Document title: Overview of Voyager magnetometer experiment
Project: VOYAGER
NDADS Datatype: VY2_MAG_IP_48S_ASCII          
NDADS Datatype: VY2_MAG_IP_SUM_VMSBIN
NDADS Datatype: VY1_MAG_IP_SUM_VMSBIN         
NDADS Datatype: VY1_MAG_IP_48S_ASCII          
Super-EID: DOCUMENT                      
There may be other documents also identified by this super-EID.
NDADS Filename: DOC_MAG_IP_VY_HIRES.TXT
TRF entry B46547.txt in NSSDC's controlled digital document library. Feb. 1998.
Document text follows:
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                                                        Version: 11/26/96 JFC

********************************************************************************

OVERVIEW OF VOYAGER MAGNETOMETER EXPERIMENT

The magnetic field experiment carried onboard the Voyager 1 and 2 missions
consists of dual low field (LFM) and high field magnetometer (HFM) systems.
The dual systems provide greater reliability and, in the case of the LFMs, 
permit the separation of spacecraft magnetic fields from the ambient fields.
Additional reliability is achieved through electronic redundancy.  The wide
dynamic ranges of +/- 0.5 G for the LFMs and =/- 20 G for the HFMs, low
quantization uncertainty of +/- 0.002 nT in the most sensitive +/- 8 nT
LFM range, low sensor RMS noise level of 0.006 nT, and use of data compaction
schemes to optimize the experiment information rate al combine to permit
the study of a broad spectrum of phenomena during the mission.  Objectives 
include the study of planetary fields at Jupiter, Saturn, Uranus and Neptune;
satellites of these planets; solar wind and satellite interactions with 
planetary fields; and the large-scale structure and microscale characteristics 
of the interplanetary magnetic field.  The interstellar field may also be 
measured.

Space Science Reviews, 21 (1977) 235-257.

DESCRIPTION OF VOYAGER MAG DATA SUBMISSIONS TO THE NSSDC

The high resolution data submitted to the NSSDC has its origins in the 
original MVS "Summary" data sets formally produced on the NSSC IBM MVS 
mainframe system.  The original data sets contain a mix of engineering, 
electron, magnetic field and plasma data.  The Voyager magnetometer (MAG)
experiment now resides on a dedicated workstation where only MAG data is
processed.  High resolution MAG data continues to use the same format as 
that of archived MVS data sets.

All Voyager MAG data submitted to the NSSDC consists wholly of high resolution 
LFM averages.  These files consist of a set of averages applied across all 
of the mission's differing telemetry modes.  In the case of the Magnetometer 
Experiment, the records contain both 1.92 second, 9.6 second and 48 second
averages.  1.92 second averages are created from the detail detail data,
9.6 second averages are created from 1.92 second averages and 48 second 
averages are created from the 9.6 second averages.  Thus, there are 25 1.92
second averages, 5 9.5 averages and one 48 second average per record.

In producing this submission to the NSSDC, the magnetic field records have 
been stripped from MVS "Summary" files and converted from MVS to VAX binary
representation.  File content remains unchanged, except that the header records
preceding and following each data record have been stripped. Since the main
function of these header records is to identify the interplanetary or
planetary coordinate system used to process the data, they are not needed
because all data in this NSSDC data set are interplanetary and in heliographic
coordinates (see below). A second, ASCII format data set containing key
components of the 48 second magnetic field data and ephemeris data has been
created allowing more convenient access to high resolution Voyager
magnetometer data.

COORDINATE SYSTEMS

Interplanetary magnetic field studies make use of two important coordinate
systems, the Inertial Heliographic (IHG) coordinate system and the 
Heliographic (HG) coordinate system.  

The IHG coordinate system is use to define the spacecraft's position.  The IHG 
system is defined with its origin at the Sun.  There are three orthogonal axes, 
X(IHG), Y(IHG), and Z(IHG).  The Z(IHG) axis points northward along the Sun's 
spin axis.  The X(IHG) - Y(IHG) plane lays in the solar equatorial plane.  The
intersection of the solar equatorial plane with the ecliptic plane defines a
line, the longitude of the ascending node, which is taken to be the X(IHG) axis.
The X(IHG) axis drifts slowly with time, approximately one degree per 72 years.

Magnetic field orientation is defined in relation to the spacecraft.  Drawing
a line from the Sun's center (IHG origin) to the spacecraft defines the X axis
of the HG coordinate system.  The HG coordinate system is defined with its 
origin centered at the spacecraft.  Three orthogonal axes are defined, X(HG), 
Y(HG), and Z(HG).  The X(HG) axis points radially away from the Sun and the
Y(HG) axis is parallel to the solar equatorial plane and therefore parallel 
to the X(IHG)-Y(IHG) plane too.  The Z(HG) axis is chosen to complete the 
orthonormal triad.  

An excellent reference guide with diagrams explaining the IHG and HG systems
may be found in Space and Science Reviews, Volume 39 (1984), pages 255-316, 
MHD Processes in the Outer Heliosphere,  L. F. Burlaga.

DATA FORMATS

Binary MAG data files usually contain two types of records. The first and last
records are header records containing a description of the parameters used to
process the Experimental Data Records (EDRs). The remaining  records contain
the field averages from the low field magnetometers (LFM). Only the LFM records
are included in the NSSDC data set.

--------------------------------------------------------------------------------

Each binary LFM record contains the following format:

VOYAGER LOW FIELD MAGNETOMETER (LFM) SUMMARY RECORD FORMAT

LFM Record Components

   1.  Header block -     32 words
   2.  MAG data block -  496 words
   3.  SEDR data block -  40 words
                         ---
                         568 words total (32 bit words)

Header Block Description

word type description
---- ---- -----------
1    A*4  Data Identifier ("HDR1" or "LFM ")
2    A*4  Telemetry Format ("ENG ","CR-1","CR-2","CR-3","CR-4","CR-5",
                            "CR-6","CR-7","CR5A","GS-2","GS-3","GS-4",
                            "GS-6","GS-7","GS-8","GS10","G10A","OC-1",
                            "OC-2","UV5A","****")
3    A*4  S/C Identification ("FLT1","FLT2","PTM ","SIM1","SIM2","UKWN")
4    I*2  Year of Data
4.5  I*2  Day of Year (Jan. 1 = 1)
5    I*2  Hour of Day
5.5  I*2  Minute of Hour
6    I*2  Second of Minute
6.5  I*2  Millisecond of Second
7    R*8  Decimal Day of Year (Jan. 1 = 0)
9    R*8  Decimal Day Count since August 20, 1977
11   A*4  Time Type (SCET = spacecraft event time, 
                     ERTS = start earth receive time)
12   R*4  Time period of data block (48 sec)
13   I*2  48 Minute Counter (2**16 sequence counter of data at time 
                             of telemetry readout.)
13.5 I*2  48 Second Counter (Modulo 60 sequence counter of data at time
                             of telemetry readout.)
14   I*2  60 Millisecond Counter (Line counter of data at time 
                                  of telemetry readout.)
14.5 I*2  Spare
15   Z*4  MAG Status Word
16   Z*4  MAG Command Word
17   I*2  Data Identifier (LFM=2,HDR1=8)
17.5 I*2  Data Rate (GS-X=0,CR-1=1,CR-2=2,CR-3=3,CR-4=4,CR-5=5,
                     CR-6=6,CR-7=9,ENG=8,CR5A=13,UV5A=13)  
                     * CR5A and UV5A are both VIM-5 modes
18   I*4  Spare
19   A*8  Data Confidence Flag (indicates reduced data confidence due to 
                                software/hardware intervention)
21   I*4  Spare
22   I*4  Spare
23   I*4  Spare
24   I*4  Spare
25   I*4  Spare
26   I*4  Spare
27   I*4  Spare
28   I*4  Spare
29   I*4  Spare
30   I*2  Number of primary data words
30.5 I*2  Number of secondary data words
31   I*2  Number of primary full words
31.5 I*2  Number of secondary full words
32   I*2  Record number in file
32.5 I*2  Number of words remaining in logical record

MAG Data Block Description

48 second averages
------------------
33   R*4  F1: 48 second average of 9.6 second norms (nT)
34   R*4  F2: Norm of 48 second component averages (nT) 
              F2 = sqrt(B1**2+B2**2+B3**2)
35   R*4  Latitudinal field angle = arcsin(B3/F2) (degrees)
36   R*4  Longitudinal field angle = 180 - atan(B2,-B1) (degrees)
37   R*4  B1: 48 second average of first 9.6 second averaged component (nT)
38   R*4  B2: 48 second average of second 9.6 second averaged component (nT)
39   R*4  B3: 48 second average of third 9.6 second averaged component (nT)
          (B1,B2,B3) = (BR,BT,BN) (HG coordinates)
40   R*4  RMS of B1 (nT)
41   R*4  RMS of B2 (nT)
42   R*4  RMS of B3 (nT)
43   I*4  Number of 9.6 averages per 48 second average (0<n<5)

9.6 second averages
-------------------
44-48  5(R*4)  F1: 9.6 second averages of 1.92 second norms (nT)
49-53  5(R*4)  F2: Norms of 9.6 second component averages (nT)
                   F2 = sqrt(B1**2+B2**2+B3**2)
54-58  5(R*4)  Latitudinal field angle = arcsin(B3/F2) (degrees)
59-63  5(R*4)  Longitudinal field angle = 180 - atan(B2,-B1) (degrees)
64-68  5(R*4)  B1: 9.6 second average of first 1.92 second averaged component (nT)
69-73  5(R*4)  B2: 9.6 second average of second 1.92 second averaged component (nT)
74-78  5(R*4)  B3: 9.6 second average of third 1.92 second averaged component (nT)
               (B1,B2,B3) = (BR,BT,BN) (HG coordinates)
79     R*4  RMS of B1 (nT)
80     R*4  RMS of B2 (nT)
81     R*4  RMS of B3 (nT)
.      .    .
.      .    .              5 sets of (B1,B2,B3) triplets
.      .    .
91     R*4  RMS of B1 (nT)
92     R*4  RMS of B2 (nT)
93     R*4  RMS of B3 (nT)
94-98  5(I*4)  Number of 1.92 averages per 9.6 second average (0<n<5)

1.92 second averages
--------------------
99-123  25(R*4)  F1: 1.92 second average of detail norms (nT)
124-148 25(R*4)  F2: Norm of 1.92 second component averages (nT) 
                     F2 = sqrt(B1**2+B2**2+B3**2)
149-173 25(R*4)  Latitudinal field angle = arcsin(B3/F2) (degrees)
174-198 25(R*4)  Longitudinal field angle = 180 - atan(B2,-B1) (degrees) 
199-223 25(R*4)  B1: 1.92 second average of first detail component (nT)
224-248 25(R*4)  B2: 1.92 second average of second detail component (nT)
249-273 25(R*4)  B3: 1.92 second average of third detail component (nT)
                 (B1,B2,B3) = (BR,BT,BN) (HG coordinates)
274     R*4  RMS of B1 (nT)
275     R*4  RMS of B2 (nT)
276     R*4  RMS of B3 (nT)
.       .    .
.       .    .              25 sets of (B1,B2,B3) triplets
.       .    .
346     R*4  RMS of B1 (nT)
347     R*4  RMS of B2 (nT)
348     R*4  RMS of B3 (nT)
349-373 I*4  Number of detail points per 1.92 second average (0<n<32)

S/C field averages
------------------
374-520 Z*4  spare words
521     R*4  RMS of first 48 second averaged s/c field component (nT)
522     R*4  RMS of second 48 second averaged s/c field component (nT)
523     R*4  RMS of third 48 second averaged s/c field component (nT)
524     I*4  Number of 9.6 s/c field averages per 48 second average (0<n<5)
525     R*4  48 second average of first s/c field component (nT)
526     R*4  48 second average of second s/c field component (nT)
527     R*4  48 second average of third s/c field component (nT)
528     R*4  Norm of 48 second s/c field component averages (nT)

SEDR Data Block Description
---------------------------
529-530 R*8  TN:  Epic day of navigation block "closest" to data time
531-532 R*8  TP:  Epic day of pointing vector block "closest" to data time
533     R*4  s/c X position component (A.U. or planetary radii *)
534     R*4  s/c Y position component (A.U. or planetary radii *)
535     R*4  s/c Z position component (A.U. or planetary radii *)
536     R*4  s/c X velocity component (km/s)
537     R*4  s/c Y velocity component (km/s)
538     R*4  s/c Z velocity component (km/s)
539     R*4  s/c distance R (A.U. or planetary radii *)
540     R*4  s/c longitude = asin(Z/R) (radians *)  
541     R*4  s/c latitiude = atan2(Y,X) (radians *)
542-550 R*4  Matrix 1 **
551-559 R*4  Matrix 2 **
560-568 R*4  Matrix 3 **

Notes:

* 
  Spacecraft position data is dependent upon the coordinate system selected
  during EDR processing. Valid coordinates are HG (heliographic), S3 (Jupiter planetary
  system), L1 (Saturn planetary system), U1 (Uranus planetary system) and
  N1 (Neptune planetary system). During cruise processing (HG), the spacecraft
  location is based upon the Inertial Heliographic system (IHG). During
  encounter processing (S3, L1, U1, N1), the spacecraft location is based upon 
  planocentric system coordinates. The specific coordinate system is usually
  indicated in header records which have been stripped from the NSSDC data set.
  The latter includes only interplanetary spacecraft coordinates in the IHG 
  system.

**
  IHG Coordinates
  ---------------

  Matrix 1 transforms three element vector from Inertial Heliographic to
           heliographic coordinates.

  Matrix 2 transforms three element vector from Earth Mean ecliptic, 
           Equinox of 1950 (EME 50) to heliographic coordinates.
           
  Matrix 3 transforms three element vector from payload coordinates to 
           heliographic coordinates.

  IMPORTANT!  Due to performance issues in the original MVS processing
              routines, matrices one and three are written as the transpose 
              of the above descriptions.  Rows and columns need be swapped
              before performing right hand matrix multiplications.

--------------------------------------------------------------------------------

VOYAGER 48 SECOND AVERAGED ASCII DATA FORMAT

field  descriptor
-----  ----------
  1.   s/c identification (FLT1=Voyager 1)
                          (FLT2=Voyager 2)

  2.   coordinate system  (PL= Payload)
                          (HG=heliographic) - used for this data set
                          (S3=Jupiter)
                          (L1=Saturn)
                          (U1=Uranus)
                          (N1=Neptune)

  3.   field magnitude (nT)
  4.   field component 1 (nT) - HG
  5.   field component 2 (nT) - HG
  6.   field component 3 (nT) - HG
  7.   spacecraft radial distance (AU)
  8.   spacecraft x position (AU) - IHG
  9.   spacecraft y position (AU) - IHG
 10.   spacecraft z position (AU) - IHG

---------------------------------------------------------------------------

Principal Investigator: 

Prof. Norman F. Ness
Bartol Research Institute
Univerity of Delaware
Newark, Delaware 19716-4793
Phone: (302) 831-8116
Fax: (302) 831-1843
Internet: norman.ness@mus.udel.edu

Data Set Contact: 

S. B. (Sandy) Kramer 
Hughes STX Corporation
Laboratory for Extraterrestrial Physics
Code 692.0
NASA Goddard Space Flight Center
Greenbelt, MD 20770
Phone: (301) 286-4957
Internet: z1sbk@lepvgr.gsfc.nasa.gov