how the radar can't see f117 A?

Answer 1

"The F-117A Nighthawk Stealth Fighter attack aircraft was developed by Lockheed Martin after work on stealth technology... The surfaces and edge profiles are optimised to reflect hostile radar into narrow beam signals, directed away from the enemy radar detector. All the doors and opening panels on the aircraft have saw-toothed forward and trailing edges to reflect radar. The aircraft is mainly constructed of aluminum, with titanium for areas of the engine and exhaust systems. The outer surface of the aircraft is coated with a Radar-Absorbent Material (RAM). " [1]

"Stealth technology (often referred to as "LO", for "low observability") is not a single technology but is a combination of technologies that attempt to greatly reduce the distances at which a vehicle can be detected; in particular radar cross section reductions, but also acoustic, thermal and other aspects specifically: ... Almost since the invention of radar, various techniques have been tried to minimize detection. Rapid development of radar during WWII led to equally rapid development of numerous counter radar measures during the period; ... The term 'Stealth' in reference to reduced radar signature aircraft became popular during the late eighties when the F-117 stealth fighter became widely known... Since [1989] it has become less effective due to developments in the algorithms used to process the data received by radars, such as Bayesian particle filter methods." [2]