ACRIMSAT (Active Cavity Radiometer Irradiance Monitor Satellite)

Overview    Spacecraft    Launch   Mission Status    Sensor Complement   References

ACRIMSAT is a NASA funded minisatellite mission to monitor the amount of total solar energy received (part of EOS program). The objective is to monitor the solar constant, or TSI (Total Solar Irradiation), with maximum precision and provide an important link in the long-term TSI database. ACRIMSAT is part of a multi-decade effort to understand variations in the sun's output and resulting effects on Earth. 1) 2) 3) 4) 5) 6)


The spacecraft builder/integrator is OSC (Orbital Sciences Corporation) of McLean, VA. The structure is based on the Ministar bus. In stowed configuration the S/C dimensions are: 79 cm in diameter x 69 cm in height (total span with solar arrays deployed = 178 cm). The spacecraft is spin-stabilized and sun pointing. It carries a flight processor used on previous missions. S/C mass = 115 kg, power = 49 W (average). The mission design life is 5 years.

The mission is managed and operated by the JPL ACRIM team via the ACRIMSAT ground station located at the JPL/Table Mountain Facility. The PI of the mission is Richard C. Willson of Columbia University. 7)


Figure 1: View of the ACRIMSAT spacecraft (image credit: JPL)


Figure 2: Alternate view of the AcrimSat spacecraft (image credit: NASA/JPL)


Launch: ACRIMSAT was launched as a secondary payload on Dec. 20, 1999 on a Taurus launch vehicle (the primary payload was KOMPSAT-1 of Korea) from VAFB, CA, USA.

Orbit: Sun-synchronous near-circular orbit, altitude = 683 km x 724 km, inclination = 98.13o, LTDN (Local Time on Descending Node) at 10:50 hours.

RF communications: The communication links are in S-band, uplink frequency at 2065.5 MHz, uplink data rate at 2 kbit/s; downlink frequency at 2250.0 MHz with data rates of 3.6, 28.8, 57.6 or 115.2 kbit/s. In addition, there is an UHF link.



Mission status:

In December 2013, AcrimSat suffered a mission-ending failure when its degrading batteries could no longer sustain operations. The spacecraft has not responded to ground commands since December 14, 2013. After several unsuccessful recovery attempts and extensive failure analysis, the mission was determined to be unrecoverable and officially terminated on July 30, 2014. 8)

- AcrimSat lasted nearly 13 years, more than twice as long as originally designed, and delivered a dataset that exceeded mission requirements, making it an unqualified success. The dataset will receive a final reprocessing using updated laboratory characterizations of all 3 ACRIM sensors; the reprocessed dataset will be available to researchers beginning August 2015. This more than 3 decade long data series exceeds the duration of any other irradiance instruments.

• March 15, 2014: The AcrimSat spacecraft went into ‘Phoenix status' during its recent (November 2013 – February 2014) eclipse orbital mode due to battery issues (contact was lost on Dec. 14, 2013). There was hope that it would have revived when it came out of the eclipse season a few weeks ago, but as of two weeks ago it had not been heard from.

- The AcrimSat battery problems occurred only months after a review recommended its continued operation. 9) 10)


Figure 3: TSI (Total Solar Irradiance) monitoring results : 1978 to present (image credit: Richard C. Wilson, Nov. 22, 2013) 11)

• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 12)

- The ACRIM-3 (Active Cavity Radiometer Irradiance Monitor-3) experiment on ACRIMSat began in 2000 as part of the Earth Observation System (EOS) of NASA's Earth science missions. It is a key NASA component of the international effort to compile an accurate and precise TSI (Total Solar Irradiance) database on climate timescales. ACRIM-3 is capable of sustaining the 34-year satellite TSI database across a potential gap resulting from the launch failure of the Glory/TIM experiment with no loss of traceability or measurement cadence.

- The primary purpose for an extended ACRIM3 mission is to ensure that there is no deterioration of the 34-year satellite TSI database across a potential gap caused by the loss of the GLORY/TIM experiment.
The ACRIMSat satellite and the ACRIM-3 instrument are 100% operational and capable of continued operation with full science performance through at least FY14–15. In fact, the technical risk rating is medium-low for the extended mission. Satellite health and performance trends indicate that ACRIMSat/ACRIM-3 observations are likely to be available to support the launch and calibration phase of the next TSI monitoring experiment, JPSS-1/TSIS, in 2017. In the event of a SORCE failure, ACRIM-3 measurements are of the quality required to continue the TSI record and should be maintained as long as possible.



Figure 4: TSI results of the ACRIMSat/ACRIM-3 mission (image credit: RC Wilson, April 7, 2013) 13)

• The AcrimSat spacecraft is operating nominally in 2013.

• The AcrimSat spacecraft is operating nominally in 2012 (on orbit for 12 years). NASA extended the AcrimSat mission for the period 2012-2013.
Note: The AcrimSat mission is considered as a backup to TIM (Total Irradiance Monitor) flown on the SORCE (Solar Radiation and Climate Experiment) mission of NASA. 14)

• The AcrimSat spacecraft is operating nominally in 2011. 15)

• The spacecraft and payload (ACRIM-III) are operating nominally as of 2010 (good health of instrumentation with very little degradation in system performance). In Dec. 2009, the mission was in orbit for 10 years. The current mission extension goes through 2011. 16) 17)

• AcrimSat completed its 5-year nominal mission life in the spring of 2005. 18)

• The ACRIM-III instrument observed a drastic drop in solar irradiance levels when Venus transited between the Earth and Sun in June 2004. 19) 20)

• The science mission began April 5, 2000, and has provided an average of 65 minutes of TSI data per orbit.

• Initial problems of the attitude control subsystem were experienced preventing proper pointing of the S/C toward the center of the sun. However, the pointing issue was resolved two weeks after launch. The science payload was intentionally left in standby mode, from January 16, 2000 through April 4, 2000, in order to assure outgassing of the spacecraft as much as possible; an outgassing lesson learned from previous missions. This has proved to be very effective in initial contamination control, which typically plagues sensor surfaces. 21)



Sensor complement: (ACRIM-III)

ACRIM-III (Active Cavity Radiometer Irradiance Monitor-III):

ACRIM-III (or ACRIM-3) was designed and built at JPL. The ACRIM-III instrument has a heritage of ACRIM-I, flown on the SMM (Solar Maximum Mission) satellite (launch Feb. 14, 1980) and ACRIM-II, flown on UARS (Upper Atmosphere Research Satellite; launch Sept. 1991). The objective is to monitor the variability of total solar irradiance (TSI) with state-of-the-art accuracy and precision, thereby extending the high-precision database compiled by NASA since 1980 by other ACRIM experiments as part of the Earth Radiation Budget Program in the National Climate Program. Note: The first ACRIM sensor was flown on the Solar Maximum Mission (Launch: Feb. 14, 1980), follow-up missions: UARS, Spacelab-1, and ATLAS. 22) 23) 24) 25)


Figure 5: Photo of the ACRIM-III instrument (image credit: NASA/JPL)


Figure 6: Illustration of the ACRIM-III instrument design (image credit: NASA/JPL, Ref. 22)

Spectral coverage

0.2 - 2000 nm

Observation coverage

Reception of 62-64 minutes per orbit of full solar disk data

FOV (Field of View)


Pointing accuracy

±0.25o on S/C to sun line

Detectors (ACR)

Three, each with 30o right-circular cone painted specular black on the sun-viewing face


0.1% of full scale

Instrument mass, power

13 kg, 10 W

Instrument dimensions

Sensor housing: 18.5 cm diameter x 36 cm in length
Central processing unit: 20.5 cm x 11.5 cm

Sampling interval

2 minutes (approx.)

Data rate

600 bit/s ( about 600 kByte of data are collected per day)

Table 1: Specification of the ACRIM-III instrument

The spacecraft is pointed toward sun using magnetic torque bars of its attitude control subsystem. ACRIM-III is rigidly attached to the spacecraft. The instrument contains three independent active cavity radiometer (ACR) solar-monitoring sensors and a sun-position sensor. One ACR monitors solar irradiance, the other two are being used to calibrate the optical degradation of the first instrument. 26) 27)


Figure 7: Total solar irradiance monitoring results: 1978 to present (image credit: R. C. Willson, June 23, 2011) 28)


Figure 8: TSI monitored by the AcrimSat/Acrim/3 mission (image credit: R. C. Willson, June 23, 2011) 29)


1) Information provided by A. McLean of NASA/JPL

2) "ACRIMSAT Science and Mission Overview," NASA/JPL. Oct. 19, 2004, URL:

3) ACRIMSAT - Measuring the Sun's Energy," NASA, JPL, Oct. 30, 2007, URL:


5) NASA Press Kit, December 1999, URL:

6) R. C. Willson, "Science Rationale for an EOS/ACRIMSAT (Active Cavity Radiometer Irradiance Monitor Satellite) Mission," The Earth Observer January/February 1995, Vol. 7, No. 1

7) ACRIMSAT - Measuring the Sun's Energy," NASA, JPL, Oct. 30, 2007, URL:

8) "Active Cavity Radiometer Irradiance Monitor Satellite (ACRIMSAT)," NASA, August 5, 2014, URL:


10) Ken Jucks, Hal Maring, "NASA HQ Update," NASA, January 2014, URL:


12) Elizabeth Ritchie (Chair), Ana Barros, Robin Bell, Alexander Braun, Richard Houghton, B. Carol Johnson, Guosheng Liu, Johnny Luo, Jeff Morrill, Derek Posselt, Scott Powell, William Randel, Ted Strub, Douglas Vandemark, "NASA Earth Science Senior Review 2013," June 14, 2013, URL:


14) Ramesh Kakar, "NASA Earth Science Division Perspective," NASA, Sept. 13, 2011, URL:

15) NASA, Jan. 26, 2011, URL:

16) Tudor Vieru, "Anniversary: AcrimSat Turns 10," Dec. 22, 2009, URL:

17) Steven A. Ackerman (chair), Richard Bevilacqua, Bill Brune, Bill Gail, Dennis Hartmann, George Hurtt, Linwood Jones, Barry Gross, John Kimball, Liz Ritchie, CK Shum, Beata Csatho, William Rose, Carlos Del Castillo, Cheryl Yuhas, "NASA Earth Science Senior Review 2009," URL:



20) Richard C. Willson, "ACRIM3 Observations and Variations of Total Solar Irradiance during solar cycles 21-23," SORCE Science Team Meeting, April 28, 2003, URL:

21) Information provided by Roger S. Helizon of NASA/JPL



24) "AcrimSat," Earth Science Reference Handbook, pp. 65-68,

25) Richard C. Willson, Roger S. Helizon, "Active Cavity Radiometer Irradiance Monitors 1, 2 & 3, S.I. Measurement Uncertainties," NASA/NIST TSI Workshop, July 18-20, 2005, URL:

26) R. C. Willson, R. S. Helizon, "EOS/ACRIM III instrumentation," Proceedings of SPIE, Vol. 3750, pp. 233-242, Earth Observing Systems IV, William L. Barnes; Ed., Sept. 1999

27) R. C. Willson, "The ACRIMSAT/ACRIM III Experiment - Extending the Precision, Long-Term Total Solar Irradiance Climate Database," The Earth Observer, May/June 2001 , Vol. 13, No 3, pp. 14-17, URL:


29) R. C. Willson, January 15, 2011, URL:

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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Serves as backup for SORCE.