## 2 Potential magnetic sensors for space applications  
As happens in almost every technological niche on ground [2], magnetic sensors are useful for many applications in the space sector [3]. Though the most representative application is the in-orbit measurement of the magnetic field, there are some others as magnetic encoders [4], angular and position sensors [5] and magnetometers or gradiometers for planetary magnetometry. Since magnetic applications are so varied, the choice of magnetic sensor can be a difficult task. Figure 1 represents in a graph the panorama of the different magnetic sensors: the most representative technologies used for magnetic sensing are represented as a function of their magnetic characteristics: minimum detectable field and dynamical range. The applications have been depicted in Ben diagrams intersecting the bars of the technologies which can be used for the particular application. This work is focused on potential COTS and small sensors for space measurements of the magnetic field or magnetic gradient. The magnetic field in-orbit can be measured for geomagnetic measurement purposes, or also inversely, to determine the relative orientation of a spacecraft in the geomagnetic field. This is the purpose of magnetic sensors in ACS - Attitude Control Systems [6]. In some missions measurement of the gradient of the field is also needed. In general terms the requirements of the magnetometers used for geomagnetic field mapping are very strict. These magnetometers are required to measure the vector and scalar magnetic fields of the Earth with resolutions typical of the degrees 40 to 60 of the harmonics expansion of the field [7], the variations due to the ionospheric interaction and other perturbations [8-10]. The sensor technology par excellence for measuring the Earth magnetic field vector is the fluxgate because it is the best trade-off between resolution, stability and power consumption, mass and volume, and also some scalar magnetometers have been used to measure the intensity of the field or to complement the measurement obtained with fluxgates. Fluxgates are based on the change of magnetic reluctance of a ferromagnetic core when it is driven by an ac saturating field in the presence of a magnetic field. The driving field is provided with the so called primary coil and the changes in the reluctance are measured by means of the secondary coil. These sensors are able to measure magnetic fields ranging from the mT to the tens of pT and in the range of frequencies from dc to the order of the operation frequency (tens of kHz). Fluxgates for Geomagnetic field mapping are usually sophisticated instruments manufactured _ad hoc_, precisely oriented (in the order of arcsec) with dynamical ranges of \(\pm\) 64000 nT, bandwidths in the order of tenths of kHz (due to the relatively slow variations of the Earth magnetic field), resolutions in the order of tenths of nT to pT with long-term stability (1 nT in the whole range of temperatures of operation) [11] and low noise density (several pT/\(\sqrt{\mathrm{Hz}}\) @ 1 Hz). The main drawbacks of fluxgates are the high mass and power consumption. Typical mass of this kind of instrument can be in the range of 0.5 kg and 2 W of power consumption. Besides, both the reduction of mass and power decrease sensitivity and stability of the sensors. Thus, fluxgates should be used in those missions where volume and mass savings are not a priority, only moderate requirements. Scalar magnetometers can be used alone (for isodynamical charts) or in combination with vector magnetometers for their calibration (absolute measurement). Proton precession (based on the principle that protons spin axis is aligned with the magnetic field), Overhauser (based on a quantum physics  
Figure 1: Magnetic Sensors Technologies: Magnetic properties (dynamic ranges and detectivities) and applications. effect in hydrogen atoms), helium and cesium vapor magnetometers (based on the quantum mechanic change of absorption of the gas) are examples of scalar magnetometers.