Saturday, April 20, 2024

When Hobbies Converge or What The Heck is Meteor Scatter?

It's no secret that I'm a radio amateur (ham radio operator); I've talked about that hundreds of times over the life of the blog.  There has been a Radio Sunday series and a specific Ham Radio Series. This will go with the ham radio series. 

Something I haven't mentioned as often, but still have several times, is that one of my hobbies as far back as I can recall has been astronomy and telescopes. I haven't been active in that for a while, but there are posts here about that I found by going back and searching the blog for specific words. I've built telescopes, reflectors from grinding the mirror from a plate of glass, and refractors from things like copier lenses. Some background here, although not related to tonight's post. 

As an amateur astronomer, I've gone to places near and far where I could observe a meteor shower that was supposed to be particularly good. People who aren't in to this hobby generally aren't aware that there are regularly recurring meteor showers several times a year. If you follow meteor showers to observe the pretty light streaks the meteors leave, you quickly learn that showers are best in the early morning hours, or between midnight and first light. If the moon is close to full and bright, it can wipe out the best of showers.  The reason for the timing is that as the Earth moves in its orbit, the rotation that brings night and day rolls the planet onto some meteors, so more are seen. The meteors have to be moving faster to catch up with the planet.

Back in the 1950s, hams discovered that the trail created in the upper atmosphere by meteors burning up acted like a radio reflector, as if the ionosphere was enhanced. The effect is most beneficial to VHF stations which typically don't have regular, predictable ionospheric propagation. In the early days, this was done with Morse code, or CW as the hams typically call it (CW for Continuous Wave). Now that we're firmly in the age of computer assisted modulation modes, most people trying to bounce signals off the meteor trails are using a computer assisted mode. The most popular appears to be MSK144 (.pdf alert).

MSK stands for minimum shift keying, a form of continuous-phase frequency-shift keying (FSK) with shift equal to half the baud rate. MSK144 uses message frames of 144 bits and modulation at tone frequencies 1000 and 2000 Hz to transmit channel symbols at keying rate of 2000 baud. The resulting audio waveform can be viewed as a form of offset quadrature phase-shift keying (OQPSK) with individual pulses shaped like the first half-period of a sine wave.

Although there are other modes still used for meteor scatter, this is the dominant mode. 

So how do you operate meteor scatter? Have contacts with people hundreds to perhaps 1500 miles or more by bouncing a signal off a meteor trail?  

This is why the talk of meteor showers. This time of year has an annual shower named after the constellation it appears to radiate from, Lyra (the lyre) and called the Lyrid shower. Meteor showers don't just happen, they ordinarily happen when the Earth goes through the trail left by comet; the Lyrids are the trail of comet called C/1861 G1 (Thatcher). That's right, it's a comet that was first named in 1861, and it's a long period comet with around a 422-year orbit. It's expected to return around 2283. Thatcher is the discoverer. 

The Lyrids are predicted to peak Monday morning. Interestingly, another shower is already starting to build, coming to its peak two weeks later on May 5th. This is called the Eta Aquariids because it appears to radiate from close to the star Eta Aquarii. This shower is dust from the trail of Halley's comet. Halley's comet is another relatively long period comet although at 76 years quite a bit shorter than Thatcher's; its last close approach to the sun or perihelion was in 1986, and its next perihelion will be in 2061. I remember taking my son to the beach to see Halley's comet; he was five. I haven't asked him if he remembers that in years. 

Given the shower to provide the meteors and the software to interface to your radio to modulate and demodulate the data stream, if your antenna is directional, it makes sense to point it toward the radiant because the trails will be denser there. Most people use a 15 second transmit period followed by 15 seconds receive. The protocol is that if you're trying to contact people to your east, you transmit on the minute and half minute marks - or 0 and 30 seconds. Those are referred to as the even intervals, so they give rise to the acronym PETE: Point East Transmit Even. If you're trying to contact people to your west, you transmit on the 15 and 45 second marks. 

You'll get the best reflections where the trails are the densest, so unlike the case with other modes, you don't necessarily point your antenna at the person your contacting. It works best if you're both pointed at the radiant. Remember, the radiant is like everything else in the sky: it raises in the East and sets in the West.  The exact position in the sky of the radiant isn't extremely important, but the astronomy observing-oriented websites help you learn where the radiant is. There's a program aimed at ham use that seems helpful, called Virgo.

It's important to know that while the astronomy websites are concerned with the peak number of meteors visible per hour, and the Lyrids aren't impressive that way, the showers are perfectly usable for radio after first light and well into the morning. The things that make a shower a highlight for visual observation don't mean much to radio use. I've played at this mode only a few times, and copied signals off the meteor trails until 9 to 10 AM. 


  1. When I was still living in Amarillo, TX, several of the members of the local amateur radio club where I was a member were also members of the local astronomy club. I went along as a guest with the astronomy club several times. There are some remarkable views, especially through the larger telescopes. The chief engineer of one of the local TV stations was also a member of the astronomy club.

    I knew of some who did moon bounce on a regular basis. A couple of them tried meteor scatter but with minimal success.

    As I understand it, the military has a special communications network setup to use the backscatter from even the dust sized meteors that enter the atmosphere.

  2. Wow that is a great post SG!
    In David Drakes military Sci-Fi trilogies Hammers Slammers, the Hammers resort to using meteors to bounce secure coms to their troops, when say a planet they are fighting on in which all sat coms are taken out, and how they developed highly complex software to increase the effectiveness of this method, using highly compressed short burst packets.
    Maybe David Drake in real life was a Radio Ham? Its such a great idea regardless. To find out its an actual thing is really cool.

  3. We need a great celestial event, an eye visible comet would work, its been a while could be we are due one, preferably something spectacular. It would fit the times thats for sure.

    1. eye visible as in a comet tail glowing cross half the nights sky, that works.

    2. The last comet bright enough to be seen naked eye on a major street here was Hale-Bopp back in the late '90s. It would be super cool to see another comet that bright, but they're rare - once or twice in a lifetime. The mechanisms that form a comet tail aren't the same as those that form the meteor trails, so I don't think it would work for that. From an old memory, the last time a comet tail crossed the Earth was before radio.

    3. Indeed that was a dandy. There was Kohotec in the mid 70's I remember as a kid,

  4. The predictions of the Lyrids being good for radio fell pretty flat this morning. I copied two 15 second transmissions between 6:15 and 8:00, both in the interval between 6:20 and 6:30. As Murphy would have it, at 8:00 I went into the shop to set up a battery test and when I came back in 12 minutes, there were a couple of dozen of those 15 second messages on screen.

    Tomorrow is the predicted peak. Might be worth another try.

  5. I remember (barely) Halley's comet, and the discussion that my grandfather was old enough at the time to have seen it twice.

    I assume if you set it up properly, you can bounce a signal off of anything.
    I've read of radars keying off streams of bullets from celebratory gunfire in the middle east.

    1. The problem with being setup properly is that we ordinarily don't get to do that. We can only operate in allocated frequency bands and under the current laws. The optimum way of bouncing a signal off a comet tail, for example, might require moving to the microwave spectrum and running high power. That would eliminate interest for the vast majority of folks.

      Getting radar returns off bullets flying a few hundred feet or yards above the ground seems like a pretty easy problem. Something like a 10 GHz radar designed for short output pulses seems like it should do it. Metal is very reflective and the rounds aren't microscopic or moving very fast.

    2. Bullets are pretty clear at 94GHz also. They show up as short-lived scintillations.