## 4 Magnetic sensors for space missions. The implementation of magnetic COTS in spacecrafts
Thus, the magnetic cleanness requirements are very strict with magnetic field lower than 10 \(\mu\)T and Power Spectral Density - PSD lower than 650 nT \(/\)\(\mathrm{\SIUnitSymbolMicro Hz}\), magnetic field gradient (proportional to the magnetic force) lower than 5 \(\mu\)T/m and PSD lower than 250 (nT/m)\(/\)\(\mathrm{\SIUnitSymbolMicro Hz}\). In the beginning of the project four fluxgates out of the cavity were to be used to measure the magnetic environment inside the cavity. Since the knowledge of the magnetic behaviour can be hardly deduced with this configuration, there is an actual trend to substitute the bulky fluxgates by an array of small solid state sensors temperature compensated [59]. But still the magnetic application where miniaturized sensors has been used is the ACS. As it has been said many satellites use magnetometers for attitude determination. Close to 63 % of the small satellites since the nineties used magnetic-based ACS or at least magnetic devices combined with sensors and actuators of another nature (GPS, Sun sensors, Earth sensors, gyrosensors, microvacuum arc thrusters, gravity gradient sensors, etc). In the beginning, these satellites inherited the fluxgate magnetometers from the conventional missions but as soon as mass, volume and power savings became more important requirements, the use of COTS magnetic sensors became a normal practice. The first microsatellite, according to the modern concept of small satellite, was the UoSAT-1 or OSCAR-9. OSCAR - Orbiting Satellite Carrying Amateur Radio - was the first program of modern microsatellites produced by the AMSAT-AMateur radio SATellite community. Their first launch (1961) consisted of a simple radio-satellite, but the subsequent missions have become more and more sophisticated with time. UoSAT-1 was the first experimental satellite from the University of Surrey in the UK. The 52 kg and 420 x 420 x 740 mm\({}^{3}\) platform, launched in 1981 by a Thor Delta launcher from the Vandenberg Air Force Base, carried research, technology demonstration and educational payloads. The magnetometer of UoSAT-1 consisted in a three-axis fluxgate that provided vector measurements of the Earth magnetic field in a boom at a rate of 6.25 vectors per second. Each orthogonal component had a range of around 33 \(\mu\)T and a resolution of 2 nT. Shortly afterwards and motivated by the success of UoSAT-1, the group developed in just half a year the UoSAT-2, a 45.5 kg and 350 x 350 x 650 mm\({}^{3}\) platform to be launched in 1984. It was thanks to these two satellites that the University of Surrey became a world leader in the small satellites field by means of the SSTL-Surrey Satellite Technology Ltd spin-off company. The example was followed by other universities leading to smaller and smaller satellites like the OSCAR-18 (WO18, WeberSat), a 16.03 kg and 150 x 150 x 150 mm\({}^{3}\) size platform, developed by Weber State University in Utah in 1990 [60-62]. The ADCS-Attitude Determination and Control System of OSCAR-18 comprises a horizon sensor using two photodiodes with 22 \({}^{\circ}\) field of view and a fluxgate magnetometer as sensing elements and permanent magnets, hysteresis rods, and black/white painted antennas for spin control (similar to a photometer) as actuators. Besides, it also carried, as payloads, a piezoelectric particle impact detector, which measures the micrometeorite environment and a light spectrometer using a CCD - Charged-Coupled Device by NEC, which measures the spectrum of sunlight reflected from the atmosphere. The satellite suffered an apparent radiation induced computer failure on August 23\({}^{\mathrm{rd}}\), 1997 and it did not recover from the failure until November 1997. The jump to COTS technologies for the ACS of these small satellites had to wait until the new millennium. At that moment there were fluxgates off-the-shelf but also there were solid state sensors off-the-shelf, which had already been exhaustively tested on-ground applications (TRL higher).