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.