A meteor's velocity components are derived from time-of-flight differences between spaced-station echoes with scalar speeds being also provided independentaly by diffraction behaviour. As an impacting meteor deposits ionisation the reflection cross section grows as more Fresnel zones contribute and the instant when the meteoroid has reached the specular geometrical condition it termed the to point. Analysis of post-¿0 echo amplitude fluctuations arising from diffraction provides speed values.
As a meteor traverses those Fresnel zones prior to the to condition large phase changes occur which provide speed values more accuratly than the amplitude oscillations later in the echo. This is because for times early in the creation of a. meteoric plasma a radar echo is not subject to the long-term processes present (especially after echo maximum) of plasma diffusion, effects of ionization irregularities, meteoroid fragmentation, and atmospheric wind-shear. These procceses can contribute to disrupting the smooth character of the plasma column with resulting distortion and often destruction of the classical phase behaviour: many 60%) meteor radar echoes do not exhibit post-¿o amplitude oscillations.
In the present upgrade independent signal phases are available at all six of the elevation-finding antennas. This permits echo elevation measurement from the relative phases (three values for each meteor echo) to provide unambiguous elevation and echo height over the height regime 60 to 150 km.
Each of the three independent antenna phases maps the Fresnel diffraction function over
Was this article helpful?