Cassini at Saturn, use of windowed ramped uplink
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The document analyzes the use of non-ramped uplinks in the context of the Cassini Rhea encounter, comparing them to ramped uplinks. It discusses the effects on spacecraft transponder performance and radio science, highlighting the benefits of using fixed frequency uplinks for stability and reduced phase noise. The analysis also includes graphical data on frequency discrepancies and implications for uplink transfers and signal integrity.
![Actual Rhea encounter used ramped predicts Uplinking used a time history of Doppler compensated XAs Each Uplink Frequency in time, arrives at S/C at near to BLF Provides Minimal Stress on S/C transponder PLL Guarantees Minimal Risk of recovery if TXR were to Glitch (I/L shut off), as the transponder is always ‘sitting’ right at XA or BLF Transfers (handovers between DSSs) could potentially occur without slewing above and below BLF, but in practice a sweep from above, through, and continuing past where BLF is, are used to guaranteed continuation of 2-way Negative impacts to Radio Science [ Tuned XA(t) compared to fixed TSF frequency ] Uplink Tuning adds additional frequency instabilities and phase noise (compared to TSF) Additional post-processing is needed to remove effect of ramped uplink Ramped vs. Non-Ramped](https://image.slidesharecdn.com/useofwindowedrampeduplinkpdx07-100627201815-phpapp02/85/Cassini-at-Saturn-use-of-windowed-ramped-uplink-2-320.jpg)
![Analysis of Rhea encounter, if had used non-ramped Uplinking Uplinking starts using a single Doppler compensated XA, then ramps to TSF The single TSF Frequency (ea. DSS), arrives at S/C at many different values (not at BLF) Provides Increasing Stress on S/C transponder PLL, depending on Doppler Profile Has increased Risk of 2-way recovery if TXR were to Glitch (I/L shut off), as the transponder has been pulled substantially away from BLF Transfers still occur at BLF, but require a larger sweep from above, through, and continuing past where BLF is, as both DSSs start at their individual TSFs (to a “common” XA, BLF) Reduced impacts to Radio Science [ fixed TSF frequency compared to Tuned XA(t) ] Fixed TSF uplink provides maximum frequency stabilities and reduced phase noise Additional post-processing is not needed (there is no tuning nor effect of ramped uplinks) Ramped vs. Non-Ramped Closest approach (Gravity well v/ t) forces causes XA-TSF to increase Larger differences between XA MAX and XA MIN , generate an “average” TSF further Doppler curve Techniques can be used to reduce the differences between XA(t) and individual TSFs Restrict Predict duration to shorter periods, but still include enough for transfer between DSSs (min overlap) Tune from TSF back to XA, Glitch uplink to load new PDX, which so loose 2-way (and Doppler)](https://image.slidesharecdn.com/useofwindowedrampeduplinkpdx07-100627201815-phpapp02/85/Cassini-at-Saturn-use-of-windowed-ramped-uplink-3-320.jpg)
Cassini at Saturn, use of windowed ramped uplink
- 1. Analysis of Possible use of Non-Ramped Uplinks, using example of Cassini Rhea Encounter David Tyner NOPE 11 Jan 2006
- 2. Actual Rhea encounter used ramped predicts Uplinking used a time history of Doppler compensated XAs Each Uplink Frequency in time, arrives at S/C at near to BLF Provides Minimal Stress on S/C transponder PLL Guarantees Minimal Risk of recovery if TXR were to Glitch (I/L shut off), as the transponder is always ‘sitting’ right at XA or BLF Transfers (handovers between DSSs) could potentially occur without slewing above and below BLF, but in practice a sweep from above, through, and continuing past where BLF is, are used to guaranteed continuation of 2-way Negative impacts to Radio Science [ Tuned XA(t) compared to fixed TSF frequency ] Uplink Tuning adds additional frequency instabilities and phase noise (compared to TSF) Additional post-processing is needed to remove effect of ramped uplink Ramped vs. Non-Ramped
- 3. Analysis of Rhea encounter, if had used non-ramped Uplinking Uplinking starts using a single Doppler compensated XA, then ramps to TSF The single TSF Frequency (ea. DSS), arrives at S/C at many different values (not at BLF) Provides Increasing Stress on S/C transponder PLL, depending on Doppler Profile Has increased Risk of 2-way recovery if TXR were to Glitch (I/L shut off), as the transponder has been pulled substantially away from BLF Transfers still occur at BLF, but require a larger sweep from above, through, and continuing past where BLF is, as both DSSs start at their individual TSFs (to a “common” XA, BLF) Reduced impacts to Radio Science [ fixed TSF frequency compared to Tuned XA(t) ] Fixed TSF uplink provides maximum frequency stabilities and reduced phase noise Additional post-processing is not needed (there is no tuning nor effect of ramped uplinks) Ramped vs. Non-Ramped Closest approach (Gravity well v/ t) forces causes XA-TSF to increase Larger differences between XA MAX and XA MIN , generate an “average” TSF further Doppler curve Techniques can be used to reduce the differences between XA(t) and individual TSFs Restrict Predict duration to shorter periods, but still include enough for transfer between DSSs (min overlap) Tune from TSF back to XA, Glitch uplink to load new PDX, which so loose 2-way (and Doppler)
- 4. Graphical Analysis if had used exactly same predicts, but had used non-ramped TSF technique (“BLF” at transfer between DSSs) See next page (5) TSF-BLF for DSS-34 is 49 kHz TSF-BLF for DSS-63 is 106 kHz Graphical Analysis if had Revised predict sets for minimal overlap See pages (6-7) TSF-BLF for DSS-34 is 25 kHz TSF-BLF for DSS-63 is 53 kHz Ramped vs. Non-Ramped Drop Lock approach, enforcing a maximum deviation from BLF See pages (8-11) Reduces stress on transponder PLL Loss of Ranging data is severe for Cassini’s RTLT
- 5. XFR at T 0 = 2245z TSF 34 = 7174.510 MHz TSF 63 = 7174.660 MHz TR MAX = (150 Hz/s) * (3600 s/hr) = 540 kHz/hr “ BLF” = 7174.559 MHz Nominal Non-Ramped Uplink Transfer at Closest Approach TSF – XA Difference = 49 kHz TSF – XA Difference = 106 kHz Uplink Transfer DSS-34 to DSS-63
- 6. TR MAX = (150 Hz/s) * (3600 s/hr) = 540 kHz/hr “ Improved” Non-Ramped Uplink Transfer at Closest Approach TSF 34 = 7174.535 MHz TSF 63 = 7174.611 MHz XFR at T 0 = 2245z “ BLF” = 7174.559 MHz By Shortening PDX Duration, We Reduce the XA MAX - XA MIN Difference, which brings TSFs closer to Doppler Curve TSF – XA Difference = 25 kHz TSF – XA Difference = 53 kHz Uplink Transfer DSS-34 to DSS-63
- 8. XFR at T 0 = 2245z TSF 34 = 7174.535 MHz TR MAX = (150 Hz/s) * (3600 s/hr) = 540 kHz/hr XFR at T 0 = 0550z TSF 63 = 7174.611 MHz “ BLF” = 7174.686 MHz 75 kHz 95 kHz TSF 14 = 7174.781 MHz
- 9. TSF 34 = 7174.535 MHz TR MAX = (150 Hz/s) * (3600 s/hr) = 540 kHz/hr 2 nd GDSCC PDX loaded to Restrict < 50 kHz “ BLF” = 7174.691 MHz 50 kHz 50 kHz TSF 14 = 7174.781 MHz “ BLF” = 7174.611 MHz TSF 14 = 7174.661 MHz LOSS OF UPINK, to LOAD PDX TSF 14 = 7174.611 MHz 1 st ORIGINAL 2 nd new PDX view period
- 10. Windowed Non-Ramped Uplinks T 0 331 / 0230 0430 0630 0830 1030 1230 1430 1630 T 0 + OWLT = T OWLT +2 +4 +6 +8 +10 +12 +14 DOWNLINK RNG OK U/L PDX Load Glitch, Loss of data in D/L “Pipeline” NEW RNG cycle U/L PDX Load Glitch, Loss of data in D/L “Pipeline” NEW RNG cycle UPLINK No RNG No RNG