On 3/29/12 6:19 PM, Tom Knox wrote:
I thought I might apologize because I didn't explain my idea very
well initially, and reiterate my original thoughts on a pulsar timing
idea. The basis of my idea was that if problems could be overcome
certain pulsars could provide not only Time but also Position info
directly to orbiting platforms.
I think that trying to figure out how to use pulsars as as timing source
is certainly worthy of contemplation. And as for resources.. who'd have
thunk 20 years ago that you could have atomic clocks (real ones, not
WWVB receivers) in your house as a hobby using surplus.
First: I assumed from the start that
this was not a DIY idea, but mainly an intellectual exercise, far
beyond the resources of all but a few governments. And most likely
beyond the scope of our current GPS system. But since I have been a
TimeNuts I have been constantly amazed by the complex problems that
have been taken on and solved by this brilliant group. So I thought I
would throw in something purely theoretical. I should have known
there would be at least one TimeNuts writing a thesis on Pulsars. It
reminds me, I have been very lucky to have worked around some
brilliant people, when I am asked about their depth of knowledge I
usually reply "They are not that smart, I can get coffee for anyone
of them and get it right three out of four times". My point is "Do
you want any coffee".
Second: What I was envisoning was collecting
data on Ultra stable pulsars including location, motion and timing
needed to use them as a defacto GPS constallation for a set of earth
based satalites.
I think that's been done.. At least there is a good list of candidates.
With this information you could ascertain the exact
position of each satellite. This idea has been under investigation
for long distance satellite navigation for some time. I see no
advantage terrestrially receiving pulsar signals as a atlernative
freq standard in place of the USNO since the cool thing about this
concept is that the pulsars would provide both time and position
directly to the orbiting platform.
and orientation. Sort of like a super star tracker all in one! (You
can see why NASA is interested..)
Third: Does observing these
pulsars from space allow reception of signals at levels and in bands
that would be blocked by the earth atmosphere.
Certainly yes for X-ray pulsars.. the atmosphere blocks them. For radio
pulsars, the attenuation through the atmosphere isn't huge (at least at
microwave frequencies, and if you don't pick something like 60GHz where
there's an absorption band).
If not could some of
the advances in superconductivity provide an amp of sufficient
sensitivity to overcome these problems.
The limiting thing on weak signals is typically noise, and cooling helps.
Forth: The problems I foresee
are can an practical algorithm accounting for the complex motion of
all these bodies be built,
Yes. It's just geometry, and probably not more difficult than any other
triangulation problem. Surveyors use a form of least squares in some
cases, and that's sort of what a Kalman filter does, as well. That's
probably the easy part.
and can Pulsar signals be received at a
high enough signal to noise ratio for this system to produce a
coherent time source.
That's the hard part.
I have really enjoyed all the topics lately
thanks all for the contributions. Clearly Time Nuts; Thomas Knox
Date: Fri, 30 Mar 2012 00:17:33 +0200 From:
mag...@rubidium.dyndns.org To: time-nuts@febo.com Subject: Re:
[time-nuts] Pulsar Source?
On 03/29/2012 01:50 AM, Jim Palfreyman wrote:
Folks, I'm currently writing my thesis on pulsars, but I need to
spend time on it rather than here. :-) But since a lot of this
discussion is right at the front of my brain, here's a summary.
Some pulsars "glitch" or speed up. The Vela pulsar (PSR
J0835-4510) does this (this is the pulsar I've been studying) and
yes these type of pulsars are bad clocks. A jump of its pulse
rate of the order of 10^-6 s/s randomly every few years is not
good. It does nearly settle back to its original rate after a few
months.
The nature of these glitches (in Vela at least) is not well
understood, but three theories have been put forward:
- An orbiting planet - but this has been discarded due to their
irregularity. - Star quakes caused by a separation of the crust
on the surface of the neutron star from its super-fluid interior.
(Not very popular any more) - The effects of tiny micro vortices
in the internal super-fluid. (If you can understand this paper -
good luck to you!)
Now faster pulsars, in particular millisecond pulsars (~700 Hz
from memory is the fastest) are quite good clocks and they do
rival atomic clocks. The hunt is on to find as many of these as
they can, well spread across the sky, so they can look at the
effects of gravitational waves on the beams of these super
accurate clocks. This is one proposed method to detect
gravitational waves.
The main problem I see from the original suggestion is that most
pulsars are quite faint and you need a very decent telescope to
see individual pulses. Vela is very bright, and the 26m telescope
I used can only just see the average pulse. I'm studying bright
pulses and we can see those easily.
So to dedicate a massive radio telescope (or two) pointing at a
millisecond pulsar just so we can re-transmit it, is probably not
sensible. However, studies of these remarkable pulsars is
ongoing.
Hmm, wouldn't the space-located antenna have a good chance of
better S/N as the antenna sees cold space and could be kept cold
itself?
I was also thinking antenna size would be a limitation. Then I was
thinking about what WMAP has achieved in measuring the background
temperature and look back at the very early years of the universe.
Cheers, Magnus
Cheers, Magnus
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