AE MESA DESCRIPTION FOR NSSDC A. OBJECTIVE The scientific objective of the AE Miniature Electrostatic Accelerometer (MESA) is to study variations in the neutral atmospheric density and to relate this data to solar and geophysical phenomena. This experiment provides density data which are valuable for understanding the structure and dynamics of the lower thermosphere and for development of atmospheric models. B. EXPERIMENT DESCRIPTION The instrument is used to measure the satellite decelerations induced by aerodynamic drag which is directly proportional to atmospheric density. The instrument sensing element consists of a proof mass which is electrostatically suspended and force rebalanced. Drag accelerations cause motion of the proof mass from its null position. This motion is detected electronically and accurately controlled voltages are generated on "forcer" electrodes to exactly balance the drag accelerations. Three instruments with mutually orthogonal sensitive axes were flown on each satellite. The instruments are mounted internal to the spacecraft. The sample time of each instrument was 0.25 seconds. A complete description of the instrument is given in reference 1. C. CALIBRATION PROCEDURES Scale factor ground calibration data are obtained by input of an accurately known component of earth's gravitational field into the instrument sensitive axis. Laser interferometer techniques are used to accurately measure the angle between the sensitive axis and the gravitational field. Absolute calibration of scale factor is estimated to be accurate to better than +/- 0.5%. D. OVERALL DATA REDUCTION PROCEDURE 1. Filtering -- Drag accelerations are extracted from "noise" accelerations utilizing digital filtering techniques. The theory and application of the filtering procedures used for the AE MESA data are completely described in reference 2. For despun orbit data variations in atmospheric drag lower than about 0.067 Hz are passed through the filter while higher frequencies are filtered out. For spinning orbit data the effect of the filter is to eliminate accelerations with frequencies higher than 0.013 Hz. 2. Density Calculation -- Atmospheric density is calculated from measured drag accelerations on one sensor using the equation: ro=(2*M*ad)/(cd*A*V**2*cos(teta)) where ro = atmospheric density, ad = measured drag acceleration, teta- is the angle between the instrument sensitive axis and spacecraft velocity vector and M, A, cd, and V are respectively the satellite's mass, cross- sectional area, drag coefficient and velocity . For spinning orbit data only the positive and negative ram points were utilized to minimize uncertainties in the filtered output, cross-sectional area, and attitude. Because the density is proportional to the magnitude of the drag the data are more accurate at the lower altitudes. For spinning orbit data the errors are estimated as follows: uncertainty in area-to- mass ratio (A/M) is +/- 1%; attitude uncertainty is negligible; the filtering error varies from negligible at perigee to +/- 2% at 200 km and +/- 5% at 250 km. For despun orbits there is an additional error of typically +/- 2%. This is because the sensor is nominally at 45 degrees to the velocity vector in this mode and attitude errors of about 2-3 degrees are encountered. For both spinning and despun data there may be a +/- 10% systematic error in the assumed free molecular flow drag coefficient of 2.2. 3. Final Data -- The density results placed into the Unified Abstract file were calculated at 15 second intervals from the complete geophysical unit files using a second order Lagrangian fit to the three data points closest to the appropriate time. 4. Thorough Data Analysis Procedures---Reference 3 completely describes the AE MESA data processing system. E. ORBITAL PERFOMANCE OF SENSORS All instruments on AE-C, -D, and -E appear to have performed normally throughout the missions. No anomalous behavior has been noted. All above comments refer to elliptical orbit data. The final processing for circular orbit data will be described in an appendix submitted at a later date. F. OTHER CONDITIONS The density data have been derived using spacecraft attitude results. Spuriously large attitude errors were obvious and these orbits have been deleted. If attitude errors of 5-10deg. are used the accuracy of the density data will be decreased accordingly. G. REFERENCES 1. Champion, K.S.W. and Marcos, F.A. (1973). The triaxial accelerometer system on Atmosphere Explorer. Radio Science, 8, 297. 2. Noonan, J.P., Fioretti,R.W. and Hass,B. (1975) Digital filtering applied to the Atmosphere Explorer-C MESA accelerometer data. AFCRL Rept; ACFRL-TR75-0293. 3. Fioretti,R.W. (ed), (1976). Atmosphere Explorer MESA accelerometer processing system. AFGL Rept. AFGL-TR-76-0137.