Minimize ACE

ACE (Advanced Composition Explorer)

A NASA solar-terrestrial space weather mission in the Explorer Program (Explorer-71) with the prime objectives to determine: the elemental and isotopic composition of matter, the origin of the elements, the formation of the solar corona and acceleration of the solar wind. The PI for mission is E. C. Stone of Caltech/JPL. 1) 2)


The ACE spacecraft was designed and built at JHU/APL, Laurel, MD. The S/C structure (decks and panels: honeycomb with aluminum alloy facesheet) has two octagonal decks, 1.6 m across and 1 m high; the overall wing span is about 8.3 m. The S/C is spin stabilized with the spin axis Earth/sun pointing (star and sun sensors). Attitude is measured by a star tracker (CT-632 solid-state scanner of BATC) and sun sensors, attitude knowledge is within ± 0.7º (goal of ± 0.5º for the magnetometer). The nominal spin rate is 5 rpm, the S/C is oriented in the Earth/sun direction.

S/C launch mass = 785 kg (includes 189 kg of hydrazine fuel for orbit insertion and maintenance), peak power = 443 W (EOL with EOL defined as 5 years) from four deployable solar panels (each 86.4 cm x 149.9 cm). A NiCd battery (18 cell 12 Ah) is being used. There are two deployable magnetometer booms (magnetometer sensors on ends of boom). Nominal life of the mission is 2 years with a five-year goal. 3) 4) 5)


Figure 1: Line drawing of the ACE spacecraft

The C&DH (Command and Data Handling) subsystem is of JHU/APL design. It utilizes a Harris RTX2010 processor executing the FORTH language. The RTX2010 is fabricated in a CMOS/SOS process that is exceptionally hard to single-event upsets (SEUs), making it suitable for operation through solar flares. Code is stored in electrically erasable/programmable read-only memory (EEPROM) and downloaded into random-access memory (RAM) for execution. The EEPROM can be reloaded on the ground, and the RAM can be patched in flight. Both RAM and EEPROM utilize error detection and correction (EDAC) circuitry to correct single errors and detect double errors. The FPGAs are designed with triple voting cells to minimize the probability of SEUs.

The ACE propulsion subsystem (of Primex) corrects launch vehicle dispersion errors, injects the spacecraft into the L1 halo orbit, adjusts the orbit and spin axis pointing, and maintains a 5 rpm spin rate. The subsystem is a hydrazine blowdown unit that uses nitrogen gas as the pressurant and is made up of four fuel tanks, four axial thrusters for velocity control along the spin axis, and six radial thrusters for spin plane velocity control and spin rate control.


Figure 2: Block diagram of the ACE observatory (image credit: JHU/APL)

For APL developed spacecraft - the 1st generation autonomy capability began with the ACE mission, during which autonomy was first separated from hard-coded software. The ACE autonomy system, in conjunction with hardware-based fault detection and reaction and together with the command and data handling (C&DH) and power subsystems, formed the overall ACE safing strategy. This autonomy system was responsible for preparing the spacecraft for first contact, monitoring component health, monitoring overall spacecraft attitude and maneuver health, and maintaining proper spacecraft component on/off configurations and other autonomous actions to support the recorder and hardware-based reactions. 6)

The ACE autonomy system, which was a facility of C&DH software, was based on a set of autonomy rules. These rules take the form of “if-then” statements that can be loaded into fixed-size memory locations known as bins. When the autonomy system is running, it scans the rules at a regular interval, evaluating each rule in turn and executing any that evaluate to “true.”

To program an autonomous behavior, the autonomy designer would construct a rule by defining the telemetry point (a section of the spacecraft’s telemetry data block representing a spacecraft sensor value), defining a mask of the telemetry point if needed, selecting the conditional type, defining the A and B values for the conditional types, defining the number of true evaluations before a command is executed, and selecting the command to issue.

The command selected to issue could be a single command or a call to a block of commands to be run in sequence. The sequence of commands could also include pauses in the sequence to provide relative timing of commands. All commands issued from the autonomy facility, whether single commands or the command sequence from a block, are executed at the same priority. Therefore, only a single autonomy rule could control the spacecraft at one time.

Development of the ACE autonomy system established the separation between rules and hard-coded autonomy at APL. Before this development, autonomous behavior was nonexistent or was directly written into the C&DH software for the spacecraft. This rule-based approach to meeting autonomy requirements allowed C&DH design to proceed, even when autonomy conditions and actions had not been fully specified at the mission level.


Launch: A launch of the ACE spacecraft took place on August 25, 1997 on a Delta II 7920 launch vehicle from the KSC (Kennedy Space Center) at Cape Canaveral, FLA, USA.7)

Orbit: Initial circular orbit parking orbit of 185 km with an inclination of 28.7º. Then transfer trajectory insertion toward the sun (with re-ignition of 2nd stage). Final halo orbit (Lissajous) about the Lagrangian (or Earth-sun libration) point L1 (250 Earth radii toward the sun, or about 1.5 million km from Earth toward the sun).


Figure 3: Schematic view of the halo orbit of ACE at L1, the point of equilibrium between the Earth and sun's gravitational fields (image credit: NASA, Caltech)

RF communications with ACE are in S-band (2097.9806 MHz for the uplink and 2278.35 MHz for the downlink.). Science and engineering data are collected during one 3-4 hour pass per day. Real-time data is transmitted at a data rate of 6.9 kbit/s or 434 bit/s (to NOAA/SEC at Boulder, CO, supporting the solar wind project). Onboard data may be recorded onto a 1 Gbit solid-state recorder (two recorders available) and played back at a rate of 78 kbit/s. The uplink data rate is 1 kbit/s. The telemetry is designed to be compatible with CCSDS (Consultative Committee for Space Data Systems) protocol standards. - The ACE mission is being monitored by NASA/GSFC. The ACE Science Center is at the SRL (Space Radiation Laboratory) of the California Institute of Technology (Caltech), Pasadena, CA.

On January 21, 1998, NOAA/SEC (Space Environment Center) at Boulder, CO, and the ACE project opened up the ACE Real Time Solar Wind (RTSW) monitoring capability to the public. The ACE RTSW network uses a beacon to deliver an operationally useful subset of its space physics data to various ground stations around the world in real-time. The intent is to provide 24 hour coverage of the solar wind parameters and solar energetic particle intensity. The position of ACE at 1.5 million km upstream of Earth offers an hour's warning time of CME (Coronal Mass Ejection) events that can cause geomagnetic storms on Earth. Four ACE instruments supply data to NOAA/SEC for RTSW processing.

The instruments are:

• EPAM (Electron, Proton, and Alpha-particle Monitor) for energetic ions and electrons

• MAG (Magnetic Field Monitor) for magnetic field vectors

• SIS ((Solar Isotope Spectrometer) for high energy particle fluxes

• SWEPAM (Solar Wind Electron, Proton, and Alpha Monitor) for solar wind ions.


Figure 4: Alternate illustration of ACE (image credit: NASA)


Figure 5: Expanded view of the ACE spacecraft structure (image credit: NASA)



Status of the ACE mission:

• In 2014, the ACE mission is operating nominally (however, without the SEPICA instrument).

Since January of 1998, ACE has been in orbit around the L1 Lagrangian point, ~1.5 million km sunward of Earth. At this location it has made measurements of the elemental, isotopic, and ionic charge-state composition of energetic nuclei from solar wind to galactic cosmic ray energies. These observations have been used to enhance our understanding of the sources, composition and processes related to the solar wind, solar energetic particles (SEPs), and galactic and anomalous cosmic rays. In addition, continuous measurements of the background and disturbed solar wind, provided by solar wind plasma, energetic particles and magnetic field instruments on ACE, are crucial for space science, as well as for space weather. 8)

In spite of being one of the “older” missions, ACE continues to make significant contributions to new and emerging scientific problems on topics related to the solar wind and ICME (Interplanetary Coronal Mass Ejections), solar and interplanetary energetic particles, cosmic rays and heliosphere/interstellar interactions, and space weather and the science behind space weather. During the next few years, new observations by ACE will be crucial for understanding changes in the solar corona as the recent unusual solar cycle conditions (weak solar minimum and solar maximum) evolve.

• Dec. 2, 2013: The last solar minimum, which extended into 2009, was especially deep and prolonged. Since then, sunspot activity has gone through a very small peak while the heliospheric current sheet achieved large tilt angles similar to prior solar maxima. The solar wind fluid properties and IMF (Interplanetary Magnetic Field) have declined through the prolonged solar minimum and continued to be low through the current "mini" solar maximum. Compared to values typically observed from the mid-1970s through the mid-1990s, the proton parameters are lower on average from 2009 through the day 79 of 2013 by: solar wind speed and beta (~11%); temperature (~40%); thermal pressure (~55%); mass flux (~34%); momentum flux or dynamic pressure (~41%); energy flux (~48%); IMF magnitude (~31%), and radial component of the IMF (~38%). These results have important implications for the solar wind's interaction with planetary magnetospheres and the heliosphere's interaction with the local interstellar medium, with the proton dynamic pressure remaining near the lowest values observed in the space age: ~1.4 nPa, compared to ~2.4 nPa typically observed from the mid-1970s through the mid-1990s. The combination of lower magnetic flux emergence from the Sun (carried out in the solar wind as the IMF) and associated low power in the solar wind points to the causal relationship between them. Our results indicate that the low solar wind output is driven by an internal trend in the Sun that is longer than the ~11-year solar cycle, and suggest that this current weak solar maximum is driven by the same trend. 9) 10)


Figure 6: Weakest solar wind of the Space Age and the current "Mini" Solar Maximum (image credit: CalTech)

Legend to Figure 6: Top — Solar wind dynamic pressure in the ecliptic plane at ~1 AU, taken from IMP-8, Wind, and ACE and inter-calibrated through OMNI-2. Means (red), medians (blue), 25%-75% ranges (dark grey), and 5%-95% ranges (light grey) are shown averaged over complete solar rotations from 1974 through the first quarter of 2013. Bottom — monthly (black) and smoothed (red) sunspot numbers and the current sheet tilt (blue) derived from the WSO radial model [Hoeksema, SSRv, 72, 137, 1995].

• The ACE mission is operating nominally (however, without the SEPICA instrument) in 2013 at L1 for over 15 years. According to SRL (Space Radiation Laboratory) of Caltech, the ACE spacecraft has enough propellant on board to maintain an orbit at L1 until ~2024 (Ref. 15). 11)

• The solar CME (Coronal Mass Ejection) that erupted from the sun on Oct. 4, 2012 at 11:24 p.m. EDT, arrived at Earth on Oct. 8 at 12:30 a.m. EDT, as observed by instruments aboard NASA's ACE (Advanced Composition Explorer) spacecraft. At Earth, when the CME connected up with Earth's magnetic environment, the magnetosphere, it caused a space weather phenomenon called a geomagnetic storm. This storm was categorized by NOAA as a G2 – on a scale from G1 to G5. A storm at this level is considered reasonably mild. Auroras did appear in the north, including Canada, due to this storm. 12)

• The ACE mission is operating nominally in 2012 - providing continuous, real-time space weather data. ACE has been at the L1 point for over 14 years (ACE was 15 years on orbit on August 25, 2012), and the spacecraft and instruments are still working very well, with the exception of the SEPICA instrument (Ref. 14).

- Due to a failure of the valves that control the gas flow through the instrument, active control of the SEPICA proportional counter was lost, and delivery of science data from SEPICA ended on Feb 4 2005. The SEPICA instrument was turned off permanently on April 20, 2011 (Ref. 14).

ACE is a crucial component of NASA's fleet, but its job as sentinel is, in fact, just a small piece of what ACE has accomplished since it launched on August 25, 1997. In its first 15 years, the spacecraft has helped determine the composition of the vast sea of flowing particles surrounding Earth. ACE also serves as a sentinel that helps measure the input — the solar wind — that drives the dynamics of the magnetosphere. 13)

• The ACE mission is operating nominally in 2011.

• The ACE mission is operating “nominally” in 2010 (after > 12 years in orbit at L1). The spacecraft and instruments are still working very well, with the exception of the SEPICA instrument. 14) 15) 16)

- ACE provides near-real-time solar wind information over short time periods. When reporting space weather, ACE can provide an advance warning (about one hour) of geomagnetic storms that can overload power grids, disrupt communications on Earth, and present a hazard to astronauts.

- The only spacecraft anomalies have been rare, inconsequential, single-bit errors in the solid-state recorders. All propellant line and tank temperatures and pressures have been within nominal limits. Although warmer than predicted, temperatures of the spacecraft sunward top deck and thermal blankets are within specifications. Solar array performance is currently declining by ~1%/year; the power output is predicted to be adequate until ~ 2025. The attitude control, propulsion, RF, and command and data handling systems have all performed nominally.

- Three types of maneuvers (attitude, orbit and spin) have been used since July 2001 to control ACE. Orbit maneuvers use ~1.5 kg/year of fuel per year and keep the spacecraft bound to the L1 libration point. Attitude maneuvers use ~ 3 kg/year and are required to maintain the HGA antenna constraint. With this strategy, fuel use is 4.5 kg/year total, and the 70 kg of fuel remaining as of October 2007 will be consumed by 2024.

- The instruments on the sun-facing deck have experienced higher operating temperatures than expected, due to degradation of the thermal blankets. However, they are still operating nominally and returning excellent science data, and the thermal blanket degradation is a process that slows over time, so we expect temperatures for these instrument to rise only a few degrees over the next 10 years (Ref. 14).

• SEPICA failure in 2005.

• In 2002 the SEPICA instrument experienced problems with the gas flow regulation of its proportional counters and with a high-voltage power supply. Two thirds of the instrument is non-functional, but the third counter is returning good science data. Data of SEPICA is only available until Feb. 4, 2005 when further problems arose resulting in a de facto retirement of the instrument.

• The spacecraft has enough propellant on board to maintain an orbit at L1 until about 2019. Lately, a fuel use strategy has been implemented that will allow continued operations through the year 2022.




To meet observing requirements and to simplify access to the instruments and spacecraft subsystems, all components except the propulsion system are mounted on the external surfaces of the body. Six of the instruments are mounted on the top (sunward facing) deck, and two are mounted on the sides. 17) 18)


Mass (kg)

Power (W)

Data rate (bit/s)

Measurement Technique

Type. Energy (MeV/nucleon)





dE/dX x E






dE/dX x E












ΔE x E x E/Q






TOF x E x E/Q






TOF thru special E-field






Electrostatic Analyzer






dE/dX x E






Triaxial Fluxgate


Table 1: ACE instrument summary


Figure 7: Overview of the sensor complement on ACE (image credit: JHU/APL)


SWIMS (Solar Wind Ion Mass Spectrometer):

PI: G. Gloeckler, University of Maryland. Objectives of SWIMS: Measurement of solar wind composition data over a wide range of solar wind bulk speeds and for all solar wind conditions. Abundances of most of the elements and several isotopes in the mass range from 4 - 60 amu (atomic mass unit) every few minutes. SWIMS uses a time-of-flight (TOF) measurement technique to determine the mass of a solar wind ion with high accuracy. SWIMS consists of the Wide-Angle, Variable Energy/charge (WAVE) three chamber parallel-plate electrostatic analyzer, the time-of flight High-Mass Resolution Spectrometer (HMRS), high-voltage supplies, and analog and digital electronics.



Figure 8: View of the SWIMS instrument (image credit: NASA)


SWICS (Solar Wind Ion Composition Spectrometer):

PI: G. Gloeckler, U. of Maryland. Objective: measurement of the elemental and ionic-charge composition and the temperature and mean speeds of all major solar wind ions from H through Fe at solar wind speeds ranging from 145 km/s (for protons) to 1532 km/s (for Fe+8). The instrument, which covers an energy per charge range from 16 - 60 keV/Q, combines an electrostatic analyzer with post-acceleration, followed by a time-of-flight (TOF) and energy measurements.


Figure 9: View of the SWICS instrument (image credit: NASA)

SWICS uses 4 components to study the mass and isotopic composition of incoming ions: a) E/q filtering by electrostatic deflection, b) post-acceleration of the filtered ions by up to 30 kV, c) TOF measurement, and d) E measurement using ion-implant solid-state detectors (SSDs).

Knowing the E/q, E, and TOF, one knows the mass (M) and the mass per charge (M/q) since E = (M/2) x v 2. SWICS was the flight spare of the Ulysses GLG experiment, launched in October 1990.


ULEIS (Ultra-low Energy Isotope Spectrometer):

PI: G. Mason, U. of Maryland, R. Gold, JHU/APL. Objective: measurement of ion fluxes over the charge range from He through Ni from about 20 keV/n to 10 MeV/n (superthermal and energetic particle ranges). ULEIS is a time-of-flight (TOF) mass spectrometer which identifies incident ion mass and energy by simultaneously measuring the time-of-flight, τ, and residual kinetic energy, E, of particles which enter the telescope cone and stop in one of the six detectors in the telescope. 19) 20)


Figure 10: View of the ULEIS instrument (image credit: JHU/APL)


SEPICA (Solar Energetic Particle Ionic Charge Analyzer):

PI: E. Möbius, University. of New Hampshire (UNH) and MPE Garching; D. Hovestadt, MPE Garching. Objective: measurement of the ionic charge state, Q, the energy, E, and the nuclear charge, Z, above 0.2 MeV/n. - Energetic particles entering the multi-slit collimator will be electrostatically deflected between the six sets of electrode plates which are supplied with variable high voltages up to 30 kV. The deflection, which is inversely proportional to energy per charge, E/Q, is determined in the back portion of the instrument (dE/dX device and a position-sensitive silicon solid-state detector). The residual energy of the particle, Eres, and the amount of electrostatic deflection is directly determined in the detector, thus yielding the energy per ionic charge, E/Q, of the incoming particle, and its energy, E. 21) 22) 23)

SEPICA consists of three independent sensor units, called ”fans”. Each of the three fans is symmetric about the plane with the high voltage deflection plate. A schematic view of one individual sensor unit is shown in Figure 11 together with the basic measuring principles. Shown is a single side of one SEPICA fan. Energetic particles enter a multi-slit collimator, which selects those incoming particles that target a narrow line in the detector plane (indicated by F, the ”focal line”). They are electrostatically deflected between a set of electrode plates. The curved plate is on ground potential, while the flat center plate is supplied with a positive high voltage up to 30 kV (to be set by telecommand).


Figure 11: Schematic view and principles of operation of the SEPICA instrument (image credit: UNH)


Figure 12: Functional block diagram of SEPICA (image credit: UNH)


Figure 13: View of the SEPICA instrument (image credit: UNH)


Figure 14: SEPICA during the hoist on ACE (image credit: UNH)


SIS (Solar Isotope Spectrometer):

PI: A. Cummings, California Institute of Technology (CalTech). Objective: measurement of elemental and isotopic composition of solar energetic particles, anomalous cosmic rays, and interplanetary particles from He to Zn over the energy range from 10 - 100 MeV/nucleon. Measurements by a technique considering a particle's energy loss ΔE in a detector (multiple ΔE versus residual energy E). SIS has a geometry factor of ~40 cm2 sr, which is significantly larger than previous satellite solar particle isotope spectrometers. It is also designed to provide excellent mass resolution during the extremely high particle flux conditions which occur during large solar particle events.

Spectroscopic observations of solar isotopes are very difficult; there are isotopic observations for only a few elements and the uncertainties are large. With its greatly improved collecting power over other instruments, it is hoped that SIS can make a major advance in our knowledge of SEP (Solar Energetic Particle) isotopic composition. 24)

Anomalous cosmic rays: During solar minimum conditions there are seven elements (H, He, C, N, O, Ne, and Ar) whose energy spectra have shown anomalous increases in flux above the quiet time galactic cosmic ray spectrum. This so-called ”anomalous cosmic ray” (ACR) component is now thought to represent neutral interstellar particles that have drifted into the heliosphere, become ionized by the solar wind or UV radiation, and then been accelerated to energies >10 MeV/nucleon, most likely at the solar wind termination shock.


Figure 15: SIS instrument illustration (image credit: NASA)

ACR measurements, free from contamination of solar and interplanetary particles at lower energy, and free from GCR contamination at higher energies, are best made in the energy interval from ~5 to 25 MeV/nucleon, where the flux is a decreasing function of energy. Similarly, SEP spectra typically decrease rather steeply with increasing energy. It follows that to maximize the number of detected particles for both of these species requires the use of thin detectors with as low a threshold for penetration as possible, combined with a large geometry factor. For this reason SIS has two telescopes composed of the largest area devices available (~65 cm2 each).

It is also of interest to extend the SEP measurements to as high an energy as possible to understand the acceleration process in these events. The SIS detector stack is composed of devices of graduated thicknesses to cover a broad energy range. There are two identical telescopes in SIS, each composed of 17 high-purity silicon detectors (Figure 16).

The first two detectors, M1 and M2, are position-sensitive ”matrix” devices (Figure 17) that form the hodoscope measuring the trajectory and energy loss of incident nuclei. The matrix detectors are octagonal in shape, 70 to 80 µm in thickness, and have 34 cm2 active areas that are divided into 64 strips. Each of the strips on M1 and M2 is individually pulse-height analyzed with its own 12 bit ADC (Analog Digital Converter) when an event occurs so that the trajectory of heavy ions traversing the system can be separated from the tracks of low energy H or He that might happen to hit one of these detectors at the same time. Detectors M1 and M2 are separated by 6 cm; the resulting rms angular resolution of the system is ~0.25 degrees, averaged over all angles.


Figure 16: Scale drawing of one of the two SIS telescopes (image credit: CalTech)


Figure 17: Illustration of matrix detectors M1 and M2 (image credit: CalTech)


CRIS (Cosmic Ray Isotope Spectrometer):

PI: A. Cummings, California Institute of Technology; T. von Rosenvinge, GSFC; R. Binns, Washington U.; M. Wiedenbeck, JPL. Objective: measurements of all stable and long-lived isotopes of galactic cosmic ray nuclei from He to Zn over the energy range from ~100 to 600 MeV/nucleon. CRIS also provides limited measurements of low energy H isotopes and data for exploratory studies of the isotopes of “ultra-heavy” (UH) nuclei. Measurements by a technique considering a particle's energy loss ΔE in a detector ( multiple ΔE versus residual energy E). CRIS is of CRRES, ISEE-3 and SAMPEX heritage. 25)

The fully assembled CRIS instrument consists of two boxes bolted together (Figure 18). The upper box contains the SOFT system, while the lower box contains the Si(Li) detector stacks with their pulse-height analysis electronics, as well as the main CRIS control electronics. The large window on the top of the SOFT box is the CRIS entrance aperture. The two smaller patches are thermal radiators for cooling the two CCD cameras.


Figure 18: Photo of the CRIS instrument (image credit: CalTech)


Figure 19: Block diagram of CRIS electronics (image credit: CalTech)


EPAM (Electron, Proton, and Alpha-particle Monitor):

PI: R. Gold, JHU/APL. The EPAM instrument is the flight spare unit of the HI-SCALE instrument flown on Ulysses. 26)

Objective: measurement of solar and interplanetary particle fluxes with a wide dynamic range and a directional coverage of nearly a full unit sphere. EPAM consists of five apertures in two telescope assemblies and an associated instrument electronics box.. The EPAM detectors consist of three silicon solid-state detector systems: 1) LEMS (Low Energy Magnetic Spectrometers); 2) LEFS (Low Energy Foil Spectrometers); and 3) CA (Composition Aperture). The LEMS/LEFS provide pulse-height-analyzed single-detector measurements with active anticoincidence. The CA provides elemental composition in an energy range similar to LEMS/LEFS, plus Helium isotope resolution.

EPAM measures ions (Ei ¿50 keV) and electrons (Ee >30 keV) with essentially complete pitch angle coverage from the spinning ACE spacecraft. It also has an ion elemental abundance aperture using a delta-E versus E technique in a three-element telescope. The telescopes use the spin of the spacecraft to sweep the full sky. Solid-state detectors are being used to measure the energy and composition of the incoming particles.


Figure 20: Illustration of the EPAM instrument (image credit:JHU/APL)


SWEPAM (Solar Wind Electron, Proton, and Alpha Monitor):

PI: D. McComas, LANL. SWEPAM is of SWOOPS heritage flown on the Ulysses spacecraft. Objective: high quality measurements of electron and ion fluxes in the low energy solar wind range (electrons: 1 - 1240 eV; ions: 0.26 - 35 keV). SWEPAM is of Ulysses mission heritage. SWEPAM makes simultaneous and independent electron and ion measurements with two separate sensors. Both sensors make use of curved-plate electrostatic analyzers which are spherical sections cut off in the form of a sector. 27) 28)


Figure 21: View of the SWEPAM instruments (image credit: NASA, LANL)


MAG (Magnetic Field Monitor):

PI: N. Ness, U. of Delaware. Objective: measurement of the three components of the magnetic field. MAG is triaxial fluxgate magnetometer, boom-mounted. MAG provides continuous data at 3, 4 or 6 vectors/sec, and snapshot memory data and FFT (Fast Fourier Transform) data based on 24 vectors/sec. acquired on board, working synchronously with blocks of 512 samples (FFT only) each. 29)

Instrument type

Twin, triaxial fluxgate magnetometers (boom mounted)

Dynamic ranges (8)

±4 nT; ±16 nT; ±64 nT; ±256 nT; ±1024nT; ±4096 nT; ±16,384 nT; ±65,536 nT

Digital resolution (12 bit)

±0.001 nT; ±0.004 nT; ±0.016 nT; ±0.0625nT; ±.25nT; ±1.0 nT; ±4.0 nT; ±16.0 nT


12 Hz

Sensor noise level

< 0.006 nT rms, 0-10 Hz

Sampling rate

24 vector samples/s in snapshot memory and 3,4 or 6 vector samples/s standard

Signal processing

FFT processor, 32 logarithmically spaced channels, 0 to 15 Hz. Full spectral matrices generated every 80 seconds for four time series (Bx, By, Bz, |B|)

FFT windows/filters

Full despin of spin plane components, 10% cosine taper, Hanning window, first difference filter

FFT dynamic range

72 dB, *-Law log-compressed, 13 bit normalized to 7 bit with sign

Sensitivity threshold

~0.5 x 10-3 nT/*Hz in range 0

Snapshot memory capacity

256 kbit

Trigger modes (3)

Overall magnitude ratio, directional max.-min. peak to peak change, spectral increase across frequency band (rms)

Instrument mass

Sensors (2): 450 g. total
Electronics (redundant): 2100 g. total

Power consumption

2.4 watts, electronics: regulated 28 V ± 2%
1.0 watts , heaters: unregulated 28 V

Table 2: Summary of MAG instrument characteristics


Figure 22: The MAG instrument (image credit: University of New Hampshire)

1) “Advanced Composition Explorer (ACE),” SRL, 2006, URL:

2) E. C. Stone, A. M. Frandsen and R. A. Mewaldt, E. R. Christian, D. Margolies, J. F. Ormes, F. Snow, “The Advanced Composition Explorer,” 1998,


4) M. C. Chiu, U. I. von Mehlem, C. E. Willey, T. M. Betenbaugh, J. J. Maynard, J. A. Krein, R. F. Conde, W. T. Gray, J. W. Hunt, Jr., L. E. Mosher, M. G. McCullough, P. E. Panneton, J. P. Staiger, E. H. Rodberg,, “ACE Spacecraft,” Space Science Reviews, Vol. 86:, 1998, pp. 257-284, URL:

5) S. B. Jacob, E. R. Christian, D. L. Margolies, R. A. Mewaldt, J. F. Ormes, P. A. Tyler, Tycho von Rosenvinge, T. B. Griswold, “Advanced Composition Explorer (ACE),” NASA brochure, 2nd edition, 2002, URL:

6) George J. Cancro, “APL Spacecraft Autonomy: Then, Now, and Tomorrow,” John Hopkins APL Technical Digest, Vol. 29, No 3, 2010, pp. 226-233, URL:

7) Information provided by D. L. Margolies of NASA/GSFC

8) William Lotko (Chair), Doug Braun, Jim Drake, Joe Fennel, Richard R. Fisher, Joe Giacalone, Tim Horbury, Bob McCoy, Mark Moldwin, Alexei Pevtsov, John Plane, Howard Singer, Charles Swenson, “Senior Review 2013 of the Mission Operations and Data Analysis Program for the Heliophysics Extended Missions,” NASA, June 13, 2013, Submitted to: Victoria Elsbernd, Acting Director Heliophysics Division, URL:

9) “Weakest Solar Wind of the Space Age and the Current "Mini" Solar Maximum,” ACE News No 165, Dec. 2, 2013, URL:

10) D. J. McComas, N. Angold, H. A. Elliott, G. Livadiotis, N. A. Schwadron, R. M. Skoug, C. W. Smith, “Weakest Solar Wind of the Space Age and the Current "Mini" Solar Maximum,” The Astrophysical Journal, Vol. 779, No 1, Nov. 14, 2013


12) Karen C. Fox, “Aurora from Oct. 8, 2012 CME,” NASA/GSFC, Oct. 8, 2012, URL:

13) Karen C. Fox, “ACE, Workhorse Of NASA's Heliophysics Fleet, Is 15,” NASA, Aug. 29, 2012, URL:

14) Information provided by Andrew J. Davis of Caltech, Pasadena, CA



17) ACE Brochure, Second Edition, Caltech, March 2002, URL:

18) Donald L. Margolies, Tycho von Rosenvinge, “Advance Composition Explorer (ACE), Lessons learned and final report,” NASA/GSFC, July 1998, URL:


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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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