. Validation plan
    • First system tests possible during Correlator SAT.
    • Test Campaign on current ESA Deep Space missions (MEX, Rosetta).  DOR results will be compared versus standard tracking techniques (Doppler tracking and ranging).
    • Possible tests with Smart-1 (equipped with a transponder generating DOR tones at  2 and  16 MHz)
    • Comparison with JPL  DOR results (in S/C orbital reconstruction) during VEX Cruise phase
  • 27. Further developments
    • BepiColombo
      •  DOR will be needed as well for BepiColombo orbit insertion
      • BepiColombo will be equipped with a transponder capable of generating dedicated DOR tones in both X- and Ka-bands.
      • The use of a much larger spanned BW will permit better results in terms of S/C position accuracy
    • JPL interoperability
      • JPL is interested in interoperability with ESA since this would permit JPL to use the CEB – NNO baseline (JPL do not have any similar baseline)
      • Lot of work is needed to have compatible data formats (at station level).
  • 28. Example of error budget
    • Main Parameters
    Station Geodesy Geodesy S/C Link Link Link Astronomy   S/C   Q
  • 29. Current schedule (IFMS-EOLP)
    • Kick-off IFMS-EOLP (31 January 2005);
    • Critical Design Review by Kick-off + 10 weeks (held on 13-04-05);
    • Pre-Acceptance Report by Kick-off + 4 months
    • Final acceptance of IFMS-EOLP, (all documents delivered and accepted by the Agency, Acceptance certificate issued) by Kick-off + 6 months.
    • IFMS upgrade in DSA1-DSA2 by Kick-off + 9 months
  • 30. Current schedule (Correlator)
    • Kick-off meeting (28 January 2005);
    • Critical Design Review by Kick-off + 12 weeks
    • End of development phase by Kick-off + 8 months
    • Final acceptance of S/W Correlator, (all documents delivered and accepted by the Agency, Acceptance certificate issued) by Kick-off + 9 months.
    •  DOR Test Campaign: December 2005
    • Venus Orbit Insertion: March 2006
  • 31.  DOR team
    •  DOR Team in OPS-GS is:
      • Ricard Abelló
      • Javier De Vicente
      • Marco Lanucara
      • Roberto Maddè
      • Mattia Mercolino
    • Many thanks to Mattia Mercolino for helping in preparing this presentation