Re: Ranging and Pioneer
- From: George Dishman <gad@xxxxxxx>
- Date: Sat, 26 Aug 2006 11:10:28 GMT
<sent via email due to ISP fault>
"Oh No" <NotI@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:mt2.1-5765-1156518139@xxxxxxxxxxxxxxxxxxxxx
Thus spake George Dishman <gad@xxxxxxx>
The idea is that whenever a direct measurement of position is done (in
this instance ranging) the quantum theory gets renormalised.
How can the choice of what is done with data in a computer
which represents digitised samples of a sine wave from an
antenna affect whether that signal is a simple sine wave
or has phase discontinuities in it? You are saying that
the shape of the waveform out of the wavguide from the
dish is latered by the kind of program the user runs on
his computer to process that received waveform.
I don't mean to say that.
That's why I warned you it was "something for you to
knock down."
This bears on the conditions under which
collapse of the wave function takes place in quantum theory. If data
exists which can be analysed in principle to give a direct measurement
of position, then collapse must have taken place and that will give one
result. If such data does not exist, then collapse has not taken place
and a different result is expected.
OK then you have to identify the point at which collapse
takes place and look at what is actually being measured
regardless of what can be inferred from that data.
The signal (a stream of photons in the quantum world) is
captured by the dish and reflected into the feed horn
then down a waveguide to the LNA. Two types were used,
I'll assume the HEMT (high electron-mobility transistor)
amplifier here. At the end of the waveguide there is a
wire which picks up the signal which is fed to the
transistor. The transistor resonds to current injected
into the base and the collector current varies in
response. Conditions on the collector have a minimal
effect on the base current so that is what is being
measured.
However, I would suggest that collapse must be slightly
earlier at the point where the photons induce the current
in the wire within the waveguide. At that point the
current is due to the aggregate effect of many photons
so you have turned many quantum values into a single
average number. It is that aggregated value which is then
measured.
The measurement rate is the sampling rate of 256Ms/s I
quoted earlier so you get a new independent measurement
about every 4ns.
To get the anomalous shift in
Pioneer transmissions I have to assume that collapse has not taken
place, and hence that there is no way to analyse the data so as to get
ranging information. The question is why, precisely, should this be the
case.
But we can get range from the series of readings. Consider
that the rotation of the earth applies a diurnal sine wave
Doppler shift to the transmitted signal and another on
reception. The time of reception is known so that from
reception can be subtracted, as can the annual motion of
the Earth. What remains is a constant frequency due to the
craft's radial speed plus a sine wave due to the motion of
the transmitting site. The phase of that therefore gives
range. Of course you aren't measuring the locaton of an
individual photon where the uncertainty principle would
apply, just inferring the location of the craft from a
long series of measurement.
<snip>
I don't think anyone was sure why it failed, perhaps
radiation damage during the Jupiter flypast.
Really I am looking for some reason in principle why this should fail.
There can be none, the method works for most craft
and is the primary means of navigation for all deep
space missions. It just broke on Pioneer.
<snip>
OK so that would mean you need at least 11 discontinuities
per second to avoid continual cycle slips but occasionally
some larger jumps to create the observed blunders. Can you
calculate from your theory the mean magnitude and rate of
phase jumps and show they produce the required frequency
offset to explain the anomaly? That seems to be what you
are claiming.
To calculate the magnitude and rate of phase jumps I need information on
the accuracy to which the position of Pioneer can be measured directly
by ranging.
As I said before the system is capable of mesuring to
about 0.5cm in theory. Lack of knowledge of effects
like refraction limit that to a few metres but if the
DSN station was on the Moon that would not apply. In
terms of quantum effects that is the value you should
use, though I think you may be using it in the sense
of knowing the position of a photon in the radio beam
to that accuracy since that is where the quantum effects
apply. However, it appears to me that that only gives a
mean rate of phase shift, not the cycle slip rate. I
think you probably need time to grasp the engineering
behind cycle slips before commenting on that though.
<big snip>
Well the data was digitised so in theory could have been
written to file and an measurement imaginable applied at
a later date. In reality the frequency was computed and
stored but anything that can be calculated from those
numbers can be done. The files are freely available so
you can write a program to find the range from them now.
If that is true, then how come JPL have not written programs to find the
range of Pioneer and compared the results to the Doppler estimates of
position?
That is what the navigational software does, it is where
the "estimates of position" come from. The programs listed
in the paper are CHASMP and ODP. Craig Markwardt wrote his
own.
<snip>
I'll carry on trying to read and absorb the information you have given
and pointed me at. There is a bit too much to comment on in one go.
I understand, you are trying to cram a significant
part of an engineering course in a few days.
I am unclear about this. Anderson talks of it taking minutes to resolve
and return a ranging signal. Does this time period not gradually
increase with distance, or is there something more fundamental I have
not understood?
The problem with Pioneer was that every time they sent the
modulation pattern the craft lost the uplink signal and all
they got back was the free-running craft oscillator.
This sounds critical. If it is stated in Anderson's paper I have missed
it. Can you point me to a reference.
It isn't in the paper or anywhere AFAIK, it is from
private emails I got from Slava Turyshev.
From Anderson I had thought that
the problem was to do with signal to noise ratio, but I don't see that
as a good answer because it is not a fundamental matter of principle -
one ought to be able to increase the power of the signal sent from
earth.
The uplink power used the emergency facility sending
400kW. Compare that to the 8W returned. The SNR was not
the problem as the technique was tried and failed soon
after the Jupiter encounter. They were still able to
communicate a decade later when the range had increased
and the signal strength was far lower.
<snip>
This is to do with the way in which the classical correspondence appears
from the quantum theory. If a range measurement is done simultaneously
then we are essentially dealing only with the classical theory. If that
were so we should get no anomalous acceleration or shift. If no such
range measurement is possible, then we need the quantum theory to model
transmissions from one reference frame to a remote reference frame.
I think you have to base this on the actual measurements
being of induced current in the antenna and at a rate
of 256M-samples/second. Everything else is inferred.
There is no specific measurement of distance, velocity or
acceleration as such.
HTH
George
.
- Prev by Date: Re: Ranging and Pioneer
- Next by Date: Re: Ranging and Pioneer
- Previous by thread: Re: Ranging and Pioneer
- Next by thread: Re: Ranging and Pioneer
- Index(es):
Relevant Pages
|
