## 7 Discussion
Both of these errors would be significant given the sensitivity of the RCS to small changes in elevation angle. On the other hand, for our application, there was little difference in the calculated elevation angles between the true (reanalysis) atmosphere for the measurement time period, the various applicable ITU standard atmospheres and the four-thirds earth radius approximation. Given its computational simplicity, we therefore recommend the use of the four-thirds earth radius approximationexcept in cases where the real atmosphere is expected to deviate substantially from the standard atmospheres. For comparing simulation results against measurements, the uncertainty in the aircraft attitude information can also be significant and is a significant source of potential error in our case. Care should be taken to choose a sufficiently accurate IMU for such measurements. The choice of solver is highly significant in terms of required computation time, but of course it also has some impact on accuracy. Using only the initial mesh for HFSS's MoM solver introduced a mean error of only 0.36 dB whilst requiring only one-fifth of the time to simulate. The approximate SBR+ solver we investigated reduced the required time only 0.27% of the time of the iterative-mesh MoM solver, but at the cost of a mean error of 3.2 dB (when PTD, UTD and creeping wave physics were enabled). This is a significantly large error, but it is more than adequate to resolve the variability in the RCS. Moreover the dramatic decrease in simulation time (and memory requirement) make it suitable for use in a wider array of situations. If, for example, it was desired to generate a large library of example targets and trajectories, the speed of an SBR-like solver would be an enormous advantage. We note that, whilst the simulations presented here were all performed in Ansys HFSS, similar solvers are available in competitors' products (FEKO, for example) and similar results should be obtainable with such products. A final important consideration is how well the model simulated represents the real aircraft. We were able to confirm that our model compares well with anechoic chamber measurements of a scale copper model, which gives us some confidence. However, we cannot say with certainty whether the copper model RCS approximates the real aircraft's RCS well since we do not have comparable measurements of the real aircraft. We find significant differences between the passive radar-simulated received-power levels and measurement, beyond that which can be explained by the known limitations of either our simulations or our measurements. In some areas of the comparison, the characteristics of the simulated and measured curves broadly agree, but there are other areas where this is clearly not the case. It is unlikely that other targets in the scene are a source of the discrepancies. Any such targets should appear at most only fleetingly in the same range-Doppler cell as the aircraft and would therefore have only a very brief impact on the measurement results. The source of the disagreement must come from at least one of three sources: either substantial differences between the simulated model and the real aircraft, unaccounted for propagation effects, or problems with the measurements or measurement processing. The SNR is generally quite low in the measurements performed, and the processing chain is intended primarily for detection rather than estimation, so it is possible that there is uncertainty in the measurements beyond that which is currently assumed. Another possible source of the discrepancy is in the propagation of the signal between the transmitter and the aircraft due to interaction with the terrain/features on the ground. We know that we have a clear direct line of sight to the aircraft, but it is at least possible that interactions elsewhere in the main lobe result in fluctuation in the incident power on the aircraft. In a future experiment, it would be helpful to have a direct measurement of the amplitude of the transmitted signal at the aircraft.