Evaluation of JSAF EM propagation prediction methods for navy continuous training environment / fleet synthetic training results and recommendations: part I – evaluation of current JSAF EM propagation modeling

摘要

The EM propagation model currently used in JSAF is “FFACTR” which is a part of the Engineers Refractive Index Prediction System (EREPS) Tactical Decision Aid (TDA) developed by what is now SPAWARS SSC San Diego in 1988. This model is no longer supported by SPAWARS or any other group and has been replaced; it is obsolete. This model is able to represent some realistic features including: (1) decrease in signal strength (increase in propagation loss) with range, (2) more interference lobes for higher elevation and higher frequency transmitters, and (3) increased surface ranges for evaporation and surface ducts, and also with greater K-Factors. However the following deficiencies were noted:1. Duct Strength (M value change) had no effect on the JSAF predictions. 2. The JSAF interference lobes caused by interaction between direct and surface-reflected radiation did not have the correct spacing..3. The effects of surface ducts were not realistically modeled. In reality, ducts create complex signal strength patterns and at the surface typically show “skip and hop” bands of increased and decreased signal strength. The JSAF predictions were unrealistically smooth and showed no skip patterns. 4. The far range (> 30 km) JSAF predictions appeared to have too strong signals and very simplified “flat” patterns. It appears that the JSAF EM model was not designed for these regions. 5. The standalone version used in this had no consideration for geographic location for predicting duct effects. Other versions do allow duct features to vary strongly over the Earth’s surface. 6. The varying “leakage’ of radiation above ducts was not captured by the JSAF predictions. 7. The radar “hole” that is usually present just above duct tops was not seen in the JSAF predictions.It is clear that there are considerable and significant weaknesses in the JSAF EM propagation prediction model which result in unrealistic range predictions, particularly in situations when ducting is present.