[time-nuts] Voyager space probe question

[jimlux]

On 11/29/20 3:53 AM, Hal Murray wrote:
> 
> jimlux at earthlink.net said:
>> I don't know about Voyager, but on the SDST and later, the auxosc is a  TCXO
>> with fair to middling performance. A good part of the "art" of  communicating
>> with an "old" spacecraft is knowing/predicting/guessing  where the "best lock
>> frequency" is, because DSN sweeps very slowly  through that frequency hoping
>> the spacexraft receiver acquires it in a  <10Hz BW filter.  If you don't see
>> the transmitter frequency (in  turnaround mode) jump when acquisition occurs,
>> you widen up the sweep,  or slow it down, or turn up the exciter power.  When
>> round trip time is  measured in hours, this is a slow process<grin>
> 
> Neat.  Thanks.
> 
> How often do you contact old spacecraft? 

That depends on a lot of factors - one of which is the budget <grin> - 
not necessarily for the DSN time, but for the people to build the 
commands and handle the telemetry.

Once a week or once a month.

I think I remember a news story about Ed Stone (who's the Project 
Scientist on Voyager, as well as former director of JPL, now a Caltech 
professor) checking on the data periodically.


  How much does the TCXO drift due to
> aging since the last time you talked to it?

Not much, by now <grin>  That's a well aged crystal.

I happen to have the data for a 49.244 MHz TCXO clock from Vectron that 
we used on a radio for the SCaN Testbed on ISS (which has now been 
discarded to burn up in the atmosphere).

It's spec was <2ppm first year, <10 ppm for 10 years (obviously, they 
didn't test it for 10 years!) at 70C
The actual aging rate was 0.024 ppm first year, epected to be no more 
than 0.030ppm for 10 years.



   How much does the temperature
> change? 

Depends on the spacecraft - for Voyager, I'll bet it stays within a 
couple degrees. Sunlight is tiny at that distance, and they're not 
changing operating modes (power dissipation) all that much.  On my LEO 
spacecraft, it was about 10-15 degrees every orbit as we went in and out 
of eclipse.

  What else changes that influences the TCXO frequency?

Radiation has a small effect. Power supply voltage (called "pushing" in 
the spec sheet, about 0.01-0.03 ppm for a 0.25V difference on my Vectron 
parts).  That could be due to a change with temperature or radiation 
dose (changes in the reference voltage in the regulator due to TID).

Load impedance changes the frequency (pulling), too.

> 
> If you are transmitting on the right frequency, how long does it take the
> receiver to lock up?
> 
> 

Seconds - if you're right on frequency it's just how long it takes the 
phase to change to match the incoming signal, and that's mostly about 
the tracking loop time constant.

Newer transponders have rapid acquisition circuitry which change the 
loop filter to be narrower after carrier lock is acquired. This, of 
course, depends on the SNR.

In most cases, one starts out transmitting pure carrier, waits long 
enough to be sure that the receiver has locked, then go into a data 
mode, which moves power from the carrier to the sidebands (if not 
entirely suppressing it), so changing the loop bandwidth after 
acquisition is a useful thing.

In precision two way ranging, the locked oscillator in the receiver is 
used to generate the downlink signal as well, so having very narrow loop 
bandwidth helps with having the downlink signal have narrow bandwidth, 
too. The uplink signal from the ground is derived from a maser.

In two way mode, the auxosc is ignored (although in modern designs, the 
auxosc is also the processor clock, so it keeps running)