**TWINS Explorer Mission of Opportunity**

**Project Data Management Plan (PDMP)**

October 2006

Document Number**: TWINS-PROJ-D008-A (REVISED)**

## TABLE OF CONTENTS

1.0. Introduction

2.0. Project Overview

3.0. Science Instrumentation

4.0. TWINS End-to-End Data Flow

##### APPROVAL SIGNATURES

David McComas, Principal Investigator

Southwest Research Institute

Lynn Friesen, Host Accommodation

The Aerospace Corporation

Jerry Goldstein, Document Editor and Science Operations Center Lead

Southwest Research Institute

Ruth Skoug, Document Originator and Co-I

Los Alamos National Laboratory

Edwin Grayzeck, Director

National Space Science Data Center

Mei-Ching Fok, TWINS Program Scientist

NASA Goddard Space Flight Center

Reginald Eason, TWINS Project Manager

NASA Goddard Space Flight Center

**TWINS PDMP Approval**

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David McComas Date

Principal Investigator

Southwest Research Institute

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\_\_\_\_\_\_\_\_\_\_\_

Lynn Friesen Date

Host Accommodation

The Aerospace Corporation

\_\_\_![](media/image1.png){width="1.5in"
height="0.6069444444444444in"}\_\_\_\_\_\_\_\_\_\_\_\_\_\_ [\_25 Oct
06\_\_]{.underline}

Jerry Goldstein Date

Science Operations Center Lead

Southwest Research Institute

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Ruth Skoug Date

Document Originator and Co-I

Los Alamos National Laboratory

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Edwin Grayzeck Date

Director

National Space Science Data Center

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Mei-Ching Fok Date

TWINS Program Scientist

NASA Goddard Space Flight Center

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Reginald Eason Date

TWINS Project Manager

NASA Goddard Space Flight Center

**[Revision History:]{.underline}**

+------+----------+-------+--------------------------------+---------+
| **   | **S      | *     | **Description of Change**      | ## Date |
| Affe | ection** | *Head |                                |         |
| cted |          | ing** |                                |         |
| Re   |          |       |                                |         |
| visi |          |       |                                |         |
| on** |          |       |                                |         |
+------+----------+-------+--------------------------------+---------+
|      | All      | All   | Changes to accommodate PI move | 11/     |
|      |          |       | from LANL to SwRI.             | 29/2000 |
+------+----------+-------+--------------------------------+---------+
|      | 3.1.3,   | Data  | Update number of direct events | 11/     |
|      | 4.3.1    | Acq   | to reflect current ISD values. | 29/2000 |
|      |          | Table |                                |         |
|      |          | 4.1   |                                |         |
+------+----------+-------+--------------------------------+---------+
|      | A        |       | New TWINS Program Scientist    | 11/     |
|      | pprovals |       |                                | 29/2000 |
+------+----------+-------+--------------------------------+---------+
|      | cover    |       | added Jerry Goldstein's name   | June    |
|      | pages    |       |                                | 2004    |
+------+----------+-------+--------------------------------+---------+
|      | 1.2      |       | Add Jerry Goldstein's name     | June    |
|      |          |       |                                | 2004    |
+------+----------+-------+--------------------------------+---------+
|      | 2.0, 2.3 |       | Revise projected launch        | June    |
|      |          |       | schedule                       | 2004    |
+------+----------+-------+--------------------------------+---------+
|      | 3.1.2.2  |       | Acknowledge probable overlap   | June    |
|      |          |       | of TWINS 1 and IMAGE missions  | 2004    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.2,   |       | Add Oracle database discussion | June    |
|      | 4.3.4,   |       |                                | 2004    |
|      | 4.4.1.   |       |                                |         |
|      | 4.4.3    |       |                                |         |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.6    |       | New section discussing         | June    |
|      |          |       | ancillary data products        | 2004    |
+------+----------+-------+--------------------------------+---------+
|      | Cover    |       | Minor changes to cover pages   | Oct     |
|      | pages    |       |                                | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 1.2      |       | Change of title for J.         | Oct     |
|      |          |       | Goldstein                      | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 2.0, 2.3 |       | Revise TWINS-1 launch and      | Oct     |
|      |          |       | projected data acquisition     | 2006    |
|      |          |       | schedule, TWINS-2 projected    |         |
|      |          |       | launch schedule.               |         |
+------+----------+-------+--------------------------------+---------+
|      | 2.3      |       | Brief mention of sunlight      | Oct     |
|      |          |       | exposure limitations.          | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 3.1.2.2  |       | Discuss IMAGE satellite        | Oct     |
|      |          |       | failure and revised TWINS-1    | 2006    |
|      |          |       | solo operations and science.   |         |
+------+----------+-------+--------------------------------+---------+
|      | 3.1.3    |       | Revise Table 3.1               | Oct     |
|      |          |       |                                | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 3.3      |       | New section added to discuss   | Oct     |
|      |          |       | sunlight exposure analysis     | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.1      |       | \"Data Delivery Hub            | Oct     |
|      |          |       | Processing\" List added        | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.1      |       | TWINS URL updated to current   | Oct     |
|      |          |       | site.                          | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3      |       | .Update Table 4.1 (for Level 0 | Oct     |
|      |          |       | data), New Table 4.2 (for      | 2006    |
|      |          |       | Level 1 data)                  |         |
+------+----------+-------+--------------------------------+---------+
|      | Various  |       | All mention of HDF format      | Oct     |
|      |          |       | removed and replaced with CDF. | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.3    |       | Update Level 0 Data            | Oct     |
|      |          |       | description.                   | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.4    |       | Update Level 1 Data            | Oct     |
|      |          |       | description                    | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.5    |       | Update Level 2 Data            | Oct     |
|      |          |       | description                    | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.3.6    |       | Update Ancillary data          | Oct     |
|      |          |       | description                    | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.4.0    |       | Update to Archiving Discussion | Oct     |
|      |          |       |                                | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.4.5    |       | Update Review Items            | Oct     |
|      |          |       |                                | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.4.3,   |       | Change numbering to Table 4.3  | Oct     |
|      | 4.4.5    |       | and Table 4.4                  | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | 4.4.2    |       | Update discussion of           | Oct     |
|      |          |       | NSSDC/SPDF                     | 2006    |
+------+----------+-------+--------------------------------+---------+
|      | Cover    |       | Add Reginald Eason as signees  | Oct     |
|      | pages    |       | (at request of Holmes)         | 2006    |
+------+----------+-------+--------------------------------+---------+

**1.0. Introduction**

This document describes the Project Data Management Plan (PDMP) for the
Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) mission. TWINS
is a Mission of Opportunity selected under NASA's Small Explorer (SMEX)
program. The TWINS PDMP is designed to be consistent with the TWINS
Program-Level Requirements Document (Appendix MO-2 of the TWINS Explorer
Program Plan).

1.1. Purpose and Scope

This data management plan describes the generation and delivery of TWINS
science data products, institutional responsibilities for data analysis,
and the transfer of archival data products (including data, data
documentation, and associated software) to the National Space Science
Data Center/Space Physics Data Facility (NSSDC/SPDF). Covered in this
plan are:

1\. Brief description of the instruments

2\. Description of the data flow

3\. Description of the science data products

4\. Processing requirements and facilities

5\. Policies for access and use of TWINS data

6\. Data product documentation

It is important to note that the TWINS telemetry system will be provided
by the spacecraft sponsor, and thus a detailed discussion of this topic
will not be included in this document.

1.2. PDMP Development, Maintenance, and Management Responsibility

The TWINS Principal Investigator (PI), Dr. David McComas and Science
Operation Center lead Dr. Jerry Goldstein, both at Southwest Research
Institute, are responsible for the development, maintenance, and
management of the PDMP throughout the TWINS mission. Data handling and
data distribution tasks are the responsibility of both LANL and SwRI
personnel. Data analysis tools and capabilities will be developed at
both SwRI and LANL. The TWINS PDMP will be modified and updated as
required in accordance with the TWINS Configuration Management Plan.

**2.0. Project Overview**

The TWINS mission was selected as a result of AO-97-OSS-03 for SMEX
missions. The Phase B study began in July 1998. The mission Preliminary
Design Review (PDR) and Confirmation Assessment Review (CAR) were held
on March 10, 1999. The mission Confirmation Review was held on April 12,
1999, and TWINS entered Phase C/D in May 1999. The mission Critical
Design Review (CDR) was held on February 23, 2000. The TWINS Flight
Model 1 (FM1) Pre-Environmental Review (PER) was held on March 27, 2002
and a combined FM1 Pre-Ship Review (PSR) and FM2 PER were held on August
24, 2004. The TWINS FM1 Mission Readiness Review (MRR) was held on
August 23, 2005. The FM2 PSR was held on February 28, 2006, and the FM2
MRR on April 5, 2006. A final TWINS Mission Readiness Brief was
successfully held at NASA Headquarters on May 23, 2006.

TWINS-FM1 was delivered to the Host contractor in early 2005 and
launched successfully into orbit in the summer of 2006. (The exact
launch date is unavailable.) As of the writing of this document,
high-voltage science data have not yet been acquired, although
instrument high voltage turn-on could occur at any time. Based on the
results of UV Sun exposure analysis (*TWINS UV Exposure Study,*
Aerospace Report No ATR-2006(7687)-1), it may be early 2007 before we
are in a optimal season for science data acquisition. (See Section 3.3.)

TWINS-FM2 was delivered to the Host contractor June 13, 2006 and is
scheduled for launch in 2007, with the two-year stereo mission beginning
in 2007 or 2008.

2.1. Science Objectives

The overall scientific objective of the TWINS project is to establish
the global connectivities and causal relationships between processes in
different regions of the Earth's magnetosphere. TWINS will address this
objective through stereoscopic neutral atom imaging of the magnetosphere
from two widely spaced, high-altitude, high-inclination spacecraft. By
imaging charge exchange neutral atoms over a broad energy range
(\~1--100 keV) using identical instruments on two spacecraft, TWINS will
enable the three-dimensional (3-D) visualization of the magnetosphere
and the resolution of large scale structures and dynamics within the
magnetosphere for the first time. In contrast to traditional in situ
measurements, TWINS will provide nearly continuous global, stereo
coverage of the magnetosphere.

Specific scientific questions to be addressed by TWINS include the
structure and evolution of the magnetosphere and the sources,
energization, transport, and sinks of magnetospheric plasma populations.
The first level of questions will be addressed by the 3-D images
themselves, and the second through inversion of the images to determine
the characteristics of magnetospheric ions.

2.2. Spacecraft Description

A description of the host spacecraft is not appropriate for the TWINS
Mission of Opportunity. We shall refer to these spacecraft as TWINS-1
and TWINS-2.

2.3. Mission Summary

A summary of the TWINS mission is given in Table 2.1. Mission operations
for TWINS will be provided by the spacecraft sponsor. Each spacecraft
will be placed in a Molniya orbit, and will be three-axis stabilized and
approximately nadir pointing.

In nominal operating mode, the TWINS instruments will acquire image data
with time resolution of 60 seconds. Between each image, a 12-second
period is required to change the actuator rotation platform travel
direction. No data will be collected during this time, giving a duty
cycle for data collection of 83%, and effective time resolution of 72
seconds.

Table 2.1. TWINS Mission Summary

+--------------------------------+-------------------------------------+
| Project Name                   | TWINS                               |
+--------------------------------+-------------------------------------+
| Number of Spacecraft           | 2                                   |
+--------------------------------+-------------------------------------+
| Orbit Description              | Molniya orbit                       |
|                                |                                     |
|                                | Inclination: 63.4˚                  |
|                                |                                     |
|                                | Apogee: 7.2 R~E~                    |
|                                |                                     |
|                                | Perigee: \~1000 km                  |
|                                |                                     |
|                                | Period: 12 hours                    |
+--------------------------------+-------------------------------------+
| Approximate Launch Dates       | TWINS-1: Summer 2006                |
|                                |                                     |
|                                | TWINS-2: Projected 2007 or 2008.    |
+--------------------------------+-------------------------------------+
| Launch Vehicle                 | Provided by spacecraft sponsor      |
+--------------------------------+-------------------------------------+
| Nominal Mission Duration       | 4 year design lifetime for each     |
|                                | instrument                          |
|                                |                                     |
|                                | 2 year stereo mission               |
+--------------------------------+-------------------------------------+
| Potential Mission Life         | Host spacecraft have 7 year design  |
|                                | lifetime                            |
+--------------------------------+-------------------------------------+
| Continuous Data Acquisition    | 50 kbps                             |
| Rate                           |                                     |
+--------------------------------+-------------------------------------+
| Attitude Control               | 3-axis stabilized                   |
+--------------------------------+-------------------------------------+

TWINS will nominally operate only during the apogee portion of each
orbit, when the spacecraft is above the radiation belts. Operations are
also planned for some perigee passes, providing low altitude
observations when the spacecraft is below the radiation belts.
Ultraviolet exposure analysis (*TWINS UV Exposure Study,* Aerospace
Report No ATR-2006(7687)-1), based in part on UV calibration (*TWINS
Calibration Procedure,* SwRI doc 1609-TCP-01) indicates that sunlight
exposure will constrain acquisition of science data such that
observation windows will fluctuate with season. (See Section 3.3.)

Telemetry for the TWINS mission will be received by the spacecraft
sponsor, who will extract raw data from the telemetry stream and send it
to the Aerospace Corporation (henceforth Aerospace) for processing into
Level 0 data. Level 0 data will then be sent to SwRI for processing of
Level 1 data (images). These data products will be posted on the World
Wide Web for use by the TWINS team, the scientific community, and the
public, and will be provided to the NSSDC for archiving. Higher level
data products will also be posted on the web and archived at the NSSDC,
as appropriate.

**3.0. Science Instrumentation**

The TWINS instrumentation consists of a neutral atom imager to obtain
images of the Earth's magnetosphere and a Lyman-α detector to measure
the density of the neutral hydrogen geocorona.

3.1. Neutral Atom Imager

3.1.1. Instrument Description

The TWINS neutral atom imager is based on the MENA imager developed for
the IMAGE mission. TWINS combines two sensor heads to provide a field of
view of ±60˚ about the rotation direction (10˚ off nadir pointing). The
instrument uses alternating biased and grounded collimator plates to
reject charged particles and set the instantaneous azimuthal field of
view. A carbon foil is used to produce secondary electrons for time of
flight (TOF) measurement, and a position sensitive anode provides
angular determination. Gold transmission gratings are used to block
ambient ultraviolet light, to which the detectors are sensitive.
Velocity analysis (from TOF measurements) combined with pulse-height
analysis provides statistical mass determination. Since the spacecraft
for this mission of opportunity are three-axis stabilized, the
instrument will be located on a rotating actuator platform to provide
360˚ azimuthal coverage. Figure 1.1 shows the layout of the TWINS
instrumentation package, including the ENA imager sensor heads, the
rotating actuator platform, and the single electronics box.

Figure 1.1. TWINS Instrument Layout

![](media/image2.jpeg){width="5.4319444444444445in"
height="3.3756944444444446in"}

3.1.2. Capabilities and Requirements

3.1.2.1. Minimum Science Requirements

The minimum performance requirements for the TWINS neutral atom imagers,
taken from the TWINS Program-Level Requirements Document, are given
below. The neutral atom imagers on both TWINS spacecraft are required to
meet these minimum requirements in order to provide a stereo neutral
atom imaging mission.

3.1.2.1.1. TWINS shall obtain at least 300 stereoscopic neutral atom
images simultaneously from two independent spacecraft platforms over a
period including at least 3 geomagnetic storms and/or substorms.

3.1.2.1.2. Each TWINS instrument shall acquire images of the
magnetosphere which cover a field of view of at least 2 steradians
centered about the nadir direction.

3.1.2.1.3. TWINS shall measure the energy spectrum of magnetospheric
hydrogen at energies from 1--50 keV with energy resolution of ∆E/E ≤
1.0.

3.1.2.1.4. TWINS shall resolve the development of substorms by measuring
neutral atom images with 12-minute resolution.

3.1.2.1.5. TWINS shall measure the spatial distribution of
magnetospheric hydrogen with resolution of at least 12˚ x 12˚ for
energies above 10 keV.

3.1.2.2. Expected Performance Levels

Table 3.1 gives the expected performance levels for the TWINS neutral
atom imagers. These specifications are in agreement with the Baseline
Science Requirements listed in the TWINS Program-Level Requirements
Document. TWINS is designed to achieve a two-year stereo imaging
mission. As stated in the Baseline Science Requirements, all scientific
goals can be met by the acquisition of at least 3,000 stereoscopic
neutral atom images simultaneously from the two TWINS spacecraft over a
period including at least 30 geomagnetic storms and/or substorms.
However, the TWINS mission plan is to collect as much data as possible
over both the two years of single spacecraft imaging and the two years
of stereo imaging, and perhaps beyond if circumstances permit. It was
originally hoped that there would be overlap between TWINS-1 and IMAGE
satellite operations, allowing stereo imaging prior to the TWINS-2
launch. However, the IMAGE satellite stopped transmitting data in late
2005. Some failure scenario analysis suggests that in late 2007 when the
IMAGE orbit precesses to bring the satellite into full eclipse behind
the Earth, a full power down and reboot will occur, and as a result
IMAGE might resume data transmission. However, given the uncertainty of
this analysis, it is likely that stereo imaging will not be possible
until TWINS-2 data are available.

3.1.3. Data Acquisition

The TWINS neutral atom imager will nominally acquire data in only one
science mode. The TWINS data set will consist of two major products: 1)
neutral atom images in energy, species, and angle space, collected and
transmitted with 60-second time resolution every 72 seconds, and 2) raw
position, time of flight, and pulse height information for individual
events (direct events data). To allow transmission of a significant
number of raw events (1253 events per 4˚ azimuthal bin (1.333 sec)), the
image data will be compressed from 16 to 8 bits before transmission,
using a lossy compression scheme with error of less than 3% for all
instrument count rates. Details of the TWINS telemetry format can be
found in Appendix D of the TWINS Interface Specification Document (ISD).
Data from the two instrument heads will be transmitted separately and
combined on the ground, as will data from the two spacecraft.

Table 3.1. Expected Neutral Atom Imager Specifications

+----------------------------+-----------------------------------------+
| Measurement Type           | Neutral atom energy and composition     |
|                            | resolved images                         |
+----------------------------+-----------------------------------------+
| Geometric Factor           | 0.020 cm^2^sr                           |
+----------------------------+-----------------------------------------+
| Polar angle resolution     | 4˚ FWHM for \> 10 keV hydrogen          |
+----------------------------+-----------------------------------------+
| Azimuthal angle resolution | 4˚ FWHM                                 |
+----------------------------+-----------------------------------------+
| Instantaneous FOV          | ±60˚ x ±2˚ FWHM                         |
+----------------------------+-----------------------------------------+
| Total FOV                  | ≥ 3.43 sr centered about axis 10º from  |
|                            | nadir                                   |
+----------------------------+-----------------------------------------+
| Energy range               | 1--100 keV hydrogen                     |
+----------------------------+-----------------------------------------+
| Energy resolution (∆E/E)   | 0.4                                     |
+----------------------------+-----------------------------------------+
| Mass identification        | H, O                                    |
+----------------------------+-----------------------------------------+
| Time resolution            | 60 sec image / 72 sec cadence           |
+----------------------------+-----------------------------------------+
| UV rejection               | 4 x 10^-8^ (MENA was in the range       |
|                            | 10^-6^)                                 |
+----------------------------+-----------------------------------------+
| Data Rate                  | \~50 kbps continuous data rate,         |
|                            |                                         |
|                            | \~1/3 for image data, 2/3 for raw       |
|                            | single events                           |
+----------------------------+-----------------------------------------+

3.2. Lyman-α Detector

3.2.1. Instrument Description

The TWINS Lyman-α Detector consists of two independent sensors oriented
at angles of ±40˚ with respect to the actuator spin axis. Each sensor
has a FWHM field of view of 4˚, defined by collimation hole baffles,
uses Lyman-α interference filters as narrow band transmissions filters,
and applies a KBr or CsI photodiode for photon detection. The Lyman-α
detector is located on the rotating actuator platform to provide full
360˚ azimuthal angle coverage. The Lyman-α detector is shown in the
instrument layout diagram given in Figure 1.1.

3.2.2. Capabilities and Requirements

The Lyman-α detector is not included in the minimum science requirements
for TWINS as defined in the TWINS Program-Level Requirements Document.
Table 3.2 gives the expected performance characteristics of this
instrument.

Table 3.2. Expected Lyman-α Detector Specifications

  ----------------------------------- -----------------------------------
  Measurement Type                    Geocoronal Lyman-α

  Aperture Area                       3.2 cm^2^

  FOV                                 4˚ FWHM

  Wavelength                          1216 Å

  Wavelength resolution               ±50 Å

  Time resolution                     60 sec / 72 sec cadence

  Sensitivity                         0.1-0.5 counts/sec/R

  Data Rate                           40 bps continuous data rate
  ----------------------------------- -----------------------------------

3.2.3. Data Acquisition

Lyman-α data are acquired in only one mode. Data from each sensor are
collected in 90 azimuthal angle bins, each 2˚ wide (0.667 sec), with two
Lyman-α azimuthal bins for each neutral atom imager azimuthal bin. As is
done for the neutral atom image data, a Lyman-α image will be collected
with 60-second time resolution every 72 seconds.

3.3. Ultraviolet (UV) Sunlight Exposure Analysis

Guided by the IMAGE MENA instrument response to sunlight at various
imaging and spin angles, some limited ultraviolet exposure experiments
were conducted as part of TWINS-1 and TWINS-2 calibration (*TWINS
Calibration Procedure,* SwRI doc 1609-TCP-01). Based on this measured
angular response to UV, it is expected that during several months of the
year, sunlight exposure will partially constrain ENA observation times,
and that this limitation will become severe for perhaps 1 to 2 months
each year. Thus, we expect that each of the TWINS satellites will have
\"good\" and \"bad\" seasons for observation.

Using TWINS calibration UV testing results (*TWINS Calibration
Procedure*, SwRI doc 1609-TCP-01) and TWINS-1 ephemeris information, UV
sunlight exposure analysis (*TWINS UV Exposure Study,* Aerospace Report
No ATR-2006(7687)-1) indicates that TWINS-1 will view the Sun over
extended intervals near apogee during the North American winter (\"bad\"
viewing season), and that North American summer (\"good\" viewing
season) is best for uncontaminated viewing. We do not have a firm
projected launch date for TWINS-2, but it is expected that there will be
sufficient overlap between the respective optimal viewing seasons of
TWINS-1 and TWINS-2 to achieve our science goals. Sunlight exposure
studies will be updated periodically during the TWINS mission.

**4.0. TWINS End-to-End Data Flow**

4.1. Overview

Figure 4.1 shows the flow of TWINS data from the spacecraft through the
archiving process. This section describes the processing steps required
for the TWINS data. TWINS data will be obtained from the spacecraft
through the spacecraft sponsor, who will forward the data to Aerospace
for processing. Further processing will then be performed at SwRI, at
LANL, and by other members of the TWINS team. The resulting products
will be made available on the TWINS web site, and will be forwarded to
the NSSDC for permanent archiving and public distribution. The TWINS
team will maintain a web page providing information about the TWINS
mission, including information about data types and accessibility. This
web page is presently located at: http://twinstest.space.swri.edu/.

The following list summarizes the various processes that will occur
whenever data are delivered from Aerospace.

> 4.1.1. Get the Level 0 Data (see Table 4.1): TWINS telemetry data, and
> Attitude and Ephemeris data will be acquired from Aerospace (every
> 7-14 days) on CD or DVD.
>
> 4.1.2. SwRI will make a copy of the Aerospace CD/DVD and ship it to
> LANL for backup purposes.
>
> 4.1.3. The Level 0 Data will be imported into the database (Section
> 4.3.2), and validated.
>
> 4.1.4. The Level 0 data will be read out of the database, put into CDF
> format and transmitted (or shipped) to NSSDC.
>
> 4.1.5. Lyman-α data files will be generated from the database.
>
> 4.1.6. Summary plots will be generated and imported into the database.

The steps 4.1.1---4.1.6 above need to be performed whenever new data are
delivered to SwRI. Individual steps will need to be performed
iteratively if we update plot times, formats, calibration look-up tables
(LUTs), etc.

4.2. Mission and Science Operations

4.2.1. Telemetry Services

Mission operations for TWINS, including spacecraft tracking, data
downlink, and uplink communications with the spacecraft, will be
provided by the spacecraft sponsor. A detailed discussion of spacecraft
telemetry is therefore not appropriate for this document.

The spacecraft sponsor will receive the raw telemetry on the ground, and
will extract the TWINS data and provide it to a dedicated TWINS data
processing system, referred to as the "TWINS Ground Station", maintained
by Aerospace. The spacecraft sponsor will also provide spacecraft
attitude, ephemeris, and timing information to the TWINS Ground Station
for inclusion in the TWINS data set. Aerospace will then be responsible
for preliminary processing of the data, consisting of organizing the raw
data by viewing direction and incorporating ephemeris and attitude data
for the TWINS instrument, and of preliminary storage and distribution of
these data. The data produced by this procedure will be defined as Level
0 data. Starting from this point, all TWINS data will be open for
scientific usage with no restrictions. Raw (pre-Level 0) data will be
stored by Aerospace for the period required to determine that Level 0
data are being generated correctly. The TWINS telemetry rate will be 50
kbps. This data rate corresponds to an annual raw data volume (not
including attitude and ephemeris data) of 197.1 GBytes per year for each
spacecraft.

4.2.2. Mission and Science Control

Because access to the TWINS spacecraft is done through the TWINS
spacecraft sponsor, the TWINS mission is designed to require minimum
contact with the ground. We intend that the instrument will function
autonomously throughout its lifetime, and that at most times will
operate in a single mode, reducing the need for instrument commanding.
Instrument commanding can be accommodated by the spacecraft sponsor as
necessary, and TWINS is designed to include test modes and safe modes,
as well as the nominal science data collection mode. During instrument
turn-on and check-out, more frequent communication with the spacecraft
will be available. Instrument health will be monitored through
examination of both the housekeeping and science data by personnel at
Aerospace, SwRI, and LANL.

Figure 4.1. TWINS Data Flow

4.3. Data Products

4.3.1. TWINS Data Set

The TWINS data set includes scientific data collected by the
instruments, instrument engineering and housekeeping data, and
instrument attitude and ephemeris data provided by the spacecraft. A
detailed description of the data products can be found in Section 5.2.2
and Appendix D of the TWINS ISD. The TWINS data products are described
in Tables 4.1 and 4.2.

Table 4.1. TWINS Level 0 data products (See Section 4.3.3.)

+-------------------+--------------------------------------------------+
| Data Product      | Description                                      |
+-------------------+--------------------------------------------------+
| ENA image data    | Sampled in 4˚ (1.333 sec) azimuthal pixels.      |
|                   | Identified by sensor head (2), species (3:       |
| (\"Onboard        | H,He,O), polar angle (32: 4˚ bins), and          |
| images\")         | quasi-logarithmic-spaced time of flight (TOF)    |
|                   | bins. Compressed from 16 to 8 bits prior to      |
|                   | transmission.                                    |
+-------------------+--------------------------------------------------+
| ENA direct events | 5-byte data containing raw position, TOF, and    |
| (\"DE\") data     | pulse height information for valid particle      |
|                   | events. Sampled in 4˚ (1.333 sec) azimuthal      |
|                   | sectors. Up to 1253 events transmitted per       |
|                   | sector in nominal operating mode.                |
+-------------------+--------------------------------------------------+
| Lyman-α data      | 16-bit data sampled in 2˚ (0.667 sec) pixels.    |
|                   | Identified by sensor (2).                        |
+-------------------+--------------------------------------------------+
| ENA singles data  | 26 16-bit counters, sampled every 1.333 sec.     |
|                   | These parameters include start, stop, and valid  |
|                   | counts, as well as overflow counters used for    |
|                   | engineering purposes.                            |
+-------------------+--------------------------------------------------+
| Instrument status | Instrument status parameters which are tightly   |
| data              | coupled to science data, and so are sampled      |
|                   | every 1.333 sec. These parameters include        |
|                   | azimuthal sector identification, sector angular  |
|                   | boundaries, and actuator voltages.               |
+-------------------+--------------------------------------------------+
| Housekeeping data | Instrument parameters required less frequently   |
|                   | than instrument status data. Sampled at least    |
|                   | once per 60-sec scan in normal operating mode.   |
+-------------------+--------------------------------------------------+
| Spacecraft        | Provided by the spacecraft to the TWINS Ground   |
| ephemeris data    | Station. Position information is required to     |
|                   | within at least 500 km for high altitude         |
|                   | observations, and to within TBD km for any       |
|                   | perigee operations, and shall be provided with   |
|                   | the highest resolution possible.                 |
+-------------------+--------------------------------------------------+
| Spacecraft        | Provided by the spacecraft to the TWINS Ground   |
| attitude data     | Station. Knowledge of the pointing direction of  |
|                   | the TWINS sensor heads is required with          |
|                   | resolution of at least 0.1˚. Data shall be       |
|                   | provided with time resolution of 1 minute.       |
+-------------------+--------------------------------------------------+

4.3.2. TWINS Data Management

TWINS data management will be accomplished using an Oracle database.
This system allows flexible storage and searching capabilities, and is
not tied to any specific data format. The Oracle database will be
accessed through a front-end user Web Interface (WI), described below.
The WI will be used for browsing data, for analysis, and for transfer of
data from SwRI to TWINS team members and the larger scientific
community. For archiving data at NSSDC, the baseline data format will be
CDF (the customary data format used by NSSDC) for digital data, and JPEG
or PNG for rasterized images. (For more details, see Section 4.4.)
Aerospace will mail DVDs of data to SwRI with an estimated latency of
two weeks or less. The format of the data will be as follows. TWINS
telemetry data will be in binary format defined by Aerospace (appendix D
of the TWINS ISD). Ephemeris data will be in ASCII text format defined
by Aerospace.

Table 4.2. TWINS Level 1 data products

  ---------------------- ------------------------------------------------
  Data Product           Description

  ENA direct events      Images created using the TWINS Image Maker (TIM)
  (\"D.E.\") Images      software, analogous to Onboard Images, but
                         created on the ground with more sophisticated
                         look-up tables (LUTs) for binning and assigning
                         fluxes, energies, and species.

  ENA D.E. Skymap Images ENA D.E. images, mapped to geophysical
                         coordinates and presented in a \"fish-eye\"
                         view, with the Earth in the center, dipole
                         magnetic field lines drawn in, and the Sun
                         direction indicated.

  ENA Onboard Skymap     Onboard images in geophysical coordinates as for
  Images                 D.E. images.

  Lyman-α Parameterized  Hydrogen geocorona model, a 30-parameter fit to
  Exospheric Model (PEM) observed Lyman-α line-of-sight intensities. PEM
                         should reproduce a full 3D distribution of
                         hydrogen density.

  Summary Plots          Various plots designed to give overviews of
                         TWINS data.
  ---------------------- ------------------------------------------------

4.3.3. Level 0 Data

The basic TWINS Level 0 data product will consist of raw instrument
counts data organized by viewing direction (both from the ENA imagers
and from the Lyman-α detectors), as well as Onboard Images (See Table
4.1). This procedure will be carried out by Aerospace, using a data
processing server dedicated to the TWINS mission. Aerospace will then
forward Level 0 data to SwRI (via DVDs or CDs, mailed) for further
processing. Aerospace will delete Level 0 data from its local servers
only upon confirmation that the current shipment of data has been
received and validated by SwRI. The Level 0 data set will also include
ENA imager single event data, instrument housekeeping data, and
spacecraft attitude and ephemeris data. Level 0 data will be provided to
SwRI as quickly as possible after transmission from TWINS to Aerospace,
with an expected latency period of no less than one day and no more than
one week. Once at SwRI, Level 0 data will be quickly incorporated into
the Oracle database and distributed to LANL and NSSDC for archiving (See
Section 4.1). Analysis will be performed to provide browse images as a
primary TWINS data product. From ingestion of TWINS calibration data,
the data ingestion process is estimated to have a latency of 10-20
minutes.

4.3.4. Level 1 Data

The goal of Level 1 data processing is to produce a data product that
can easily be used for scientific studies. Level 1 processing will take
raw (Level 0) data and create ENA images and a Lyman-α model.

4.3.4.1. ENA Images

Level 1 processing will take raw ENA direct events (D.E.) data and
create ENA images. LANL and SwRI have developed a software package
called TWINS Image Maker (TIM) that uses look-up tables (LUTs) to bin
D.E. data by angle, energy, and species, and to incorporate the
instrument response function. (The LUTs are based on calibration data,
and will be updated as necessary, as the mission proceeds and we learn
more about the data.) TIM creates images in instrument coordinates, and
then the skymap procedure maps TIM images onto a geomagnetic coordinate
view containing the Earth in the center, with dipole magnetic field
lines drawn in and the Sun direction indicated. TIM skymap images can be
easily compared to each other and to previous and complementary data
sets.

4.3.4.2. Lyman-α Model

Level 0 Lyman-α data will be used to constrain a 30-parameter fitting
function designed by the University of Bonn to emulate the Hodges
\[1994\] hydrogen density model of the terrestrial exosphere up to about
60,000 km (or about 10 Earth radii). The coefficients of this
Parameterized Exospheric Model (PEM), along with the full 3D
distribution they describe, will constitute Level 1 Lyman-α data.

4.3.4.3. Level 1 Data Access and Processing

Level 1 data will be accessible through the Web Interface (WI) to the
Oracle database. Also accessible through the WI will be analysis and/or
visualization software developed/used to create images. TWINS team
members who wish to download data from the database will have a choice
of formats for such transfer from the following list: ASCII (text), CDF,
JPEG and/or PNG, and Postscript (PS) format plots generated by the WI
software. Level 1 data processing will be performed at SwRI by accessing
the architecture of the Oracle database, using a data processing server
dedicated to the TWINS mission, and will occur immediately upon
ingestion into the database of Level 0 data at SwRI.

4.3.5. Level 2 and Higher Level Data Products

Higher level TWINS data products are being designed to illustrate
magnetospheric structures and dynamics. In particular, higher level
processing at SwRI and LANL will combine data from the two TWINS
spacecraft to produce stereo images of the magnetosphere. It is also
expected that many of the TWINS Co-Investigators will routinely generate
additional higher level data products. For example, the PEM model will
be combined with ENA D.E. Images (See Table 4.2) to constrain linear
inversion codes designed by Co-Investigators and collaborators at Johns
Hopkins Applied Physics Lab (JHUAPL) and Auburn University. These
inversions can yield global ion pressure and field-aligned current
distributions from single images, and can yield pitch-angle
distributions from stereo images.

These data products, and the analysis software developed to create them,
will be accessible through the Web Interface. SwRI will ensure that
analysis software is optimized for use by the server. SwRI will provide
software to TWINS team members on an as-is basis, with no promise to
support or modify the software for TWINS team members to use on their
own platforms.

4.3.6. Ancillary Data Products

The TWINS database, accessed by TWINS team members through the Web
Interface, will also include several useful ancillary data products such
as solar wind, geomagnetic indices, etc. Ancillary data will be ingested
into the database periodically via an automated updating routine. The
list of ancillary data products will evolve with usage and suggestions
by the TWINS team members. Ancillary data will be stored for periods of
time covered by the TWINS mission, and shall be acquired and updated
periodically by enabling automated routines to check for availability of
updated data on various remote servers.

4.4. Data Archiving and Distribution

4.4.0 Overview

The goal of archiving is to ensure that TWINS data are available to
interested researchers in perpetuity. Therefore, we have adopted a plan
in which TWINS data are archived at three separate locations:

> SwRI: Aerospace Level 0 data CDs and DVDs will be physically stored at
> SwRI. Electronic copies of the data will be stored in the Oracle
> database.
>
> LANL: Copies of the Aerospace CDs and DVDs will be shipped to LANL for
> physical storage.
>
> NSSDC: TWINS science data will be archived at NSSDC with the following
> format scheme:
>
> Level 0: CDF
>
> Level 1: CDF for digital data
>
> JPEG/PNG for pre-made images
>
> Level 2: CDF for digital data
>
> JPEG/PNG for pre-made images
>
> Compressed ASCII for software

More details of archiving are given in the following sections.

4.4.1. SwRI Data Archive

SwRI will be responsible for distribution of data to other members of
the TWINS team and for making the data available to the scientific
community and to the public. We anticipate that most data distribution
(to the public and scientific community, and to TWINS team members) will
take place through web access to the TWINS Oracle database system. TWINS
data archiving at SwRI will be through the database, in binary format.

All data will be archived at SwRI as soon after processing and
validation as possible. This time frame will range from hours to months
depending on the complexity of the specific data product. Data archiving
will include both raw, validated data (Level 0) and histograms corrected
for instrument response (Level 1). Higher level data products will also
be archived, including both single spacecraft and stereo images.

4.4.2. NSSDC/SPDF Archive

The TWINS Level 0, Level 1, and higher level data products will be
provided to the NSSDC/SPDF for permanent archive and additional public
distribution of the data. Our plan is to use the first two years of
single spacecraft observations as a testing and preparation period. By
the time the second spacecraft is launched and the primary mission phase
begins, we anticipate that delivery of Level 0 and Level 1 data to the
NSSDC/SPDF will occur within one month of collection, and that we will
meet or exceed this specification throughout the TWINS stereo mission.
Higher level data products will be provided with a time lag appropriate
to the individual product. The baseline format for submissions to the
NSSDC/SPDF is CDF.

Products provided to the NSSDC/SPDF will include not only the data
themselves, but also documentation as to the contents of the data files,
and, if appropriate, software to read and/or display the data.
Documentation for all products described in Section 4.3 will be provided
as necessary to ensure the long term independent usability of the data.
CDF format was selected because CDF files can be read by standard
software packages, and CDF is the current standard format employed by
NSSDC/SPDF

4.4.3. Archival Volume

Table 4.3 shows the estimated volume of data acquired by TWINS over the
nominal mission lifetime. The estimated raw data volume for TWINS, based
on the expected telemetry rate of 50 kbps, is 197.1 GBytes per year for
each spacecraft. Assuming a nominal four-year lifetime for the first
spacecraft and a two-year lifetime for the second spacecraft, the total
Level 0 data volume for the two TWINS spacecraft will be \~1185 GBytes.
Level 1 data products are estimated at 40% of the Level 0 volume, and
higher level products at 20% of the Level 0 volume. Based on these
estimates, the total TWINS data volume to be archived will be
approximately 1900 GBytes over the nominal mission lifetime.

Table 4.3. TWINS data volume estimates.

  ---------------------- ---------------- ---------------- ----------------
                         Level 0          Level 1          Higher Level

  TWINS data volume      1185 GBytes      475 GBytes       240 GBytes
  ---------------------- ---------------- ---------------- ----------------

This data volume will easily be accommodated by the TWINS Oracle
database, in terms of data storage, processing, and archiving. At the
nominal data 50 kbps data rate, Level 0 data is estimated at 540 MBytes
per day, requiring approximately 1--2 DVDs (or their equivalent) per
spacecraft per week for storage. The data volume will be reduced
somewhat since the instrument does not operate continuously (is off in
the radiation belts, for example), but attitude, ephemeris, and higher
level data products will offset this reduction. As a baseline, we plan
that a significant fraction of TWINS data will be continuously stored on
hard drives connected to SwRI computers, and will be accessible in this
form to the TWINS team, the NSSDC, and the scientific community. Data
will additionally be made available to both the TWINS team and the NSSDC
on DVDs, as described in Section 4.4.0.

It is expected that where possible, data transfers will be done
electronically, either through the WWW or using FTP. Although the data
volume is large, the sizes are comparable to files currently being
transferred. For example, daily files from the ACE spacecraft are \~100
MBytes in size, and require at most a few minutes to transfer. With
expected improvements in technology, transfer of the 5 times larger
TWINS files is not expected to require a significant amount of time. For
the occasional transfer of very large amounts of data, such as
submission of early mission data to the NSSDC, DVDs will be the
standard.

4.4.4. Data Rights and Rules for Data Use

All TWINS data (Level 0 and higher) are open to all scientists and the
public without restrictions of any kind. There are no proprietary
periods associated with any of the TWINS data products. All TWINS data
will be provided to the NSSDC for archiving.

4.4.5. Review Items

Because of the extended time period between the launch of TWINS-1 and
TWINS-2, various aspects of data processing and management will need to
be reviewed and (where necessary) updated on a rolling basis. Table 4.4
lists these items, all of which were discussed above.

Table 4.4. TWINS data system items to be reviewed after obtaining
TWINS-1 science data.

+-----------------------------------+----------------------------------+
| Item                              | Baseline Plan                    |
+-----------------------------------+----------------------------------+
| TWINS look-up tables (LUTs),      | Based on calibration data, but   |
| etc., including:                  | will be updated after launch to  |
|                                   | take account of knowledge gained |
| \- Culling                        | from flight data.                |
|                                   |                                  |
| \- Geometric factors              |                                  |
|                                   |                                  |
| \- Head-matching                  |                                  |
|                                   |                                  |
| \- angle LUTs                     |                                  |
|                                   |                                  |
| \- energy fits in TIM code        |                                  |
+-----------------------------------+----------------------------------+
| TWINS browse plots,               | We will review usefulness and    |
|                                   | optimize displays & formats      |
|                                   | (e.g., color tables).            |
+-----------------------------------+----------------------------------+
| TWINS inversion techniques        | Test and refine codes provided   |
|                                   | by, for example, JHUAPL and      |
|                                   | Auburn University                |
+-----------------------------------+----------------------------------+

We note that following the launch of TWINS-1, we have reviewed a number
of the items listed in previous versions of this document. Data will be
stored in an Oracle database, with a web interface allowing access to
data in ASCII or CDF formats, as well as to processed images. CDF format
has been selected for file transfer to the NSSDC. Electronic formats
will be used for data distribution where possible, with CD/DVD for
larger data volumes.