Ranging and Pioneer

George Dishman wrote:
"John (Liberty) Bell" wrote in message
oups.com...
Craig Markwardt wrote:
...
I refer you to my communications with Jonathan Silverlight, and to
gr-qc/0104064 for confirmation that spacecraft transmissions were
indeed "switched off." repeatedly (and switched on again successfully).

I quote you, personally (from the reference provided by Jonathan
Silverlight):

"Turyshev admitted two things: the round trip signal time is recorded
in some form, but it is not precise enough to constrain the anomaly.
Second, he said they did not use that form of measurement technique
anyway.

As I've already gone into in a different post, I wanted to investigate
this more deeply, so I went to a primary document, the DSN procedures
manual. I already referred to the table in that manual, where they
describe that it takes a certain amount of time to acquire a signal
lock, somewhere in the range 0-4 seconds, but perhaps more time,
depending on the pre-acquisition bandwidth.

Thus, the signal receive time cannot be known to a precision better
than a few seconds. Hence, satellite distance discrepancies of 0.5
light seconds or smaller would not be measurable with such a system."

Your first paragraph appears to confirm that means to control
spacecraft transmissions from Earth do, in fact, exist.

Your second paragraph appears to confirm that the lack of accuracy in
the already recorded light time data is due to the time taken to
achieve a signal lock.

The transmitter can be switched off and on but
after switching on it can take a long time to
lock on. At extreme range a narrow receiver
bandwidth has to be used on the craft and the
uplink signal is swept through the range of
frequency where the craft might be listening
hoping it will lock. The time taken depends
on the particular frequency hence on the
absolute accuracy of the on-board reference.

This would seem to suggest that, once a signal
lock has been achieved, the primary obstruction to obtaining more
accurate ranging data has already been overcome.

Yes, a better approach is to consider the switch
off transition.

My point exactly.

Once the uplink and downlink have
been locked, there should be a clear indication
if the downlink is switched off. One problem is
that the downlink keeps lock by using a very
narrow bandwidth, probably less than 1 Hz so the
response time may be more than a second.

More important than the response time of ground based hardware, which
can be determined by hands on engineering tests, is the uncertainty in
that response time (which can again be determined by successive
iterations of the same method).

Another
problem is how it is achieved. For normal
operational purposes, they probably had software
that would switch the transmitter off and on at
specified times to match the ground station
schedules. Using that requires an accurate clock
on the craft and synchronisation becomes
significant. There may not be a command to do an
immediate switch off, but again if there were
then software response times would need to be
deterministic and known.

All these points are valid, or potentially valid. The salient question
remains:- to what extent can these factors be modelled, and how tightly
can we constrain the uncertainties in that model? This question can
only be unambiguously answered by a team with precise knowledge of the
relevant NASA/JPL engineering details. Only then will we know for
certain the actual distance at which such a ranging test becomes
unambiguously feasible.

Since the links use
forward error correction and possibly repeated
commands, it may not be certain which copy of
the command caused the transmitter to go off.

No problem. You simply arrange for that specific instruction repeat
time to be larger than the sum of the engineering and theoretical
distance uncertainties.

Whilst it is true that signal levels are, by now, already below the
noise threshold, that problem too can potentially be overcome, via the
development of still lower noise detectors.

Not necessarily, the problem can be that the signal
falls below the galactic background. The only way
to extract it then is to narrow the bandwidth so
that all the signal power is seen but less wideband
noise is allowed through.

Not so. I had originally considered external as well as internal noise,
and concluded that both were controllable in principle by appropriate
engineering. (Given any engineering solution, it can always be improved
on). However, it was only today that I realised precisely how to
achieve that objective. The solution is sufficiently elegant, and of
potentially broad applicability, that I have decided it would be
prudent to snip the relevant details from this response, to comply with
British (and International) Patent Law.
Suffice it to say, in the public domain, that preliminary calculations
indicate even the simplest embodiment of that invention will bring
Pioneer 10 back into range for a further 20 years. (Barring exhaustion
of the Pioneer power source, of course)

Of course reducing the
bandwidth has the problem of increasing the response
time and hence measurement uncertainty.

Without knowing the precise engineering details of the relevant
electronic hardware used by NASA, I would imagine that the input stage
behaves as a high quality (Q) tuned filter. Maximising the Q of the
circuit results in 'ringing' i.e. the circuit continues to oscillate
at the resonant frequency after the input stimulus is removed. This I
suspect is equally pertinent with the current configuration.

Nevertheless, it is still merely an (admittedly tedious) experimental
engineering exercise to determine the extent of that ringing, its rate
of decay, and the uncertainty in that decay, given a known input signal
level and noise threshold, taken over an (ideally controlled)
temperature range.

[[Mod. note -- In practice, I think a major issue is that power cycling
(and thus thermal cycling) any hardware carries some risks of it breaking.
When you've spent 30+ years getting the hardware where it is, you do
_not_ want to do anything which would endanger it.
-- jt]]

This was certainly a major concern the first time the amplifier was
switched off, for a 15 minute comscan manoeuvre in the cold of deep
space. However:
1) That procedure has been repeated numerous times subsequently without
problem. In electronic engineering terms, the equipment has thus passed
a simplified 'burn in' test to weed out 'infant mortalities', and can
thus now be considered substantially more reliable than was originally
believed.
2) The proposed experiment only requires a switch off for a matter of
seconds, which would result in far less thermal shock than a further
comscan manoeuvre.

John Bell
http://global.accelerators.co.uk
(Change John to Liberty to respond by email)