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The excitation of a parametric decay cascade and of the oscillating two-stream instability (OTSI) have been predicted earlier by one-dimensional and two-dimensional weak turbulence approximations for observations with the 430-MHz radar for ionospheric modification experiments at Arecibo, for heights 1-2 km below the reflection height of the pump wave. For the same height range the present numerical simulations, using a version of the one-dimensional driven and damped Zakharov system of equations, are shown to lead over a limited range of pump powers to the simultaneous excitation of a weak turbulence decay cascade and of the OTSI or modulational instability. The linearized form of the Zakharov equations is used to derive the functional dependence of the transfer of power from one Langmuir wave to another on the parameters of the two waves. Comparison of that function with the more correct version derived from kinetic theory was then used to guide the choice of the damping coefficient of ion acoustic waves. With that choice the simulations show that for a pump power that exceeds the threshold for excitation of the parametric decay instability by a factor less than about 3, the OTSI is not excited in the steady state which only shows the excitation of the parametric decay cascade. For pump powers that exceed the threshold power by a factor greater than about 3 but are not great enough to lead to a Langmuir condensate and ultimately to strong cavitational Langmuir turbulence, the simulations show the simultaneous excitation of a weak turbulence cascade and of the OTSI. These conclusions agree qualitatively with the conclusions reached on the basis of earlier weak turbulence approximations. SPRAGUE AND FEJER: EXCITATION OF PARAMETRIC DECAYS 23,961

AREPS is the U.S. Navy's designated software tool ** for radar performance simulation and analysis. For well over 20 years it has been used to provide accurate propagation predictions in realistic refractive environments for terrestrial radar systems from VHF (~100 MHz) to Q-band (~60 GHz). Recently, the range of applications for AREPS has been extended by the addition of a communications prediction capability at high frequencies (HF, ~2 MHz-30 MHz) which includes both surface wave and ionospheric sky wave coverage for beyond line-of-sight propagation paths. In this paper we describe the latest addition to AREPS, which uses refractive height profile input along with a newly developed ray trace capability to provide signal strength estimates for earth-to-satellite communications. By including atmospheric refractivity the model becomes especially useful for determination of antenna pointing angles in strongly refracting environments and for low-elevation satellites for which ray paths through the atmosphere may be very long. The model includes an orbital location prediction capability which uses two-line element sets (TLE) which are widely available on the internet for non-classified systems and generally available for classified systems for designated users. The model provides a transparent homing procedure which determines the launch angle for the direct ray connecting transmitter to satellite and, in some situations, for the earth reflected mode. Possible ray blockage by terrain for low elevation satellites can be investigated by including DTED terrain elevation data in the analysis.

Develop electromagnetic propagation models, that perform equally well over land and sea and in the presence of anomalous propagation conditions for both surface and airborne emitters, for use in operational or engineering propagation assessment systems.