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NEXRAD ANOMALY: SPOKES / SPIKES/ SPARKS
The anomaly above, which has the appearance of "spokes" on a wheel, is almost always
some kind of radio frequency interference, usually caused by other nearby NEXRAD sites
reflecting off of obstacles such as mountains or other obstructions or those signals
being pulled back down to the ground by atmospheric conditions. It can also be caused
by a problem with the radar itself, such as a calibration or alignment problem.
NEXRAD operates by sending a pulse of energy at a specific frequency and listening for
its echo as it bounces off of rain, dust, mountains, etc. When its own echo is returned
it can determine the distance to those objects. However, if the timing is such that
another NEXRAD site is transmitting and its signal reflects off of distant mountains, or
atmospheric conditions bend the signal back towards the ground, and into the receiver
of the local NEXRAD site, that interference will be picked up as an echo at all
distances. This results in a line (sometimes quite think) in the direction from the
object that reflected the other NEXRAD's signal.
These anomalies can be sporadic, based on chance atmospheric conditions, or may be
quite constant at a given site. For example, the above anomaly can be observed
rather frequently at Billings MT (as of the
date of this writing. 2/10/2004). In the case of Billings, the source of the interference
is the NEXRAD sites at Great Falls MT and
Riverton WY.
A full investigation of the Billings anomaly can be found here.
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