Monday, January 10, 2011

Meteor Scatter

Meteor scatter communications are also known as Meteor burst communication. It's a types of signal bounce possible only under specific circumstances. When a meteor enters the atmosphere of Earth an area of the E-layer is ionized.  This area is roughly cylindrical and like the tail of the meteor, it is long. When it first forms it is able to reflect and scatter radio signals.This ionization is brief, lasting from a few seconds to about a minute. The effect is observable from about 25 MHz up to about 10 GHz. Meteor skip allows a radio signal to bounce upwards of 900 miles.

 The earliest recorded observation of meteor scatter was by Hantaro Nagaoka of Nagasaki, Japan. He was then the chief physicist at the Institute of Physical and Chemical Research in Tokyo.  In 1929 he published a paper on his findings titled "The Possibility of radio Transmissions Being Disturbed by Meteoric Showers."  It sounds dry, but the 1929 Volume 5 issue of the Imperial Japanese Academy of Tokyo became a classic. The NRAO described his breakthrough
Nagaoka was impressed with the coincidence of height (~100 km) between the ionospheric layer and where visual meteors occur. Not only might meteors directly cause quick ionization, but he argued that long-lived fine dust resulting from disintegration of the original meteor particles would act as nuclei for collecting ions or electrons from the surroundings, thus causing a region of less ionization, or a "disturbance"
In 1931, Greenleaf Whittier Pickard made his own observations on the phenomena. He noticed that bursts of long distance propagation occurred during meteor showers. He published his paper "A Note on the Relation of Meteor Showers and Radio Reception" in July 1931.
 At about the same time Albert. Melvin Skellett a researcher at Bell Labs was theorizing a relationship between the kinetic energy of a meteor and the ionization of the ionosphere. Skellett spent 2 years testing his ideas with his colleagues. In November of 1932, during the Leonids meteor shower they tested the meteoric effect. More here. He assumed three crucial and correct details:
  1. Meteors as a class could contribute, along with cosmic rays, the bulk of the energy needed to maintain charged particles in the ionosphere.
  2. A large meteor could produce as many as 1,000,000 electrons and a critical mass of ions. (enough to effect radio) 
  3. The variation in the strength of shortwave radio signals indicates turbulence in the ionosphere caused by the constant mixing action of the meteors.
The first signifigant attempt to use meteor scatter for anything useful was by the Communications Defense Centre in Canadia (CRC)  In the 1950s they launched project, "JANET."  It was code named JANET for Janus, the Roman god of the doorways, beginnings, endings and time. Janus is often depicted with two heads, facing opposite directions. I take that as a metaphor for transmission and reflection. In JANET they sent bursts of data when conditions were ideal trying to catch those short partial minute windows. to detect the window they broadcast a 90 MHz carrier wave and monitored it for sudden changes in strength.  They bounced signals from their radar research station in  Saskatchewan all the way to Toronto, a distance over 600 miles. The system was operational from 1955 until about 1960. The average character error rate was only 1.5 percent!  the Friends of CRC sumarized their data and bandwidth allocation:
"It may seem that the Janet technique was somewhat wasteful of spectrum space because of the relatively high instantaneous signaling rates which are required to achieve modest average information rates. This defect was offset by the fact that Janet systems, using the same frequency assignments, can be located much closer to each other than can other systems operating over comparable distances."
It is generally the case that frequencies between 50 to 80 MHz are ideal for meteor scatter communication. The FM band itself  (88.1 – 107.9 MHz) also works quite well. During meteor showers, laypeople can still perform basic experiments and get recognizable results. All you need is to know when the metoer showers are. Here's what fun to look for in 2011
  • Quadrantids - January 3
  • Lyrids - April 21
  • Eta Aquarids -May 5
  • Perseids - August 13
  • Draconids - October 8
  • Orionids - October 21
  • Leonids - November 17
  • Geminids - December 13