• ray tracing;
  • density distribution;
  • direction finding

[1] High-frequency (HF) direction-finding (DF) systems measure the angles of arrival of signals at selected frequencies. With this information, ray tracing can accurately determine the location of the HF transmitters if the three-dimensional (3-D) electron density (Ne) distribution between the DF site and the transmitters is known. The usual approach is to use an ionospheric model like the International Reference Ionosphere (IRI) as a proxy for the density distribution. We describe a more realistic approach developed in cooperation with Codem Systems in Merrimack, New Hampshire. A collocated digisonde at the DF site measures the vertical electron density profile and the local ionospheric tilt, providing, in real time, the inputs for the construction of the 3-D Ne distribution. The vertical profile is automatically obtained from the Automated Real Time Ionogram Scaler with True Height (ARTIST)-scaled ionogram and the local tilt from the sky maps recorded after each ionogram. The characteristics of each layer, for example, critical frequencies and peak heights, are expressed as a function of latitude λ and longitude Ψ. In the neighborhood of the DF site each characteristic, for example, foF2, is given as foF2(λ, Ψ) = foF2m (1 + C7Δλ + C8ΔΨ) (1 + CλΔλ + CΨΔΨ). The coefficients C7 and C8 for any given azimuth direction are determined with the use of the Union Radio Scientifique Internationale/CCIR coefficients (which are also used in IRI), and the calculation of Cλ and CΨ makes use of the measured ionospheric tilt data; foF2m is the local, measured foF2 value. When the measured density profile and tilt data are available, the derived 3-D density distribution represents the instantaneous ionosphere structure near the site. The numerical ray tracing includes the effects of the magnetic field and properly treats the spitze effect, making the ray-tracing program especially useful for small distances. Ray tracing through simulated tilts shows that the differences in ground distances for one-hop high-frequency (HF) propagation vary from about 1 to 100 km depending on the assumed tilts and distances. Operational tests for distances up to approximately 100 km have demonstrated good results in determining the transmitter location in real time and have illustrated the importance of using the actual ionospheric profiles and tilts in the ray tracing.