Ranging and Pioneer

"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. 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. 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. 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.

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

George


[[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]]