Frequency-Agile Distributed-Sensor System (FADSS) Deployment in the Western 
United States: VLF Results 
Donald D. Rice, J. Vincent Eccles, Jan J. Sojka, Robert D. Hunsucker, and John 
W. Raitt 
 
A network of inexpensive, frequency-agile, beacon monitors is currently being 
developed to provide a real-time description of space weather effects on 
ionospherically-dependent systems. This array of software radios is dynamically 
programmed to measure GPS variations and received signal strengths from select 
beacons from VLF through HF along a multitude of propagation paths.   The 
real-time network collects the information and augments it with geophysical data 
(GOES, various indices.)  Processing provides information on the prevailing 
ionospheric weather conditions, and allows the scheduling of sensor observations 
to be optimized. 
 
We describe the VLF signal strength results of our initial six-station 
deployment. The initial FADSS frequency selection includes Navy VLF stations in 
Washington, North Dakota, Hawaii, and Maine.  The sensors are deployed in the 
Western United States:  KFO (Klamath Falls, Oregon), BLO (Bear Lake Observatory, 
Utah), PRV (Providence, Utah), LGN (Logan, Utah), TUC (Tucson, Arizona), and SEC 
(at SEC office in Providence, Utah.)  Additional beacon monitors are being 
fabricated for temporary observing campaigns. In this study these measurements 
are augmented by almost two years of monitoring from Providence, Utah by 
receivers of the Stanford IHY program.  The data show strong seasonal effects on 
the D-region during solar minimum conditions.  In summer, daytime signal levels 
are quite constant except during Sudden Ionospheric Disturbances (SIDs) related 
to solar x-ray flares.  At night, signal levels are highly variable with gravity 
wave periods of 1-2 hours dominating.  In winter, daytime VLF signal strengths are 
modulated with periods consistent with planetary waves of several days to more than 
a week. This modulation also affects signals at low HF frequencies through D-region 
absorption. The multi-point FADSS measurements are used to deduce the scale lengths 
of the D-region structures. This information is used to improve ionospheric 
specification as well as generate knowledge on the scale sizes of these space 
weather-related ionospheric structures.