GB3MBA · 50.408 MHz · forward scatter

Sherwood Meteor Zoo

what we have measured simple version

Two minutes of training first. Every picture below is a real meteor, confirmed by cameras on the ground — so you are learning from the genuine article, not a drawing.

First, what you are looking at

A six-second spectrogram with a meteor echo

Six seconds of radio

The newest moment is at the top, the way the live display scrolls, so the picture reads upward in time. Across is frequency — how far the echo is shifted from the beacon's own note.

Head echoes are horizontal. Tail echoes are vertical. That one line will get you most of the way.

The reason is worth knowing, because it makes the odd cases obvious. A head echo is the meteor itself, moving at 30 to 70 km a second. That speed shifts the note it returns, and because the meteor is travelling so fast the shift races across the frequency scale in a fraction of a second — many frequencies, one instant, which draws a horizontal line.

A tail echo is the glowing trail left hanging in the air behind it. The trail is not going anywhere, so it returns the same note the whole time it lasts — one frequency, many instants, which draws a vertical line. It fades as the trail cools. Either way you are looking for a line, never a dot.

Two things are ours, not the sky's. A bright band repeating exactly once a second, six or seven times up the picture, is our clock pulse — it lies the same way round as a head echo, so tell them apart by the even repeat, not by the direction. It is far wider than a meteor streak and usually spans most of the picture, though at some sites it stays nearer the centre, so trust the repeat rather than the width. A steady line straight down the middle is the beacon itself. Neither is ever a meteor.

The four shapes worth telling apart

Three more you will meet, which we have no confirmed picture of yet

E

Aircraft

A long, steep, straight line crossing the whole picture. Aircraft are the thing most often mistaken for a meteor.

G

Interference

Man-made noise getting into the receiver: lines that slide or waver in frequency, a burst of hash across the picture, or a stack of lines that is not evenly spaced. Beware the band that repeats once a second — that is our own timing pulse and belongs in F, not here. Measured across the queue, about six clips in ten carry it, so it is the normal state of the picture rather than something wrong.

H

Unknown

Clearly something, but you cannot say what. A real and useful answer — far better than forcing it into a box that does not fit.

Drag a box round each thing you can see and press its letter. Mark as many as you like. If something is there but you cannot name it, mark what you can and use …and there’s more I can’t name — that is a better answer than a confident one, because it stops the leftovers being counted as empty sky. If you are unsure about the whole frame, press S and it goes to somebody else — guessing helps nobody.