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Ranging and Pioneer



 
 
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  #81  
Old September 24th 06, 11:35 PM posted to sci.physics.research,sci.astro.research
George Dishman[_1_]
external usenet poster
 
Posts: 2,509
Default Ranging and Pioneer


"Oh No" wrote in message
...
Thus spake Spud
...
gr-qc/0104064

Many thanks. If I understand Anderson correctly it is merely an
engineering constraint. With a (perhaps greatly) amplified signal it
should be possible to reduce integration times to achieve correlation so
that the signal can be returned without a substantial range delay.


The problem was more fundamental. Reading between the lines,
and from some personal correspondence, the rate of change of
the carrier frequency was I believe limited in choice by the
design of the exciter or correlator equipment. The narrow
bandwidth required to get an acceptable SNR at the craft
meant the minimum sweep rate was too fast and the craft lost
lock on the uplink. Increased gain at the craft wouldn't help,
only more transmit power, but that was in the 100's of kW
already and being routed through their largest (70m) dishes.

Anyway that is the basis on which I am working at the moment.

But I am not an engineer, and I was hoping this might be confirmed. My
arguments would take quite a different form if this was a fundamental
constraint.


It was purely an equipment limitation, possibly exacerbated
by radiation damage during the Jupiter flyby though that is
my speculation. BTW, I am a digital and systems engineer in
a company working significantly in HF comms.

George


  #82  
Old September 24th 06, 11:35 PM posted to sci.physics.research,sci.astro.research
George Dishman[_1_]
external usenet poster
 
Posts: 2,509
Default Ranging and Pioneer

test sending via ISP

"John (Liberty) Bell" wrote in message
ups.com...
wrote:
John (Liberty) Bell wrote:
wrote:

Finally you have perhaps the biggest problem of
measuring the switch-off of the signal with millisecond
accuracy through a receiver chain with a bandwidth of
less than 1 Hz which is what was needed to hold lock
on the carrier. That applies whichever method of timing
you use at the transmitter.

Notwithstanding my prior comment on this point, it is obvious that many
positions in this receiver chain have much higher frequency responses
than 1 Hz.


True but the siganl is far les than the noise prior to
the filter so not accessible. More below.

You are confusing me here. We seem to be talking about several filters
simultaneously.


There are several filters involved and you seemed to be
discussing tapping off a signal from earlier in the chain
where the bandwidth was wider.

Furthermore, in a feedback loop, it is no longer
meaningful to discuss earlier and later, only physical points in the
loop.


Again you were discussing "positions in this receiver chain
[which] have much higher frequency responses". Those are
before the phase-locked loop where the narrowband filter
resides.

Before taking your points separately, the manuals
can be found he

http://deepspace.jpl.nasa.gov/dsndocs/810-005/

The index lists them by number on the various pages, the
100 series numbers are on the "Space Link Interfaces"
page and the 200 series on "Station Data Processing".

Clearly, at one point we must have a high Q bandpass filter which is
tuned to the carrier frequency of ~ 2 gig. Its frequency response is,
presumably, the carrier frequency + - 1 Hz


It isn't that simple. The dish has a low-noise amplifier
(LNA) in the pedestal room which amplifies the whole S-band
range from 2270 MHz to 2300 MHz. A structural diagram is
shown in manual 203 figure 15 on page 34 and functional
block diagrams are given in manual 101, pages 26/27.

The "D/C" in manual 203 figure 15 is a 'downconvertor'
which is further detailed in the manual 209. Read para
2.3 on page 7 and look at Figure 2 on page 10. The
amplified signal from the LNA is heterodyned to an IF
frequency of 300MHz, then it is filtered with a bandwidth
from 265MHz to 375MHz (centred at 320 MHz) and that is
then heterodyned to a centre of 64MHz. That signal is
then digitised at 8 bit resolution and a sample rate of
256 M-samples/s and fed to the digital down-convertor
(DDC) which extracts subchannels of 16MHz width.

Clearly at another point we must have a low pass filter providing a
control signal. Its effective frequency response is, presumably, 0 to
1Hz

I would be happy to stand corrected on this, but I don't think these
two 1Hz figures are the same thing.


This very narrow band filtering is done in the digital
domain and is described in manual 207. Start with section
4 on page 11, mainly the first two paragraphs. You can
probably skip para 4.1 though the first three paras on
page 13 add useful background to the process for
determining when the system is "in lock".

Go on to para 4.2 and read the first two paragraphs,
and note the graph of Figure 4 on page 19.

It is this loop which can go down to 0.01 Hz but note
the first para of 4.2 says "Once phase-lock is declared
at this bandwidth, the loop bandwidth is gradually
reduced to its final value - the desired operational
value - in a manner consistent with the retention of
phase-lock." The trouble is I don't know what the
'desired operational value' was at any time. In the
latter days at extreme range I am sure it was 0.01Hz
but in previous years they might have selected a
wider value. Anyway that means the filter would ring
with a decay time constant of 16s, 1/(2 pi bandwidth).

As you said above, there were earlier stages with up
to 30MHz analogue bandwidth or 16MHz digitized but
these would contain vastly more background noise than
signal.

George


  #83  
Old September 25th 06, 10:13 AM posted to sci.astro.research
George Dishman[_1_]
external usenet poster
 
Posts: 2,509
Default 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
  #84  
Old September 26th 06, 12:22 AM posted to sci.physics.research,sci.astro.research
John (Liberty) Bell
external usenet poster
 
Posts: 242
Default Ranging and Pioneer


George Dishman wrote:
"Oh No" wrote in message
...

The position of Pioneer was calculated from Doppler information. Ranging
was not available. Can anyone explain why ranging could not be used? Is
this just a limit on available technology, or is there a more
fundamental reason?


To get a range measure, the DSN applies a "ramp" to the
uplink. I believe that means very slow sawtooth FM. The
time delay to the corresponding downlink modulation via
the transponder gives the range. Because of the extreme
distance and low signal levels, the bandwidth had to be
very narrow both at the craft and the DSN. From personal
emails from one of the team, I believe that they did
attempt to apply the modulation soon after the Jupiter
flyby but it always caused the spacecraft to loose lock
with the uplink.

I would just like to add to this that one of the earlier refs, or a
link therefrom, does indicate that the cause was a failure within the
spacecraft, quite possibly radiation damage. However, it does seem that
both Pioneer 10 and 11 suffered the same fate in this respect, which is
suggestive of a design flaw. Ranging on other missions worked fine.

John Bell.

  #85  
Old September 26th 06, 12:22 AM posted to sci.physics.research,sci.astro.research
Oh No
external usenet poster
 
Posts: 433
Default Ranging and Pioneer

Thus spake George Dishman

"Oh No" wrote in message
k...
Thus spake Spud
...
gr-qc/0104064

Many thanks. If I understand Anderson correctly it is merely an
engineering constraint. With a (perhaps greatly) amplified signal it
should be possible to reduce integration times to achieve correlation so
that the signal can be returned without a substantial range delay.


The problem was more fundamental. Reading between the lines,
and from some personal correspondence, the rate of change of
the carrier frequency was I believe limited in choice by the
design of the exciter or correlator equipment. The narrow
bandwidth required to get an acceptable SNR at the craft
meant the minimum sweep rate was too fast and the craft lost
lock on the uplink. Increased gain at the craft wouldn't help,
only more transmit power, but that was in the 100's of kW
already and being routed through their largest (70m) dishes.


Considering that on Earth we pick up a signal of 8kW from Pioneer, it
doesn't sound to me as though the amplitude of the signal sent to
Pioneer was really too low. Loss of lock on frequency modulation sounds
interesting. I might be helpful to understand this more precisely. Can
you give more detail on how the signal is encoded.

Anyway that is the basis on which I am working at the moment.

But I am not an engineer, and I was hoping this might be confirmed. My
arguments would take quite a different form if this was a fundamental
constraint.


It was purely an equipment limitation, possibly exacerbated
by radiation damage during the Jupiter flyby though that is
my speculation.


I am slightly sceptical about the equipment failure theory, because it
would mean the equipment from both pioneers failed in the same way at
much the same place. I would also be a little surprised if radiation
from Jupiter is energetic enough to cause such damage. After all other
space craft have flown close to the sun and survived.

Equipment limitation seems probable. It would be more satisfying if one
could identify a cause for the limitation, since clearly it was not
expected from the design parameters. Your response has caused me to
wonder if there is an unmodeled effect in the uplink which might
contribute to loss of lock. Clearly there is a Doppler variation in the
signal received by Pioneer due to orbital motion of the Earth of
30,000m/s which must have been taken into account. But how narrow is the
bandwidth, and how accurately does this need to be predicted? Could an
unmodelled effect as small as _+-2m/s (varying annually) contribute to
loss of lock?

BTW, I am a digital and systems engineer in
a company working significantly in HF comms.


It's clear you know a lot about this kind of thing, and your input is
much appreciated.


Regards

--
Charles Francis
substitute charles for NotI to email

  #86  
Old September 26th 06, 08:49 AM posted to sci.astro.research
John (Liberty) Bell
external usenet poster
 
Posts: 242
Default Ranging and Pioneer

George Dishman wrote:
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.


This response is remarkably similar to your original response of Fri,
Aug 4 2006 8:26 pm . Are we starting to go round in ever diminishing
circles here?

[Mod. note: a number of postings that George made earlier this year
and that were lost in his ISP's system have now reappeared and a few
were approved by your inattentive moderator before he noticed. This, I
suspect, is one of them. Apologies for any confusion caused -- mjh]
  #87  
Old September 26th 06, 12:42 PM posted to sci.astro.research
[email protected]
external usenet poster
 
Posts: 96
Default Ranging and Pioneer

John (Liberty) Bell wrote:
George Dishman wrote:
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.


This response is remarkably similar to your original response of Fri,
Aug 4 2006 8:26 pm . ...


I probably sent it on Aug 4th, see the mod's note.
There are a few others like that too. Just ignore
them or reply if you choose.

[Mod. note: a number of postings that George made earlier this year
and that were lost in his ISP's system have now reappeared and a few
were approved by your inattentive moderator before he noticed. This, I
suspect, is one of them. Apologies for any confusion caused -- mjh]

  #88  
Old September 26th 06, 09:25 PM posted to sci.physics.research,sci.astro.research
[email protected]
external usenet poster
 
Posts: 96
Default Ranging and Pioneer

Oh No wrote:
Thus spake George Dishman

"Oh No" wrote in message
k...
Thus spake Spud
...
gr-qc/0104064

Many thanks. If I understand Anderson correctly it is merely an
engineering constraint. With a (perhaps greatly) amplified signal it
should be possible to reduce integration times to achieve correlation so
that the signal can be returned without a substantial range delay.


The problem was more fundamental. Reading between the lines,
and from some personal correspondence, the rate of change of
the carrier frequency was I believe limited in choice by the
design of the exciter or correlator equipment. The narrow
bandwidth required to get an acceptable SNR at the craft
meant the minimum sweep rate was too fast and the craft lost
lock on the uplink. Increased gain at the craft wouldn't help,
only more transmit power, but that was in the 100's of kW
already and being routed through their largest (70m) dishes.


Considering that on Earth we pick up a signal of 8kW from Pioneer,


That is 8 W, not 8 kW. The anomaly is equivalent to
the radiation pressure of a signal of just 63 W, a
light bulb would do it.

it
doesn't sound to me as though the amplitude of the signal sent to
Pioneer was really too low. Loss of lock on frequency modulation sounds
interesting. I might be helpful to understand this more precisely. Can
you give more detail on how the signal is encoded.


AIUI they had a dedicated piece of equipment that
they used. It provided an output which was used to
modulate the uplink carrier and it also then looked
at the downlink frequency and tried to run a
correlation. Once it found a match, it would measure
the time delay.

I don't know the exact modulation scheme but I
believe it involved "ramping" the transmitter frequency,
possibly by generating commands to the local signal
generator rather than producing a frequency itself. The
ATDF format includes "ramp" records and that facility
was used as part of the signal acquisition strategy.

My guess it that linear frequency ramps were used
possibly with variable ramp durations to resolve any
ambiguity.

Anyway that is the basis on which I am working at the moment.

But I am not an engineer, and I was hoping this might be confirmed. My
arguments would take quite a different form if this was a fundamental
constraint.


It was purely an equipment limitation, possibly exacerbated
by radiation damage during the Jupiter flyby though that is
my speculation.


I am slightly sceptical about the equipment failure theory, because it
would mean the equipment from both pioneers failed in the same way at
much the same place. I would also be a little surprised if radiation
from Jupiter is energetic enough to cause such damage. After all other
space craft have flown close to the sun and survived.


Jupiter is worse because the craft get much closer and
at the time they were shocked at the levels. Remember
the craft were some of the first to encounter these fields
and subsequent craft have had much better protection
as a result.

Equipment limitation seems probable. It would be more satisfying if one
could identify a cause for the limitation, since clearly it was not
expected from the design parameters. Your response has caused me to
wonder if there is an unmodeled effect in the uplink which might
contribute to loss of lock. Clearly there is a Doppler variation in the
signal received by Pioneer due to orbital motion of the Earth of
30,000m/s which must have been taken into account.


The effect from the rotation of the Earth is more significant
due to the shorter period.

But how narrow is the
bandwidth, and how accurately does this need to be predicted? Could an
unmodelled effect as small as _+-2m/s (varying annually) contribute to
loss of lock?


For the initial search I know the DSN uses FFTs over
a fairly wide rang - 16kHz from memory with a check
in adjacent bands. The diurnal Doppler is around 8 kHz
(and is known of course) but produces a rate of change
in the kHz per hour range. The FFT then told a PLL
where to look with an initial bandwidth in the low Hz
which then dynamically reduced to as low as 0.01Hz
as lock was obtained. However, the craft hardware
was probably much simpler. I'm sorry I can't help
more on that but I don't have details on the craft
receiver design.

Against that, the uplink power was 400 kW compared
to just 8 W on the downlink so I would have expected
a much stronger lock. Anyway the bottom line is that
I expect it was the rate of change of frequency that
would break the uplink lock. It's a shame they couldn't
hack the correlator box to run a smaller frequency
deviation but since it was probably dedicated hardware
in those days, it might not have been practical.

BTW, I am a digital and systems engineer in
a company working significantly in HF comms.


It's clear you know a lot about this kind of thing, and your input is
much appreciated.


Thanks, I don't pretend to know everything but I hope
it is helpful.

George

 




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