That would be correct for ranges above 50km or for systems that we're first fielded in the 60's. With modern system at ranges below 50km you can get speed, iden

That would be correct for ranges above 50km or for systems that we're first fielded in the 60's. With modern system at ranges below 50km you can get speed, identification and distance. But it all depends on weather, heat signature and contrast against backround. For anything you can fit in a missile, that 50km is close to current max range of detection and we have all the problems you mention. But direction is all that missile needs for most of the flight time. Modern missiles have so good kinematic performance that they might begin to "lead the target" only one second before impact. At mach 3 thats about 1km. Digital radio frequency jamming can hide size and distance from a radar too. The downside is that you need your own radar to send the masking signals. But it can't hide direction. https://en.wikipedia.org/wiki/Digital_radio_frequency_memory https://en.wikipedia.org/wiki/Digital_radio_frequency_memory Gripens Skyward-g IRST is said to weight about 60kg, which is light for such system. Gripen also has light weight radar, that weights 150kg. Other 4th gen figthers seem to sport 150kg IRST and 400kg radar. So you could theoretically replace the radar with 3 or 4 IRST, while not gaining any weight.