The ray tracing tool determines the specific propagation path of each ray from a given radar and beam direction. The ray tracing is calibrated to mimic radar operation: rays are launched over an elevation range from 5° to 55°, at the selected operating frequency. Fig. 1 below illustrates a ray-tracing run for a single beam from the Blackstone SuperDARN radar.

**Fig. 1**(Left) Blackstone radar field of view with beam 11 highlighted. (Right) Ray tracing for beam 11 of the Blackstone radar at 11MHz on Jan 16, 2001.

The ray paths are computed using a 2D scheme based on work by Coleman 1998 and integrated using an adaptive Runge-Kutta Cash-Karp numerical method. Ionospheric profiles are generated by the latest International Reference Ionosphere (IRI-2011), and the non-collisional transverse Appleton-Hartree formula is used to compute refractive indices. The computation time for a full day with 30-minute time resolution is between 10-20 seconds.

The ray tracing can be used to predict the occurrence of backscatter, both from the ionosphere and from the ground. The basic idea is to count the number of rays scattering within a given 45 km range gate. In the ionosphere, each scattered ray is weighted by N2/R3 where N is the background electron density and R is the slant range. The code outputs power distribution, reflection altitude (virtual and real), elevation angle, refractive index in scattering volume and aspect conditions. An example of a parameter plot is shown in Fig. 2, with labeling for scatter type.

**Fig. 2**Range-time plot for beam 7 of the Blackstone radar.

*References*:

- Coleman, C.J. [1998], A ray tracing formulation and its application to some problems in over-the-horizon radar, Radio Sci., 33(4), p. 1187-1197
- Ponomarenko, P.V., J.P. St-Maurice, C.L. Waters, R.G. Gillies, and A.V. Koustov [2009], Refractive index effects on the scatter volume location and Doppler velocity estimates of ionospheric HF backscatter echoes, Ann. Geophys., 27, p. 4207–4219
- IRI-2011: iri.gsfc.nasa.gov