Transpon­der lite

Pilot - - AIRMAIL -

Your con­trib­u­tor (‘Air­mail’, Septem­ber) is to be com­mended for sug­gest­ing a method for pro­vid­ing fea­tures sim­i­lar to a transpon­der but with­out the equip­ment over­heads. Un­for­tu­nately, the way that mo­bile phone tech­nol­ogy works is that the hand­set needs to be in cov­er­age of a ter­res­trial base sta­tion, so air­borne re­li­a­bil­ity will be sig­nif­i­cantly af­fected. Over the sea or un­pop­u­lated ar­eas, and even above an al­ti­tude of only a few thou­sand feet, the base sta­tion sig­nal might not reach the hand­set. Even if the hand­set can re­ceive the base sta­tion, the hand­set sig­nal may not reach the base sta­tion.

There may be lim­ited sit­u­a­tions where the use of a mo­bile phone would work but, on the whole, the cur­rent cov­er­age ar­eas would be too re­strict­ing to use it as an ef­fec­tive SSR re­place­ment. Martin Jewell (telecom­mu­ni­ca­tions sys­tems ar­chi­tect), Lon­don

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