Dynamics Explorer 2
Retarding Potential Analyzer (RPA)
and Ion Drift Meter (IDM)
Developed at the University of Texas at Dallas, the Retarding Potential Analyzer and Ion Drift Meter were flown aboard NASA's Dynamics Explorer-2 spacecraft. This picture shows the two DE spacecraft stacked in their launch configuration (DE-2 on top, DE-1 on the bottom).
Description of Spacecraft:
The Dynamics Explorer 2 spacecraft was one of two satellites launched for the Dynamics Explorer program. The two spacecraft were launched together into coplanar polar orbits for the purpose of studying coupling between the magnetosphere, ionosphere, and the atmosphere. The DE-2 spacecraft was placed in a low elliptical orbit whereas the DE-1 orbit was highly elliptical.
Instruments aboard the DE-2 spacecraft were: magnetometer, vector electric field instrument, neutral atmosphere composition spectrometer, wind and temperature spectrometer, Fabry-Perot intereferometer, ion drift meter, retarding potential analyzer, low altitude plasma instrument, and Langmuir probe.
Orbit Information:
Because the Delta launch vehicle did not complete a full burn the DE-2 satellite was placed in a lower than anticipated polar orbit, initially
1012 by 309 km. The orbital period was 98 min. The DE-1 and DE-2 satellites were launched by the same vehicle so that their orbits would be coplanar, allowing occasional two-point measurements along magnetic field lines. The
DE-2 spacecraft spun once per orbit and the spin axis was perpendicular to the orbital plane so that one axis of the satellite always was aligned with the center of the earth.
Performance:
The DE-2 spacecraft performed well through its lifetime. Power limitations forced the duty cycle to be limited to an average which was originally targeted at 30%. The lifetime of the spacecraft was shorter than anticipated because of the less than nominal performance of the launch vehicle. The launch was on August 3, 1981 and the DE-2 satellite reentered the atmosphere on February 19, 1983, with the last contact the day before.
Investigation Objectives:
Among the science objectives for the DE program was the study of the electrodynamics of the ionosphere and its application to ionosphere- magnetosphere interactions. The RPA/IDM objectives were to provide a measure of the vector ion drift velocity in the ionosphere and the ion temperature, concentration and composition. This data is used in studies of the ionospheric electric field and ionosphere-thermosphere interactions.
Instrument Attributes:
A. Description of Instruments
The Retarding Potential Analyzer (RPA) consists of two planar sensors that view approximately along the spacecraft velocity vector. One sensor provides a measure of the total ion flux entering the sensor aperture from which structure in this parameter can be measured with extreme sensitivity.
The other sensor has internal grids that can be stepped through positive voltage waveforms to modulate the incoming ion flux that reaches the collector. The resulting ion-flux versus retarding potential measurements are used to derive the ion temperature, the ion drift velocity along the sensor look direction ,and the ion composition. ( See Hanson et al., Space Sci. Instrum. 5, 503, 1981)
The Ion Drift Meter (IDM) consists of two sensors that view approximately along the spacecraft velocity vector. Each sensor has a square entrance aperture to a gridded region in front of a collector with four quadrant sectors. By comparison of the ion currents to each sector it is possible to measure the angle of arrival of the ions in two mutually perpendicular directions perpendicular to the look direction of the sensor. From this angular information, knowledge of the ion velocity along the sensor look direction from the RPA and knowledge of the spacecraft attitude, it is possible to derive the ambient ion drift along the two mutually perpendicular directions. ( See Heelis et al., Space Sci. Instrum., 5, 511, 1981)
B. Operational Modes:
The nominal modes of operation for the RPA provides a measure of the total ion flux 64 times per second and a measure of the ion flux versus retarding potential every second. The retarding voltage wave form may contain a high frequency "wiggle" from which a derivative signal can be obtained. The steps comprising the retarding voltage waveform could be changed to obtain data more efficiently. The IDM provides absolute measures of the ion arrival angle 4 times per second and measurements of the angle relative to an absolute value, established every 8 seconds, at the rate of 32 samples per second.
C. Measured Parameters:
The RPA provides a measure of the total ion concentration by incorporating the spacecraft velocity into the measured ion flux. The ion flux versus retarding potential data provide the input to a non-linear least squares fitting procedure that derives the ion temperature. The ion drift velocity along the sensor look direction is also derivable from this procedure if either the spacecraft potential is known or, as is usually the case, if two widely spaced masses comprise the ion composition.
Using the velocity of the spacecraft relative to the ambient plasma, it is possible to use the angular information provided by the IDM to compute the ion velocity along the mutually perpendicular directions (nominally horizontal, perpendicular to the spacecraft's orbital plane, and vertical).
D. Performance of the Instrument:
The RPA data provide high resolution measurements of the ion concentration for the entire duration of the DE-2 lifetime. Due to a failure in the instrument memory system, measurements of the ion flux versus retarding potential are unavailable from 81317 06:26:40 UT to 82057 13:16:00 UT. The component of ion drift along the look direction of the sensor, the ion temperature and the ion composition are not available during this period. The IDM provided data on the ion arrival angle for the entire duration of the DE-2 lifetime. Interpretation of this data in terms of ion drifts requires the assumption of zero ram drift during the period of RPA failure. This is usually not a serious limitation.
Data Description and Formats:
Data Resolution:
Three different types of geophysical data files have been produced for inclusion in our data sets. The first is the RPA geophysical data file which is produced from a combination of RPA and IDM data. The resolution is determined by the basic RPA retarding voltage sweep time and the efficiency of the least-squares analysis. RPA geophysical data files provide ALL geophysical parameters measured by the RPA and IDM at a resolution of usually 2 seconds and sometimes
4 seconds.
The second type of file is the DUCT geophysical data file which provides measurements of the total ion concentration every 64 times per second. These files are designated DUCT.
The third type of file is the IDM geophysical data files that provides absolute measurements of the cross track ion drift velocity 4 times per second.
Data Parameters:
RPA data files contain time tagged orbit data and geophysical data for each analyzed sweep.
DUCT data files contain geophysical data in 8 second blocks corresponding to the standard segment of data on the DE spacecraft. Each 8 second block of DUCT data, generally containing 512 ion concentration measurements, is accompanied by some basic orbit position information.
IDM data files contain geophysical data in 8 second blocks corresponding to the standard segment of data on the DE spacecraft. Each 8 second block of IDM data containing 32 pairs of mutually perpendicular ion drift velocities in spacecraft coordinates, is accompanied by some orbit position information.
Data Set Quality:
RPA data files have been nominally screened for obvious errors in least- squares analysis. Some errors are, however, quite subtle and caution should be exercised in interpretation of temporal discontinuities in the ion temperature or ion drift velocity along the spacecraft look direction. As much data as possible has been retained in the data files and the rms error is available for each entry. The accompanying IDM data for each entry has not been interpolated for the time tag given as the mid-point of the RPA sweep.
Thus time shifts of 1/8 second may exist between the specified and actual time for the cross track drifts.
IDM data is of high quality since only absolute ion drift velocities have been retained in the data files. This higher resolution data has not been corrected for the presence of ion drifts along the sensor look direction. If such a drift Vr exists then the value in the file should be corrected by the factor (Vs +Vr)/Vs, where Vs is the spacecraft velocity along the sensor look direction. This correction factor can be as high as 20%.
DUCT data is of high quality since there is little interpretive work required in its conversion from ion current. The spacecraft velocity along the look direction of the sensor is used to convert ion flux to ion concentration.
Apparent variations in ion concentration of order 10% may result from variations of order 1 km/sec in the ambient ion drift velocity along the look direction of the sensor. Such changes should be apparent in the RPA data files.
A. Data Processing Cycle:
Raw telemetry segments were reduced to RPA mission analysis files (MAF's) utilizing production processing on the project computer. The cpu commitment to RPA analysis could not be retained during the periods when the TM was initially available. Thus a situation in which TM file promotion was required for RPA analysis evolved. This quickly resulted in a backlog of unprocessed TM which still exists. Though data files for most data segments exists some files do not exist at all and others require complete reprocessing due to inadequate least-squares analysis.
B. Data Processing History
All data was originally reduced on project computers at GSFC. More recently an attempt to transfer telemetry files to local computers at the investigators site has been undertaken. Some data files originate from reduction of telemetry on local computers. This data is usually of higher quality and higher temporal resolution owing to the resources that can be applied to it.
It is not identified in any way as originating from this source.
Data Organization:
Geophysical data files exist for each DE2 telemetry segment. Generally there will be a DUCT file corresponding to each RPA file. These data files may be identified by orbit number or by approximate UT start time.
Naming Convention:
RPA, IDM, and DUCT files are named according to the U.T. starting date and time of the data in the file. Example: RPA821141206.DAT indicates that this is an RPA file for the year (19)82, day 114, hour 12, minute 06.
File Time Coverage:
The length of a file can vary from less than 1 minute to approximately 1 complete orbit of the DE-2 satellite (approximately 100 minutes).
Data Availability
DE-2 data from the RPA (RPA and DUCT files) and the IDM are available through the
National Space Science Data Center at NASA's Goddard Space Flight Center.
Network connection can be made to the data center through either the direct WWW link above or a terminal session on NODIS (NSSDC On-line Data and Information Service: Log in with a user ID of "NODIS").
Data Usage:
Usage of the geophysical data is not formally restricted. Users are urged to
be cautious in interpretation of structure and discontinuities in the RPA data.
In many cases such structure is real, but it may also be caused by the
production of false minimums in the least-squares analysis. Review of data by
the scientific contacts below is desirable to ensure it is used correctly.
Scientific Contacts:
Dr. R. A. Heelis or
Dr. W. R. Coley
William B. Hanson Center for Space Sciences
University of Texas at Dallas, M.S. Fo2.2
P. O. Box 830688
Richardson, Texas 75083-0688
972-883-2851
SPAN Address - UTSPAN::UTADNX::UTDSSA::HEELIS
SPAN Address - UTSPAN::UTADNX::UTDSSA::COLEY
Internet Address: heelis@utdallas.edu
Internet Address: coley@utdallas.edu
This page maintained by
W. R. Coley.
Last modified 8/16/96.