  The Alpha Magnetic Spectrometer (AMS) was flown in june 1998 on board of the
space shuttle Discovery (flight STS-91) at an altitude ranging between 320 and
390 km. This preliminary version of AMS included an Aerogel Threshold Cherenkov
detector (ATC) to separate antiprotons from electron background, for momenta
less than 3.5 GeV/c. In this paper, the design and physical principles of ATC
will be discussed briefly, then the performance results of the ATC will be
presented.
