IMEX proposal http://ham.space.umn.edu/spacephys/imexg.html
For the science section of the IMEX proposal, click here

The Inner Magnetospheric Explorer (IMEX):
A NASA University Explorer Mission to Investigate the Dynamics
of the Major Geomagnetic Storms during the Coming Solar Maximum

Principal Investigator: John Wygant, University of Minnesota
(612) 626-8921 wygant@ham.space.umn.edu

Project Manager: Keith Goetz, University of Minnesota
(612) 624-3520 goetz@waves.space.umn.edu

Mission Description

The Inner Magnetosphere Explorer (IMEX) is designed to investigate the physical processes which produce the rapid acceleration of charged particles to very high energies, as well as their transport and eventual loss, during major geomagnetic storms. These acceleration processes are the response of the Earth's magnetosphere to coronal mass ejections (CMEs) in the solar wind, fast-slow interface regions, and their associated interplanetary shocks. IMEX, in a geosynchronous transfer orbit, will make high resolution in-situ measurements in the equatorial plane of the evolution of particle populations and the electric and magnetic fields generating that evolution. This orbit makes radial cuts through the inner magnetosphere where the acceleration processes are the strongest over orbital periods capable of resolving the main and recovery phases of the storms. Launch is planned for June, 2001 near the peak of the next solar maximum when these storms are the most intense and frequent.

Mission Implementation

The IMEX mission to explore the inner magnetosphere will follow on the successful University program called Student Nitric Oxide Explorer (SNOE). Like SNOE, the IMEX spacecraft will directly involve university level students in its design, construction, integration and operation. The SNOE program was conducted by the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP) in Boulder. SNOE was the first student built spacecraft in the STEDI program and, having recently been launched, is on its way to being a complete success.

IMEX will study the dynamics of the inner magnetosphere of the earth with a focused suite of modern instruments. We have selected a geosynchronous transfer orbit which will allow frequent passages through the regions of most interest. Building on the success of SNOE, IMEX will use a similar spacecraft bus, attitude control system, solar arrays and associated flight hardware. IMEX will be able to use much of the SNOE spacecraft management and operations teams and will inherit the “corporate” memory and infrastructure of the SNOE project. An experienced team, the use of proven designs of spacecraft, subsystem and instruments will allow high scientific return with minimal risk.

IMEX will be launched in mid-2001 (nominally 30 June 2001) from the Cape on a Titan 4 launcher into a geosynchronous transfer orbit (GTO). The IMEX team has identified a military payload which is bound for geosynchronous orbit. This payload will use the Titan IV to go to GTO with an Inertial Upper Stage (IUS) to raise its perigee to geosynchronous orbit. Because the launch capabilities of the Titan far exceed the primary payload (see Gorney letter, Appendix C), considerable mass and volume is available for a secondary payload such as IMEX. We will attach the 350 pound IMEX spacecraft to the side of the IUS where it will easily fit within the Titan’s faring (see figure G1). After the Titan has delivered the combined stack to GTO (350km by 6.6 Re), IMEX will be released from the IUS where it will begin its two year mission in the GTO orbit. A short time after the IMEX release, the IUS will propel the military payload into its desired geosynchronous orbit.

IMEX Spacecraft

The IMEX spacecraft mass is 350 pounds and is shown in figure G2 with its booms deployed and in figure G3 in its stowed for launch configuration. The IMEX spacecraft bus is a six sided structure 36 inches across from point to point. It will be approximately 30 inches tall and will consist of two major instrument decks and a mating ring for attachment to the launch vehicle. figure G4 shows an exploded view of the spacecraft with various spacecraft subsystems identified. The spacecraft will deploy a pair of opposing hinged booms, one boom will support the DC magnetometer and the other supports the triaxial search-coil magnetometers. Four wire antenna deployment units will deploy thin wire electric field antennas in the spin plane. The IMEX spacecraft will be spin stabilized at 10 rpm. Its spin axis will be controlled to point at the sun (±15°) which provides constant illumination for the six primary solar array wings. This orientation is chosen to allow roughly constant solar illumination on the electric field antennas. This prevents periodic shadowing of the wire antennas since the resulting enormous photoelectric signals can make low frequency electric field measurements impossible. IMEX uses magnetic torquers for attitude control. Attitude control manuevers will be performed during the twice daily perigee passages (when in the strongest part of the earth’s magnetic field). The spacecraft will be equipped with dual patch antennas and a 5 Watt S-band transmitter.

The structure selected for IMEX is very similar to the SNOE structure. The primary components are the adapter structure including a separation assembly, the central equipment and instrument mounting plates, solar array assemblies, and antennas. An additional mounting plate has been added to the SNOE design for the instruments. The on-orbit mass of SNOE was 254 pounds. With its additional instruments, shielding and mounting deck, conservative mass estimates give IMEX a weight of 350 pounds.

The spacecraft subsystems on IMEX will be very simple and, as a result, inexpensive to procure and test. We will use a proven CCSDS packet telemetry formatter system. The command processor will allow time-tagged commands such that both maneuvers and science commands can executed at times when the spacecraft is not in contact with the ground.

The operation of the IMEX spacecraft will be controlled by the Central Data Processor (CDP). The C+DH system which is the same as the one on SNOE mission, provides the commanding, verification, checking and decoding functions, science and housekeeping telemetry collection and formatting, command memory storage, solid state mass memory for TM data storage and the encoding and conditioning circuitry. The computer chosen for the IMEX mission is a system developed by the Aerospace Corporation for a radiation environment. It can be shielded to meet any additional mission requirements for accumulated total dose radiation and has minimal SEU and latch up vulnerability. The interface board will be a modified SNOE unit and provides a single interface to the science DPU. The CDP communicates with the instrument DPU via a simple RS-422 interface.

LASP will develop the flight software for the CDP. SNOE software will be reused because of the very similar mission operation requirements. A hardware watchdog timer will prevent the processor from getting stuck due to programming errors or the effects of the environment (e.g. discharges or radiation). New flight software for the IMEX mission will be written, tested and integrated in modular pieces to reduce development risk

The SNOE thermal design has been selected to provide the required operating environments over all the beta angles encountered during the mission. As the instruments will be operated in 100% duty cycle, the driving thermal requirement is operation in both eclipse and daylight. The design for the IMEX mission is a low-risk passive approach employing the use of blankets and paints. An allowance for resistive heater power is carried in the power budget, and the need for heaters will be examined during the study phase.

IMEX instruments


Subsystem

Mass

(kg)

Power

(Watts)

bit-rate

(bits/s)

IMEX DPU

3.5

3.5

10

FIELDS

4.3

6

0

Normal

0

0

1,288

Burst

0

0

1,000

Antennas (4)

8

0

0

Preamps (4)

.8

1.2

0

Searchcoils (3)

.75

.2

0

HEPEX

12

3

6,500

ESA

2.5

2.5

6,000

MAG

2.5

2

1,920

Total Estimated

34.35

18.4

16,718

Total Budget

40

25

17,000

The instruments will be assembled onto their own structural plate and mated to the spacecraft bus with a simple mechanical interface. The instruments are in the same thermal environment as the spacecraft bus subsystems. The instruments will be integrated on their structure at the University of Minnesota along with the instrument DPU. This separate integration allows co-evolution of the spacecraft and instruments and minimizes the interfaces between the spacecraft and the instrument complement. This concept was used on another successful LASP mission, Solar Mesosphere Explorer.

The various instruments (FIELDS, HEPEX, ESA and MAG) will interface to the central science DPU using simple interfaces of serial and parallel logic levels. These interfaces will be defined during the phase A study to allow instrument and DPU development to proceed as soon as the study phase is complete. The DPU will use a simple serial interface to send telemetry packets to the on board data handling computer (CDP) and another serial interface to receive commands words from the CDP.

A central DC-DC converter will provide regulated DC voltages to the various instruments. This approach will lead to a very simple spacecraft integration which reduces risk and enhances program resilience. The spacecraft power system will provide several switched power lines from the unregulated 28 Volt power bus. The common power converter will produce and regulate the required voltages for the operation of the science instruments. High voltages will be produced in the instruments which require them. Provisions will also be made for voltage and current monitoring.

We have used ample subsystem design and component heritage in a balance with mission science objectives to reduce mass, cost, and risk where appropriate. No new technology developments are required in the spacecraft subsystems. With the exception of the HEPEX magnet design, no new technology or development is required for the science instruments.

Although the instruments have equivalent or superior time resolution, sensitivity and field of view when compared to previous instruments in this region of space, a number of important compromises have been made to stay within the UNEX cost cap. First, we have traded mass for cost. For example, the mass spectrometer design uses less expensive technology with a heavy magnet. The number of instrument modes are extremely limited and focused on the science goals. This limits the costs associated with the DPU and development of both instrument and ground software. Our mass margin allows us to use shielding generously and limit rad-hard electronics purchases to the most crucial and sensitive devices. Many of the instruments, particularly FIELDS and ESA have a sounding rocket heritage. The developers of HEPEX also include team members with extensive sounding rocket backgrounds. The electric field experiment does not require high sampling rates or the complex swept frequency receivers found in more expensive instruments. Complex instrument software used in previous instruments (e.g. neural network plasma line tracking) is not required. As a result, the instruments proposed will focus explicitly on the specific mission goals at relatively low cost.

Communications

The NASA compatible communication subsystem, consists of a receiver/demodulator, transmitter/baseband unit, hemispherical patch antennas, coupler, RF switch and filter. The comm subsystem provides a traditional yet low cost approach used successfully on the SNOE spacecraft. The telecommunications subsystem will broadcast science and housekeeping telemetry and receive commands during all mission phases and all vehicle attitudes. The approach employed uses a pair of SNOE flight proven S-band patch omni-directional antennas, and a 5-watt S-band amplifier.

The design goal for the IMEX instruments requires an average bit-rate of 17,000 bits per second. Telemetry will be collected and stored in a solid state recorder (SSR) on the spacecraft. The recorder will be on the order of 1.5 Gbit to allow somewhat more than 24 hours of telemetry to be stored for later downlink. Ground contacts will be arranged once per day by the IMEX mission operations center (at LASP) to collect the data stored in the SSR. With no need for real-time contact, automatic contact handling software and contacts scheduled during daytime hours, the operations center will continue existing operations staffing with minimal incremental cost.

IMEX will use two low gain patch antennas rated at 5 Watts. Because the GTO orbit covers a wide range of distances and slant ranges, we will build IMEX with two downlink bit rates. When the spacecraft is close to the earth and slant ranges are less than 20,000km, the high bit rate mode can be used. Using high bit rate will give 400k data bits per second with Reed-Solomon and convolutional encoding. The downlink margin is conservative (3dB) with an S-band downlink into an 8 meter TOTS antenna with a bit error rate better than 1 in 106 bits. For contacts where the spacecraft is farther than 20,000km away, a low bit rate mode will provide 100k bits per second. The downlink rate of 400k allows a full day's data to be downlinked from the SSR in something less than one hour of ground contact time each day.

IMEX will require only a modest amount of command uploading. The spacecraft stored command table (SCT) will consist of a few hundred commands per day, most of which are used for attitude control. Science operations require only a few dozen commands in the SCT each day but will also require occasional microprocessor memory loads. Such memory loads are expected to be infrequent (on the order of a few per month) and will typically be between 100 and a few thousand commands. An uplink rate of 1000 bps will allow all spacecraft commanding to be completed (with substantial margin) within the typical one hour contact period.

Power

The six fixed IMEX solar arrays will produce 108 Watts of total electrical power at the beginning of the mission and, with a 20% reduction during the two year mission, will produce and estimated 86.4 Watts of power at the end of mission. Spacecraft average operating power (transmitting and collecting data but without heaters) is estimated to be 46.6 Watts which affords substantial margin for full operation at the end of the mission. During the study phase, we will examine various configurations of size, type and number of panels to optimize the configuration.

Power storage is needed during the periods of eclipse in the Earth’s shadow and is provided by 2 flight proven 4-amp-hr NiCd batteries. A typical earth eclipse time is 60 minutes giving a battery depth-of-discharge of approximately 30% which provides margin for longer eclipse times as well. This level will allow the batteries a predicted life of over three years. The EPDS is a simple direct energy transfer system to the bus subsystems. Battery charge is controlled by a 4 level V-T controller. Undervoltage and overcurrent monitoring is performed by the power control unit and controlled by programmable resistors. Non-essential bus loads are relay connected and removed in the event of an undervoltage/overcurrent condition. This straightforward design is both cost effective and simple, and has been flight proven by LASP on the SNOE and SME missions.

Launcher

IMEX will get to GTO as a secondary payload on the Titan 4 launch of a military payload. After the Titan insertion into GTO, IMEX will be released and the military payload will continue to a geostationary orbit. With the military payload, the launcher has a large amount of mass margin which allows up to 400 pounds of additional payload. The 350 pound IMEX spacecraft and its required support structures will have no difficulty fitting into the envelope. The Titan’s faring envelope is much larger than the IUS giving ample clearance. The 30 inch IMEX spacecraft will fit easily within the available shroud space. The primary military payload is scheduled for launch in mid-2001 which fits perfectly with the IMEX program. Preliminary studies suggest that the IMEX payload is a good match with this Titan launch. More detailed studies will be conducted during phase A (see Gorney endorsement letter in Appendix C).

Attitude Control

When it is released into the appropriate orbit by the Titan/IUS, IMEX will determine its attitude using a combination of sun and horizon sensors as well as an ACS magnetometer. With ground control of magnetic torquers during its first perigee passes, IMEX will orient itself with its solar array wings directed to the sun. Secondary solar arrays may be provided on all faces of the spacecraft to allow sufficient battery charging to allow for operation of the torquers during early orbit and attitude adjustment maneuvers. The optional secondary solar cells would also provide limited power should IMEX lose its attitude at some point during the mission but would not provide power in normal circumstances. Once it achieves the proper attitude with respect to the sun, IMEX will use the next several perigee passes to spin the spacecraft to the desired boom deployment spin rate. After deploying the magnetometer and search coil booms and adjusting the spin rate, the wire electric field antennas will be deployed. These deployments will reduce the spin rate to the desired on-orbit rotation rate of 10 rpm. Torquers will be used throughout the mission during perigee passes to adjust the spin rate as needed and to maintain the IMEX spin axis orientation with respect to the sun.

During the operational part of the mission, the spacecraft must be rotated about one degree per day to maintain solar pointing. This will be done with time tagged commands executed by the spacecraft stored command table to the magnetic torquer coils during the two perigee passes each day. Since these attitude control maneuvers will upset normal science measurements, a signal will be provided to the science DPU indicating that maneuvers are under way and measurements may be invalid. Power margins are sufficient to allow instruments to remain powered-on during maneuvers.

Preliminary analyses indicate that all of the attitude determination and control modes for the IMEX mission can be satisfied with the use of off-the-shelf, demonstrated SNOE flight hardware. Furthermore, the LASP operations center is currently operating two satellites in geosynchronous transfer orbits from its mission control center. The same techniques used for ACS on these satellites will be used for the IMEX mission.

In-flight attitude knowledge is also important for the IMEX instruments. The attitude determination and control system will be equipped with sun and horizon sensors such that the spacecraft can generate a spin phase pulse (e.g. 4096 clocks per spin) and a sun pulse at all points in the IMEX orbit. These spin phasing signals will be provided to the science DPU in order to coordinate the science measurements with the spacecraft attitude.

Radiation Model

Since IMEX will spend essentially all its time in the radiation belts, radiation is a serious concern. The instruments on which the IMEX instruments are based have flown successfully on spacecraft in similar radiation environments. Our studies have shown that the radiation spectrum in this environment is one where shielding can be quite effective – the equivalent of about 6 mm of aluminum has a substantial benefit in reducing total radiation dosage. We plan for a 40krad environment for most instruments after structural shielding and will procure parts accordingly. Since radiation hardened parts can be difficult or expensive to obtain we will use our considerable mass budget to build the spacecraft with substantial aluminum decks. We will fabricate instruments with more substantial structures and we will use tantalum spot shielding on our radiation sensitive parts. We will place our most sensitive components at locations and orientations on the spacecraft which afford the most protection. Co-I Blake of the Aerospace Corporation is a world expert on the subject of radiation effects on space systems. Minnesota maintains a radiation test facility with both Cesium and Cobalt sources and so we will be able to easily test parts for radiation damage as well as screen parts for radiation tolerance.

Since the normal operating mode is to store up to 24 hours of IMEX telemetry in the solid state recorder and since the recorder memory will be affected by the harsh radiation environment, we will incorporate a “memory scrubbing” algorithm in flight software to constantly clean the memory, correcting for single bit errors. Furthermore, both flight processors will include watchdog circuitry to provide a hardware reset in the event a processor becomes “stuck” for any reason. The spacecraft command handling system (running in software) will be augmented by a small bank of hardware commands. These commands will execute regardless of the state of the on-board command and data handling system. This will allow power switching commands to execute in the event the microprocessor latches-up.

Parts

Because of the severe radiation environment, we expect to use radiation hard or radiation tolerant parts in many areas of the spacecraft and the instruments. Parts such as processors, ROMS, RAMS, gates arrays and glue logic will be needed in several mission subsystems. As early as possible in phase B we will identify a small list of electronic parts which are common to several of the mission subsystems. When possible and especially when economically compelling, we will arrange for a “common buy” of such parts in order to reduce the cost and time needed to procure and test these important components.

Integration and Test

The IMEX instruments will be fabricated and tested at their home institutions. Following the completion of testing, calibrations and unit level environmental testing, the instruments will be delivered to Minnesota where they will be integrated with the flight data processing unit as well as the flight power converter. Instrument level I&T will continue until all instruments are fully integrated with the DPU. At this point, the integrated set of instruments will be ready for spacecraft integration.

The IMEX spacecraft will be assembled at the LASP facilities in Boulder, Colorado. After the spacecraft subsystems are integrated on the spacecraft, the instruments will be delivered and the integration and test phase will begin. After a short period of instrument I&T, the spacecraft assembly will be completed (solar arrays, blankets and close-out panels installed). Since the instruments will be delivered ready to fly (in terms of environmental testing), we plan a brief spacecraft level environmental test program consisting of EMC, TV/TB and vibration/acoustic. These spacecraft level tests will be performed at LASP facilities or at the subcontracted facilities at Ball Aerospace (also in Boulder).

LASP will validate the spacecraft performance through a formal systems-level performance verification and environmental test program. This will be performed on the configured spacecraft to further reduce risk through detection and correction of possible design or workmanship flaws not found during previous testing. LASP will verify, validate and fully characterize spacecraft performance at specified environments. Hardware and software will be verified both separately and as an integrated system.

LASP uses the Operations and Science Instrument Support (OASIS) system to perform I&T. OASIS is a LASP software product which will also be used for IMEX instrument development and mission operations. Inclusion of LASP operations personnel, software and equipment during I&T will allow a side by side verification test of the ground system. This will validate the IMEX ground system and provide training for LASP operations engineers. This method of using a unified software environment for development, test, and operations brings significant efficiencies to the program, as was demonstrated on WIND, SME and SNOE.

Spacecraft Cleanliness

The four instruments proposed are sensitive to various kinds of possible spacecraft interference. Electromagnetic interference (EMI), electromagnetic compatibility (EMC), electrostatic cleanliness (ESC) and magnetic cleanliness (MAG) are all serious concerns.

We will not develop a substantial, time consuming and expensive specification of spacecraft requirements for cleanliness. Instead, the needed cleanliness requirements will be defined by a small team of scientists and engineers. Formed in the study phase, this team will have at least one representative from each of the four instrument teams and several from the spacecraft builder. Mario Acuna of GSFC will participate as the magnetometer representative. Mario has long championed the idea that a good science spacecraft doesn't need to be expensive. Wygant, McFadden and Clemmons will attend to electrostatic cleanliness issues. Wygant and Goetz will define the electromagnetic interference plan. The PI and PM will make final decisions in matters regarding cost or schedule.

Working with science and spacecraft teams together we'll be able to build a spacecraft that allows the measurements by the most sensitive instruments in the world while at the same time remaining on schedule and under budget.

Launch Campaign

After the IMEX preship review (PSR), the spacecraft will be shipped to the Cape where launch processing will be completed. The IMEX instruments will be completely tested (for functionality) after shipment. When IMEX is ready for launch (and irrespective of the primary Titan payload), we will conduct a launch readiness review. On our current schedule, this is to take place about three months before the scheduled launch dates which gives us three months of scheduled slack. IMEX will then be mated to the IUS after which it will move to the pad for integration with the TITAN launch vehicle. On or about 30 June 2001, we expect to launch IMEX on its mission to GTO. While IMEX's science goals include observations during the period of maximum solar activity in 2001 and beyond, no other launch constraints exist for this mission. The launch date will be driven almost entirely by the primary military payload.

Payload Operations Center

The IMEX control center will be located at the University of Colorado LASP facility. The spacecraft will be controlled from LASP’s existing mission operations center in the University of Colorado Research Park in Boulder. LASP has acquired substantial experience over the past two decades in spacecraft mission operations. LASP successfully operated the Solar Mesosphere Explorer spacecraft from 1981 to 1989 and is currently operating the twin Space Technology Research Vehicles STRV-1A and STRV-1B. LASP has also developed the mission operations system for the SNOE spacecraft and is operating SNOE in its current space mission. The IMEX mission operations system will be built by the same team that developed the operations systems for STRV and SNOE, and IMEX will reuse much of the hardware and software from the current missions. This includes systems for real-time monitor, control and display; planning and scheduling; command generation; orbit prediction and determination; attitude determination and control; data management; and spacecraft engineering data analysis.

LASP traditionally involves students in all facets of mission operations because one of the best ways to learn about spacecraft is to operate them. IMEX will be no exception: while mission operations will be managed by LASP professionals, the IMEX mission operations team will comprise nine to twelve part-time shared graduate and undergraduate students for each year on orbit.

The IMEX Payload Operations Center (POC) will be responsible for arranging and conducting daily contacts with the spacecraft. These contacts will require some command uploading as well as real time and recorded telemetry acquisition. Using existing command generation software, the student/operators will build daily loads as many as several days in advance. Spacecraft ranging will also be performed by the tracking facility during these contacts. The operations center will monitor the real time data stream for spacecraft health and safety. Procedures will be established at the POC for anomalous events. The operations center will then produce a variety of daily distribution products (level-0 telemetry files (200MB/day), orbit and attitude files) for distribution to the IMEX Science Operations Center in Minnesota.

The POC will be equipped with the same client/server data access and display software used during instrument and spacecraft development. In this way, health and safety monitoring can be done on orbit as it was during integration and test. Furthermore, should anomalies develop, the web based client/server system will allow IMEX scientists or engineers as well as spacecraft contractor personnel to access real time displays from any remote location. Because all users of level-0 IMEX data will access the data in the same way and also because of the client/server nature of the ground system, additional resources can be brought to bear at the POC during critical operations or anomalies. Scientists or engineers simply arrive at the POC with their favorite computing platform and plug-in to the telemetry stream or they can connect into the POC server from their remote location. This means the core POC facilities do not need to be sized for the worst possible case.

Science Operations Center

The IMEX science operations center (SOC) will be established at the University of Minnesota. Daily instrument command loads will be generated at Minnesota based on the overall mission plan and input from the various Co-I institutions. These command files will be sent to the payload operations center at LASP where they will be verified and merged with spacecraft subsystem and attitude control commands. Daily 200Mbyte level-0 telemetry files and associated ancillary files will be received from the POC and distributed to the IMEX team electronically. The level-0 data will be kept on-line (65GB/year) and will be available to the public as it is received. Key parameter data (both high and low resolution) will be generated and distributed automatically. After a few days for validation, the key parameters will be provided to the NSSDC for long term archival and distribution. Once again, students will play a key part in the IMEX mission, as they are involved in the day to day science analysis and processing.

Trade Studies

We recognize there are some things which are uncertain at the time of this proposal. We currently plan to use GaAs solar arrays. These cells are more expensive but more efficient than conventional Si cells. Since our power situation at the end of mission is quite good, we could consider switching to less expensive Si cells. We could also consider reducing the size or number of solar array panels. This decision will be made during the phase A study. We will also consider the addition of solar cells on other faces of the spacecraft. This could allow IMEX to survive in a situation where the main solar arrays are not pointed to the sun. This level of failure protection seems beyond the scope of a UNEX mission but bears some consideration.

Since we use magnetic torquers for attitude control, part of the ACS system includes a magnetometer. Since our science payload also includes a magnetometer, it is possible that the science magnetometer could be used in place of the ACS system saving mass, power and money. The drawback here is the communication required between the science DPU and the ACS subsystem. During phase A we will consider the feasibility of a single magnetometer for both science and ACS.

IMEX requires a solid state recorder for storage of its telemetry data between the daily one-hour contacts. While we plan to connect the SSR to the on-board data handling computer, it would be possible to attach it to the science DPU instead. This would allow the science DPU to better control the data which is saved for later telemetry to the ground. The final location of the SSR will be determined during phase A based on the costs and complexities of the two solutions

Naturally, it is essential to know the position of the IMEX spacecraft in space (with respect to the earth) throughout its mission. One of the daily data products will be a file containing the spacecraft position and attitude (e.g. GSE coordinates) as a function of time. While the spacecraft location can be determined with ranging as part of the tracking function, it is possible to install a Global Positioning System (GPS) receiver on the spacecraft. This would give the spacecraft its coordinates as well as the time of spacecraft events (UT). We will determine in phase A if this is an affordable and reliable option.

Descope options

Even though costing methods, contingency funding and scheduling are reasonable, it is possible that we will need to consider descoping parts of the IMEX mission. One of the first things we consider for descoping is science capability. Unfortunately, large reductions in our science capabilities result in relatively small savings of time or money. Nonetheless, there are reductions which could provide some cost savings. Rather than procure potentially long lead time rad-hard parts, we could use our considerable mass margin to shield less expensive rad-tolerant parts. We would consider reducing the amount of data collected from IMEX. Because our science focus is major geomagnetic storms, we could consider collecting telemetry data only after major storms. This could reduce our tracking time and costs by up to a factor of three with a serious but acceptable reduction in science. Another option could be to reduce the length of the active science mission from two years to one year. This could save about $1M but would have an impact on our scientific scope.

Risk and Resilience

One of the recognized risks to the IMEX program is the number of components on the instrument package. Risk mitigation has driven the size and complexity to the absolute minimum configuration required to meet the focused science goals. The electrical interface has been simplified by controlling all four instruments with a single central DPU which minimizes the command and telemetry interface to the spacecraft. This central DPU and its interfaces represents a simplifying element in the plan but it is essential that its development stay on schedule. Minnesota produced a similar DPU for the WIND mission which handled interfaces with five receivers and understands the complexities involved in such a development. Breadboard DPU units will be produced very early in the program for delivery to the hardware institutions including the spacecraft builder. These simple breadboard models will allow development of flight hardware and software in a high fidelity environment well ahead of instrument or spacecraft integration.

The central science DPU provides several cost benefits and time savings for the IMEX program at large. As opposed to isolated software development efforts for each instrument, only one integrated flight software package is needed to provide instrument control and telemetry formatting for all sensor instruments on-board. In addition to the reduced complexity of a single electrical interface to the spacecraft, integration and test costs are reduced substantially with only one DPU-S/C interface. Because the DPU is light, rugged, and easy to carry, it will travel easily to each of the instrument institutions for sensor interface testing. The early delivery of breadboard DPUs is an essential part of keeping instrument development on schedule.

The LASP program approach to risk management is based on lessons learned from 26 successful space flight missions. First, the team for IMEX is a small but experienced group that works well together and the lines of communication are established. This team is firmly committed to providing the best spacecraft and science and finishing the program on schedule and within budget. The subsystems for the spacecraft have very similar models with recent successful space flight heritage. All the long lead items will be ordered early in Phase B. Adequate cost and schedule reserves are built into the plan.