In 2022, ALTIUS should be circling around the Earth, looking permanently at the sun-lit atmospheric limb, and keeping a close eye on the amount of ozone in our air. Deeply hidden in the body of the ALTIUS spectrometer is a piece of electronics hardware that truly creates the heartbeat of this unique instrument. Pulsating at a rhythm millions times faster than our own heart, using radio frequency signals, it allows the very eyes of ALTIUS—the Acousto-Optical Tunable Filters—to capture light at any place in the spectral domain.
The ALTIUS instrument
ALTIUS is a small autonomous Earth observation mission allowing for atmospheric sounding in many different geometries. The name ALTIUS is given as well to the PROBA-like spacecraft as to the unique payload instrument on board.
The main science objective of the mission is to monitor the distribution of ozone in the atmosphere with high resolution by looking with a 2D imaging spectrometer to the limb of the Earth.
The ALTIUS instrument (fig. top) is composed of three spectral channels, of which two, the infrared and the visible channel, contain an Acousto-Optical Tunable Filter (AOTF) (fig. 2). An AOTF is a band-pass filter made of an optically transparent material (e.g. tellurium dioxide). When an acoustic wave is pushed through the crystal, it alters its properties (index of refraction).
Depending on the wavelength of the acoustic wave, a different small portion of the spectrum will diffract out of the crystal. All the other light will be blocked inside. This technique is extremely useful to look at dedicated spectral absorption features in the atmosphere.
Creating the acoustic wave requires that a piezo-electric transducer is attached to the side of the crystal to which a radio-frequent signal can be applied. Varying the radio-frequency will change the center of the spectral passband of the AOTF.
A radio-frequent (RF) electronic system
The AOTF has to be driven by a well-controlled and stable radio-frequency signal that has sufficient power. For the infrared channel, the frequency range is 46 to 95 MHz, for the visible channel 72 to 183 MHz.
In a first block, a low power radio-frequent signal is produced in an RF generator (fig. 3). Although also fully digital solutions are possible for radio-frequent signal generation, here an analog approach is used, based on the phase-locked loop (PLL) principle. A PLL is a regulation system that matches (the phase of) its divided output signal to a high precision reference frequency.
At its exit the PLL has a voltage controlled oscillator (VCO) that is regulated from a phase comparator (a frequency synthesizer) inside the loop.
The stable high-frequency output from the PLL is scaled down to the appropriate frequency of the ALTIUS channel and passed to a second block, a radio-frequent amplifier (fig. 4).
In the amplifier the low power signal from the generator is amplified in order to reach the appropriate power level needed in the AOTF to produce an efficient acoustic wave.
RF generator, RF amplifier, crystal transducer and AOTF form the core of the NIR and VIS channels of ALTIUS. The radio-frequent beating of this system defines which molecules will be observed in the Earth’s atmosphere.