The on-ground thermal vacuum tests and commissioning data showed that BAM data are more complex than the pure interference of two perfect Gaussian beams.3 This is probably the combination of several effects, such as accumulated aberrations along the BAM optical path, partial clipping by some mirrors or interference effects within the detector itself (fringing). At the fringe level, these effects manifest as being neither purely plane-parallel nor equispaced.
An ad-hoc processing has been carried out for selected time intervals. Basically, for each fringe and AC column, cosine fits have been carried out to determine the AL position, which is a 2D map of the fringe pattern. Row by row subtraction provides the fringe periods spatial distribution. Low order 2D polynomial fits have |  | FoV1 white fringe index median FoV2 white fringe index median FoV1 white fringe index 2D fit FoV1 white fringe index 2D fit |

[Figure: Figure 13. Basic angle variation sensitivity to heat pulses in various service (Top and Middle) and payload (Bottom) module components. This experiment was carried out during two days before a payload decontamination event in June 2015.]

[Figure: Figure 14. BAM signal decomposition for telescope 1. Linear combination of different sets of service module temperatures can reproduce the overall shape of the basic angle variations. Left: phase-array antenna and launch vehicle adapter ring. Right: deployable sun-shield panels.]

[Figure: Figure 15. Basic angle and station keeping manoeuvres. Left: schematic geometry of the forces applied to the spacecraft during one station keeping manoeuvre. Centre/Right, associated transient response in the basic angle as seen by the BAM. Plots: Airbus DS.]

[Figure: Figure 16. BAM interferogram detailed analysis. Left: for a given telescope 1 pattern, the AL location for each fringe and AC column has been determined via cosine profile fitting, producing this staircase graded pattern. Middle: a low order 2D polynomial fit to the data seems similar at first glance. Right: row by row subtraction of the left 2D map provides the distribution of the fringe period over the interferogram, revealing significant structure.]

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[Figure: Figure 17. BAM white light fringe. Left: effect of a decentred white light fringe plus a change in the fringe period mimicking a true pattern shift. Image: Airbus DS. Right: apparent location of the white light fringe for each field of view and several tests.]

also been computed for comparison. Fig. 16 shows some sample fit results for telescope 1 (the one with more elements in the BAM optical path). The inhomogeneity of the fringe period over the interferogram is evident, which raises concern on summarising it as a single and homogeneous quantity. This effect should be considered in future missions relying in very precise laser interferometry.
There have been efforts to determine the location of the white light fringe during the on-ground thermal vacuum tests and in-orbit. Basically, the white fringe location was determined during payload integration using a non-monochromatic light source. It was thus verified that, for each interferogram, the white fringe is well placed in the central region (good control of the optical path). The reasoning behind this requirement was to avoid aliasing between true pattern shifts due to line of sight variations and the combined effect of a change in the fringe spacing and a decentered white fringe, which would provide a centre of mass shift of the interferogram, see Fig. 17.
There have been efforts during the on-ground thermal vacuum campaigns, in-orbit commissioning and even some tests running during the nominal mission to re-determine the location of the white fringes. The objective is to verify that they are still well centred and to estimate the potential effect of aliasing with typical nominal mission fringe period changes during a revolution, which are small, ∼1 ppm but not zero.
The BAM has two laser sources (nominal and redundant), but no white light source. Therefore, different tests were carried out making use of the known laser wavelength sensitivity to temperature (major effect) and current intensity (minor).