Die Huygens-Mission und das Doppler-Problem

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1 Die Huygens-Mission und das Doppler-Problem (Salvaging the Huygens-Mission from a Receiver Implementation-Flaw) L. Popken European Space Agency

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3 Die Zeit Dez 2000

4 Übersicht Seit seiner Entdeckung in 2000 hat das Doppler Problem des Huygens Telemetrie Empfängers viel Aufsehen erregt in den Medien; ungerechtfertigte Schuldzuweisungen sowie falsche Interpretationen verursacht. Was war tatsächlich passiert? Und wie wurde das Poblem gelöst?

5 Cassini releases Huygens Probe (25 Dec 2004)

6 Huygens Signal Spectrum 132 khz Subcarrier 2040 MHz Carrier Symbol Rate of 16 ksymb/sec (8 kbit/sec Data Rate)

7 Transmitter on Huygens Probe

8 Relay-Link Receiver In-Orbit Test (Feb 2000) Status of Relay-Link Receiver monitored via Cassini X-Band Telemetry

9 v (km/sec) 10 db, 5.7 km/sec 14 db, 5.7 km/sec 16 db, 5.7 km/sec 11 db, 3 km/sec Wir haben ein Problem! E s /N o (db)

10 Trouw 3 Nov 2000

11 132 khz Subcarrier 2040 MHz Carrier Symbol Rate of 16 ksymb/sec (8 kbit/sec Data Rate) Doppler Effect of (1+v/c) increases Frequencies and stretches Frequency-Bands

12 Data Transition Tracking Loop (DTTL) Data-Transition (P t ) dependence symbol stream Concept

13 In-phase Integration Midphase Integration Output only when Symbol- Transition

14 too small negligible noise effect Doppler Probe Influence

15 g (λ) g(λ) = -g(-λ) g(λ) = g(λ +n), n = Integer E s /N o = 0, 3, 6, 30 db λ = ε / T

16 F (ppm) P t Discontinuities by AGC Model Predicting the Tracking-Range E s /N o (db)

17 Automatic Gain Controls Coherent-AGC Gain affects DTTL Loop-Gain

18 v (km/sec) Model Validation by In-Orbit Tests : Sync-Lock : No Sync-Lock : Intermittent Sync-Lock E s /N o (db)

19 F (ppm) Model Validation by In-Orbit Tests : Sync-Lock : No Sync-Lock : Intermittent Sync-Lock Mission Profile E s /N o (db)

20 PRT-4

21 log(ser) F=9ppm F=16ppm Error-Rate along Sawtooth 2-nd E S /N o -Switching-Point Detection Performance degraded by Offset in DTTL E s /N o (db)

22 PRT-4 Loss of 0.28 % : 2 Frames Lost 8 Frames Not Decoded

23 Revised Relay-Link Geometry Periapsis

24 F (ppm) Uncertainty- Band in E s /N o minimum E s /N o mean Es/No maximum E s /N o E s /N o (db)

25 A (LHCP) B (RHCP) Comment Probe Tx RF POWER dbw CIRCUIT LOSSES db Probe ANT GAIN dbi Reference Case Probe EIRP dbw FREQUENCY GHz DISTANCE 1000*km Reference Distance SPACE LOSS db Probe ANT AXIAL RATIO db Reference Case HGA AXIAL RATIO db POLARIZATION LOSS db Total PROPAGATION LOSS db Cassini RX ANT GAIN, peak dbi HGA POINTING LOSS db Reference Case G/T db/k Es/No REFERENCE db Extract from Reference Link Budget

26 Gain (dbi) variation or uncertainty of actual gain max mean min Probe-Aspect Angle (deg)

27 Original Mission Profile 19 ppm (5.7 km/sec) + F F (ppm) Uncertainty- Band in E s /N o minimum E s /N o mean Es/No maximum E s /N o E s /N o (db) Example of a Mission Profile in the Frequency-Offset vs. E s /N o Chart.

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30 Lessons Learned Implement sufficient onboard Reconfigurability for Recovery from Design-Problems or Hardware-Failures: Design for Parameter-Selection or -Modification by Telecommand: Tracking-Loop and Synchronization Parameters, Thresholds. Allow for Algorithm-Adaptation by Software-Patch. Transmission of Telemetry Data should never be blocked intentionally. No conditioning on Data-Integrity or Correct Data-Alignment. Sophisticated Data Recovery can be applied on ground.

31 Warum lieferte Kanal-A keine Daten?

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34 RUSO was not switched ON

35 Final Remark Copies of IEEE Paper available to take home: Model of Receiver Design Flaw Crucial for Huygens Space Mission Recovery, IEEE Aerospace and Electronics Conf., March 2004, Big Sky, MT.

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