## 2 Potential magnetic sensors for space applications
All of them are absolute sensors of the magnetic field and can improve one order of magnitude the resolution of the fluxgates. The first mission with a magnetometer onboard was the Sputnik 3 (1958), which carried a fluxgate. There were also fluxgate magnetometers in the Lunik 1 and 2 (1959) devoted to the measurement of the magnetic field of the Moon. In the American side Ranger 1 and 2 (1961) had rubidium scalar magnetometers and the Mariner 4 (1964) to Mars and the Mariner 5 (1967) to Venus carried helium magnetometers. Explorer 10 (1961) already used a combination of magnetometers separated a certain distance (dual technique): a rubidium scalar magnetometer and a fluxgate to measure the interplanetary field. With this technique the contribution of the platform to the magnetic field can be compensated. Besides scalar a vector magnetometers have complementary advantages. For example scalar ones can be used for calibration and vector ones can give more information about the sources of the field. This solution has been adopted by the big geomagnetic missions like MAGSAT (1979), which carried a cesium scalar magnetometer and a fluxgate vector sensor [12], Orsted (1999) [13] and CHAMP (2000) with an Overhauser scalar and a fluxgate vector magnetometer, SAC-C (2000) with a helium magnetometer and a fluxgate [14], ST-5 [15] or the future SWARM in which the magnetic field will be not only measured at different points but also at different moments. The Attitude Control System (ACS) of a spacecraft is that devoted to determine and control the orientation of the satellite. That is to sense and correct the relative orientation of the satellite within an inertial reference frame. Attitude determination for a spacecraft is in general a difficult task since it is continuously changing. To determine and control the attitude, spacecrafts normally use several sensors that work in a closed loop with actuators or torquers for the correction of the orientation of the spacecraft. These torquers can also be of many types: gas propulsion thrusters, magnetocoils, gravity gradient torquers, inertial wheels, gyros, etc. In the ACS applications missions incorporating an optical camera, of Earth observation, military missions, or missions devoted to the development of accurate measurements of the geomagnetic field, like the ones mentioned above, have strict pointing needs respect to an inerical frame or reference and thus, (solid state) magnetic sensors, which can only achieve accuracies in the determination of 0.5\({}^{\circ}\), are normally replaced by more accurate devices as star sensors (arc sec accuracies) together with accurate inertial sensors (gyroscopes) for a continuous quantification of the deviations in the attitude. But there are other missions like some communications satellites, even with omnidirectional antennae, test-beds for technological experiments, etc, with lower requirements in which other alternatives can take place. In these missions the ACS requirements are more relaxed and they often use a combination of sensors for the determination of the attitude. This combination can include a magnetic sensor and other sensors as solar, planetary limb or gravity vector sensors as measuring elements. As actuators they can use for instance magnetic torques. For this kind of application other magnetic sensors apart from fluxgate are potentially good candidates. Micro, nano, picosatellites and some minisatellites are in general representative examples of this situation. Requirements of the different magnetic applications are often relaxed, and have a philosophy headed to save mass and volume. In these frames, commercially available technologies (COTS) [16] like AMR-Anisotropic MagnetoResistance, a mature technology, and the brand new GMR-Giant MagnetoResistance, MI-MagnetoImpedance and TMR-Tunnel MagnetoResistance or SDT-Spin-Dependent Tunnel technologies (Table 1), can be plausible alternatives for magnetic purposes providing in most of the cases a miniaturized solution [17, 18].