OT, talking of physics

[Brian Gaff]

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

[Martin Brown]

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.

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

[Terry Fields]

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.

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

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.

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

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

Hope this helps.

[Brian Gaff]

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

Brian

[Brian Gaff]

Yes, I know about that, but a picture is of no use to me of course, and the scanner does not have the 50khz bandwidth, but if you use a narrow filter and a bfo, you can resolve individual sub carriers in the signal and yes you can tell which side you are on easy enough. the olde sats are just slowish fax transmissions, but the newer ones are more like drm kind of data. The one on 149.9 is a kind of multiplexed rtty signal with a very low time to vanish and huge doppler shifts, so do not know what that is. There also seem to be some blank carriers around400 mhz as well, but no idea what they do.

Brian

[Brian Gaff]

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