AE-C Retarding Potential Analyzer/Drift Meter (RPA) 


   The RPA/Drift Meter is a passive instrument mounted on a large ground
plane such that its look direction corresponds to the +X direction of 
the spacecraft coordinates. The spacecraft +X axis points nominally 
along the spacecraft velocity vector in a circular orbit. For an elliptical
orbit this condition is true only at perigee and apogee. 
The orientation of the spacecraft  axes with respect to the earth is 
supplied with the  AE data base
   The RPA/Drift Meter consists essentially of two sensors - one, the RPA,
which measures the energy distribution of the thermal ions entering the
sensor and another, the Drift Meter, which measures the angle of arrival
of the ions relative to the sensor. A description of the instrumentation
and detailed descriptions of the procedures used to determine the 
geophysical parameters may be found in Hanson et al. (1973) and Hanson
and Heelis (1975).
   The RPA consists of a circular entrance aperture to a gridded region
which is terminated by a solid gold collector. One of the grids has a
positive retarding potential applied to it which determines the energy
of the ions that reach the collector. The retarding potential is swept 
from +32 (or +22) volts to 0 volts in 0.75 seconds and the resulting 
collector current versus retarding potential curve is fit to a theoretical 
expression using a least squares technique. The geophysical parameters 
obtained from this procedure appear in the UA file as
the ion temperature = Ti in K
the total ion concentration = Ni in ions/cm**3
the ion drift velocity along the +/-X axis of the spacecraft= Vx in m/s
the vehicle (ground plane) potential = psi volts

   In addition, the RPA can be operated in a "duct" mode where variations
in the total ion current at zero retarding potential are monitored for 
three-second periods with a sensitivity that automatically adjusts to 
the magnitude of the variations. The geophysical parameter obtained 
from this operation and placed in the UA file is:
total ion concentration roughness dNi/Ni = RO in %

The ion Drift Meter consists of a square entrance aperture to a gridded
field free region which is terminated by a segmented gold collector. The
angles of arrival of the ions with respect to the +/- Y and +/-Z axes of
the spacecraft are determined by measuring the asymmetry in the ion current
between different collector segments. There is a direct relationship between
the ion drift velocities along the +/-Y and +/-Z axes, the tangents of the 
angles of arrival with respect to these axes and the total ion velocity 
with respect to the sensor along the +/-X axis.
   It is assumed that the ambient ion drift velocity along the +/-X axis
of the spacecraft is zero so that the total ion velocity in this 
direction is equal to the spacecraft velocity in this direction. 
The contributions of the corotation velocity are removed from all the ion
velocity  components and the geophysical parameters obtained from this 
operation and placed in the UA file are
The ion drift velocity along the +/-Y axis of the spacecraft = Vy in m/s
The ion drift velocity along the +/-Z axis of the spacecraft = Vz in m/s
Again it should be emphasized that Vx, the RPA derived ion drift 
velocity, is assumed to be zero for the computation of Vy and Vz.

   In addition to measuring the asymmetries in the collector currents,
on AE-D and AE-E the total ion current is also monitored to provide a measure
of the variations in total ion concentration. This parameter is particularly
useful when the spacecraft is spinning and RO from the RPA is unavailable. The
parameter total ion concentration roughness dNi/Ni= ROd in %
is determined by the Drift Meter and placed in the UA file for AE-D and AE-E.
   The three components of the ion drift velocity Vx, Vy and Vz
are directed along the +/- X, +/- Y and +/- Z axes of the spacecraft. Space-
craft turnarounds change the directions of the Y and Z axes but leave the
X axis direction unchanged. There is a sign convention attached to the
drift velocities Vx, Vy and Vz as they appear in the UA file, which makes
their directions and signs independent of the changing axes' directions.
1. Positive Vx is directed along the X axis in the negative X direction.
2. Positive Vy is directed along the Y axis away from the earth.
3. Positive Vz is directed along the Z axis in the direction - Vx x Vy.

                       Averaging & Filtering

     The UA data presented here have undergone substantial averaging
and filtering. The RPA and Duct data Ti, Ni, Vx, PSI and RO are obtained
every 15 seconds from 30 second running weighted averages of the individual
data points. The drift meter data Vy, Vz and ROd are represented by the
individual data point nearest to the time indicated. In this case the
time difference between the data point and the time indicated is usually 
less than 2/3 sec.

                         Consistency Checks

Both the RPA and Drift Meter data have undergone a number of checks
to ensure that obviously erroneous data do not appear in the UA File. These
checks cannot of course cover all possible cases, but we believe that any
erroneous data appearing in the UA file will be immediately recognizable
in the form of "flyers."
   Both the RPA and the Drift Meter suffer a degradation of their
data at very low altitudes owing to a phenomenon caused by neutral particle
impact on the instrument collectors. While the data degradation on the
drift meter is immediately recognizable and is simply a function of the ambient
total ion concentration, this is not the case for the RPA. For the RPA data
the phenomena is rather difficult to detect and even at large ambient ion
concentrations the shape of the retarding potential versus collector current
curve can be seriously compromised. The RPA data below 200 km altitude does
not appear in the UA file when this phenomena is likely to affect the data
quality.
   Similar distortions of the retarding potential versus collector
current curves result when the angle between the look direction of the
sensor (i.e. the spacecraft X axis) and the spacecraft velocity vector
exceeds about 20 degrees RPA data to not appear in the UA file when this angle
exceeds 20 degrees.
   The drift meter data are seriously compromised by any phenomena
other than ion drift that may deliver an asymmetric current to the
collectors. The neutral particle impact experienced by the RPA at low
altitudes is one such phenomena. Drift Meter data below 200 km altitude
is removed from the UA file when this phenomena is known to affect the
data.
   Photoemission from the grids and grid holders can also produce
asymmetric currents when the sun shines into the sensor. Again the Drift
Meter data are removed from the UA file when this condition is known to
be serious.
   Significant concentrations of light ions (H+ and He+) can affect
the sensitivity of the drift meter. The data, which is calibrated correctly
for O+ and heavier ions, do not appear in the UA file when an adjustment
for sensitivity change is known to be required.
   Much of the data which does not appear in the UA file is retrievable
with careful consideration of the ambient conditions existing at the time
the data were taken. The user is encouraged to contact the investigator
at the University of Texas at Dallas, from whom further extensive information
can be obtained.

                         References

Hanson, W. B., D. R. Zuccaro, C. R. Lippincott, and S. Sanatani, The
       retarding potential analyzer on Atmosphere Explorer, Radio Sci., 8,
       333, 1973
Hanson, W. B. and R. A. Heelis, Techniques for measuring bulk gas-motions
       from satellites, Spa. Sci. Instrum., 1, 493, 1975