Cassini at Saturn, use of windowed ramped uplink

Interplanetary Network Directorate

1Deep Space Network Operations & Maintenance

Analysis of Possible use of Non-Ramped Uplinks,

using example of Cassini Rhea Encounter

David TynerNOPE

11 Jan 2006



• 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



• 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)


• Closest approach (Gravity well ∆v/∆t) forces causes XA-TSF to increase● Larger differences between XAMAX and XAMIN , 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)



• 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


• Graphical Analysis if had Revised predict sets for minimal overlap

– See pages (6-7)




• 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



XFR atT0 = 2245z

TSF34 = 7174.510 MHz

TSF63 = 7174.660 MHz

TR MAX = (150 Hz/s) * (3600 s/hr) = 540 kHz/hr

“BLF” = 7174.559 MHz

TSF – XA Difference= 49 kHz


Uplink Transfer DSS-34 to DSS-63

Nominal Non-Ramped Uplink Transfer at Closest Approach




By Shortening PDX Duration, We Reduce the XAMAX - XAMIN Difference,

which brings TSFs closer to Doppler Curve

“Improved” Non-Ramped Uplink Transfer at Closest Approach

TSF34 = 7174.535 MHz

TSF63 = 7174.611 MHz

XFR atT0 = 2245z

“BLF” = 7174.559 MHz



Uplink Transfer DSS-34 to DSS-63



XFR atT0 = 2245z

TSF34 = 7174.535 MHz


XFR atT0 = 0550z

TSF63 = 7174.611 MHz

“BLF” = 7174.686 MHz

75 kHz

95 kHz

TSF14 = 7174.781 MHz



TSF34 = 7174.535 MHz


2nd GDSCC PDX loaded to

Restrict < 50 kHz

“BLF” = 7174.691 MHz

50 kHz

50 kHz

TSF14 = 7174.781 MHz

“BLF” = 7174.611 MHz

TSF14 = 7174.661 MHz

LOSS OF UPINK, toLOAD PDX

TSF14 = 7174.611 MHz1st ORIGINAL

2nd new PDX view period



Windowed Non-Ramped Uplinks

T0

331 / 0230

0430 0630 0830 1030 1230 1430 1630

T0+ OWLT

= TOWLT +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

Cassini at Saturn, use of windowed ramped uplink

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