**5. Experimental implementation of FSI based sensors**

In order to demonstrate the capabilities of FSI based sensors, two implementations are described in this section, one for FSI and another for Dual FSI, both tailored for space applications (i.e. long distances, no atmosphere refraction errors are considered, specific operational requirements associated to measurement rate and accuracy).

The first absolute distance sensor, based in FSI, was designed for a space mission comprising a multiple aperture optical telescope (coherent array), to measure the OPD between two sub-telescopes and the combining-telescope with an uncertainty at the 10 µm level at a 10 Hz measurement rate with the lowest possible technical complexity.

The second sensor, based in Dual FSI, was defined in view of a coronagraph mission composed of two spacecraft in a free-flying formation. The requirements on the absolute distance metrology sensor prototype were a measurement uncertainty below the 100 µm level for a distance around 50 m at a 10 Hz measurement rate.

In both sensors, the Laser and the FP are the key items. Complex tasks normally implemented in real time electronics were assigned to data processing, thus moving system complexity to the software area.

A drift compensation model using consecutive measurements was implemented and validated experimentally. Fig. 5 shows the principle behind the compensation technique using two consecutive measurements with different signs on the sweep range (Δν) and equal durations, corresponding to a symmetrical triangular shaped frequency sweep (Δ*t2* = Δ*t1* = Δ*t* and Δν*2* = -Δν*1* = Δν *r2* = *r1* = *r*).

Fig. 5. Principle of the Drift compensation model using two consecutive measurements with different signs on the sweep range and equal durations corresponding to a symmetrical triangular shaped frequency sweep.

Note that the frequency of the dynamic mode measurement can still be the same as it would be in the static mode single measurement as the first measurement can always be the same as the second measurement of the previous pair of sweeps. Thus, every new measurement enables another dynamic mode measurement (in this case, the characteristics of the two sweeps would swap from measurement to measurement). With this approach, we are not only correcting the length measurement but also providing a measurement of the drift speed, using Eq.(36).
