Well this seems to be the subject of off topic at the moment, perhaps you
can help me with this one.
I have a radio scanner, and often pick up data from orbiting satellites.
Most notably on the band from 137-138Mhz and around 149.9 Mhz.
There is of course a Doppler shift, and as I do not know the actual spot
frequencies in use. it is hard to tell what is going on.
My understanding is that if an object is moving with a constant speed
relative to me, the shift will be constant and higher in frequency as it
moves toward me. But if its moving away, then it should be lower.
In an inclined orbit, as most of these objects are, and with the world also
turning the effect seems to be counter intuitive to my scanner. IE put in
the bfo and beat that with a subcarrier or the carrier when no data is
present, and as it comes above the horizon, the rate of speed changes as one
would expect as the ground track speed is constantly changing, but hang on,
it always getting lower in frequency, ie to preserve the beat pitch the
actual tuning needs to go to lower and lower frequencies. To me, surely, it
should be going higher at the start, and lower as it heads for another
horizon.
I suspect it has to do with the constantly varying relative speed and the
turning of the planet, but its just not what I expect, but it happens,
nonetheless. I suspect if I knew the actual spot frequency,to he problem
would go away as one is measuring the rate of change not the actual velocity
without knowing that fact.
Brian

--
From the Sofa of Brian Gaff Reply address is active

On 11/11/2014 22:06, Brian Gaff wrote:
Well this seems to be the subject of off topic at the moment, perhaps you
can help me with this one.
I have a radio scanner, and often pick up data from orbiting satellites.
Most notably on the band from 137-138Mhz and around 149.9 Mhz.
There is of course a Doppler shift, and as I do not know the actual spot
frequencies in use. it is hard to tell what is going on.
My understanding is that if an object is moving with a constant speed
relative to me, the shift will be constant and higher in frequency as it
moves toward me. But if its moving away, then it should be lower.
In an inclined orbit, as most of these objects are, and with the world also
turning the effect seems to be counter intuitive to my scanner. IE put in
the bfo and beat that with a subcarrier or the carrier when no data is
present, and as it comes above the horizon, the rate of speed changes as one
would expect as the ground track speed is constantly changing, but hang on,
it always getting lower in frequency, ie to preserve the beat pitch the
actual tuning needs to go to lower and lower frequencies. To me, surely, it
should be going higher at the start, and lower as it heads for another
horizon.

That is what you would expect. The satellites component of velocity
towards you varies from almost its entire orbital speed approaching to
the same receeding and is on its true frequency (give or take
gravitational shifts) when it is closest to you with maximum rate of
change. The frequency starts high and gets lower with time.

The characteristic meow sound as a police car on the opposite
carriageway whizzes past.

Although the orbit is probably elliptical the eccetricity and the
Earth's spin merely alter the time of the point of closest approach when
its component of velocity along your line of sight is zero.

I suspect it has to do with the constantly varying relative speed and the
turning of the planet, but its just not what I expect, but it happens,
nonetheless. I suspect if I knew the actual spot frequency,to he problem
would go away as one is measuring the rate of change not the actual velocity
without knowing that fact.
Brian

You can work out the spot transmission frequency by plotting frequency
against time and looking for the steepest point on the curve.

--
Regards,
Martin Brown

Ah that is only possible if you can see a curve of course, but I get your,
erm drift I think.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Martin Brown" wrote in message
...
On 11/11/2014 22:06, Brian Gaff wrote:
Well this seems to be the subject of off topic at the moment, perhaps you
can help me with this one.
I have a radio scanner, and often pick up data from orbiting satellites.
Most notably on the band from 137-138Mhz and around 149.9 Mhz.
There is of course a Doppler shift, and as I do not know the actual
spot
frequencies in use. it is hard to tell what is going on.
My understanding is that if an object is moving with a constant speed
relative to me, the shift will be constant and higher in frequency as it
moves toward me. But if its moving away, then it should be lower.
In an inclined orbit, as most of these objects are, and with the world
also
turning the effect seems to be counter intuitive to my scanner. IE put
in
the bfo and beat that with a subcarrier or the carrier when no data is
present, and as it comes above the horizon, the rate of speed changes as
one
would expect as the ground track speed is constantly changing, but hang
on,
it always getting lower in frequency, ie to preserve the beat pitch the
actual tuning needs to go to lower and lower frequencies. To me, surely,
it
should be going higher at the start, and lower as it heads for another
horizon.

That is what you would expect. The satellites component of velocity
towards you varies from almost its entire orbital speed approaching to the
same receeding and is on its true frequency (give or take gravitational
shifts) when it is closest to you with maximum rate of change. The
frequency starts high and gets lower with time.

The characteristic meow sound as a police car on the opposite carriageway
whizzes past.

Although the orbit is probably elliptical the eccetricity and the Earth's
spin merely alter the time of the point of closest approach when its
component of velocity along your line of sight is zero.

I suspect it has to do with the constantly varying relative speed and
the
turning of the planet, but its just not what I expect, but it happens,
nonetheless. I suspect if I knew the actual spot frequency,to he problem
would go away as one is measuring the rate of change not the actual
velocity
without knowing that fact.
Brian

You can work out the spot transmission frequency by plotting frequency
against time and looking for the steepest point on the curve.

--
Regards,
Martin Brown

Brian Gaff wrote:

I have a radio scanner, and often pick up data from orbiting satellites.

There is of course a Doppler shift, and as I do not know the actual spot
frequencies in use. it is hard to tell what is going on.

My understanding is that if an object is moving with a constant speed
relative to me, the shift will be constant and higher in frequency as it
moves toward me. But if its moving away, then it should be lower.

That is correct. Technically, the Doppler shift is dependent on the
radial velocity along a line between satellite and observer. That
is, if you draw an imaginary line between you and the satellite, the
Doppler shift is dependent on the velocity along that line.

In an inclined orbit, as most of these objects are, and with the world also
turning the effect seems to be counter intuitive to my scanner.

The only thing that matters for Doppler is the radial velocity between
you and the satellite, so Planet Earth can be ignored.

IE put in the bfo and beat that with a subcarrier or the carrier when no data is
present, and as it comes above the horizon, the rate of speed changes as one
would expect as the ground track speed is constantly changing, but hang on,
it always getting lower in frequency, ie to preserve the beat pitch the
actual tuning needs to go to lower and lower frequencies.

I'm not sure how one could beat a BFO with a subcarrier when the
satellite is below your local horizon. You'll receive nothing from the
satellite in this condition; there will be no sub-carrriers
receivable. This might mean that your radio isn't correctly tuned at
the time the satellite rises locally to you.

I used to play with radios in my younger days, but might not have
this fully correct, but the only way I can see that this is
happening is that your BFO is set 'high' with regard to the carrier
you are monitoring. In this case, the actual Doppler shift will go
higher, but that's where your BFO is set, so the beat note will get
lower. If your radio has selectable sidebands, you might be
listening using the 'wrong' one.

Imagine holding up the index fingers of both hands, separated by a few
inches but parallel to each other. The left-hand's finger represents
the satellite carrier frequency, and the right-hand's one is the BFO
frequency. Frequency increases from left to right. The frequency of
the beat note you hear depends on the spacing of your two fingers.
Now imagine the satellite carrier increasing in frequency as it
emerges over your local horizon, this is the same as your LH finger
moving towards your RH (BFO) finger - the beat note is getting lower
even though the actual satellite Doppler is increasing.

But reverse the roles of the two fingers, and as the satellite, now
represented by your RH finger, moves away from your LH (BFO) finger,
the beat note increases.

You could prove this was the case by listening for a complete pass of
the satellite over your position - if your radio is set up to receive
on the 'high' side of the satellite carrier, and the initial Doppler
beat goes lower, then as the satellite passes it's nearest point to
you where the radial velocity is zero, the received Doppler beat
note will start to increase in frequency even though the Doppler is
going lower.

I suspect it has to do with the constantly varying relative speed and the
turning of the planet, but its just not what I expect, but it happens,
nonetheless.

No, the Doppler depends only on the rate of change of velocity along
the radius vector between you and the satellite.

I suspect if I knew the actual spot frequency, the problem
would go away as one is measuring the rate of change not the actual velocity
without knowing that fact.

I think that this, possibly combined with a mis-tuned radio, is giving
you the Doppler problem.

Hope this helps.

--
Terry Fields

No I've sorted all that out ok, thanks. and no I was meaning that when it
pops up it is heard, and when it gets too low it goes away of course. See my
other posts for the rest.

No I think I have it. The point is as I surmised that the beat only tells
you the frequency is changing. If its heading toward you more than either
up down or from side to side the frequency will be high, but as it comes
nearer it will get lower in frequency as the speed is more pararell than
toward you making it appear to slow and get lower, and of course the wsame
will happen once it passes abeam or overhead so its really always going to
happen that way.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"Terry Fields" wrote in message
...
Brian Gaff wrote:

I have a radio scanner, and often pick up data from orbiting satellites.

There is of course a Doppler shift, and as I do not know the actual spot
frequencies in use. it is hard to tell what is going on.

My understanding is that if an object is moving with a constant speed
relative to me, the shift will be constant and higher in frequency as it
moves toward me. But if its moving away, then it should be lower.

That is correct. Technically, the Doppler shift is dependent on the
radial velocity along a line between satellite and observer. That
is, if you draw an imaginary line between you and the satellite, the
Doppler shift is dependent on the velocity along that line.

In an inclined orbit, as most of these objects are, and with the world
also
turning the effect seems to be counter intuitive to my scanner.

The only thing that matters for Doppler is the radial velocity between
you and the satellite, so Planet Earth can be ignored.

IE put in the bfo and beat that with a subcarrier or the carrier when no
data is
present, and as it comes above the horizon, the rate of speed changes as
one
would expect as the ground track speed is constantly changing, but hang
on,
it always getting lower in frequency, ie to preserve the beat pitch the
actual tuning needs to go to lower and lower frequencies.

I'm not sure how one could beat a BFO with a subcarrier when the
satellite is below your local horizon. You'll receive nothing from the
satellite in this condition; there will be no sub-carrriers
receivable. This might mean that your radio isn't correctly tuned at
the time the satellite rises locally to you.

I used to play with radios in my younger days, but might not have
this fully correct, but the only way I can see that this is
happening is that your BFO is set 'high' with regard to the carrier
you are monitoring. In this case, the actual Doppler shift will go
higher, but that's where your BFO is set, so the beat note will get
lower. If your radio has selectable sidebands, you might be
listening using the 'wrong' one.

Imagine holding up the index fingers of both hands, separated by a few
inches but parallel to each other. The left-hand's finger represents
the satellite carrier frequency, and the right-hand's one is the BFO
frequency. Frequency increases from left to right. The frequency of
the beat note you hear depends on the spacing of your two fingers.
Now imagine the satellite carrier increasing in frequency as it
emerges over your local horizon, this is the same as your LH finger
moving towards your RH (BFO) finger - the beat note is getting lower
even though the actual satellite Doppler is increasing.

But reverse the roles of the two fingers, and as the satellite, now
represented by your RH finger, moves away from your LH (BFO) finger,
the beat note increases.

You could prove this was the case by listening for a complete pass of
the satellite over your position - if your radio is set up to receive
on the 'high' side of the satellite carrier, and the initial Doppler
beat goes lower, then as the satellite passes it's nearest point to
you where the radial velocity is zero, the received Doppler beat
note will start to increase in frequency even though the Doppler is
going lower.

I suspect it has to do with the constantly varying relative speed and
the
turning of the planet, but its just not what I expect, but it happens,
nonetheless.

No, the Doppler depends only on the rate of change of velocity along
the radius vector between you and the satellite.

I suspect if I knew the actual spot frequency, the problem
would go away as one is measuring the rate of change not the actual
velocity
without knowing that fact.

I think that this, possibly combined with a mis-tuned radio, is giving
you the Doppler problem.

Hope this helps.

--
Terry Fields