The MMS instrumentation consists of three major systems:
- An air motion sensing system to measure the velocity of the air with respect to the aircraft, i.e., the true air speed.
- An inertial navigation system to measure the velocity of the aircraft with respect to the earth, i.e., the ground speed.
- A data acquisition system to sample, process and record the measured quantities.
The air motion sensing system consists of sensors, which measure temperature, pressures, and airflow angles (angle of attack and yaw angle). The Litton LN-100G Embedded GPS Inertial Navigation System (INS) provides the aircraft attitude, position, velocity, and acceleration data. On the DC-8, the Trimble TANS Vector provides secondary attitude and navigation data. The TANS Vector utilizes the GPS carrier phase shift between multiple antennas to derive independent aircraft attitude. The Data Acquisition System samples the independent variables simultaneously and provides control over all system hardware.
The instrumentation of the ER-2 MMS is described by Scott et al. . Fundamentally, subtracting the measured aircraft ground velocity from the true air speed vector produces the 3-dimensional wind vector. Details of the wind equation are documented in Lenschow [1972, 1986]. The ground speed vector is measured by the Inertial Navigation System, and the MMS air motion sensing system determines the true air speed vector. The calculation is difficult because the vertical wind (» ± 0.1 ms-1) is several orders of magnitude smaller than the aircraft velocity and air velocity (» 220 ms-1). Small error in the aircraft and/or air velocity results in large error in the wind data. A contributing error source is that the instrumentation measures quantities with respect to the aircraft frame of reference. These measurements are then transformed to the earth coordinate system. The transformation depends on the accuracy of the attitude (pitch, roll, and heading) data. The attitude data are given by the Inertial Navigation System (INS), but they are susceptible to imperfect INS installation. In the ideal limit, the INS is installed squarely with respect to the aircraft frame. In practice, there are always residual offsets, which are determined during system calibration. For the typical air speed of 200 ms-1 on the ER-2, the desired 1ms-1 wind accuracy requires ± 0.3° angular measurements.
The true airspeed vector depends on air data measurements, including static pressure, static temperature, pitot pressure, and air flow with respect to the fuselage. Accurate measurements of these quantities require judicious choices of sensor locations, repeated laboratory calibrations, and proper corrections for compressibility, adiabatic heating and flow distortion. The ground speed vector is derived from the integration of acceleration data using the appropriate numerical constraints and compensation. For example, the vertical acceleration data includes the compensation for distance above the surface (1/R2), centrifugal effects, and non-spherical earth effects. The integration is constrained by an altitude derived from the hydrostatic equation.
The system calibration of the MMS consists of:
- Individual sensor calibrations.
- Sensor dynamical response tests.
- Laboratory determination of the dynamic behavior of the inertial navigation system.
- In-flight calibration.
- Comparison with radiosonde and radar-tracked balloons.
Individual sensors are routinely re-certified to NIST standard by their respective calibration laboratories. Specific MMS sensor configuration are tested rather than the nominal set-up. For example, a temperature bath calibration is determined for a specific platinum sensor matched to a specific signal conditioner. Another example is that the recovery correction for a specific temperature probe is re-established by wind tunnel testing.
Particular attention has been given to the dynamic response of the MMS measurements. Both time shifts and the frequency response of various quantities have been determined by direct measurement and/or by calculations. The calibrations include effects due to filter response, gyros and accelerators response, pressure transducers (including plumbing) and temperature. For example, a chopped air stream that is directed at the plumbing line leading to a pressure transducer induces a step response output. Propagation delay as well as the transducer characteristic is determined from the analysis. A simple physical pendulum with measurable angular rate was constructed to determine the time delays of the INS data. Using the Lissajous plots to analyze the pendulum data, time delays as small as 0.01 second are detectable.
The MMS in-flight calibration is a self-consistency system test, requiring that the computed winds have minimum leakage from the aircraft motion. More fundamentally, the position error of the static pressure curve on the fuselage is determined by direct comparison between pressure altitude and the integrated altitude data. The winds computed during the maneuvers determine the residual phase delays between the variables and the angular offset between the air speed and the ground speed vector.
Comparison of MMS measurements with Vaisala radiosonde and radar tracking of balloons and the ER-2 aircraft was conducted in 1986, reported by Gaines et al . Comparison of the wind data with radar tracked "Jimsphere" balloons were conducted in 1989. In both cases, the results support the MMS measurement accuracy.
Power spectra of the measured quantities are finally analyzed to determine the resolution and noise figure. Independent spectral and fractal analysis of the MMS data are documented in Bacmeister  and Tuck .
DC-8 MMS Instrument Improvement
To increase system performance and reliability, we propose to improve the following DC-8 MMS sub-systems:
- The current dynamic and static pressure measurements include very long pneumatic lines, which span the width and half the length of the DC-8 fuselage. The long lines have proven to be unreliable and prone to leakage. The long transmission line system shows significant signal propagation time delays, oscillations, and frequency dependent damping. Dedicated transducers will sample the outside pressure with a 12-inch connecting line. The payload integration will require minor mechanical and electrical efforts for the transducer installations.
- We also plan to replace these transducers with vibrating quartz digital pressure sensor, with temperature compensation from ParoScientific. The ParoScientific transducers have significantly better accuracy (0.01% compared to Rosemount's 0.1% full scale) and lower long-term drifts. The improvement in the pressure measurements translates to higher accuracy in static pressure, static temperature, and wind products.
- We propose to update the data system to operate autonomously similar to the ER-2 MMS without an operator on-board during a science flight. An operator at the console, in the passenger cabin, is still a requirement during the testing and validation phase. The instrument is operated with a simple on/off switch. This improvement reduces the seat requirement during science flights and reduces the number of field personnel.
- Special pilot induced flight maneuvers are required to calibrate and to validate system performance on each flight. The calibrating maneuver typically includes 5 sinusoidal ± 2° of pitching, 5 sinusoidal ± 2° of yawing, and 360° square box pattern. The pitch and yaw maneuvers consume negligible flight time; they are usually executed in transit. It takes the DC-8 about 10 minutes to complete the four 90-degree of the box pattern. We have developed a new calibration algorithm and reduced the box pattern frequency by half.
ER-2 MMS Instrument Integration
The ER-2 MMS last performed in January-March 2000 during the Sage Ozone Loss Validation Experiment. The instrument is flight ready and only needs installation on to the CAMEX-4 ER-2 payload. Because the CAMEX payload utilizes a special nose for the ER-2 Doppler Radar (EDOP), the MMS temperature sensors and airflow angle system need to be installed. We have made a preliminary assessment with the ER-2 Engineering Technical Lead at Dryden Flight Research Center.