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Barometric Altitude and Density Altitude Teodor Lucian Grigorie, Liviu Dinca, WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Jenica-Ileana Corcau, Otilia Grigorie Aircrafts’ Altitude Measurement Using Pressure Information: Barometric Altitude and Density Altitude TEODOR LUCIAN GRIGORIE Avionics Department University of Craiova 107 Decebal Street, 200440 Craiova ROMANIA [email protected] http://www.elth.ucv.ro LIVIU DINCA Avionics Department University of Craiova 107 Decebal Street, 200440 Craiova ROMANIA [email protected] http://www.elth.ucv.ro JENICA-ILEANA CORCAU Avionics Department University of Craiova 107 Decebal Street, 200440 Craiova ROMANIA [email protected] http://www.elth.ucv.ro OTILIA GRIGORIE Carol I, High School 2 Ioan Maiorescu Street, 200418 Craiova ROMANIA [email protected] Abstract: - The paper is a review of the pressure method used in the aircrafts’ altitude measurement. In a short introduction the basic methods used in aviation for altitude determination are nominated, and the importance of the barometric altitude is pointed. Further, the atmosphere stratification is presented and the general differential equation, which gives the dependence of the static pressure by the altitude, is deduced. The barometric and the hypsometric formulas for the first four atmospheric layers are developed both in the analytical and numerical forms. Also, the paper presents a method to determinate the density altitude with an electronic flight instrument system. A brief review of the flight altitudes is performed, and the calculus relations of the density altitude are developed. The first two atmospheric layers (0÷11 Km and 11÷20 Km) are considered. For different indicated barometric altitudes an evaluation of the density altitude, as a function of non-standards temperature variations and of dew point value, is realized. Key-Words: - standard atmosphere; atmospheric layers; barometric formula; density-altitude; evaluation 1 Introduction measurement of the altitude can be made directly, with Seen like one of the most important parameters for the the altimeters, or indirectly, situation in which it is flight safety, the altitude can be defined as the distance calculated by means of complementary systems, such as, between the aircraft centre of mass and the point when for example, Air Data Computer (ADC), Inertial the local vertical intersects the surface of Earth [1]. The Navigation System (INS) or Global Positioning System (GPS) [2]÷[5]. Showing altitude information on board ISSN: 1109-2734 503 Issue 7, Volume 9, July 2010 Teodor Lucian Grigorie, Liviu Dinca, WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Jenica-Ileana Corcau, Otilia Grigorie can be done by the measuring system, either through an point on the Earth surface is positioned on a baric Electronic Flight Instrument System (EFIS). If it is surface. If the reference pressure is 760 mmHg then it determined using a GPS than it can be defined as aircraft corresponds to the mean sea level as it is defined by the elevation from the surface of the reference geoid WGS International Standard Atmosphere (ISA) [7]. 84 (World Geodetic System) [6]. A particularly important role that barometric altitude Relative to the position of the ground point taken as a plays is to provide information to the Air Traffic Control reference or to the corrections introduced in the systems (ATC) in order to avoid the collisions between measuring system, the flight altitude behaves different aircrafts. This is achieved through the transponder names [5]: (Transmitter-responder), which is a dedicated radio • true altitude (Hnm): is real altitude of aircraft above equipment representing the on board part of the Air mean sea level (MSL); Traffic Control Radar Beacon System (ACTRBS). The • relative altitude (Hr): is the altitude reported at an transponder sends a response in the form of a pulse at airfield level on which the aircraft performs take off 1090 MHz when it is interrogated with a radio frequency or landing manoeuvres; its value depends on the signal of 1030 MHz by the secondary surveillance radar altimeter adjustment. (SSR) of the ATC. Developed for the civil air traffic • absolute altitude (Ha): in this case the reference point control, the SSR is based on military technology is considered at the intersection of the local vertical Identification Friend or Foe (IFF); currently the two and the Earth surface, so its calculation takes into technologies are compatible [8]. account the overflowed forms of relief. This altitude Civil flights operating modes of the transponder are A, C is considered to have the greatest importance for and S. The mode A contains the identification of aircraft flight safety. and is based on a 4-digit code containing numbers between 0 and 7 assigned by ATC and set by pilot. The • barometric altitude (pressure altitude) (Hb): this is the altitude indicated by the altimeter when this is tuned mode C transmits the barometric altitude information. on a basic pressure, so the reference point is This information is generally received by transponder positioned on a baric surface. If the reference pressure from a system of barometric altitude coding, which has a is 760 mmHg it corresponds to the mean sea level in barometric pressure sensor connected to the static the International Standard Atmosphere (ISA) [7]. pressure bus on aircraft. In most cases the information on the identification and barometric altitude of the • density altitude (Hρ): is the barometric altitude corrected for non-standard temperature variations. aircraft are transmitted together; the mode of the This corresponds to the altitude at which the density transponder is known as 3A/C or A/C. Each of the 2 of air is equal to ISA air density evaluated for the modes is represented by 12 bits, which means that the current flight conditions. maximum number of ID codes available for assignment • indicated altitude (H): is the altitude displayed on the at a time is 4096 [8]. The third mode S has been dashboard. designed in order to avoid over-transponder Besides the altitude indicators, on board the aircraft are interrogation areas crowded with too many radar stations used also different systems for measuring and [9]. Furthermore, a transponder heaving this mode transmitting information of altitude. Usually called allows the communication of the carrying aircraft with altitude transmitters, these systems provide information other aircraft equipped with a Traffic Alert and Collision relating the altitude to the on board automated Avoidance System (TCAS) [10]. On the basis of equipment used in flying, navigation and not only. The barometric altitude and distance obtained from other transmitted information is under the form of electrical aircraft, TCAS may determine by extrapolation, signals, in digital or analogue format. independently of ATC, if it is possible a collision Altitude measurement can be accomplished using between the two aircrafts. This mode allows the several methods; some of them are often implemented representation of the identification code on 24 bits, simultaneously on board the aircraft: trilateration method which represents 16.777.214 individual codes. Thus, (GPS system), inertial method (INS), radio method and each aircraft can be assigned with a unique identification barometric method [1]÷[5]. code that can be generated according to the country in Barometric method is most commonly used on board to which it was registered and the registration number. So determine the altitude of flight. This is based on the this aircraft can be addressed selectively by ground dependence of the static pressure p by altitude. The stations or other aircrafts. altitude measured with this method is called the Also, extremely useful for flying is so-called Altitude Alerting System. During the ascent or descent aircraft barometric altitude (Hb), and the measurement devices are called barometric altimeters. This kind of altitude can phases the pilot enter the desired altitude in the altitude be defined as the altitude indicated when the altimeter is selector, after that being warned by visual and/or sound set on a pressure basic value; in this case the reference signals to have enough time to do the manoeuvres ISSN: 1109-2734 504 Issue 7, Volume 9, July 2010 Teodor Lucian Grigorie, Liviu Dinca, WSEAS TRANSACTIONS on CIRCUITS and SYSTEMS Jenica-Ileana Corcau, Otilia Grigorie necessary to maintain the respectively ceiling. Also, this • lift decreasing and the need to increase the flight system warns the pilot in a situation in which deviate speed to maintain the desired lift; from the required altitude. Such an alerting system uses • decreasing of the propulsion systems power, and, the same primary information as the altitude measuring thus, decreasing of the thrust forces; system, exactly the static pressure. Its basic operation • runway acceleration decreasing because of the low can be described using the diagram in Fig. 1 [11]. thrust; • increasing of the take off distance and decreasing of Approaching alert threshold the climb speed; Alert at the deviation from the imposed • decreasing of the aircraft maximum flight altitude, altitude (visual and sound signals) Deviation threshold affecting their capacity to fly above the higher Imposed obstacles. altitude Deviation threshold 2 Atmosphere Stratification y Approaching alert threshold Since the experimental data collected from the earth's or ct je Alert at approaching of the atmosphere have suggested that the dependence of the ra T imposed altitude (visual signals) static pressure by the altitude changes is based on a number of factors (latitude, seasons, weather, switching Fig. 1 Basic operation of the altitude alerting systems from day to night ...), was necessary a statistical processing of them to establish a standard dependency When approaching the required altitude, in an ascent or relation. This relation must allow the design and descent phase, usually around 300-400 meters before calibration of the measuring instruments. After the reaching it, is triggered a visual signal. Before reaching statistical processing the International Standard this required ceiling, with approximately 60-100 m, the Atmosphere (ISA) resulted. It is a model of the earth’s signal disappears.
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