MESSENGER EPPS EPS
EDR-to-CDR Processing

Version 1A





7 January 2010





Prepared by
Lawrence Brown and Lillian Nguyen
The Johns Hopkins University
Applied Physics Laboratory

Table of Contents
1  PURPOSE  3
2  INTRODUCTION   3
3  CALIBRATION IMPLEMENTATION 3
4  CALIBRATION PROCEDURE   4

1
Purpose
This document provides a preliminary description of the conversion of
MESSENGER Energetic Particle and Plasma Spectrometer (EPS) Experimental Data
Records (EDRs) to Calibrated Data Records (CDRs). The processing steps
described in this document represent the state of knowledge at the date of
this document.

2 Introduction
The Science EDRs are the raw data records used to derive data used for
scientific analysis. The processing steps from the EDR to the CDR level are
described in this document.

3 Calibration Implementation
See the EPPS CDR SIS for a detailed description of instrument design issues
relevant to calibration.  We present below the specific calibration algorithm
used in the production of EPS CDRs, including the Geometry/Efficiency factors
used.  These represent a current "best guess" by the instrument team but
these will be refined as more data is taken and understanding of the
instrument response is improved.  Note that by reporting "differential" flux,
we are implicitly "pretending" that the flux is constant across the energy
band.
In practice, the CDR files are generated using the same Mission Independent
Data Layer (MIDL) codebase which also underlies the EPS Instrument Team's
science analysis software.  The rates are read in from the EDR files as
number of counts in an accumulation interval.  The rates in the CDR are then
just derived by the simple formula:
       Rate = Counts/deltaT
      deltaT = Accumulation Time
For channels with a double-bounded pass-band, the flux is calculated by:
       Flux = Counts/(deltaT*deltaE*G)
      deltaE = Upper Energy Bound - Lower Energy Bound
      G = Geometry Factor = 0.0007 cm2-sr (large pixel)
                                             0.000035 cm2-sr (small pixel)
Whether the instrument is operating in large or small pixel mode is
determined by the following:  Before 2007:249 - small pixel mode except for
2005:214:13:16:1 -2005:214:15:42:59.  After 2007:249 pixel mode is read from
the EPS_ION_PIX_SIZE or EPS_ELEC_PIX_SIZE field in the Long Status EDR.

4 Calibration Procedure
What follows is a summary of the procedure for applying the algorithm from
the previous section to produce CDR rates from EDR rates.
DT = Integration time (column 2 in the EDR)
C = The "Rate" value from the EDR.  This is in raw counts accumulated over DT.
U = Upper energy bound in KeV.  Get this from the table in the CDR SIS for
the Rate you're calibrating.
L = Lower energy bound in KeV. Also from the CDR SIS table.
G = Geometry factor (assumed energy independent for now).  See previous
section for how to find.
Flux (in Counts/keV-cm^2-sr-sec) = C/(DT*(U-L)*G).

- 4 -