The Ghost of Fireballs Past
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Updated: June 18th, 2018
Meteoroids like the Leonids travel at tremendous velocities. When they strike Earth's atmosphere at speeds of 100,000 mph or more they ionize the air in their path. These luminous ionized trails, like the one pictured below, are visually striking. But that's not all -- they also reflect radio waves. During a shower like the Leonids, radio signals from TV stations, RADAR facilities, and AM/FM transmitters are constantly bouncing off meteor trails. The echoes can be heard around the world.
One of the most powerful transmitters on Earth is the Navy Space Surveillance Radar (NAVSPASUR for short). Located in Kickapoo, TX, NAVSPASUR transmits 800 kW of continuous-wave (CW) radio power into an east-west oriented fan beam at 216.98 MHz. The radar's primary mission is to track satellites and space debris for the US Space Command. It can detect objects as small as 10 cm orbiting 15,000 km above the earth's surface. Although meteoroids measuring a mm or less in size are generally too small to be detected, their ion trails produce strong echoes that can be heard on simple ham radio equipment.
The WAV-format audio files linked below contain recordings of NAVSPASUR radar echoes from Leonid and Geminid meteors, and from the space shuttle Endeavour during the recent ISS assembly mission. Each file was extracted from an audio recording provided by Dr. Steven Bienvenu, M.D.
Leonid meteor echo: This 25s audio file begins with 3 brief satellite echoes in rapid succession followed by a 15+ second Leonid meteor radar reflection. The data were obtained on November 17, 1998 during the Leonid meteor shower.
Geminid meteor echo: This 10s audio file includes a radar echo from a Geminid meteor lasting approximately 3 seconds. The recording was made during the Geminid meteor shower on December 14, 1998.
STS-88 radar echo: This 5s audio file contains a radar echo from space shuttle Endeavour obtained 14 Dec 1998 during the first ISS assembly mission. The reflection from Endeavour lasts approximately 1.5 sec.
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Meteor trails are longer-lasting. They usually persist for a few seconds, but in extreme cases they can last for 10 or more minutes. They sound less like blips, and more like long-winded warbling whistles. Meteors, satellites, and spacecraft have even more distinctive signatures in the dynamic spectra of NAVSPASUR radar returns. A full discussion of radar spectra is beyond the scope of this article. More information, plus colorful echo spectra of meteors and the space shuttle Endeavour may be found here. Reports from Dr. Bienvenu and other amateur radio operators indicate that the November 1998 Leonids were far better radio meteors than December's Geminids. The Leonids were more numerous and their reflective tails lasted longer. One amateur radio operator reportedly exchanged calls with 14 other hams during a 9-minute long Leonid "burn." In contrast, the 1998 Geminids featured very few "long burners."
Most meteor echo enthusiasts listen for reflections of distant radio transmitters at frequencies between 40 MHz and 100 MHz. These are the best bands for such work because weakly ionized meteor trails reflect signals most efficiently at lower frequencies, below 100 MHz. Although 216 MHz would be considered by most to be a poor frequency for meteor observations, because it is too high for efficient reflections, the tremendous power of the Naval Space Surveillance radar transmitter more than compensates for its less-than-optimum observing frequency.
Amateurs in much of North America and in the Caribbean should be able to detect NAVSPASUR echoes from bright meteors and the larger space satellites using relatively modest ham radio equipment. For example, Dr. Bienvenu's NAVSPASUR observing setup was relatively simple. He used a ICOM R7000 radio receiver in USB mode tuned to 216.980 MHz, with a slight frequency offset to center the NAVSPASUR signal in the narrow 2.5 kHz receiver band pass. The antenna was a common TV/FM Yagi-style aerial. During the Leonid and Geminid meteor showers he pointed the antenna toward azimuth 275 degrees, the direction of the NAVSPASUR transmitter as seen from his location in Louisiana. Meteor trails are formed just 80 km above Earth's surface, so he pointed the Yagi just 8 degrees above the horizon.
He recorded the audio output of his receiver at a rate of 11,000 samples per second using the sound card in his 120 MHz Pentium PC.
This approach to meteor observing could prove fruitful for showers that are difficult to observe visually, including daytime meteor showers, and showers that take place during the full moon. The upcoming Quadrantid meteor shower is a perfect example. As many as 120 meteors per hour are expected during the shower's peak on January 3, 1999, but the full moon will make observations of all but the brighter fireballs difficult. It may be a good year to forgo the cold outdoors and listen to the Quadrantids on the radio instead. Web Links
(or, How to distinguish meteor and satellite echoes in one easy lesson)
One way to distinguish between satellite and meteor echoes is by simply listening. Satellites sound like "blips" or "pings". Meteors are longer-lasting warbling whistles.
A more clear-cut approach is to plot the power spectrum of the radar echo as a function of time. In a "dynamic spectrum" the signature of a meteor is clearly different from that of a satellite or spacecraft.
Satellites race through the NAVSPASUR radar beam at velocities between 1 and 10 km/s, depending on the details of their orbit. Not only is the reflection brief, it is also Doppler shifted as the orientation of the satellite's velocity vector changes with respect to the radar transmitter. In a dynamic spectrum a satellite reflection appears as a slanted, nearly vertical line (see below).
This dynamic spectrum spans the first 11 seconds of the discussed in the body of this article. Power is displayed in false color. Blue is low power; red is high.
Meteoroids move at much higher velocities than low Earth orbit satellites, so the Doppler shift from a meteoroid echo would be even greater than that of a satellite. However, it is not the fast-moving meteoroid that causes the radar echo. The radio signal bounces off the ionized air left behind when the meteoroid disintegrates in the atmosphere. The velocity of the ionized trail is low, typically no more than 0.02 km/s, compared to 30 - 70 km/s for the meteoroid. As a result there is no observable Doppler shift in the meteor echo. The meteor echo appears in the dynamic spectrum as a nearly horizontal line exhibiting no drift in frequency.