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3.4 End-to-end measurement concept

DWE is the only Huygens investigation with "science hardware" on both the Probe and within the Probe Support Avionics (PSA) of the Probe Support Equipment (PSE) on the Cassini Orbiter. An end-to-end block diagram of the DWE experimental configuration is shown in Fig. 6.

 
Figure 6: End-to-end configuration of the Doppler Wind Experiment.

The DWE-TUSO on the Huygens Probe is the primary signal generator used to drive the PRL of Transmitter A (Tx A). In case of failure, the driver signal can be switched to an internal TCXO oscillator with a frequency stability approximately 1000 times worse than the TUSO. The choice of oscillators for Tx A will be made prior to probe-orbiter separation, as based on the performance of the TUSO and TCXO during the regular cruise phase checkouts.

The 10 MHz output of the TUSO is upconverted to the PRL frequency of 2040 MHz, at which point it is transmitted via one of two routes to PSA Receiver A (Rx A) on the Cassini Orbiter. During cruise checkouts the signal is attenuated and sent via the radio frequency built-in-test equipment (RF-BITE) across the Umbilical Separation Mechanism (USM). During the Titan descent the signal is amplified for free-space RF transmission via the PTA to the Cassini HGA. The center frequency of PSA Rx A is tuned to the nominal Tx A output frequency at 2040 MHz in checkout mode and is shifted by + 38.5 kHz for descent mode.

All timing and signal generator requirements for Rx A are controlled by the DWE-RUSO, which is virtually identical to the TUSO on the Probe. Similar to the probe side, switching to a back-up TCXO is possible in case the RUSO fails. After downconversion the PSA receiver phase locks onto the PRL signal, the loop control being governed by a numerically controlled oscillator (NCO). The digitized values of the changes in NCO frequency required to maintain phase lock, an "NCO control word" of 20 bits, is written to the PSE housekeeping (HK) data at a rate of 8 samples per second. The PRL signal level is monitored at the same sample rate by recording the PSA receiver automatic gain control AGC (length: 16 bits). The frequency resolution due to the data digitization is governed by the internal PSA receiver clock and is given as 60 mHz, which is very close to the least significant bit written into the NCO control word (48 mHz). The range of Doppler shifts accomodated by the 20-bit NCO control word is 12.69 kHz ( 1866 m/s). The NCO control word and AGC data are routed to the solid state recorders on the NASA side of the Cassini Orbiter for later playback to Earth. The "redundant" data from PSA receiver B (Rx B) are generated and received using standard TCXOs and thus not expected to yield information about the probe's drift motions. Nevertheless, these data will also be recorded for the AGC information, thereby providing a control value for comparison with the data from Rx A.

The only DWE data transmitted with the PRL telemetry are three temperature measurements (8 bits each) and one bi-level lock status bit, which are recorded every 16 seconds as part of the probe HK telemetry blocks. The associated DWE "bit rate" of these HK-data is thus only 1.6 bps. Analogous HK-data are recorded at 25 bps on the PSE side from the RUSO. The quantities comprising DWE science data, the NCO control word for the frequency and the AGC for the signal level of the PRL, are recorded at 288 bps. Assuming these data are recorded for the longest possible Huygens mission of 3 hours (including 30 minutes on the surface), the total amount of cumulated DWE data for one chain will be 425 kByte. The Tx/Rx chain B, to be analyzed primarily for its AGC information (no RUSO data), will generate another 391 kByte. Noting that these data will be stored redundantly for later playback to Earth, we arrive at a grand total for the DWE data volume of 1.632 MByte.

The TUSO will be powered on before Titan atmospheric entry at the time - 17min46sec, in order that it be given sufficient warm-up time to achieve the required frequency stability. The RUSO, which will be turned on at - 30min, is less critical because of its comparitively safe location on board the Orbiter. The DWE-USO frequency stability will be monitored every six months during the regularly scheduled Probe cruise checkouts en route to Saturn. The last such rehearsal will occur during the initial Saturn orbit, about 10 days before probe separation.



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Next: 4. DWE Instrumentation: Ultrastable Up: 3. DWE Concept and Previous: 3.3 Doppler Wind Recovery


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