The P-3B Turbulent Air Motion Measurement System (TAMMS)

Geoffrey Considine, Bruce Anderson, and John Barrick

Atmospheric Sciences Division

NASA Langley Research Center

Hampton, VA 23681

The NASA Wallops Flight Facility Lockheed P-3B Orion aircraft was outfitted by our group with the TAMMS just prior to the PEM Tropics A mission. The TAMMS is composed of several subsystems including: (1) distributed pressure ports coupled with absolute and differential pressure transducers and temperature sensors, (2) aircraft inertial and satellite navigation systems, (3) a central data acquisition/processing system, and (4) water vapor instruments and potentially other trace gas or aerosol sensors. Table 1 contains a listing of the more significant parameters provided directly by the TAMMS whereas Table 2 includes parameters which are derived from the TAMMS data set. The attached block diagram indicates the placement of selected TAMMS sensors aboard the P3-B and the paragraphs below provide further descriptions of the individual system components.

Vector Air Flow - The angle of ambient air flow relative to the aircraft is determined using the five-hole pressure port technique as described by Brown et al (1983) and Larson et al. (1980). For this technique, five flush pressure ports (each with a diameter of ~ 0.6 cm) have been integrated into the P-3B radome in a cruciform pattern. Flow angle measurements, angle of attack and sideslip, are obtained from differential pressure measurements made between the pair of vertical aligned ports and horizontally aligned ports, respectively. The center hole is linked to existing static pressure ports on the side of the fuselage to provide required dynamic and total pressure measurements. Hemispherical flow-direction sensors (Armistead and Webb, 1973; Hagen and DeLeo, 1985) are mounted on the top and port side of the fuselage just aft of the cockpit as a backup for the radome airflow angle measurements. Static and dynamic pressure measurements are also made with precision sensors tapped into the aircraft total probe and static ports. All transducers are installed in a structurally designed housing open to the cabin environment and mounted on the forward cockpit bulkhead. This minimizes the length of the pressure tubes and allows the transducers to be maintained in a pressurized and thermally stable environment.

Air temperature measurements needed to determine true air speed, Ua , as well as heat flux were made within a non-deiced total air temperature Tt sensor housing using a fast-response platinum sensing element (E102E4AL) with a nominal 50 ohm resistance (DeLeo and Werner, 1960; Stickney, Shedlov, and Thompson, 1990). These type of sensors exhibit a measured temperature Tm of (0.995Tt. Experimentally obtained values of the recovery factor r for the temperature probe indicates a value of 0.98 could be used over the speed range of the airplane with minimal error (Barrick et al., 1996). A second temperature sensor will be installed prior to PEM Tropics B to provide redundant fast-response measurements in case the primary sensor becomes fouled with salt (a problem on one flight during PEM Tropics A) or otherwise fails.

Platform motion and position - A Litton Model LTN-72RH gyro-stabilized inertial navigation system (INS) was retro-fitted to the P-3B aircraft from the Electra aircraft for the present generation TAMMS. The RH model has been primarily developed for scientific applications with stringent requirements such as those needed for accurate air motion measurements. It provides an update rate of approximately 25 data frames/s on both binary and binary-coded decimal (BCD) bus in ARINC 561 format. The position error drift is approximately 0.4 km/hr (personal communication from Charles Robinson of Litton Industries, Inc.). It is mounted inside the P-3B radome within an environmentally controlled housing. The heading alignment of the INS has been certified to within 0.1( by surveying the center line of the aircraft fuselage as transferred to the ground surface.

Aircraft attitude angles provided by the INS are electrically fed through a 16-bit synchro-to-digital converter to yield angular resolution of 0.005( (for an airplane speed of 100 m-s-1, an angular resolution of 0.06( is required to provide 1 cm-s-1 precision in vertical velocity). The vertical velocity of the airplane wp is derived by integrating the vertical acceleration output of the INS and bounding it by the third-order barometric-inertial loop algorithm as suggested by Lenschow (1986). The long-term accuracy of the horizontal velocities up and vp are dictated by INS drift rate. A thorough discussion of inertial systems and the errors present in the resultant velocity measurements are presented by Broxmeyer (1964) and Kayton and Fried ( 1969). Lenschow (1972) gives a general discussion on the types and orders of the magnitude of errors associated with inertial systems.

In addition to the LTN-72RH data, measurements of attitude, acceleration, and position from the aircraft primary navigation system--a Honeywell laser-gyro, inertial reference unit—are recorded as a backup. This system does not have the accuracy or long-term stability of the Litton unit, but does provide more rapid parameter updates (50 Hz as opposed to 24 Hz) and real-time output of horizontal winds that can be compared with TAMMS calculated winds for periodic sanity checks. If need be, data from this system can be used to determine three-dimensional winds, however, preliminary calculations suggest resulting vertical wind velocities are about a factor-of-two lower in precision. The TAMMS also includes a global positioning system (GPS) which provides Universal Time to +1 usec accuracy and three-dimensional position to +100 m accuracy. These data are used as the primary time standard as well as to correct the long-term drift in the INS position measurements.

Data Acquisition System - Signals from the distributed sensors/instruments are routed to a high-speed computer data acquisition system for filtering, recording and processing. During PEM Tropics A, the TAMMS the central processing system consisted of a dual-processor SUN SPARC-20 workstation coupled to a VXI-bus chassis containing analog-to-digital, ARINC 429, synchro-to-digital, memory, and GPS modules. The SUN workstation communicated to the modules across a 6-foot cable without shared memory capability which severely limited acquisition speed and the potential for real-time processing of data. We recently replaced the SUN with a 200 MHz Pentium PC that mounts within the VXI crate beside the above interface modules. The system is thus capable of communicating directly to the interface modules across the VXI backplane which, along with replacement of the previous non-buffering analog-to-digital converter modules, has yielded about a factor of 100 increase in data transfer rates. The resulting system is also designed specifically for harsh environments and we anticipate it will be much more reliable than the previous SUN-based system.

During PEM Tropics B, we plan to generate a 66 Hz data set of filtered parameters which can subsequently be averaged to produce ~22 Hz winds and species measurements. In terms of filtering, analog signals will either be routed through an external 10 Hz Bessel filter or be greatly over-sampled (~6.6 KHz) then averaged to remove noise and unwanted high frequency components. We are in the process of porting our post-flight data reduction programs onto this system with the intent of broadcasting calculated three-dimensional winds along with running estimates of species flux across an ethernet connection to other investigators aboard the aircraft. The data system improvements as well as real-time data processing and output will be tested during the P-3B flight tests scheduled for summer 1998.

Water Vapor Measurements - A Lyman-Alpha hygrometer (Buck,1976) manufactured by Atmospheric Instrumentation Research, Inc (model AIR-LA-1AC) is used to provide fast response water vapor measurements. A slower response General Eastern 1011B hygrometer designed for airborne applications is mounted in close proximity and used to normalize the Lyman-Alpha signal.

System Calibration – Calibration/correction factors and coefficients for the TAMMS system are determined from in-flight maneuvers and measurements. As noted above, the recovery factor for the air temperature sensor was obtained by repeated passes by an instrumented tower a varying airspeeds [Barrick et al., 1996]. The aircraft static pressure measurement error was evaluated during a dedicated calibration flight in which a long tube connected to a drogue was trailed behind the aircraft to acquire comparative ambient pressure measurements outside the influence of the airframe. For the radome and 858Y angles of attack and sideslip measurements, "k" factors were determined by varying the aircraft pitch and yaw angles in "porpoise" and "crabbing" maneuvers, respectively, at a relatively constant altitude in quiescent air. Angle of attack "k" factors were refined by performing very gradual speed variations at constant altitude to systematically change the aircraft pitch angle without perturbing its bow pressure wave at constant altitude. Sideslip "k" factors were fine-tuned by the constraint that calculated cross-track winds be equal as the aircraft was flown in reverse headings over the same ground track. Similarly, along-track winds from these reversed-heading maneuvers were used to verify correction factors derived for the aircraft static and dynamic pressure measurements. The correction/calibration factors for the TAMMS are checked during missions by examining calculated winds from turns and reversed headings and by periodically performing porpoise and yaw maneuvers.

Displayed Parameters - In addition to display of the parameters listed in Table 1, we intend to calculate and provide three-dimensional winds in near real time aboard the aircraft. Work is underway to incorporate the aircraft true airspeed and vertical velocity correction algorithms into the TAMMS data acquisition software so that winds of relatively high accuracy can be calculated from the raw input signals. We plan to provide wind information at 0.1 second intervals to facilitate direct calculation of meteorological fluxes (see below) as well as possible collection of air samples for analysis and use in determining species flux via the eddy-accumulation technique.

Flush Radome Pressure System