A Frequency Agile, Distributed Sensor System (FADSS) to address space weather 
effects upon ionospherically-dependent systems 
Jan J. Sojka, Don Rice, Vince Eccles, Bob Hunsucker, and John W. Raitt 
 
 	The outstanding problem faced by operational systems that either utilize the 
ionosphere or need to mitigate its effects is that ionospheric weather 
variability is comparable to the better-understood ionospheric climate 
variability.  Unfortunately, while present day knowledge of the climate is very 
good, that of the weather is qualitative at best, with weather forecasting being 
reduced to a climatological probability forecast.  For over five decades 
ionospheric weather measurements have been made; however, even today the number 
of such simultaneous measurements is orders of magnitude too few to resolve the 
weather scales that are impacting systems.  A very current example of this is 
found in the Federal Aviation Administration (FAA) design and deployment of 
their Wide Area Augmentation System (WAAS) that deployed more than twenty dual 
frequency GPS receivers across the United States to capture ionospheric weather 
in real time.  WAAS was designed to provide ionospheric correc 
 tion information with an “unavailability” of only a few seconds per year.  Over 
the past half decade, the almost annual major geomagnetic storms have rendered 
this system unavailable for hours at a time.  This problem demonstrates that 
even at mid-latitudes ionospheric weather structure needs observational systems 
that have considerably more than twenty sensors deployed over the United States. 
 
	In this paper we describe a passive, and extremely affordable, sensor that can 
be readily distributed in very large numbers to form an ionospheric sensor 
network.  The types of measurements from this system can be dynamically 
optimized to the prevailing ionospheric weather conditions through a distributed 
network guided by ionospheric software running in real-time on the operations 
center host computers. Operating at a 15 minute cadence, this system is able to 
track regional changes in the D-, E-, or F-regions, and make appropriate 
modifications to the ionospheric specification and user products.  The heart of 
the system is the “software radio” which enables controlled frequency management 
of the sensor over the lower portion of the radio spectrum. Each software radio 
has a menu of radio beacons that it monitors and records signal strength.  Its 
frequency agility enables this menu to be dynamically changed either by its 
local operator or via the systems network in response t 
 o evolving ionospheric conditions.  The FADSS frequency range covers VLF 
through HF plus GPS.  A prototype of FADSS has been deployed with 6 fixed 
locations, and a further two mobile sensor systems being developed.