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Satellites move through the NAVSPASUR beam rapidly, usually in
less than a second. As a result they sound like the "blips"
and "pings" that are stereotypical of radars and sonars.
(In the three audio recordings above, only the Leonids WAV file
contains echoes of fast-moving satellites.) 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. |
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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. |
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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 | |
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NASA Liftoff meteor shower pages - learn the basics about meteor showers. Includes tutorials, Java animations, and educational activities. NASA Liftoff orbital velocity calculator - enter the altitude of a satellite or spacecraft and find out its velocity and orbital period using this handy JavaScript calculator. External Links: Navy Space Surveillance Radar -- basic facts about NAVSPASUR Navy Space Surveillance Radar -- history and technical specifications Richard Horne's Spectrogram 4.2.10 Home page -- Free software to produce dynamic spectra from audio signals. Used to create the meteor spectra in this article. The Leonids -- from Gary Kronk Meteors and Comets web site The Geminids -- from Gary Kronk Meteors and Comets web site December's generous Geminids -- Sky &Telescope article Meteor scattering tutorial -- from the International Meteor Organization International Meteor Organization -- home page |
Related Stories: 15 Dec. 1998: Bunches
& Bunches of Geminids
-- Video of the 1998 Geminid meteors. |
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(or, How to distinguish meteor and satellite echoes in one easy lesson)
 This dynamic spectrum spans the first 11 seconds of the Leonid meteor echo WAV file discussed in the body of this article. Power is displayed in false color. Blue is low power; red is high.
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return to Space Science News Home Author: Dr.
Tony Phillips |
December 21, 1998: A little more
than a month ago the Leonid meteors put on a dazzling show of
shooting stars and fireballs for sky-watchers around the world.
While most meteor enthusiasts were outside watching the show,
a few stayed inside and tuned into the shower on their radios.
What they heard may surprise you. 
