Find a Security Clearance Job!



The Naval Security Group (NAVSECGRU) worldwide CLASSIC BULLSEYE (now FLAGHOIST) network is part of the DOD Worldwide High Frequency Direction Finding System for strategic intelligence collection and emitter location. High-Frequency Direction-Finding [HF-DF, popularly known as huff-duff] intercepts and locates voice and message traffic transmitted on short-wave channels. HF-DF systems operate at frequencies where the signals are reflected from the earth's ionosphere [skywave signals].

HF-DF stations detect radio signals from aircraft or ships, and calculate the direction, or line of bearing, of the radio transmitter from the direction finding antenna. When the same signal is received by two or more antennae, the intersection of the lines of bearing mark the transmitter's location, using either precision single station location (SSL) capability, or in a network of DF stations using both multi-station azimuth triangulation and SSL. High Frequency Acquisition (AQ) and Direction Finding (DF) operations are performed with the Narrowband System (NBS) and Wideband Direction Finding (WBDF) Subsystem in support of normal and degraded communications modes, using both adaptive reception and super-resolution direction finding techniques.

CLASSIC BULLSEYE stations consist of the AN/FRD-10 circularly disposed antenna arrays (CDAA), popularly known as elephant cages. With a nominal range between 150 to 5000 kilometers, these arrays consist of two rings of HF antennae. The inner ring, for monitoring longer longer wavelength signals, is typically some 230 meters in diameter with some containing 40 folded dipoles. The outer ring for monitoring shorter HF wavelengths is is about 260 meters in diameter and contains some 120 sleeve monopoles. Inside each ring is a large wire screen, supported by 80 towers, to shield antennae on the other side of the array from HF signals from crossing the array, which would interfering with geolocation operations. A horizontal ground screen about 400 meters in diameter surrounds the entire site. The station's intercept operators work in an operations building in the center of the array.

Operating Locations

NSGA San Diego

NSGA Northwest

NSGA Sugar Grove [1]

NSGA Sugar Grove [2]

NSGA Winter Harbor

Join the mailing list

Unconventional Threat podcast - Threats Foreign and Domestic: 'In Episode One of Unconventional Threat, we identify and examine a range of threats, both foreign and domestic, that are endangering the integrity of our democracy'

Page last modified: 28-07-2011 00:48:22 ZULU