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SIMBA (Sun-earth IMBAlance)

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ESA’s SIMBA CubeSat is a tiny mission with a big ambition: to measure one of the fundamental drivers of climate change in a new way. The 3U CubeSat will turn from Earth to space to the Sun and back again, to calculate our planet’s overall energy budget. 1) 2) 3) 4) 5)

SIMBA was developed for ESA by a consortium led by Belgium’s Royal Meteorological Institute (RMI) with the University of Leuven and ISIS (Innovative Solutions in Space) of Delft, the Netherlands.

“This is the kind of scientific instrument we’d otherwise place on a full-size satellite platform,” explains Stijn Nevens, Simba principal investigator at RMI.


Figure 1: SIMBA is a 3-unit CubeSat mission to measure the TSI (Total Solar Irradiance) and Earth Radiation Budget climate variables with a miniaturized radiometer instrument (image credit: RMI)

“But if we can make this work on a smaller, cheaper CubeSat, then we might be able to build and fly multiple versions of this instrument in the future, to cover the entire planet for the equivalent cost of a single traditional mission. That is important because the variable we aim to measure is crucial.

“The main origin of climate change is that an increasing amount of heat from the Sun is being retained within the atmospheric system. To quantify that directly we need to measure how much solar energy Earth is receiving – we call this the total solar irradiance – then how much of this is being reflected by the Earth’s surface and atmosphere, or being radiated out as longer-wavelength heat energy.

“Subtracting the second from the first, we end up with a figure for Earth’s radiation budget – the amount of energy our planet holds onto rather than reflects or radiates away.

“We already have a class of instruments to measure irradiated energy, called radiometers, which convert it into electrical power for measuring purposes. Downward-looking radiometers are flying for instance on Europe’s Meteosat satellites in geostationary orbit, as well as the US family of CERES instruments in lower orbits. Then there are Sunward-facing radiometers on satellites like SOHO and PROBA-2.

“But while their results have high relative accuracy, they require a lot of additional modelling to take account of factors such as diurnal differences and surface variations. They accordingly come with a large margin of error, while the instruments themselves possess inherent biases. For sharper climate change modelling we need to do better.”


Figure 2: Simba for solar irradiance measurements. Simulated results from the Simba CubeSat mission, which will employ a radiometer to measure solar irradiance levels across Earth's surface to help study meteorology and climate change (image credit: ESA)


SIMBA is a 3U CubeSat with dimensions of 30 x 10 x 10 cm and with a mass of 4 kg. The satellite was to be originally part of the QB50 constellation, but eventually dropped out.

The SIMBA nanosatellite consists of 3 main building blocks:

• Satellite generic functions (avionics), specified by RMI, and assembled by ISIS (Innovative Solutions In Space BV, Delft, The Netherlands.

• ADCS (Attitude Determination and Control System) of the Catholic University of Leuven.

• Payload, developed by RMI.

Simba is equipped with a specially-developed CubeSat-optimized ADCS, contributed by the University of Leuven. This includes an experimental star tracker camera to fix its position against the star constellations in the sky and ‘reaction wheels’ whose shifting rate of spin cause the nanosatellite to shift its attitude in reaction.

Dr. Nevens adds: “This ADCS will give Simba a pointing accuracy of 0.1 degrees, which also enhances the overall accuracy of our data. We will achieve traceability, being able to know precisely where and at what we are looking at any one time.”

Simba has been supported by BELSPO (Belgian Science Policy Office) through the ‘Fly’ element of ESA’s GSTP (General Support Technology Program), readying promising technologies for space.


Figure 3: Simba in launch configuration. The Simba CubeSat in closed state ready to be fitted into its launch pod (image credit: RMI)

Launch: A launch of SIMBA is scheduled for 21 June 2020 on the inaugural flight of Vega SSMS (Small Spacecraft Mission Service) along with dozens of other CubeSats and small satellites. Vega will lift off from Europe's Spaceport in French Guiana carrying 53 satellites on its new dispenser SSMS. 6)

Target orbit for the 46 nanosatellites: Sun-synchronous orbit, altitude at separation of 530 km, inclination =97.51º.

Sensor complement (Radiometer)

The idea with SIMBA is to achieve a high absolute accuracy by employing the same instrument for the very first time to measure irradiance from both the Sun and Earth. The CubeSat will turn from our planet to deep space – for calibration purposes – then to our parent star.

“We’re using a broadband, wild field of view instrument, meaning we’re measuring the total outgoing flux from the whole Earth,” adds Dr. Nevens. “Simba is based on a cavity radiometer, which is basically an internal space on the other side of a very small hole, totally painted black. We are measuring how that cavity warms up.

“Imagine a house with central heating that you want to keep warm. On a summer day you don’t have to do any heating, but on a winter’s day you’ll lose a lot of heat and need to actively warm it. So we’ll be measuring how much extra energy we need to put in to maintain a fixed temperature.

“To get our baseline we’ll begin the mission by looking down at Earth for a long time, to see what temperature it stabilizes at. Then we’ll swivel out to deep space, just a few degrees from absolute zero, to learn the maximum level of heat we need to apply to keep it there. Then we will turn to the Sun in turn, measuring the amount of radiation coming in.”

SIMBA measurement concept

The SIMBA Sun-Earth measurement concept is illustrated in Figure 4. In order to be able to measure the small imbalance which is believed to be driving climate change – of the order of 0.5 W/m2 – the difference between the incoming solar radiation – of the order of 340.5 W/m2 – and the outgoing terrestrial radiation – of the order of 340 W/m2 has to be measured. When the incoming and outgoing radiation are measured by separate instruments – as is the traditional approach – the small imbalance to be measured will be overwhelmed by the sum of the calibration errors of the separate sun and earth measurements. The new approach proposed by Simba is to measure with the same instrument the Sun and the Earth radiation, such that in principle the instrument calibration error is the same for the Sun and for the Earth measurement and cancels in the differential Sun-Earth measurement.


Figure 4: SIMBA Sun-Earth measurement concept (image credit: RMI)

Even with this differential sun-earth measurement concept, several challenges remain in order to have an accurate intercalibration of the sun-earth measurements:

1) The Sun is almost a point source covering an angle of 0.5°, compared to the earth which covers a large field of view of 140° when viewed from horizon to horizon from a low earth orbit. Therefore Simba will need a wide field of view with a uniform angular sensitivity. As main SIMBA sensor we choose a wide field of view cavity radiometer, as has been used on the Nimbus-6, and -7 and ERBE satellites.

2) The Sun's surface is approximately a blackbody of 5800 K, emitting SW (ShortWave) radiation with wavelengths up to 4 µm. The Earth is approximately a blackbody of 300 K, emitting LW (LongWave) radiation above 4 µm. At night the Earth leaving radiation is only LW radiation, during the day it is composed of both SW reflected solar radiation and LW emitted thermal radiation. The SIMBA cavity needs to have a flat spectral response. As auxiliary sensors for the separation of Earth's SW and LW radiation, black and white flat spectral sensors are foreseen. The black sensor will absorb SW and LW radiation, the white sensor will mainly absorb LW radiation.

3) The Earth-leaving radiation is highly variable in space and in time due to the daily weather variation. Therefore the SIMBA MSF (Main Sensor Face), containing the cavity radiometer, will be pointed nearly continuously towards Earth, in order to maximize the Earth sampling. Only sporadically, e.g. once every three months, the MSF will be pointed towards the sun for a solar calibration. As the solar variation is well known (Figure 5), such a sporadic sampling of the sun is in principle sufficient. The SIMBA pointing modes are illustrated in Figure 6. During nadir pointing, the MSF is pointed towards Earth. During zenith pointing, the satellite is flipped over by 180°, and the MSF is seeing deep space for calibration at night, and sees the sun sweeping trough its field of view during the day, allowing a basic solar measurement. In a dedicated solar pointing mode, the angle of the MSF relative to the sun is kept fixed, for an improved solar calibration. As auxiliary sensors, the project foresees a second set of black and white sensors on the face opposite the MSF, and a wide angle visible camera on the MSF. The former allows solar and deep space calibration measurements during nadir pointing, the latter allows a high resolution characterization of the reflected solar radiation.


Figure 5: RMIB TSI composite measurements (red: daily measurements, green: 121 day running mean) and regression model (blue: 121 day running mean model) based on Mount Wilson magnetograms [image credit: RMIB (Royal Meteorological Institute Belgium)]

4) The diurnal cycle of the radiation, which is particularly strong in the tropics, is only sampled twice per day from LEO (Low Earth Orbit). The full sampling of the diurnal cycle combined with global measurements requires sampling with a constellation of satellites. While such a constellation would be expensive and slow to put in place with conventional satellite programs – e.g. the NASA ERBE program - it becomes feasible thanks to the relatively low cost and fast CubeSat approach. To measure the Earth radiation imbalance with a constellation of satellites is the long-term goal of the freshly started RMIB SIMBA program, to be compared with the measurement of the Solar Constant that was achieved in a 30 year program starting with the first RMIB space flight in 1983. The first SIMBA satellite to fly with QB50 in 2016 is intended as the technology demonstrator satellite of the SIMBA program.


Figure 6: SIMBA pointing modes. MSF = Main Sensor Face. RAM = flight direction (image credit: RMIB)

1) ”Simba CubeSat to swivel from Earth to Sun to help track climate change,” ESA Enabling & Support, 15 June 2020, URL:

2) Tjorven Delabie, ”SIMBA Sun Earth Imbalance mission,” KU Leuven, November 2015, URL:

3) S. Dewitte, A. Chevalier, E. Janssen, N. Clerbaux, M. Meftah, A. Irbah,T. Delabie, O. Karatekin, “The Sunā€Earth Imbalance Radiometer for a Direct Measurement of the Net Heating of the Earth,” Proceedings of the 4S (Small Satellites Systems and Services) Symposium, Port Petro, Majorca Island, Spain, May 26-30, 2014

4) Steven Dewitte, “Long term solar changes and their effect on Earth,” STCE (Solar-Terrestrial Center of Excellence) Workshop, Brussels, Belgium, May 19, 2014, URL:

5) Steven Dewitte, “Radiation Measurements at the RMIB - Past, Present and Future,” September 27, 2013, URL:

6) ”Watch Vega's rideshare launch live,” ESA Enabling & Support, 17 June 2020, 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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