[PPRAANet] More on Cassini-Huygens

[W0rw at aol.com]

The staff scientist who spearheaded the project in New Mexico is Dr. Jon

Romney.  The following is the observatory information he released this

morning, and passed on  by his permission, with comments where

needed. 



On Fri, 14 Jan 2005, Jon Romney wrote:

> Last night, the VLBA and GBT participated in an observation to track the 

> Huygens Probe's parachute descent through the atmosphere of the Saturnian 

> satellite Titan, as part of a large international VLBI network.  In 

> addition, four VLBA stations and the GBT hosted visiting teams from JPL 

> who recorded the Huygens signals on their own instrumentation to make 

> precise measurements of the Doppler shift.


GBT = the 300 ft. Green Bank Telescope

VLBA = Very Long Baseline Array antennas (88 ft. dish antennas)

VLBI = Very Long Baseline Interferometer - basically linking the radio

telescopes of several observatories together for a common observation 


> This project appears to have been extraordinarily successful.  The VLBI 

> observations ran without any serious problem, as detailed below.  The JPL 

> group at the GBT was able to detect the carrier signal from Huygens when 

> it appeared, right on schedule, and monitored the transmission until 

> Huygens set in Green Bank.  There was a brief 25-minute gap from that 

> time until Huygens rose at the Parkes telescope in Australia, where 

> another JPL Doppler receiver was stationed.  The latest information from

> the spacecraft controllers in Darmstadt, Germany, predicted Huygens 

> would reach the surface of Titan during that gap, but five minutes after

> Parkes acquired the signal, they noted a sudden change in the Doppler

> shift, indicating that Huygens had stopped descending.  This allowed the

> time of landing to be fixed, at 12:45  UT.  Huygens continues to 

> transmit, and a reanalysis of its power system's performance predicts 

> that it may continue to do so for seven more hours.


Huygen's batteries were designed to support all of the science instruments

and transmitters for only a few minutes after landing on Titan.  As of this

afternoon, the batteries were still powering all of the onboard equipment,

long after the design life, for continued unexpected data and images.  The

batteries are approaching exhaustion, however. 


> Some fairly complex scheduling was necessary to set up the VLBA baseband 

> converters to support both the VLBI and the Doppler measurements, and this 

> appeared to work perfectly. 


The Baseband Converters (BBCs) downconvert the final IF to 0-16 MHz baseband

slices with programmable bandwidths from 16 MHz to 62.5 KHz.  The local

oscillators for the BBC's are tunable in 10 KHz steps at the NRAO

facilities, but more coarsely at some of the other VLBI antennas, causing

some concern how well everything would track.  After all, these BBCs are

located at radio telescopes from Green Bank, WV to Australia - AND, all of

the local oscillators have to be "crossing zero" at the same instant for

all the radio telescopes to be "phased up" and in sync. 


> There were marginal weather conditions at two 

> sites: continuous strong, gusty winds at Mauna Kea generally remained just 

> below the auto-stow limit; and extremely cold temperatures (-20C) at North 

> Liberty left an azimuth brake frozen at the beginning of the run.


Generally, when wind speeds exceed 35-40 mph, the antennas will "auto-

stow," pointing straight up, to protect the mechanical and servo systems. 


> Band-limiting filters had to be bypassed in the S-band receivers at Fort 

> Davis, Los Alamos, North Liberty, and Owens Valley, to be able to receive 

> the 2040-MHz Huygens carrier, which is fairly far outside our normal 

> observing range. 


Now think about this.  The 10W signal left the Huygen probe at 2290 MHz,

and were received by the NRAO antennas on earth at 2040 MHz.  Now that's a

doppler shift!!! - caused mostly by the relative motion between the Earth

and Saturn. The doppler due to the Earth-Saturn velocity had to be

deducted, such that the small doppler remaining could be used to monitor

the velocity of the spacecraft and movements due to atmospheric winds. 


The S-band transmitter is actually a precision 115 MHz oscillator,

multiplied by 20 via a comb generator to produce the 2290 MHz carrier

frequency.  This is a typical approach to millimeter wave electronics. 


> These filters were put in place to protect those normal 

> observations from interference, and it was expected that serious 

> interference would be experienced at those four stations without the 

> filters.  This turned out to be true only at North Liberty, due at least 

> in part to specific requests to some occasional interference sources.


Namely, a UHF TV station in the Iowa City/Cedar Rapids area. There is an

intermodulation distortion signal being generated by their transmitter that

was very close to the Huygen signal frequency.  Also a couple of commercial

transmitters (like Taxi companies) with a harmonic close to the 2040 MHz

frequency.  These people were kind enough to halt transmissions during

the hour window that Huygens passed through Titan's atmosphere and landed

to not cause interference.  It should be noted, their transmission are

fully FCC compliant, but we are talking VERY WEAK signals entering the

North Liberty, IA VLBA antenna, and just a little power could have driven

the receivers into gain compression.  You know, that P1dB thing!  I don't

know what the P1dB is of the VLBA receivers, but I know the P1dB of the

L, S and C-band upconverters I designed for the EVLA are about -12dBm at

2-4 GHz based on wideband noise.  CW would probably take it out sooner

than that! 


> A more thorough report, including credits to the many NRAO staff members 

> who made this success possible, will be issued next week.       

> 

> Jon Romney


I went upstairs after lunch to chat with Dr. Romney again about this,

but he had gone home to sleep.  The poor guy had been up since yesterday

morning coordinating everything, some 30+ hours on his feet.  I wanted

to ask him what the received signal strength was of the Huygen as

received by our antennas.  I'm wondering how much power was left from

that 75,000,000 mile per watt signal.  My guess, is we still had plenty of

gain headroom to go on our receivers yet before we would have lost the

signals. 


And, for the record, I myself had very little to do with this.  I am on

the Expanded VLA engineering team, not the Cassini-Huygens project.

But of course for me, this all happened just upstairs less than 100 feet

away, so had a pretty good seat to see and learn what was going on. 


Sorry for the extreme bandwidth, but hopefully most of you found it

interesting, and some of the stuff you'll never find out anywhere else.


72, Paul NA5N 


PS - The above information is being released, by permission, by an

employee of the observatory. However, this is NOT an official NRAO

statement or press release.  If cited, the source is to be credited to

Paul Harden, NA5N, and not to the NRAO, and is considered to be my own

views, and not that of the observatory. 


Also, any similarity between the landing on Titan and todays solar

flares is purely coincidental :-) 

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