# An Effective Sensor Architecture for Full-Attitude Determination in the HERMES Nano-Satellites
## 3 Sensor Selection
### Magnetometers  
Magnetometers are the other sensors that are needed to fully determine the attitude states of the spacecraft. In fact, at least two independent vector measurements are needed  
Figure 9: Light measurements with CSS photodiode.  
to estimate the full-attitude state. Moreover, the geomagnetic field knowledge is often useful on-board to drive magnetic actuators or to manage the spacecraft's magnetic dipole. For the nano-satellite application, MEMS magnetometers are commonly employed in place of classical fluxgate components. Despite the former being less accurate with respect to the latter, these sensors are extremely small, light, and power-efficient, and the accuracy level is in the order of 1 deg, which is compliant with the imposed requirements.  
The proposed architecture requires two redundant magnetometers, which were selected looking for the noise level performance. In fact, the biases, the non-orthogonality, and the scale factor errors can be corrected with the on-board calibration algorithm described before. In these regards, a lower noise in the magnetic measurements would result in better performance of the calibration functions and, in general, in improved quality of the whole attitude determination and control subsystem. The selection identified the PNI RM3100 geomagnetic sensor as a good candidate for primary magnetic measurements with its military-grade performance. In fact, this sensor, shown in Figure 10, is based on magneto-inductive MEMS sensor technology, which provides high resolution, low power consumption, no hysteresis, and low noise, if compared to other miniaturized components. Its noise level is around 15 nT, which is at a maximum 0.075 % of the measured geomagnetic field value in LEO. Moreover, this value is in the order of the sensitivity of the instruments, that is 13 nT. Thus, the magnetic measurements are extremely clean with this component, as is evident from the uncalibrated results reported in Figure 11, which were sampled at 10 Hz through an I2C data interface in laboratory environment conditions. Table 5 reports the most-relevant specifications of the PNI magnetometer retrieved from the testing activities. This magnetic sensor is commercially available with an integrated temperature sensor and an I2C digital interface from GOMSpace with the product name NanoSense M315.  
The secondary magnetometer is embedded in the Memsense MS3025M IMU, since the effort of minimizing the mass, volume, and power of the overall sensor architecture suggested the selection of a nine-axis IMU component. The choice of this sensor was mainly driven by the gyroscope performance, but the quality of the magnetic measurements is anyway good for a secondary redundant component. From the uncalibrated results presented in Figure 11, it is evident how the secondary magnetometer has a greater noise level and biases, but it allows in any case achieving the desired attitude determination goals. The reported measurements were sampled in parallel with those of the PNI magnetometer at the same sampling rate. Moreover, note that Figure 11 reports the raw measurements, without any postprocessing calibration applied. In this case also, the most-important specifications are listed in Table 6