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IN the autumn of 1986, a Plessey Microwave Landing System (MLS) ground station was installed at runway 28R at Heathrow for participation in UK operational trials. At the same time, British Airways began evaluation in a Boeing 757 aircraft of both the system itself and receivers of several manufacturers. The first of these were from the Bendix Corporation and initiated trials which are due to last around 18 months. The MLS system, like ILS, uses ground based transmitters and airborne receivers to determine the aircraft's position in terms of azimuth and elevation angles with respect to the transmitters. The transmissions themselves may carry some or all of the information relating to approach azimuth and elevation angles (analagous to ILS Localiser and Glide Slope, respectively) as well as back azimuth and auxiliary data such as meteorological information. All of these functions operate on the same radio frequency using time division multiplexing whereby time slots for each function follow each other in a sequence. Each function transmission starts with a preamble which tells the receiver which function is being used thus enabling it to select the correct decoding format.