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I believe ATC still uses heading and indicated speeds for comparison purposes. There are a large number of Aircraft out there that still don't have GPS. The most recent planes out of Boeing now have GPS. Most of the 75''s and 76's have RNAV using DME/DME with INS as a backup. Most of the 727's and DC-9's have neither GPS or RNAV. The controllers then must use the lowest common denominator, i.e. heading and IAS.
Regards. Dave Link.
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