The generally accepted models for imaging and range performance modeling of thermal imagers have not been able to properly model under-sampled systems, i.e. staring focal plane arrays (FPAs). The ruling STANAGs 4349 and 4350 on measurement and modeling of Minimum Resolvable Temperature Difference (MRTD), by definition only deal with properly sampled systems and thus cannot address performance beyond the Nyquist frequency. This includes the FLIR92 model which is based on the models defined in STANAG 4350.
Range performance modeling, defined through STANAG 4347, is based on MRTD and thus likewise limits performance to below Nyquist frequencies. Practical experience has long shown that this limitation is not valid and development of new modeling techniques to address these problems has been performed e.g. in Germany, the TRM3 model, and in the US, with the NVTherm model. TRM3 addresses the under-sampled systems by introducing a concept of Minimum Temperature Difference Perceived (MTDP) which replaces MRTD for frequencies beyond Nyquist. NVTherm instead introduces a modified MRTD function through the concept of MTF squeeze. Typically, range performance predictions from NVTherm will increase ranges by some 15% over Nyquist resolution based predictions, and TRM3 based predictions exceed Nyquist ranges by up to 30%. A study is done to compare modeling results from these two models with laboratory measurements (MRTD) on QWIP long wave staring FPA based thermal imagers and finally relate these to empirical data from range performance field trials against actual targets.