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Old 1st Feb 2008, 12:59 pm   #21
Al (astral highway)
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Default Re: How to hear NASA satellites...

Upload problem fixed. Thanks ppppenguin (Jeffrey) for your PM.

Recording 1: frequency of satellite 141.00 Mhz exactly
Recording 2: freq. of satellite 145.971 Mhz

The identity of this satellite is a mystery. The closest matches are149.970 which my list tells me is Cosmos 2389, a navigation and comms. satellite.

However, this is supposed to be FM time-and-position information. Have a listen and you'll see that this isn't what we're hearing.

The next closest match is Oscar, used for ionospheric modelling by the US military. However, this is listed as just a CW carrier and what we're hearing doesn't match that.

ARGOS is at 150.012 and is used for USAF space experiments - that's a doppler tracking beacon.

I'm going by the full Zarya list: http:www.zarya.info/frequencies/FrequenciesAll.php

I don't know when that was last updated.

As I observerved yesterday, one explanation for the difficulty in pinning down to a precise frequency is the doppler effect -- you pick up a satellite with 3places accuracy after the decimal point on your frequency meter, but you hear the doppler effect and have to retune to track the satellite.

So for example, I picked one up yesterday at 140.213

Within seconds I had to retune: 140.208

And again after another few seconds: 140.201

And once more after that: 140.190...

before finally losing the modulation and just getting noisy carrier. That's a 23K/c apparent drift in just a few seconds. Of course there's no real drift, but I can't account for this phenomenon. Anyone care to help?

Both recordings were sometime between 12.30 am and 1.00 am on 31 January
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Old 2nd Feb 2008, 7:46 pm   #22
Andy Green
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Default Re: How to hear NASA satellites...

It's called Doppler shift, and is the same phenomenon that causes and approaching emergency siren to have a higher pitch as it approaches and a lower one as it disappears.
the satellite you are listening to is not in a synchronous orbit, and is approaching you at quite a velocity, which is changing with respect o your position, hence the apparent frequency drift.

It is a common phenomenon, especially with amateur radio satellites - the operators either have to manually tune their radios, or some are automatically tuned by a PC programme

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Old 3rd Feb 2008, 9:20 pm   #23
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Default Re: How to hear NASA satellites...

Quote:
Originally Posted by Andy Green View Post
.... the satellite you are listening to is not in a synchronous orbit, and is approaching you at quite a velocity...
Hi Andy. Yes:

Radius of orbit is 22,300 miles, so
Diameter =44,600 and by Pi *D, circumference of orbit is appx 140,000 miles.

Do they complete one orbit in 24 hours? So that's just under 6,000 MPH, then.
Quote:
Originally Posted by Andy Green View Post
...which is changing with respect to your position

And to make things more complicated, at the rate of 5.75 uS/mile, it takes a lengthy 0.13 seconds for the signal to reach earth.

Quote:
Originally Posted by Andy Green View Post
... hence the apparent frequency drift.

But are you please able to provide an explanation of how doppler works with radio frequencies and how the 'true' frequency can be determined despite these difficulties?
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Old 3rd Feb 2008, 10:54 pm   #24
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Default Re: How to hear NASA satellites...

Al ,

I would read wilkpedia for the doppler , it is not straightforward although the approximate equations are close enough.You are starting to move into the realms of the theory of relativity.

Mike
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Old 4th Feb 2008, 1:20 am   #25
dave walsh
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Default Re: How to hear NASA satellites...

Interesting Mike. After following this thread, I had decided not to mention relativity [perhaps too obscure and OT] even though I was reminded about recent BBC4 programs that used Doppler to try and explain atomic theory. [Please note that I am not claiming to understand the concept re satellites, the universe or anything! I can't do the maths.].
We did have the same thought though and radio theory is a part of the same deal. Dave
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Old 4th Feb 2008, 11:39 am   #26
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Default Re: How to hear NASA satellites...

It really is interesting stuff indeed Dave, I would love to know more , apparantly Einstein had trouble coming up with a complete theory for Doppler.

I must say Al and his project has got me interested in having a Go and as you quite rightly say this and radio theory is the same deal.
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Old 5th Feb 2008, 8:43 am   #27
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Default Re: How to hear NASA satellites...

Quote:
Originally Posted by MichaelR View Post

I must say Al and his project has got me interested in having a Go...
That's great, Mike. PM if you want me to send you the full text of the article.
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Old 5th Feb 2008, 7:02 pm   #28
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Default Re: How to hear NASA satellites...

I've tweaked the converter a bit more and am completely happy with it now. In the second pic you can see final anode tank coil, close-wound with 11 turns and no former. I tried about 20 variations to get to this best-performing candidate.

One problem I found and fixed after poking about was that there was too much stray 12.888 MHz floating about in the circuit. This is the crystal's fundamental and it was getting where it shouldn't be and causing trouble.

I fixed the problem by pulling some copper tape off a junked pulse transformer from a scrap TV circuit board and wrapping and grounding the case with the copper tape. The result is the golden 'lozenge' in the centre of the picture. You'll also see that the other half of this tape came in handy to screen the underside of the Nuvistor circuitry from the adjacent coils. I used a dab of silicon to stop the screen from moving.

Finally, forget the twisted wire cap. coupling to the control grid of the mixer. The signal is too weak. A 200 pF silver mica cap does a much better job.

Tuned to the IF, there is now just a 17Khz difference between the comms receiver frequency counter reading and a Marconi SG test signal, so 136.000000 Mhz. on the Marconi reads 20.017 Mhz on the Lowe comms receiver. I'm giving the benefit of the doubt to the Marconi, and will adjust readings accordingly.

You might be amused by my current dipole. When we bought this house last summer, this piece of brass was lying with tons of other junk at the bottom of the garden.

It scrubbed up okay and when I measured it, was exactly the right length to chop into two and make a temporary 136 MHz dipole for the study.

It has elements of slightly asymmetric height, but it only took twenty minutes to 'make', fits in the only space available and was able to pick up a satellite at 136.9 MHz about an hour ago. It'll do 'til I have the cash to buy something longer-term for the loft.
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Old 7th Feb 2008, 1:27 am   #29
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Default Re: How to hear NASA satellites...

Here the Nuvistors were very popular for several years in the RCA TV sets, (New Vista series tuners), Ham converters (AMECO and others) and in some audio gear.

They are always easy to find at the hamfairs, so far.
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Old 9th Feb 2008, 12:32 pm   #30
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Default Re: How to hear NASA satellites...

As there seems to be quite an interest in these older satellites, I've dug out a recording from the period. This one is of Sputnik, no less, which used to transmit at a much lower frequency (around 20 Mhz, which is now merely the IF of the circuit I'm now using.)

Sputnik, 'Спутник-1' of course, had the distinction of being the first satellite to be put into geocentric orbit, much to the chagrin of the other side.

This recording was made by Roy Welch in Dallas, TX on October 7, 1957. He used a AN/FRR3A HF receiver (mil. surplus) to pull in this signal. Apparently, he used to play these signals while simultaneously watching the booster rockets tracking overhead at night.

Now that's what I call an AV display!
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Old 9th Feb 2008, 7:14 pm   #31
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Default Re: How to hear NASA satellites...

When I found out about all this in the sixties I was looking at adverts from the period for a "Sputnik Special" receiver ie ex govt R208 covering 10-60 megs. [Info and pics on the web]. I think there were some of these still about but not so easily obtainable and out of my price range. Never came across one subsequently but then I found a pretty beat up set ten years ago which [eventually] I recognised as one of these. Anything claiming to be a VHF set in those days was pretty rare hence the popularity and I guess that some regular sets may not have been considered sensitive enough at 20 megs and above. I think some Grammar school somewhere got a lot of publicity through a project to tune in to Sputnik and may have used the 208. Dave
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Old 11th Feb 2008, 6:19 pm   #32
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Default Re: How to hear NASA satellites...

Quote:
Originally Posted by HMV 1120 View Post
Any chance of picking up 'Prospero', Britians only self-launched satelight? I gather it is still active between 137-138MHZ, although only it's timer signal can be recieved.
I've done some basic research into this and from what I can see, Prospero was last picked up and logged in 2000. Does anyone know any better?
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Old 25th Feb 2008, 7:46 pm   #33
dave walsh
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Default Re: How to hear NASA satellites...

Further to this interesting thread, BBC4's "The Satellite Story" is repeated on BBC2 tonight at 11-20pm. Dave W
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Old 26th Feb 2008, 7:33 pm   #34
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Default Re: How to hear NASA satellites...

Paul Williams of AMSAT kindly answered a query of mine about the orbital path of TIROS and similar satellites. It corrects my misassumption about this in an earlier post in this thread:

'...the satellite designers choose a low circular orbit that goes near the North and South poles ...(a polar orbit.)

What does it mean for such an orbit to always see the same local time of day? It means that the angle between the satellite's orbit and the sun's rays is fixed -- the angle of the sun's rays on the Earth's surface is the definition of "time of day". So far, so good.

Now, consider what happens over the course of a year. Ideally, the orbit plane follows the Earth around the sun but is always parallel to itself.

That means the subsatellite time-of-day will shift, because the sun's rays come from a different direction but the angle of the satellite's orbit plane is unchanged. In order to avoid the time-of-day shift, we want the orbit plane to drift slowly too, spinning a full 360 degrees over the course of a year, so that it's always at the same angle with respect to the sun's rays. Luckily, it's easy to get the orbit plane to drift at a fairly constant rate. Since the Earth isn't a perfect sphere, an orbit that isn't perfectly polar will experience some small gravitational forces that tend to twist the orbit plane. All that's necessary to get the desired drift rate is to choose an orbit plane that's a little bit off the perfectly polar orientation.

It works out that an inclination of about 98 degrees (where 90.0 degrees is a perfectly polar orbit) creates the correct drift rate.

So, a sun-synchronous satellite is a low orbiter with an inclination of about 98 degrees, such that its orbit plane spins one rotation per year, staying over the same local time-of-day continuously. And 100 minutes is very typical for a low orbit, an orbit that's just barely high enough to be fairly permanent.'
Thank you to Paul.
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Old 26th Feb 2008, 10:43 pm   #35
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Default Re: How to hear NASA satellites...

Hi Al,

Thanks for sharing that, I think I might do a little more reading on this subject. The TV program was excellent .

Mike
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