rbeville%tekig5.pen.tek.com@RELAY.CS.NET (09/08/89)
From: rbeville%tekig5.pen.tek.com@RELAY.CS.NET I'll try to contribute to this topic of sonar and resolving of targets... I speak from the '60s so someone more current may have to correct me in places... A phenomenom of nature in air and oceans where there is unusually low attenuation of radio waves or sonics is called "ducting". In WW2 it was responsible for a number of fighter missions being scrambled to intercept incoming 'bogies'... only to find when the fighters got on location (where their targets seemed to be by the range markers of the radar set...) there was no one there. A radar set flying in the "duct" will display distant targets as close, as well as a ground set whose beam swept into the "duct". Another problem was the periodicity of the 'pinger' where a distant target echo would arrive after the (n+1)th or (n+i)th ping and be interpreted as nearby... there is anecdotal evidence of this in an old _READER's DIGEST_ Humor in Uniform joke about a submarine cruising on the surface at night when a 'echo' appeared on the radar off to the east on the horizon. A crash dive was ordered... the sub resurfaced minutes later after the Captain verified the echo by periscope... it was the moon! Another aggravation is "multipath reception". This is when sounds or radiowaves arrive at the hydrophone or receiver after (or before) the straight line path. I flew during 55 launches at the Cape on 'Refract' missions... recording this phenomenon of differences of the Index of Refraction of the airspaces over the launch path-guidance path of the MISTRAM long baseleg tracking station. Accuracy of the MISTRAM was predicated to be 3 yards at 3000 miles IF the atmosphere IR was uniform, but over the mainland, Indian River, Atlantic and Gulf Stream the thermals and clouds produce much "multipath". A ground guidance system with this problem will detect that a rocket/missile is further out than it should be and correct its path in closer. In the ocean, something like the above also happens; hydrophones have trouble resolving the source of a "dispersed" echo or sound due to the "multipathing" by thermoclines(sharp temperature gradient) and subtle differences in water temperature and salinity... This is why a sub sometimes has to do a 'triangulation' run to get a better fix on a target. A problem like this is found in Tom Clancy's novel _RED_STORM_RISING_ with some degree of authenticity. that's -OWARI- from GLOWWORM-7-9-4 best regards, rbeville@tekig5.PEN.TEK.COM Bob Beville, Tektronix, Inc., Beaverton, OR 97077