Simulation with Hardware-in-the-Loop (HIL) expands design and improvement possibilities of onboard radar systems, by allowing examine impact of the physical effects of the propagation and reflection of radar signals from target and/or underlying surface. The paper discuss the known technology of digital radio frequency memory (DRFM) that can be accompanied by DSP which turn the radar probing signal into echo in such a way that, when it is re-emitted to the receiving radar antennas, it is as similar as possible to natural echoes. To do this, simulator reproduces the propagation delay, corresponding attenuation, Doppler frequency shift and spectral power distribution (SPD). The calculating requirements of this problem face with real time constrains. Utilization of HIL technology for earth surfaces with correlation radar systems is difficult because of lack of known devices able to process several receiving channels with variable parameters and accounting of surface backscattering in the real time. Such a task may be directly modeled only with huge amount of calculation resources. The paper proposes a simulation model, able to form two channels of signals reflection with varying cross-correlation while accounting basic trajectory and isotropic flat surface parameters. A set of modeled isorange rings, illuminated by the radar with burst probing signal is used for modeling. The results of mathematical modeling with the two channel version of the proposed retransmitting FPGA DRFM simulator are presented.