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Means are described for reducing the complexity of the electronics usually required in airbore navigational computing systems to determine true free-air temperature and true air speed. The vortex thermometer, when equipped with a linear resistance-temperature element, permits use of a linear potentiometer in a simple digital null-balancing servo. The vortex thermometer eliminates corrections which are otherwise needed for dynamic heating due to the speed of the plane. If true free-air temperature is measured along with the temperature of a probe having a known higher recovery factor, the difference may be combined directly using linear resistance thermometers to give an indication of true air speed without use of either pitot or static pressure measurements. Mach number and altitude may also be computed from these temperature data, together with pitot pressure, but without measurement of static pressure which is usually subject to considerably more error than pitot pressure.