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The following document covers
Two Rosemount Model 102 de-iced Platinum-resistance total temperature probes. One of them is equipped with a fast response open-wire Pt-100 element, the other one with an encapsulated, robust but slower Pt-500 element. The Pt-wire measures the temperature of air flowing through a special housing, which is formed in a way in order to avoid direct hits of water droplets on the Pt-wire. As the airflow is slowed when passing the sensor element, temperature is increased by a certain amount depending on the airspeed, which is corrected during data evaluation.
Three different sensor types are installed permanently:
The Falcon is equipped with a 1.8 m length nose-boom for wind measurements. The 3-dimensional wind vector, Vw, is derived from the difference between the velocity components of the aircraft relative to the ground, Vk, and the velocity components of the aircraft relativ to the air, Va, i.e.
Vw = Vk - Va
where V stands for a vector with components u,v,w.
Va is measured with a Rosemount 5-Hole probe model 858AJ on the tip of the nose-boom. The 5-Hole probe consists of a 2.54 cm diameter half-sphere on a cylindrical shaft of the same diameter which is conically thickened 12.8 cm behind of the tip. One port at the front center of the sphere is used to sense the pitot pressure, pt. The other four pressure ports on the sphere are arranged symmetrically at 45 degrees vertical upwards/downwards and left/right from the center port. A ring of ports around the cylindrical shaft of the probe 9 cm downstream of the tip allows the measurement of the static pressure. The vertical and lateral pair of the 45-degree ports serve to calculate the angles of attack (alpha) and sideslip (beta), respectively. In a first approximation this is accomplished by measuring the differential pressure between each two adjacent ports and normalizing this with the impact pressure, qc, which is the difference between the pitot pressure and the static pressure. However, because of the flow disturbance induced by the aircraft body and wings, the exact calculation is more complicated.
The static pressure is measured with a Rosemount pressure transducer Model 1201F2, the impact pressure with a Model 1221F2AF and the differential pressure with two Models 1221F2VL.
The airspeed is calculated from the impact pressure, the static pressure and the static temperature (plus a minor correction taking into account the influence of humidity). The 3-dimensional flow vector is calculated from the two flow angles and the airspeed, and transformed into the earth-fixed coordinate system by use of the heading and the two attitude angles (i.e. pitch angle and roll angle) provided by the Inertial Reference System (next section).
The temperature has been corrected from the near-adiabatic heating inside of the housing.
The measurements of the Lyman-Alpha and the Vaisala have been transformed to outer air conditions assuming constant mixing ratio during the pass through the measurement channel.
The drift and offset of the Lyman-alpha has been removed using a dynamic base-lining in periodic and small steps using the Vaisala data as a reference. The base-lining is performed by adjusting a fictive zero- humidity detector output voltage (which is part of the exponential relation between humidity and measured voltage) a little bit towards that value that would theoretically fit the Lyman-alpha and the reference data. The adjustment is re-calculated every 5 seconds but only 5% of the needed correction for a perfect match between Lyman-Alpha and reference is added per cycle. By this, no significant artificial steps in the data occur (after the initial swing-in phase) and the high frequency response of the Lyman-Alpha is not too much damped by the slow Vaisala. On the other hand a reasonable fast coupling (effective time constant about 100 s) to the long term stability of the Vaisala remains. This is superior to a simple constant shift of the finally calculated absolute humididity, as the later one would not respect the exponential relation between sensor output and humidity. Synchronous to the baselining, the contribution of the absorption by oxygen, which grows to a dominant factor at high altitudes, has been taken into account.
The measured static and impact pressure have been corrected from the positional error of pressure measurements at the nose-boom and the true airspeed calculation also takes into account the humidity of the air.
The horizontal velocity components of the IRS have been corrected by use of the FMC position data as a reference (Flight Management Computer, part of the aircraft's avionic system, see Baumann et al.(1990)) position data as a reference. The time-series of the position difference between the two sources have been fitted to an error model of typical IRS-errors and the time derivation of this position error has been substracted from the velocity components (Quante et al. 1995). By this, the error has been reduced by a factor of 3-5 to probably less than 0.5 m/s for each component.
On principal the vertical velocity and altitude cannot be determined directly with any IRS without an additional independent measurement of the altitude, because of the instability of the navigation equations in the vertical coordinate. On-line the auxiliary altitude comes from a standard atmosphere barometric height calculation. For off-line data evaluation altitude has been calculated more accurately by integration of the hydrostatic equation
dH = - dp / (g * rho)
where dp is the change of static pressure from one time step to the next, g is the local gravity and rho is the actual mass density of the air. This height (called 'true height' in contrast to 'standard atmosphere pressure height') also has been included in the provided dataset.
The data in this dataset have been averaged to 1 second means, but most of the turbulence related parameters (wind components, Lyman-Alpha data and fast Pt-100 Sensor) could also be processed with full 100 Hz sampling resolution on demand.
Further informations on algorithms, calibration procedures and definitions of parameters are given in Bögel and Baumann (1991), Baumann (1994) and Meischner (1985).
| parameter | units | absolute accuracy(1) |
|---|---|---|
| static temperature | °C | 0.5 |
| abs. Hum. (Lyman-alpha) | g/m3 | 0.5 (2) |
| rel. Hum. (Vaisala) | % | 8 (@8km)/12 (@10km)(3) |
| pressure | hPa | 1.0 |
| horizontal wind | m/s | 1.0 (4) |
| vertical wind | m/s | 0.5 |
(1) relative accuracy, i.e. dynamic variation of the current error during short periods of time (some minutes) would be much lower (typically by one order of magnitude), but this number depends on the stability of mean temperature, mean height, and the duration of the time period considered.
(2) For ascending/descending aircraft. During straight level flight the absolute accuracy is as good as for the Vaisala (i.e. the value in g/m3 is much smaller at high altitudes) plus an additional uncertainty of ±5% of the maximal amplitude of fluctuations.(3) Strongly depends on height, e.g. near ground accuracy is as good as 2% r.h.
(4) Estimated (preliminary) accuracy with the correction by use of FMC-Positions.
A overview of intercomparison publications is given in Fimpel (1991). Recent intercomparison results are given e.g. in Quante et al. (1993), Quante et al. (1995) and Ström et al. (1994).Between 16:46:40 and 16:59:00 the Lyman-Alpha data missed a reasonable correspondence to the Vaisala signal (the signal partially felt below zero humidity) and therefore has been marked as invalid for this period in the distributed dataset (replaced by value '99.999')
Baumann, R., 1994: Calculation Scheme for Wind- and Turbulence Data of
the DLR Research Aircraft. DLR-Institutsbericht IB 553-1/94. 16 pp.
Busen, R. and A.L. Buck, 1995: A High-Performance Hygrometer for
Aircraft Use: Description, Installation, and Flight Data. J. Atmos.
Oceanic Technol., 12, 73-84.
Meischner, P., (Ed.), 1985: Nutzerhandbuch für das FALCON-System. DFVLR
Mitteilung 85-08, 131 pp.
Corresponding author's address:
Institut für Physik der Atmosphäre
DLR Oberpfaffenhofen
82234 Wessling
Germany
email: robert.baumann@dlr.de
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