 In the case of ASV this was not a problem ; aircraft would not approach a ship on the surface more closely than its altitude perhaps a few thousand feet , so a longer pulse width was fine . But in the AI role , the minimum range was pre @-@ defined by the pilot 's eyesight , at 300 m or less for night interception , which demanded sub @-@ microsecond pulse widths . This proved very difficult to arrange , and ranges under 1 @,@ 000 feet were difficult to produce . 
