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Possibilities of Increasing the Low Altitude Measurement Precision of Airborne Radio Altimeters

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Possibilities of Increasing the Low Altitude Measurement Precision of Airborne Radio Altimeters electronics Article Possibilities of Increasing the Low Altitude Measurement Precision of Airborne Radio Altimeters Ján Labun 1 , Martin Krch ˇnák 1, Pavol Kurdel 1 , Marek Ceškoviˇcˇ 1, Alexey Nekrasov 2,3,4,* and Mária Gamcová 5 1 Faculty of Aeronautics, Technical University of Košice, Rampová 7, Košice 041 21, Slovakia; [email protected] (J.L.); [email protected] (M.K.); [email protected] (P.K.); [email protected] (M.C.)ˇ 2 Institute for Computer Technologies and Information Security, Southern Federal University, Chekhova 2, Taganrog 347922, Russia 3 Department of Radio Engineering Systems, Saint Petersburg Electrotechnical University, Professora Popova 5, Saint Petersburg 197376, Russia 4 Institute of High Frequency Technology, Hamburg University of Technology, Denickestraße 22, Hamburg 21073, Germany 5 Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, Košice 042 00, Slovakia; [email protected] * Correspondence: [email protected]; Tel.: +7-8634-360-484 Received: 7 August 2018; Accepted: 9 September 2018; Published: 11 September 2018 Abstract: The paper focuses on the new trend of increasing the accuracy of low altitudes measurement by frequency-modulated continuous-wave (FMCW) radio altimeters. The method of increasing the altitude measurement accuracy has been realized in a form of a frequency deviation increase with the help of the carrier frequency increase. In this way, the height measurement precision has been established at the value of ±0.75 m. Modern digital processing of a differential frequency cannot increase the accuracy limitation considerably. It can be seen that further increase of the height measurement precision is possible through the method of innovatory processing of so-called height pulses. This paper thoroughly analyzes the laws of height pulse shaping from the differential frequency presented by the number that represents the information about the measured altitude for this purpose. This paper presents the results of the laboratory experimental altitude measurement with the use of a so-called double-channel method. The application of obtained results could contribute to the increase of air traffic safety, mainly in the phase of the aircraft approaching for landing and landing itself. Keywords: airborne radio altimeter; methodological error; increase of measurement accuracy; height pulses; dual-channel method 1. Introduction High precision of a flight height measurement is required at low altitude, mainly in the phase of aircraft approaching for landing and landing. For this purpose, an FMCW airborne radio altimeter (radar altimeter) is used for measuring the low altitudes [1]. When the flight height is measured, the principle of radiolocation with the evaluation of the time delay t between the transmitted and received signal of a radio altimeter is used [2,3]. It can be presented that the flight altitude H directly corresponds to the delay of a received signal. The time delay is not evaluated directly for radio altimeters, but it is performed indirectly through the differential frequency Fd. The differential frequency is created between the immediate value of the Electronics 2018, 7, 191; doi:10.3390/electronics7090191 www.mdpi.com/journal/electronics Electronics 2018, 7, x FOR PEER REVIEW 2 of 9 Electronicsand it can2018 ,be7, 191presented that the flight altitude indirectly corresponds also to the differential2 of 9 frequency. frequencyIn this of way, the transmitted the complex and measurement time-delayed receivedof the very signal. short The times dependence when measuring is linear and the it cansmall be presentedheights is thattransformed the flight altitudeinto a indirectlysimple measuremen correspondst alsoof the to thedifferential differential frequency. frequency. The relation betweenIn this the way, measured the complex altitude measurement and the differentia of thel very frequency short timesis defined when by measuring a so-called the “basic small heightsformula is of transformed radio altimeter” into a simple measurement of the differential frequency. The relation between the measured altitude and the differential frequency8Δf F is defined by a so-called “basic formula of F = M H , (1) radio altimeter” d c 8D f FM Fd = H, (1) where Δf is the frequency deviation, FM is the modulationc frequency c is the speed of light. whereForDf ais classical the frequency altitude deviation, measurementFM is method, the modulation the measurement frequency accuracyc is the speed is defined of light. by the value of so-calledFor a classical critical altitudealtitude measurementΔH of a radio method,altimeter. the The measurement critical altitude accuracy presents is defined a minimal by the altitude value ofrange so-called which critical can be altitude registeredDH byof a radio altimeter.altimeter and The displayed critical altitude for a pilot. presents The a critical minimal altitude altitude is rangealso connected which can bewith registered the radio by aaltimeter radio altimeter measurement and displayed accuracy for a pilot.±ΔH. TheIt is critical possible altitude to derive is also connectedmathematically with thethat radio the altimeter critical measurementaltitude depends accuracy on the±D Hfrequency. It is possible deviation to derive of mathematicallya transmitted thathigh-frequency the critical altitudesignal according depends to on the the following frequency formula deviation [3]. of a transmitted high-frequency signal according to the following formula [3]. Δ = c DHH= .. (2) (2) 8DΔff The dependenceThe dependence can also can be also presen be presentedted graphically graphically [4,5] [(Figure4,5] (Figure 1). 1). Figure 1. Dependence of the critical altitude DΔH on the frequency deviation DΔff.. From FigureFigure1 1 we we can can see see that that at at the the low low frequency frequency deviation deviation of of 10–20 10–20 MHz MHz the the critical critical altitude altitude of 4–2of 4–2 m takesm takes place. place. This This value value is relativelyis relatively significant. significant. Therefore Therefore the the oldest oldest radio radio altimeters altimeters (ALT (ALT 1 in 1 Figurein Figure1) provided 1) provided the measurement the measurement accuracy accuracy of about of ±about2.2 m ±2.2 only. m The only. medium The frequencymedium frequency deviation ofdeviation 20–40 MHz of 20–40 provides MHz a relativelyprovides lowera relatively critical lower altitude critical of 2–1 altitude m. Some of older 2–1 typesm. Some of radio older altimeters, types of usedradio untilaltimeters, recently used (ALT until 2), providedrecently (ALT the accuracy 2), provided of about the ±accuracy1.5 m. Recent of about radio ±1.5 altimeters m. Recent use radio the ratheraltimeters high use frequency the rather deviation high frequency of about 50deviation MHz at of which about the 50 critical MHz at altitude which isthe about critical 0.75 altitude m that is ratherabout 0.75 low. m Therefore, that is rather the recent low. typesTherefore, of radio the altimetersrecent types (ALT of 3)radio provide altimeters the measurement (ALT 3) provide accuracy the ofmeasurement about ±0.75 accuracy m [6,7]. of about ±0.75 m [6,7]. From a historicalhistorical viewpoint, the accuracy is increasedincreased for approximately fiftyfifty percent at each stage of radioradio altimetersaltimeters development. Going Going the the same same way, way, to decrease the critical altitude of the ALT 33 stagestage inin twicetwice toto 0.3750.375 mm thethe frequencyfrequency deviationdeviation of 100 MHz is required to reach a new stage (ALT(ALT 4)4) forfor thethe measurementmeasurement accuracyaccuracy ofof aboutabout ±±0.30.37575 m. From this viewpoint, the paper offers a more simplesimple solutionsolution on on how how to to obtain obtain the the precision precision of theof the ALT ALT 4 stage 4 stage maintaining maintaining the recently the recently used valueused value of frequency of frequency deviation deviation of 50 MHzof 50 MHz [8,9]. [8,9]. Electronics 2018, 7, 191 3 of 9 Electronics 2018, 7, x FOR PEER REVIEW 3 of 9 2. Materials and Methods Critical altitude is presentedpresented byby minimalminimal altitudealtitude differencedifference whichwhich thethe radioradio altimeteraltimeter cancan recognize.recognize. As As the the measurement measurement accuracy accuracy depends depends on on the the value value of ofthe the critical critical altitude, altitude, it should it should be beconsidered considered in more in more detail. detail. One stablestable heightheight pulsepulse NN permanently develops (at increase) or terminates (at decrease) in thethe rangerange ofof measurementmeasurement of of the the critical critical altitude. altitude A. A total total number number of permanentof permanent height height pulses pulsesN carries N carries the informationthe information about about the measured the measured altitude. altitude. It can beIt can understood be understood so that ifso we that divide if we the divide total measured the total altitudemeasuredH intoaltitude the criticalH into the altitude critical sections; altitude their sections; number thei willr number correspond will correspond to the measured to the altitudemeasured as Naltitude= H/D Has. TheN = presentedH/ΔH. The number presented of pulses numberN develops of pulses (or terminates)N develops within (or terminates) one modulation within period one TmodulationM and modulation period T frequencyM and modulationFM. The resulting frequency evaluated FM. The differentialresulting evaluated
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