P3.17 A STUDY OF BAROMETRIC ALTIMETER ERRORS IN HIGH LATITUDE REGIONS

Young Yee* Erik Yee

Mkey Technologies 401 Avenida Ascension, Ste 100-3 Santa Teresa, New Mexico 88008

ABSTRACT

Present day barometric altimeters calculate pressure altimeter is a that atmospheric heights by assuming the ICAO measures changes in (International Civil Organization) and through a series of assumptions and Standard Atmosphere, which is an average, computer algorithms, converts these piece-wise continuous, mid-latitude changes, and displays an . This temperature profile of the earth's conversion process assumes standard atmosphere. The ICAO temperature profile atmospheric conditions. Therefore, when has been used in the aviation field for over the atmospheric conditions deviate from the 30 years. Nevertheless, seasonal changes standard atmospheric conditions, errors in in conjunction with latitudinal variations can determining the correct altitude will result. produce temperature profiles that can significantly differ from the ICAO standard atmosphere, especially for geographical regions located at high latitudes during winter seasons. This study investigates altimeter errors that occur due to cold temperatures at high latitude locations. If the temperature profile is colder than the standard atmosphere, the altimeter will read an altitude that is higher than the true altitude of the . These errors are very critical in mountainous terrain areas. Figure 1. Typical Barometric Altimeter Climatological temperature profiles from a Gauge. high latitude region will be used to conduct an altimeter error analysis. The case study 1.2 Present day barometric altimeters will highlight key issues in using barometric calculate atmospheric heights by assuming altimeters in these situations and help the International Civil Aviation Organization identify problem areas. Recommendations Standard Temperature Atmosphere (U.S. on improving or enhancing current ICAO Standard Atmosphere, 1958, 1962, 1966, barometric altimeter operations will be 1976), which is an average, piece-wise discussed and future studies will be outlined. continuous, mid-latitude temperature profile of the earth's atmosphere. The ICAO temperature profile has been used in the 1. INTRODUCTION aviation field for over 30 years (figure 2). Nevertheless, seasonal changes in 1.1 The standard aircraft altimeter (figure conjunction with latitudinal variations can 1) is a barometric device that measures produce temperature profiles that can pressure and not altitude directly. The significantly differ from the ICAO standard atmosphere, especially for geographical *Corresponding author address: Young Yee, Mkey regions located at high latitudes during Technologies, 401 Avenida Ascension, Ste 100-3, winter seasons. This study investigates Santa Teresa, NM. 88008. EMAIL: [email protected]. altimeter errors that occur due to cold temperatures at high latitude locations. If the only way to traverse much of the State. temperature profile is colder than the These challenges increase the risks to safe standard atmosphere, the altimeter will read operations” (LaBelle, 1999). an altitude that is higher than the true altitude of the aircraft. These errors are very 1.4 Alaskan cities are located at latitudes critical in mountainous terrain areas. from 51.88 degrees N (Adak, AK) to 71.8 Climatological temperature profiles from a degrees N (Barrow, AK). Global high latitude region will be used to conduct climatological temperature profiles show an altimeter error analysis. The case study dependence on latitude and season which will highlight key issues in using barometric may result in significant errors in altimeter altimeters in these situations and help readings that can further mislead pilots identify problem areas. Recommendations when flying under severe weather on improving or enhancing current conditions. barometric altimeter operations will be discussed and future studies will be outlined. 2. SOURCES OF ERRORS

Because the standard aircraft altimeter calculates altitude from specific meteorological measurements, it is subjected to several sources of errors.

2.1 For these studies, the most relevant meteorological data types are the temperature and the pressure versus height. The next section reviews the meteorological instrumentations that will either measure or infer these parameters.

2.2 The accuracy of aircraft altimeters is subject to the following factors:

Figure 2. Standard Atmospheric 1. Nonstandard temperatures of the Temperature Profile. atmosphere (addressed in this study). 2. Nonstandard atmospheric pressure. 1.3 The motivation for this study came 3. Instrument error. from an article about flight accidents in Alaska in which James LaBelle was quoted 2.3 The type of altimeter errors due to as saying “Between 1990 and 1998, there nonstandard temperatures is dependent on were 1,510 aviation accidents, an average the temperature profile rather than the of one accident every 2 days, that took the surface temperature only. Even though the lives of 355 people. The commercial aviation altimeter may be set for current surface accident rate in Alaska is three to four times conditions, it may be incorrect at higher greater than that of the other 49 States. levels. If the air is warmer than the standard Indeed, we were saddened to learn of the for the flight altitude, the aircraft will be most recent commuter airline accident that higher than the altimeter indicates; if the air occurred just last Tuesday 50 miles from is colder than standard for flight altitude, Bethel, Alaska. That accident took the lives the aircraft will be lower than the of 6 people. It is also significant to note that altimeter indicates. Thus, "If it's cold, look aircraft accidents are the leading cause of out below" or to quote an old saying: "GOING occupational fatalities in Alaska.Flight FROM A HIGH TO A LOW, LOOK OUT operations in Alaska are diverse, with a BELOW." challenging environment, such as rough terrain, adverse weather, and unique air 3. TEMPERATURE PROFILING METHODS transportation requirements. Due to the large geographic area and lack of other Temperature profiling of the atmosphere can forms of transportation, aviation is often the be measurements either indirectly by techniques or by point microwave radiometer incorporates a measurements such as balloon borne frequency agile local oscillator permitting instrumentations. Ground based frequency tuning in the 22-30 GHz and 51- temperature measurements can be obtained 59 GHz regions. The tunable frequencies by microwave, acoustic, or infrared sensors. cover the oxygen band, water line, and Temperature profiles can also be obtained window region thereby allowing through based sounders such as measurements of temperature, water vapor, the TIROS Operational Vertical Sounder and liquid water. Past field experiments (TOVS) system and the GOES satellite. A have shown the instrument to be reasonably brief description of these instruments is reliable and portable. given. Table 1 is a tabulation of various meteorological sensors and their associated 3.2 SURFACE METEOROLOGICAL characteristics. SENSORS.

MET MEASUREMENT METHOD RANGE The surface meteorological sensors SENSOR measure pressure, relative humidity, Microwave Ground- near surface temperature, wind speed and wind direction, Radiometer Temperature based, to 10 km, Remote vertical and radiation. From these surface Sensing, parameters, there are computer models that Continuous sampling can estimate atmospheric temperature (minutes) profiles in the lower boundary layers. Surface Met Temperature, Ground- limited to Sensors Pressure, based, extrapolation Relative Point algorithms 3.3 GOES SATELLITE Humidity, measurement, near the Winds Continuous surface sampling The GOES I-M Sounder is a 19-channel (minutes, radiometer that senses specific data seconds) Sounders on Inferred Satellite- Top of parameters for atmospheric temperature GOES temperature and based, Atmosphere and moisture profiles, surface and cloud top Satellite winds from Remote to Surface temperature, and ozone distribution Infrared/Microw Sensing, depending on ave Radiances Geostationary the spectral (Menzel, 1995; Hayden and Schmit, 1994). , channel The satellite Sounder system is able to Hemispheric sampling measure profiles over remote locations (hourly) where ground-based balloon systems are Sounders on Inferred Satellite- Top of absent. The Sounder has 4 sets of NOAA temperature and based, Atmosphere Satellite winds from Remote to Surface detectors: Infrared/Microw Sensing, depending on visible (.7 micrometer) ave Radiances Polar the spectral Orbiting, channel long wave IR (12-14.7 micrometers) 1-2 medium wave IR (6.5-11 micrometers) passes/day Radiosonde Temperature, Balloon- Ascension short wave IR (3.7-4.5 micrometers). Pressure, based, from surface Relative Point to Humidity, measurement, stratosphere Winds 2 depending on 3.4 NOAA SATELLITE Launches/day wind drift operationally The TIROS Operational Vertical Sounder

(TOVS) system is used to infer temperature Table 1. Table of meteorological sensors for and winds. The system consists of three atmospheric temperature measurements. separate instruments: High Resolution

Infrared Radiation Sounder (HIRS),

Microwave Sounding Unit (MSU) and 3.1 MICROWAVE RADIOMETER Stratospheric Sounding Unit (SSU). The

HIRS instrument is a step-scanned multi- The ground based microwave radiometer is channel spectrometer with 20 channels capable of measuring temperature profiles primarily in the infrared region of the from the surface up to 10 km height spectrum. These 20 spectral bands permit (Radiometrics, 1997; Measure and Yee, the calculation of the vertical temperature 1992; Yee and Measure, 1992). The profile from Earth’s surface to about 40 km. Multispectral data from one visible channel (0.69 micrometers), seven shortwave channels (3.7 to 4.6 micrometers) and twelve longwave channels (6.5 to 15 micrometers) are obtained from a single telescope and a rotating filter wheel containing twenty individual filters. The instantaneous FOV (Field Of View) is 1.3 to 1.4 degrees from an altitude of 833 kilometers, encompassing an area of approximately 19-20 kilometers at nadir on the Earth. The swath width is about 2300 km. (Kidder & Vonder Haar, 1995)

3.5 RADIOSONDE Figure 3. Climatological Temperature The radiosonde is a balloon-borne Profile for January, 60 degrees N Latitude. instrument for the simultaneous measurement and transmission of meteorological data producing vertical 5. CASE STUDY RESULTS profiles of atmospheric pressure, temperature, humidity, and wind speed and A depiction of the temperature effects on direction. Newer radiosondes employ GPS altimeter readings is illustrated in figure 4. If (Global Positioning ) to derive the the atmospheric temperatures were warmer winds as the balloon ascends. The balloon than the standard atmosphere, the indicated ascends from the surface up to 20-30 km altimeter readings would be lower than the height over approximately 1-2 hour period. actual height. On the other hand, if the atmospheric temperatures were colder than the standard atmosphere, the indicated 4. CASE STUDY altimeter readings would be higher than the actual height. In the example shown in the For this case study, a representative figure 4, the airplane is flying on the 800mb Alaskan latitudinal belt was selected i.e. 60N pressure level. If the standard atmosphere latitude for a winter season. A collection of is assumed, the pilot's altimeter would read climatological temperature profiles for the 1843 meters but if the temperature profile region during the winter month of January follows the more realistic January 60N was assembled and an averaged latitude temperature profile, the pilot would temperature profile was calculated (Figure be flying at 1689 meters. 3). From the 60N latitude temperature profile, based on atmospheric pressure were calculated. For comparison, the altitudes that a standard pressure altimeter would indicate assuming a standard atmospheric temperature profile were calculated. Altimeter errors as a function of pressure readings were determined.

Figure 4. Depiction of Temperature Effects on Altimeter Errors.

For this case study, an altimeter correction graph was calculated as shown in figure 5. Extreme caution should be exercised when These results indicate that altimeter errors flying in proximity to obstructions or terrain in can be as high as 340 meters when the low temperatures and pressures. This is pressure altimeter reads 280 mb. Because especially true in extremely cold of the non-linear characteristics of the temperatures that cause a large differential January 60N latitude temperature profile, the between the Standard Atmospheric Day altimeter corrections do not exhibit a linear temperature and actual temperature. This relationship. circumstance can cause serious errors that result in the aircraft being significantly lower than the indicated altitude. Future studies will be to investigate other latitudinal regions for different seasons.

7. RECOMMENDATIONS

• Collect and assemble climatological temperature profiles for other latitudinal belts for the different seasons to compile more detailed pressure altimeter errors due to non-standard atmosphere conditions. • Perform test and evaluation at a specific geographical location Figure 5. Altimeter Corrections for January, • Investigate the use of ground based 60 degrees N Latitude, temperature profile. microwave radiometers to correct for temperature profiles in high latitude regions

6. CONCLUSION REFERENCES

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