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RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)

Overview    Spacecraft    Launch    Mission Status    Sensor Complement   Ground Segment   References

RHESSI is a NASA SMEX (Small Explorer) solar mission, selected in Oct. 1997, and managed for NASA/GSFC by the Space Science Laboratory (SSL) at the University of California, Berkeley (UCB). The overall objective is to explore the basic physics of particle acceleration and energy release in solar flares. The prime observations performed are simultaneous, high resolution imaging and spectroscopy of solar flares from 3 keV X-rays to 17 MeV gamma rays with high time resolution. RHESSI (the original name was HESSI) is a collaboration between the following institutions: GSFC, UCB (PI: Robert P. Lin), PSI (Paul Scherrer Institut, Villigen, Switzerland), and ETH Zürich (HESSI Experimental Data Center - HEDC). 1) 2) 3) 4)

Background: The former HESSI mission was formally renamed to RHESSI in April 2002. This renaming is in recognition of the enormous contribution that Reuven Ramaty made to gamma-ray astronomy in general and to the HESSI program in particular. Reuven Ramaty died in 2001, after a long and distinguished career in the Laboratory for High Energy Astrophysics at NASA/GSFC, Greenbelt, MD. Ramaty was a pioneer in the field of solar-flare physics, gamma-ray astronomy and cosmic rays. - RHESSI is also known as Explorer 81.


Figure 1: Artist's view of the RHESSI spacecraft viewing the sun (image credit: NASA, UCB)


The S/C bus was designed and built by Spectrum Astro Inc. of Gilbert, AZ (note: in July 2004, General Dynamics acquired Spectrum Astro; Spectrum Astro is now part of General Dynamics C4 Systems of Scottsdale, AZ). RHESSI is a sun-pointing and spin-stabilized S/C spinning at 12-20 rpm (15 rpm nominal). The bus consists of the structure and mechanisms, the power system (including the battery, solar panels, and control electronics), the attitude control system (ACS), thermal control, command and data handling (C&DH), and telecommunications.

The S/C structure is 1.1 m in diameter (at base) and 2.1 m in length. Its attitude and control subsystem employs sun sensors (fine and coarse) and a magnetometer for attitude sensing and magnetic torque rods as actuators. The S/C is capable of performing autonomous sun acquisition and spin-up from any orientation. Sun pointing (precession) control is < 0.2º provided by SAS (Sun Aspect System). The on-orbit mass properties adjustment direct the sun pointing error measurement to about 0.05º.

The SAS, built by the Paul Scherer Institute, consists of three identical lens-filter assemblies mounted on the forward grid tray to form full-sun images on three 2048 x 13-µm linear diode arrays mounted on the rear grid tray. Simultaneous exposures of three chords of the focused solar images are made every 10 ms by each of the arrays. A digital threshold algorithm is used to select four pixels that span each solar limb for inclusion in the telemetry. These digitized pixel outputs allow six precise locations of the solar limb to be obtained on the ground by interpolation, thus providing knowledge of sun center in pitch and yaw to 1.5 arcsec (3σ).

The SAS provides several functional services to the ACS such as:

• High-resolution, high-bandwidth aspect information for image reconstruction

• Real-time aspect error signals for spacecraft pointing

• Monitoring of the relative twist of the two grid trays

• Full-sun white-light images for co-alignment with ground-based images.

The S/C mass is 293 kg, power = 400 W. The power is provided by four deployable solar wings; in addition there is an NiH2 battery energy of 15 Ah. The nominal S/C design life is two years with a goal of three years.


Figure 2: Photo of the spacecraft bus (image credit: NASA)


Figure 3: Schematic showing the location of instrument and spacecraft components on the RHESSI spacecraft (image credit: NASA, UCB, Ref.17)

Legend to Figure 4:

- The acronyms in the top view are: FSS (Fine Sun Sensor), SSR (Solid State Recorder), CPC (Cryocooler Power Converter), IPC (Instrument Power Converter), IDPU (Instrument Data Processing Unit)

- The acronyms in the bottom view are: RAS (Roll Angle System ), PMT RAS (Photomultiplier Roll Angle System), IAD (Inertial Adjustment Device), SEM (Spacecraft Electronics Module).

The ACS (Attitude Control Subsystem) enables RHESSI to follow the sun over time autonomously with a 3σ pointing accuracy of 0.14º (8.4 arcmin). The primary attitude sensor is an Adcole Inc. FSS (Fine Sun Sensor) with a ±32º field of view and 0.005º resolution, that is mounted to the front of the imager tube. The pointing error measured by the FSS, together with local magnetic field measurements made by the spacecraft magnetometer, are inputs to the ACS control algorithms in the flight software. This runs on the RAD6000 flight processor in the C&DH (Command and Data Handling) subsystem to drive three orthogonally-mounted Ithaco Inc. 60 Am2 electromagnetic torque rods to maintain the spacecraft attitude. Finally, a set of eight coarse sun sensor cells (two mounted on each solar array wing) allow the ACS subsystem to acquire the sun from any initial attitude after separation from the launch vehicle (Ref. 17).


Figure 5: Block diagram of the RHESSI spacecraft bus (left) and the instrument (image credit: NASA, UCB)

C&DH (Command and Data Handling) subsystem: The SEM (Spacecraft Electronics Module) houses the CCB (Charge Control Board), the PCB (Power Control Board), and the ADB (Auxiliary Driver Board) for the EPS (Electrical Power Subsystem); and CIB (Communications Interface Board), the PACI (Payload and Attitude Control Interface) board, and the flight computer (CPU) board of the C&DH subsystem. A separate SSR (Solid State Recorder), built by SEAKR Engineering, provides 4 GB of solid-state memory for science data storage.

The IDPU (Instrument Data Processing Unit) provides formatted telemetry packets of science data directly to the SSR recording high-speed parallel interface. Science data are played back from the SSR for downlink via a high-speed parallel interface with the CIB, the command and telemetry interface for the SEM to the RF transponder. The CIB is powered from the essential bus and is operational at all times. It provides command decoding capability for critical functions including the reset and power control of the flight computer, control of the telemetry transmitter, and adjustment of the battery charge control parameters. This hardware command decoding capability of the CIB provides an operational backup for faults which result in the shutdown of CPU or software.

The PACI (Payload and Attitude Control Interface) board is responsible for telemetry encoding and data acquisition. It digitally encodes analog voltage, current and temperature data, and formats telemetry frames for downlink and on-board storage. It provides serial communications interfaces for control and monitoring of the SSR and the IDPU. The PACI board is powered by the essential power bus and is always producing hardware state of health telemetry packets; whenever the transmitter is powered on these packets are transmitted to the ground. This feature along with the CIB hardware command decoding, allows problems to be diagnosed and fixed from the ground, even without the CPU or software running.

The CPU board is a radiation-hardened RAD6000 processor made by BAE Systems. It contains 128 MB of DRAM for data memory storage and cache memory storage, and 3 MB of EEPROM for code memory storage. The CPU board controls the operation of all of the other boards in the SEM. The SEM also houses DC/DC power converters and an OCXO (Oven-Controlled Crystal Oscillator). The essential bus +28 V power provided by the power subsystem is used to generate secondary +5 V, and ±15 V services which power the SEM boards. The OCXO provides a stable clock signal at a frequency of 222 Hz, which is divided by the CIB to produce clock signals at 1 Hz and 220 Hz (approximately 1 MHz). These signals are distributed to the CIB, the PACI, and the IDPU, where they are used to time-stamp data acquisition and frame transmission times.

EPS (Electrical Power Subsystem): The EPS utilizes four triple-junction gallium arsenide (GaAs) solar array wings, each producing 133.5 W for a total of 534 W at 3 years end-of-life. Energy for eclipse operations is stored in a 15 Ahr battery comprised of eleven common pressure vessels, each containing two nickel hydrogen cells. The battery can operate at 50% depth-of-discharge for the full three year design life, and provide up to 280 W during the nominal 35 min eclipse duration. The CCB (Charge Control Board) uses a direct energy transfer system and is better than 95% efficient. The amount of current produced by the solar array is controlled by pulse-width modulating FET switches between the eight solar cell circuits and the power bus. Unused solar array power is dissipated in the solar array, not in the spacecraft. The CCB uses a temperature-compensated battery voltage algorithm to set the battery charge current. The PCB distributes power to the spacecraft components and provides switched power to those components requiring unregulated power at 28+7/-4 V. It also provides current sensors for telemetry monitoring and over-current protection for the power bus and under-voltage load shedding. The ADB (Auxiliary Driver Board) provides drive signals for the Inertia Adjustment Devices and the electromagnetic torque rods, and controls the solar array wing deployments.

RF communication is in S-band. The downlink frequency is at 2215 MHz with selectable data rates of 4 Mbit/s, 1 Mbit/s or 125 kbit/s, with NRZ-M and BPSK data modulation. The uplink frequency is 2039.6458 MHz, the data rate is 2 kbit/s. Continuous S/C operations are supported through a UCB ground station and the Mission/Science Operations Center. The data are distributed to SDAC (Solar Data Analysis Center) at GSFC and to HEDC at Zürich. There is also a complementary ground-based program supported by observatories throughout the world.


Launch: An air launch on a Pegasus XL vehicle took place on Feb. 5, 2002, about 180 km east-southeast of Cape Canaveral, FL, at a launch height of about 11.8 km above the ocean (use of Orbital Sciences' Stargazer L-1011 aircraft).

Orbit: circular orbit, altitude = 600 km, inclination = 38º, period = 96.98 minutes.



Mission status:

• July 2015 - The RHESSI mission is extended to 2017: The RHESSI mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The RHESSI mission will be invited to the 2017 Heliophysics Senior Review. 5)

- RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) is an imaging spectrometer which provides high resolution X-ray and gamma ray imaging spectroscopic observations over the range of energy from 3 keV to 17 MeV. This range covers soft X-rays emitted by hot thermal plasma, hard X-ray and gamma-ray bremsstrahlung emitted by energetic electrons, and nuclear gamma-ray lines produced by energetic ions. RHESSI was launched in February 2002, has a few arcsec angular resolution, and 1–few keV spectral resolution. It is presently in a 510 km roughly circular orbit (at an inclination of 38º), with expected operations through at least 2017.

• June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 6)

- For the next mission extension, the RHESSI team proposes to collaborate extensively with the other HSO (Heliophysics Systems Observatory) missions, in particular with the newer missions such as IRIS (Interface Region Imaging Spectrograph) Observatory. RHESSI's overall science goal of understanding solar flare energy release and particle acceleration is elucidated through four Primary Science Goals: (1) the evolution of SEEs (Solar Eruptive Events), (2) the acceleration of electrons, (3) the acceleration of ions, and (4) the origin of the thermal plasma. Two additional objectives are listed which take advantage of RHESSI's capabilities for observing (5) the optical Sun and (6) X-ray and gamma-ray sources of both terrestrial and astrophysical origin. As the sole mission to observe high-energy solar emissions, the RHESSI science plan focuses on the expectations of this declining solar cycle phase. Additionally, in the past three years RHESSI data have been used to challenge the basic thick target flare model, dating from the 1970s and heretofore the essential description of the dominant non-thermal processes in the impulsive phase of a flare. Evidence has emerged for bulk acceleration in the corona, and, unexpectedly, the presence of non-thermal particles in the lowest layer of the solar atmosphere, close to the photosphere itself. The proposal anticipates major revisions to this paradigm, facilitated by new complementary data becoming available.

- RHESSI science strengths: RHESSI observations are both unique and highly complementary to data sets from other instruments in the HSO; this includes not only SDO (Solar Dynamics Observatory), STEREO (Solar-Terrestrial Relations Observatory) and Solar-B/Hinode but most recently, IRIS; astrophysics missions such as Fermi/LAT have also been recently involved with coordinated observing efforts. The proposal additionally describes ample motivation to coordinate with ground-based facilities, as well—in particular with radio observations due to the very complementary diagnostics they provide.

- The maturity of the RHESSI mission does provide an extensive database of observations over more than a full solar cycle, and its continued operation should provide additional observations of larger events which sometimes occur in a cycle's declining phase, as in the next extension. As the first and only imaging spectroscopic instrument in its energy range, and especially now with a number of additional observational resources and facilities with which to collaborate (highlighted above), there is very high expectation of further breakthroughs in topics of high priority for Heliophysics research. While the overall theme of RHESSI research focuses on solar flares and energetic events, the proposal does outline additional non-flare (and non-solar) topics that again, will benefit from another mission extension due to the availability of new complementary observing facilities.

- RHESSI spacecraft / instrument health and status: Although the proposal demonstrates conclusively that RHESSI is currently capable of continued productive activity, it is unlikely that this will continue without severe degradation until the next review cycle. There is no clear and unambiguous set of criteria provided to determine objectively when degradation will render the data effectively useless. The primary issue is the need for detector annealing, which strains the cryocooler, and can probably not be repeated.

• The RHESSI spacecraft and its instruments continue to functions nominally in the summer of 2013 - in its present orbit of 554 km x 533 km (from an initial orbit of 600 km). Except for the gradual loss of efficiency of the cryocooler, RHESSI has no other expendables and the relatively low level of solar activity means that its orbital decay has been less than predicted; consequently, its useful life should extend well beyond the next two years. RHESSI's useful life should extend at least into 2018. 7)

RHESSI has been providing unique diagnostic observations of high-energy processes in solar flares for over 11 years. These observations address the key Heliophysics goal of understanding the fundamental processes of particle acceleration and energy release in solar eruptions, both flares and coronal mass ejections (CMEs). The resulting photon emissions and accelerated particles directly affect our Home in Space, and are especially important for the Journey Outward. 8)

RHESSI is designed for imaging spectroscopy of hard X-ray (HXR) and gamma-ray continua emitted by energetic electrons, and gamma-ray lines produced by energetic ions. The single instrument makes imaging and spectroscopy measurements with a few arcsecond angular resolution and one- to a few- keV energy resolution at energies from soft X-rays to gamma-rays (3 keV - 17 MeV). No other current or planned observatory has this ability to provide direct quantitative information on the energetic electrons and ions that carry such a predominant part of the released energy in a flare.

Over 70,000 events are included in the RHESSI Flare List, over 14,000 of them with detectable emission above 12 keV, and 35 above 300 keV. Twenty-seven events show gamma-ray line emission. All the data and the analysis software have been made immediately available to the scientific community.

The value of future RHESSI observations is now greatly enhanced by improved complementary observations compared with those available during the first eight years of RHESSI's operational lifetime. Groundbreaking observations of thermal plasmas, magnetic fields, and heliospheric effects are now being provided on a regular basis by instruments on the SDO (Solar Dynamics Observatory ), Hinode (Solar-B), STEREO, and other components of the HSO (Heliophysics System Observatory), and instruments on the Fermi astrophysics mission are providing X-ray and gamma-ray spectroscopy (limited imaging and modest energy resolution) from ~10 keV to GeV energies. Interpretation of data from RHESSI, especially in conjunction with data from other instruments, forms a key part of "developing a comprehensive scientific understanding of the fundamental physical processes that control our space environment and that influence our Earth's atmosphere" (Ref. 8).

• The RHESSI SMEX spacecraft and its instruments are operating nominally in 2012. On Feb. 5, 2012, the RHESSI spacecraft was 10 years on orbit. During this time, RHESSI has observed more than 40,000 X-ray flares, helped craft and refine a model of how solar eruptions form, and fueled additional serendipitous science papers on such things as the shape of the sun and thunder-storm-produced gamma ray flashes. 9) 10)

- RHESSI's monitoring of gamma rays throughout the sky also made it a prime tool to measure what are called terrestrial gamma-ray flashes (TGFs), bursts of gamma rays emitted from high in the Earth's atmosphere over lightning storms. The first of these had been spotted before, but RHESSI showed that they are more common and more luminous than previously thought. - With RHESSI's help, scientists soon realized they occurred upwards of 50 times/day. Indeed, current numbers suggest there may be as many as 400 TGFs daily from thunderstorms at different locations around the world.

- The original mission was only for two years and we quickly achieved our initial science goals - but RHESSI didn't stop there. The mission has been extended several times, and this small mission just keeps going and going, collecting great data. In 2009, NASA extended the mission yet again. Now, scientists are working to integrate RHESSI flare observations with data from other solar telescopes such as STEREO (Solar TErrestrial RElations Observatory), SDO (Solar Dynamics Observatory), SOHO (SOlar and Heliophysics Observatory), and Solar-B/Hinode as they watch the sun's activity rise toward yet another solar maximum, currently predicted for 2013 (Ref. 9).

• The RHESSI spacecraft and its instruments are operating nominally in 2011.

• The RHESSI spacecraft and its instruments are operating nominally in 2010 (after completing 8 years of observations). - Already in Feb. 2004, the mission had reached its design life of 2 years. So far, mission extensions were granted by NASA. It is expected that observations can be obtained through the next solar maximum which is expected between 2010 and 2012.

• As of May 2009, RHESSI has detected over 800 terrestrial gamma-ray flashes, providing the largest statistical database for characterizing these events.

• The RHESSI spacecraft and its instruments are operating nominally as of 2008. All detectors were annealed at ~90ºC for 7 days in November 2007. As a result, about half the effects of radiation damage on the energy resolution and sensitive volume was removed. 11)

• A number of "first time" observations of solar processes have been obtained (hard X-ray imaging spectroscopy, high resolution spectroscopy of solar gamma-ray lines, etc.). Early observations with RHESSI have revealed information on flare energetics, timing and spatial structure which stimulated renewed efforts to model and understand flares and magnetic reconnection on the sun.
On Dec. 27, 2004, a gamma ray flare, in fact the brightest explosion of high-energy X-rays and gamma rays recorded so far, was detected by at least 15 satellites and spacecraft between Earth and Saturn, swamping most of their detectors. Some of the best observations were recorded by the RHESSI instrument. 12)

• In 2003, the NASA Senior Review Panel gave RHESSI the highest rating of any of the 14 SEC (Sun-Earth Connection) missions. The rating indicates that the eleven voting panelists regarded RHESSI as "clearly superior" with "compelling science and relevance to the SEC mission." 13) 14)

• RHESSI was the first satellite to accurately measure terrestrial gamma-ray flashes that come from thunder storms, and RHESSI found that such flashes occur more often than thought and the gamma rays have a higher frequency on average than the average for cosmic sources.

• RHESSI can also see gamma rays coming from off-solar directions. The more energetic gamma rays pass through the spacecraft structure, and impact the detectors from any angle. This mode is used to observe GRBs (Gamma Ray Bursts).

• Many emission processes that can generate gamma-ray photons can also result in the linear polarization of those photons. The level of polarization, however, may depend on the precise emission geometry. In addition, the energy-dependence of the polarization can provide clues to the emission mechanisms that may be operating.

• On Dec. 6, 2002, RHESSI caught an extremely bright gamma-ray burst in the background, over the edge of the sun, revealing for the first time that the gamma rays in such a burst are polarized. The result indicates intense magnetic fields may be the driving force behind these awesome explosions. 15)


Figure 6: The deployed spacecraft with some component allocations (image credit: SSL/UCB)



Sensor complement: (RHESSI)

RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager):

The instrument name is identical to the spacecraft name. The objective is to obtain high fidelity color movies of solar flares in X-rays and gamma rays [imaging of solar flares in energetic photons from soft X-rays (about 3 keV) to gamma-rays (up to about 17 MeV) and to provide high resolution spectroscopy up to γ-ray energies of about 17 MeV].

Earth's atmosphere absorbs radiation over a large portion of the electromagnetic spectrum. It so happens that the atmosphere is completely opaque to X-ray radiation, i.e. to photons with energy levels above about 100 eV. Hence, observations of incoming X-ray radiation can only be done by instruments on spacecraft.

The instrument employs two new complementary technologies: fine grids (molybdenum and tungsten grids with slits as fine as 20 μm wide) to modulate the solar radiation, and germanium detectors to measure the energy of each photon very precisely (about 1 keV FWHM). The ITA (Imaging Telescope Assembly) consists of the telescope tube, grid trays, SAS (Solar Aspect System), and RAS (Roll Angle System). It was constructed, assembled, aligned, and tested at the Paul Scherrer Institut in Switzerland.


Figure 7: Illustration of the 9 Germanium detector assembly (image credit: SSL/UCB)

The spectrometer contains nine germanium detectors that are positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by Ortec of Perkin Elmer Instruments. When they are cooled to cryogenic temperatures (~75 K) and a high voltage is put across them (up to 4000 V), they convert incoming X-rays and gamma-rays to pulses of electric current. The amount of current is proportional to the energy of the photon. Germanium provides not only detections by the photoelectric effect, but inherent spectroscopy through the charge deposition of the incoming ray.

RHESSI is a FTS (Fourier Transform Spectrometer) device using a set of 9 RMCs (Rotational Modulation Collimators) or grid pairs (as opposed to conventional mirrors and lenses in the optical spectrum). Each RMC consists of two widely-spaced, fine-scale linear grids, which temporally modulate the photon signal from sources in the field of view as the S/C rotates about an axis parallel to the long axis of the RMC. The modulation can be measured with a detector having no spatial resolution placed behind the RMC. The modulation pattern over half a rotation for a single RMC provides the amplitude and phase of many spatial Fourier components over a full range of angular orientations but for a small range of spatial source dimensions. Multiple RMCs, each with different slit widths, can provide coverage over a full range of flare source sizes. An image is reconstructed from the set of measured Fourier components in exact mathematical analogy to multi-baseline radio interferometry. 16) 17) 18) 19) 20) 21) 22)

Energy range

3 keV to 17 MeV (soft X-rays to gamma-rays)

Energy resolution (FWHM)

< 1 keV at 3 keV, increasing to ~5 keV at 5 MeV

Angular resolution

- 2.3 arcseconds from 3 to 100 keV, 7 arcseconds to 400 keV,
- 36 arcseconds above 1 MeV

Temporal resolution

2 s for detailed image, tens of ms for basic image

FOV (Field of View)

full Sun (~ 1º)

Effective area (photopeak)

~ 10-3 cm2 at 3 keV, ~32 cm2 at 10 keV (with attenuators out), ~60 cm2 at 100 keV, ~15 cm2 at 5 MeV


9 germanium detectors (7.1 cm diameter x 8.5 cm), cooled to < 75 K with Stirling-cycle mechanical cooler


9 pairs of grids, with pitches from 34 µm to 2.75 mm, and 1.55 m grid separation

Aspect system

Solar Aspect System: Sun center to < 1 arcsec, Roll Angle System: roll to ~1 arcmin

Number of flares expected

~1000 imaged to >100 keV, ~ tens with spectroscopy to ~10 MeV

Instrument mass, power

131 kg, 143 W

Instrument size

- Grid support structure: 45 cm diameter, 1.7 m long
- Detector/cryocooler enclosure: 1 m diameter x 30 cm deep

Data storage capability

16 Gbit in 10 minutes SSR (Solid State Recorder)

Table 1: RHESSI instrument specification


Figure 8: Detector arrangement of the RHESSI spectrometer (image credit: NASA, UCB)

Legend to Figure 8: A cutaway view of the Spectrometer, showing the location of the germanium detectors under each grid (by number). The Sunpower Stirling-cycle mechanical cooler is below the cold plate holding the detectors. The thermal radiator faces anti-sunward to reject the heat of the cryocooler. The attenuators are automatically moved in when the counting rate exceeds thresholds (commandable from the ground).


Figure 9: Schematic illustration of the RHESSI grid pair alignment (image credit: B. R. Dennis, NASA/GSFC)

The detectors are the largest currently available (as of 2000) hyperpure (n-type) germanium detectors of size: 7.1 cm in diameter and 8.5 cm in length. They are cooled to 77 K by a single stage electro-mechanical cryocooler (an integral counterbalanced Stirling cycle cooler (built by Sunpower, Inc.) which provides up to 4 W of cooling at 77 K, with an input of 100 W). The cryocooler houses the 9 germanium detectors. The Ge detectors are segmented, with both a front and rear active volume (Figures 9 and 10). Low-energy photons (below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering.

The detectors cover the entire X-ray to gamma-ray energy range from 3 keV to 17 MeV. The keV spectral resolution of germanium detectors is necessary to resolve all of the solar gamma-ray lines (with the exception of the neutron deuterium line, which has an expected FWHM of only 0.1 keV).

The critical alignment requirement for the metering structure is to maintain the relative twist of the finest grid pair to within one arcminute. The metering structure is based on the TDU (Telescope Demonstration Unit) provided by GSFC.


Figure 10: Illustration of the forward grid tray (right) and the aft grid tray (left), image credit: SSL/UCB

RHESSI achieves the alignment feat by using tungsten and molybdenum grids with extremely fine slits, some as fine as 20 µm wide. The manufacture of these grids has been made possible by newly developed microfabrication techniques.


Figure 11: View of the single-stage Stirling cryocooler for the detector array (image credit: NASA)


Figure 12: Schematic of the RHESSI imaging technique (image credit: NASA)



GDS (Ground Data System):

RHESSI is operated from the highly integrated and automated MOC (Mission Operations Center) located at SSL/UCB (Space Sciences Laboratory of the University of California at Berkeley). The MOC also supports the FAST (Fast Auroral Snapshot Explorer). Co-located with the multi-mission MOC are the RHESSI and FAST SOC (Science Operations Center) and the BGS (Berkeley Ground Station), the primary ground station to support RHESSI on-orbit (Ref. 17).


Figure 13: Overview of the RHESSI ground data system (image credit: NASA)

RHESSI is operated in store-and-dump mode. The spacecraft transmitter is turned on and off by time sequence commands stored on-board. These commands and many others related to configuring instruments for various phases of the orbit are part of an ATS (Absolute Time Sequence) load generated with the MPS (Mission Planning System). Command loads are uploaded to the spacecraft every two days and cover 4–5 days in advance.

The spacecraft command and control system for RHESSI is the ITOS (Integrated Test and Operations System). Since ITOS was also used during mission integration and testing, members of the Berkeley Flight Operations Team were trained early on operating the spacecraft. This approach allowed for a smooth transition from spacecraft integration and testing to normal on-orbit operations.

Flight dynamics and mission planning products are generated by the Berkeley Flight Dynamics System, which is based on the SatTrack Suite V4.4. SatTrack also has heritage with various NASA missions and is used to generate all flight dynamics products such as ground station view periods, link access periods, terminator, high-latitude region, and SAA (South Atlantic Anomaly) crossings, and other orbit events needed as input to MPS. Other tools in the SatTrack Suite are employed to distribute real-time event messages to various ground data system elements such as ITOS and the BGS in an autonomous client/server network environment. SatTrack also provides a multitude of related automation functions as well as 2-D and 3-D real-time orbit displays.

All RHESSI space and ground systems are tied into the SERS (Spacecraft Emergency Response System),which is a data base system that regularly parses through log files and automatically checks for yellow or red limit violations. It also acts on warning and error messages received from various GDS subsystems via electronic mail. In case an anomaly is detected, the on-call operations team member is alerted via 2-way email pager in order to assess and resolve the situation. SERS completes the autonomous ground system and adds a high degree of reliability.




3) Information provided by Brian R. Dennis of NASA/GSFC

4) RHESSI Receiving Review, May 6, 2002, URL:

5) "NASA Response to the 2015 Senior Review for Heliophysics Operating Missions," NASA, July 10, 2015, URL:

6) "The 2015 Senior Review of the Heliophysics Operating Missions, NASA, June 11, 2015, URL:

7) Information provided by Gordon D. Holman of NASA/GSFC, Greenbelt, MD, USA

8) Samuel Krucker, Brian Dennis, Manfred Bester, Laura Peticolas, "Heliophysics Senior Review 2013, The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," April 24, 2013, URL:

9) Karen C. Fox, "NASA Small Explorer Mission Celebrates Ten Years and Forty Thousand X-Ray Flares," Space Daily, Feb. 13, 2012, URL:


11) "RHESSI Detectors Successfully Annealed," Jan. 16, 2008, URL:

12) R. Sanders, "RHESSI satellite captures giant gamma-ray flare," Feb. 18, 2005, URL:

13) "Senior Review Rates RHESSI Highest," NASA, URL:

14) Wolfgang Baumjohann, David S. Evans, Priscilla Frisch, Philip R. Goode, Bernard V. Jackson, J. R. Jokipii, Stephen L. Keil (Chair), Joan T. Schmelz, Frank R. Toffoletto, Raymond J. Walker, William Ward, "Senior Review of the Sun-Earth Connection Mission Operations and Data Analysis Program," NASA, August 3, 2003, URL:

15) D. Savage, B. Steigerwald, R. Sanders, "RHESSI's Lucky Break May Lead To Secret Of Ultimate Explosions," May 28, 2003, URL:

16) R. P. Lin, B. R. Dennis, G. J. Hurford, G. J. Hurford, D. M. Smith, A. Zehnder, P. R. Harvey, D. W. Curtis, D. Pankow, P. Turin, M. Bester, A. Csillaghy, M. Lewis, N. Madden, H. F. Beek, M. Appleby, T. Raudorf, J. MyTierman, R. Ramaty, E. Schmahl, R. Schwartz, S. Krucker, R. Abiad, T. Quinn, P. Berg, M. Hashii, R. Sterling, R. Jackson, R. Pratt, R. D. Campbell, D. Malone, et al. "The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," Solar Physics, Vol. 210, 2002, pp. 3-17, URL:

17) R. P. Lin, B. R. Dennis, et al., The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)," Solar Physics, Vol. 210, 2002, pp. 18-32, URL:

18) M. L. McConnell, J. M. Ryan, D. M. Smith, R. P. Lin, A. G. Emslie, "RHESSI as a hard X-ray polarimeter," Solar Physics, Vol.. 210, 2002, pp.125-142

19) M. L. McConnell, D. M. Smith, A. G. Emslie, G. J. Hurford, R. P. Lin, J. M. Ryan, "Hard X-ray solar flare polarimetry with RHESSI," Advances in Space Research, Vol. 34, 2004, pp. 462-466

20) M. L. McConnell, P. F. Bloser, "Status and Future Prospects for γ-ray Polarimetry," arXiv:astro-ph/0508315 v1, Aug 14, 2005, published in: Chinese Journal of Astronomy and Astrophysics, Supplement, Volume 6, Issue S1, 2006, pp. 237-246





The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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