SIM3D is a new generation, room temperature radiometer with four channels, which can be operated independently or in groups of two for internal comparisons. SIM3D has electrically calibrated cavity receivers. Two cavities are operated together. One is closed and the other is operated with cyclic shutter phases of 50s open and 50s closed. Phase sensitive detection at the fundamental shutter frequency is used to analyze the signal. In order to cope with the large change in solar irradiance from 1 to 0.2 AU SIM3D uses three different apertures.
4-Channel radiometer with an uncertainty of <500 ppm Apertures
: 8, 4, and 2 mm, corresponding areas 0.5, 0.12 and 0.03 cm2; half-field view angle 4.3°; slope angles varying between 1.4 ° and 3.6 ° Volume
: LxWxH 210x150x220 mm with the 210x150 mm face looking to the sun; in front there will be a radiation shield Mass
: 4.0 kg
: 7 W. Normal data rate
: 750 b/s; during tests up to 3500 b/s
The answers to these fundamental questions are not yet known: “Solar irradiance variations – flux redistribution or luminosity variations?” “Is the radiant output at the poles different from the value we observe in the ecliptic?” “Why is the irradiance variability of the Sun a factor of three smaller than that of Sun-like stars?” Observations from the solar orbiter platform yields the solar irradiance from different angles and different solar latitudes. The viewing aspect is expected to be the key to answer these questions.
Figure 1. Total solar irradiance (TSI) composed from normalized observations of radiometers on different satellites.
Continuous measurements of the electrical power are submitted to a frequency analyzer. The amplitude components at the fundamental shutter frequency and up to about the 8th harmonics are transmitted to ground. Only the in-phase signal of the fundamental shutter frequency is needed to calculate the irradiance. The operation is manly digital, thus eliminating analogue components, which are prone to aging. There are only two analog circuits: the amplifier of the thermistor bridge and the highly stable heater power source, that is used to feed the shuttered cavity.
Figure 2.Principle of a substitution radiometer. Optical power absorbed in a black cavity is substituted by electrical power during a closed phase of the shutter. The heat flow from the cavity into a heat sink is kept constant by an electronic control loop that modulates the heater power, which is fed to the cavity synchronously with the shutter movements.
Figure 3. Exploded view of SIM3D