MESSENGER Magnetometer 

EDR-to-CDR Processing



Version 2d











14 June 2011









Prepared by

Haje Korth and Brian Anderson

The Johns Hopkins University

Applied Physics Laboratory

Laurel, MD 21784

USA



Change Log



DATE SECTIONS CHANGED REASON FOR CHANGE REVISION 6/14/11 Change Log Added

change log. V2d 6/14/11 12 Added information on changes incorporated in

processing code versions 1.4, 1.5, and 1.6.  V2d

Table of Contents

1  Purpose  4

2  Introduction   4

3  Coordinate Systems   4

3.1   Sensor and Spacecraft Coordinates   4

3.2   J2000 Coordinates 5

3.3   Mercury Solar Orbital (MSO) Coordinates   5

3.4   Mercury Body Fixed (MBF) Coordinates   5

3.5   Radial-Tangential-Normal (RTN) Coordinates   6

4  SPICE Kernels  6

5  Time Latency Correction 7

6  Heater Correction 8

7  Absolute Calibration and Relative Alignment  9

8  UTC Conversion 10

9  Spacecraft Position Determination   10

9.1   Cartesian Coordinates   10

9.2   Spherical Coordinates   11

10 Coordinate System Transformation 11

11 Data Quality   12

12 Version History   13

13 References  14



1 

Purpose

This document provides a preliminary description of the conversion of

MESSENGER Magnetometer 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. This document will be

updated as appropriate when major increments in CDR generation occur and

which warrant re-delivery to PDS and will eventually be replaced by the peer-

reviewed in-flight calibration description.



2 Introduction

The Science EDRs are the raw data records used to derive magnetic field data

used for scientific analysis. The Science EDRs contain 3-axis field samples

from the magnetometer at the commanded sample rate as well as the Mission

Elapsed Time (MET) and a range flag indicating the dynamic range the

magnetometer operated in at the time of the observation. There are two

dynamic ranges, a fine range of ?1,530 nT (range flag 0) and a coarse range

of ?51,300 nT (range flag 1). Before the science data can be used for

scientific analysis, the count rates in the EDRs must be converted to

physical units and the data must be transformed into meaningful physical

reference systems. This conversion yields calibrated data which are stored in

Calibrated Data Records or CDRs. The processing steps from the EDR to the CDR

level are described in this document and include:

(1) the accounting of time latency between the registered and actual times of

the observation and conversion from spacecraft mission elapsed time (MET) to

UTC;

(2) subtraction of the magnetometer DC offsets for the three axes; 

(3) conversion from engineering units to physical units; 

(4) coordinate transformation from sensor to spacecraft coordinates and to

other physical interplanetary and planetary reference frames;

(5) assignment of a data quality flag to the observations. 



3 Coordinate Systems

The calibrated data are transformed into several coordinate systems necessary

for scientific analysis: J2000 inertial, Mercury Solar Orbital (MSO), Mercury

Body Fixed (MBF), and Radial-Tangential-Normal (RTN). These coordinate

systems are defined as follows. Position data in each system is included in

the CDR products for J2000, MSO, MBF, and RTN coordinates.

3.1 Sensor and Spacecraft Coordinates

Sensor and spacecraft coordinates are defined as shown in Figure 1. In the

spacecraft coordinate system, the Y-axis is parallel to the magnetometer boom

axis with +Y directed from the spacecraft toward the MAG sensor; the X axis

is parallel to the solar array rotation axis; the Z axis is orthogonal to X

and Y and +Z points outward from the spacecraft adapter ring, and the +X

direction completes the right handed system. The sensor axes are oriented

nearly parallel to the spacecraft axes. The transformation between the two

coordinate systems is reported in the magnetometer instrument paper [Anderson

et al., 2007]. CDRs are provided in both sensor and spacecraft coordinates.

The CDR products also include the magnetometer range and sample rate.



3.2 J2000 Coordinates

In this coordinate system, the +X-axis points toward the mean vernal equinox,

+Z points along the mean rotation axis of the Earth on 1 Jan 2000 at

12:00:00.00 Barycentric Dynamical Time (TDB), which corresponds to JD

2451545.0 TDB, and Y completes the right-handed system.



3.3 Mercury Solar Orbital (MSO) Coordinates

In this coordinate system, the +X-axis points from Mercury center toward the

Sun, +Z points northward perpendicular to Mercury's orbit plane, and Y

completes the right hand system nominally directed opposite Mercury's orbital

velocity around the Sun. SPICE constructs the MSO coordinate system in the

following manner: it computes the position of the Sun as viewed from Mercury

and labels this as the +X-axis. Then it computes the projection of the

velocity of Mercury as viewed from the Sun into the plane normal to the

already defined X-axis and forces this to be the -Y-axis of the MSO frame.

The Z-axis is defined by completing the right-handed axes triple (i.e. the

cross product of X with Y). The Venus-centered Venus solar orbital (VSO)

coordinate system is similarly defined.



3.4 Mercury Body Fixed (MBF) Coordinates

The MBF coordinate system is defined by the planetocentric position,

Cartesian X, Y, Z coordinates related to the planetocentric distance, r, the

latitude, ?, measured positive northward from the equator, and the longitude,

?, measured positive eastward from the prime meridian. The cartesian X, Y, Z

coordinates are: xMBF = r*cos(?)*cos(?), yMBF = r*sin(?)*cos(?), zMBF =

r*sin(?). The unit eMBF components, exMBF, eyMBF and ezMBF are defined simply

as: exMBF = XMBF/r, eyMBF = YMBF/r, ezMBF = ZMBF/r.

3.5 Radial-Tangential-Normal (RTN) Coordinates

In RTN coordinates, R points from Sun center to the spacecraft. T is formed

by the cross product of the solar rotation axis and R and lies in the solar

equatorial plane. N is formed by the cross product of R and T and is the

projection of the solar rotational axis on the plane of the sky.



4 SPICE Kernels

The MESSENGER project has adopted the SPICE information system to assist

science and engineering planning and analysis. SPICE is developed by the

Navigation and Ancillary Information Facility (NAIF) under the directions of

NASA's Science Directorate. The SPICE toolkit is available in FORTRAN and C

at the NAIF web site (http://naif.jpl.nasa.gov). Interfaces to higher-level

data analysis software in the Interactive Data Language (IDL) and Matlab are

also provided. The MESSENGER MAG CDR processing routines are written

exclusively in IDL using the toolkit provided by NAIF.



The primary SPICE data sets are kernels. SPICE kernels are composed of

navigation and other ancillary information structured and formatted for easy

access. SPICE kernels are generated by the most knowledgeable technical

contacts for each element of information. Definitions for kernels include or

are accompanied by metadata, consistent with flight project data system

standards, which provide pedigree and other descriptive information needed by

prospective users.



The following SPICE kernel files will be used to compute the UTC time and

geometric quantities for the MESSENGER MAG instrument:

1. MESSENGER spacecraft ephemeris file, also known as the planetary

spacecraft ephemeris kernel (SPK) file. The entire mission trajectory of

reconstructed and predicted positions is contained in

msgr_20040803_20120401_od###sc.bsp, where ### is the version number of the

kernel.

2. MESSENGER spacecraft orientation file, also known as the attitude c-kernel

(CK) file. There is one c-kernel for each day contained in the files named

msgryyyyddd.bc, where yyyy is the 4-digit year, and ddd is the 3-digit doy.

3. MESSENGER reference frame file, also known as the frame kernel. The kernel

msgr_v###.tf, where ### is the 3-digit version number, contains the MESSENGER

spacecraft, science instrument, and communication antennae frame definitions.

The coordinate system required for scientific analysis MSO and MBF are

defined as MSGR_MSO and MSGR_MBF in the kernel msgr_dyn_v###.tf, where ### is

the 3-digit version number. The VSO coordinate system is defined as MSGR_VSO.

4. MESSENGER instrument kernel (I-kernel). The kernel msgr_mag_v###.ti, where

### is the 3-digit version number, contains references to mounting alignment,

operating modes, and timing as well as internal and field of view geometry

for the MESSENGER Magnetometer.

5. MESSENGER spacecraft clock coefficients file, also known as the spacecraft

clock kernel (SCLK) file. The clock kernels are named messenger_###.tsc,

where ### is the 3-digit version number.

6. Planetary constants file (*.tpc), also known as the planetary constants

kernel (pck) file. The current planetary constants kernel is pck00008.tpc.

7. NAIF leap seconds kernel (LSK) file. This kernel is used in conjunction

with the SCLK kernel to convert between Universal Time Coordinated (UTC) and

MESSENGER Mission Elapsed Time (MET). The current leap seconds kernel is

naif0008.tls.



Those SPICE kernel files that reflect the spacecraft trajectory, attitude or

variable instrument states will be generated throughout the mission with a

file-naming convention specified by the MESSENGER project. The variable

kernels are updated frequently, and to obtain the most current information

for time conversion, coordinate transformation, and the spacecraft position,

the most current kernels are loaded prior to processing the EDR files. An

automated procedure is used to identify the most current kernel set from

information in the kernel file names. This kernel set is listed in the

release notes for the produced CDR.



5 Time Latency Correction

As described in Anderson et al. [2007], the time stamps in the EDR records

are delayed with respect to the actual time of the magnetic field

observations due to onboard filtering of the data and intrinsic delay in the

instrument feedback response. The net time lag depends on the sample rate and

is given in Table 1 from Anderson et al. [2007]. These values listed in the

"net lag" column are subtracted from the MET associated with the vector

sample in the EDR.



6 Heater Correction

A contamination signal associated with the magnetometer heater operation is

superimposed on the magnetometer data. An extensive discussion of the origin

of this contamination signal is beyond the scope of this document and will be

covered in the instrument in-flight calibration paper. The contamination is

manifested as an offset shift with the periodicity of the 100-s heater

control period. The magnitude and waveform of the offset variations is

different for each of the three axes and additionally depends on the heater

duty cycle. At the date of this document, the heater correction has been

characterized only for the nominal heater duty cycle with the sensor in

darkness behind the spacecraft relative to the Sun.



During cruise the sensor is in darkness and the heater typically operates at

the duty cycle of 40%, i.e., the heater operates at full power for a 40-s

period, which is followed by a 60-s interval with the heater turned off. To

allow correction of the heater contamination, the heater state (on/off) is

transmitted in the MAG Science Header Data (SHD) EDRs at a time resolution of

one second corresponding to the one-second command iteration at which this

state can be changed. Using these heater state data, the waveform of the

contamination signal in all three axes for this particular duty cycle has

been determined using superposed epoch analysis and is subtracted from the

count rates in the Science EDRs. The correction waveforms are provided in the

file sc_htr_fits.txt. Time in these files is given by  xhtr which ranges from

-1 to 1, corresponding to the time between consecutive heater state

transition from off to on. The correction waveforms are applied with a

temporal offset to account for a delay between the MAG flight software

requesting a change in the heater state and execution of the request by the

spacecraft. The magnitude of the timing offset is 10 s plus the latency

associated with the current sampling rate (see Section 5). For final

calibration of the data, a detailed study of the contamination signal as

function of duty cycle will be carried out during cruise.



7 Absolute Calibration and Relative Alignment

The conversion from counts to physical units of nano-Tesla is described by

Anderson et al. [2007]. Denoting the readings (counts) in the X, Y, and Z

axes by cx, cy, and cz and the fixed offsets as cx0, cy0, and cz0, the

magnetic field may be expressed in sensor coordinates as



where kx, ky, and kz are the gain coefficients for each axis and ?, ?, and ?

measure the contributions of X in the Y axis, X in the Z axis, and Y in the Z

axis, respectively. The parameters from the ground calibration are shown in

Table 2 [Anderson et al., 2007]. The instrument offsets are expected to vary

over the course of the mission. For this reason, the fine range offsets are

regularly assessed in-flight. The offset evaluations and respective METs are

listed in Table 3. The time-dependent offsets are captured in a look-up table

for CDR processing. Presently the offsets change abruptly at the METs given

in Table 3. In the future it is planned to let the offset vary continuously

to avoid discontinuities in the data set.















Table 3: MAG fine range offsets. MET Offset (cx0, cy0, cz0) [nT] 000000000.0

3.5,  0.3,  424.2 131133844.0 0.7, -22.3, 422.9

8 UTC Conversion

The UTC conversion from MET to UTC is handled by the SPICE toolkit using the

following IDL commands:



       cspice_scs2e, sc_id, met_str, et

       cspice_timout, et, 'YYYY DOY HR MN SC.###', 21, time_str

      

The routine cspice_scs2e converts the MET into ephemeris time et native to

SPICE, whereby the spacecraft ID for MESSENGER is -236. The routine

cspice_timout then convert the ephemeris time et to a UTC time string which

is associated with the observation.



9 Spacecraft Position Determination

9.1 Cartesian Coordinates

The spacecraft position for each MET is returned by the SPICE toolkit using

the following IDL commands:



       cspice_scs2e, sc_id, met_str, et

       cspice_spkpos, target, et, frame, correction, observer, position, ltime



The routine cspice_scs2e converts the MET into ephemeris time native to SPICE

using the MESSENGER spacecraft ID which is -236. The routine cspice_spkpos

then acquires the position of the spacecraft for the ephemeris time et given

the target, frame, light-time correction, and observer. Since we are

interested in the UTC at the spacecraft, the position of MESSENGER with

respect to the Sun in J2000 coordinates is obtained without light-time

correction using:



cspice_spkpos, 'MESSENGER', et, 'J2000', 'NONE', 'SUN', position, ltime



The position of MESSENGER in the Mercury-centric MSO coordinate system

without light-time correction can be obtained using:



cspice_spkpos, 'MESSENGER', et, 'MSGR_MSO', 'NONE', 'MERCURY', position, ltime



The position of MESSENGER in Mercury body-fixed coordinates without light-

time correction can be obtained using:



cspice_spkpos, 'MESSENGER', et, 'IAU_MERCURY', 'NONE', 'MERCURY', position,

ltime



9.2 Spherical Coordinates

When transforming the magnetic field samples to Radial-Tangential-Normal

(RTN) coordinates, the spacecraft coordinates are more appropriately

represented in the following spherical coordinates: (1) radial distance of

the MESSENGER spacecraft from the Sun in units of km; (2) northward latitude

of the MESSENGER spacecraft above instantaneous ecliptic plane in units of

degrees; (3) Azimuth angle of the MESSENGER spacecraft in the instantaneous

ecliptic plane with respect to the Earth-Sun line in units of degrees,

positive in direction of the Earth's orbital motion. The components are

computed as follows:



Radial component:



cspice_spkpos,'MESSENGER',et,pframe,'NONE',observer,m_pos,ltime

pos_r=cspice_vnorm(m_pos)

        

Northward Latitude:



cspice_spkezr,'EARTH',et,pframe,'NONE',observer,state,ltime

cspice_vcrss,state[0:2],state[3:5],es_norm

pos_lat=90-cspice_vsep(es_norm,m_pos)/!dtor

        

Azimuth:



cspice_vperp,state[3:5],state[0:2],ve_espl_proj

cspice_psv2pl,state[0:2],state[0:2],state[3:5],es_plane

cspice_vprjp,m_pos,es_plane,m_espl_proj

sign=sgn(cspice_vdot(ve_espl_proj,m_espl_proj))

pos_az=sign*cspice_vsep(state[0:2],m_espl_proj)/!dtor



10 Coordinate System Transformation

The coordinate transformation from MESSENGER MAG sensor coordinates to J2000,

MSO, and MBF coordinates is handled by the SPICE toolkit using the following

IDL commands:



cspice_scs2e, sc_id, met_str, et

cspice_pxform, frame1, frame2, et, xform

cspice_mxv, xform, b_sensor, b_rot



The routine cspice_scs2e converts the MET into ephemeris time native to

SPICE, using the spacecraft ID for MESSENGER, -236. The routine cspice_pxform

gives the transformation matrix between the coordinate system of origin,

frame1, and the target coordinate system, frame2, at a given ephemeris time,

et. The coordinate system of origin for MAG after boom deployment is

MSGR_MAG. The coordinate system of MAG prior to boom deployment is accessed

using MSGR_MAG_STOWED. The target coordinate systems for the CDRs are J2000,

MSGR_MSO, IAU_MERCURY, and MSGR_RTN for transformations into J2000, MSO, MBF,

and RTN coordinates, respectively. Finally, the cspice_pxform routine

performs the actual coordinate system transformation by multiplying the

transformation matrix with the observations in the coordinate system of

origin. For example, the transformation from the MAG sensor coordinate system

into MSO coordinates is executed using:



cspice_pxform, 'MSGR_MAG', 'MSGR_MSO', et, xform

cspice_mxv, xform, b_sensor, b_mso



11 Data Quality

The final step in the conversion of experimental to calibrated data records

is the assignment of the data quality flag to the observations. The MAG data

quality flag is a three digit code, denoted as SHC, which is defined in Table

4. S indicates the configuration of the sensor. H indicates the sensor

survival heater control mode being used. C indicates the presence of

contamination in the data and whether contamination, if present, is judged to

be correctable to meet the science requirement of 1 nT. For example, a data

quality flag of '100' indicates: that the boom was deployed with the sensor

facing the sun, that the heater operated in hardware regulation, and that no

contamination signals are known to be present. The data quality flag will be

assigned via a lookup table, which is maintained during validation of the

data set.





Table 4: MAG data quality flag definitions. S: Sensor Configuration

Definition 0 Sensor stowed - prior to boom deployment 1 Boom deployed - SC +Y

axis to Sun - sensor in sunlight 2 Boom deployed - SC -Y axis to Sun - sensor

in shadow  H: Heater Mode Definition 0 Hardware regulation (MAG FSW V8) 1

Software regulation version 1 (MAG FSW V9) 2 Software regulation version 2

(MAG FSW V10)  C: Contamination Definition 0 No contamination signals known

to be present 1 Uncorrectable contamination signals present 2 Contamination

signals present but correctable

12 Version History

The conversion from the EDR to the CDR level is evolutionary. CDR files will

be reprocessed if (1) the MAG calibration changes, (2) the MAG team finds it

necessary or desirable to change the file format, or (3) when bugs in the

processing software are identified. The reprocessed CDR files and the MAG

EDR2CDR processing software are both given new version number reflecting the

changes. Version numbers for CDRs and processing software may differ from

each other. For this reason the version number of the processing software is

captured in the CDR label. Below is the summary of the version history of the

MAG EDR2CDR processing software.



v1.0: 

* Initial Release



v1.1: 

* Increased precision of position angles for RTN files from F6.1 to F12.7.



v1.2: 

* BUG FIX: The drift between MAG and SC clocks resulted in falsely identified

sample rate changes and assigned latencies.

* Modified format of MAGTable_Offsets.dat and associated read routine.

* Added file names of SCI, SHD, and LAC EDRs to release notes file.

* Added MAG offset determination from M2 MAG rolls.



v1.3:

* Updated msgr_kernel_getcurrent.pro to deal work with 3 and 4-digit version

numbers.

* Fixed heater correction to include time lag between heater-on request and

actual heater turn-on. Affected routines are: msgr_mag_edr2cdr_sci.pro,

msgr_mag_correct_latency.pro, and msgr_mag_correct_heater100s.pro.

 

v1.4:

* Added processing of burst and AC channel data.

* Added handshake with SOC via semaphore file.



v1.5:

* Added handling of subversioned C-kernels.

* Added attitude rate change information to release notes.



v1.6:

* Added mechanism to automatically reprocess CDR files on arrival of updated

C-kernels.



13 References

Anderson, B. J. et al., The Magnetometer instrument on MESSENGER, Space Sci.

Rev., 131, 417-450, 2007.





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