The Gates Learjet airplanes are certificated under the F.A.R. 25 Transport Category airplane rules, the same as most other business jets, and all the Jumbos and Wide-Bodied jets. Take-off and landing field lengths must be measured in the certification process and had been measured at Learjet since its beginning with a portable phototheodolite owned by the FAA. The search for a modern replacement for that device progressed through the expensive photographic devices in use currently by the large airplane manufacturers to a relatively inexpensive system; completely electronic, acceptably accurate, and simple to use. The system consists of a Del Norte Technology, Inc., Trisponder for measuring horizontal distance and a Sperry AA-220 Radio Altimeter for measuring height. The outputs of those devices are recorded on a magnetic tape data acquisition system. The data are computer reduced and plotted much faster than any photographic reduction. The only system component which was not in the Gates Learjet inventory at the time was the Trisponder, so the theodolite replacement system was operational for a cost of about $19,000, which is less than half of what a complete photographic system would have cost.