JuanCastillo29/space-mission-intelligence-data huggingface dataset:
The main force driving the Gaia design and architecture is to achieve maximum stability by purely passive means. In this way, there is no active thermal or mechanical control neither within the payload nor the service module, except for survival purposes.
The spacecraft architecture is structured to provide maximum isolation of the payload module with respect to the ambient changes, see Fig. 1. The payload is composed of two telescopes, a focal plane and a spectrometer mounted on a circular optical bench, the torus. The latter is connected to, but isolated from the service module via three bipods, which have double struts: carbon fibre and glass fibre reinforced polymers. The carbon fibre struts provide rigidity and resistance against the launch forces, but have high thermal conductivity. They are thus decoupled after launch, leaving the good insulator glass fibre struts as the main interface between the service and payload modules.
The service module is composed of a main body, a thermal tent and a deployable sun shield. The main body provides structural integrity through a CFRP truncated cone, support for the equipment (communications, propulsion, electric power, computers, atomic clock, ...) and serves as base for the payload. The thermal tent encapsulates the payload module and provides further stability. The deployable sun shield avoids direct illumination of the payload while the spacecraft rotates around the spin axis at a constant sun aspect angle of 45◦, which results in a constant irradiation. It also provides an appropriate operational temperature of ∼−110◦C for the detectors.

The general principle is that Gaia has no moving parts, which means using laser gyros instead of mechanical units, and a phase array beam steering antenna, instead of mechanical joints and a moving dish. The only exception is the focusing mechanism, see Sect. 3, which is very rarely operated. All components susceptible to consume significant and variable amounts of power, such as the computers, transponders and even the atomic clocks, are located in the service module.
commissioning (March 2014) to March 2016. Major payload decontaminations and telescope refocuses are indicated.]

[Figure: Figure 3. M2MM overview (left) and WFS sample pattern (right). The M2MM provides five degrees of freedom (three translations and two rotations) control over the Gaia telescope secondary mirrors. They are only operated during the rare refocusing events. Image: Airbus DS. Two Shack-Hartmann WaveFront Sensors (WFS) provide low-order sampling needed during the first focusing attempts in early commissioning and differential measurements for fine tuning during the nominal mission.]

[Figure: Figure 4. Straylight evolution for selected spacecraft revolutions. Left: VPU2, PEM06. Right: VPU7, PEM06. The x-axis shows the heliotropic spacecraft spin phase. The times when the Sun is closer to telescopes 1 and 2 are indicated by black circles. The left panel shows a CCD whose main straylight contributor is the Sun, while additional sources, mostly the Galaxy, are also a concern for other detectors. Images: Airbus DS.]

Optimum performance has been regained afterwards. This strategy is thus the baseline for the remaining of the Gaia mission: payload decontamination followed by small z-axis refinements, when needed.
The analysis of the basic angle variations comprises the bulk of this work. Sect. 5.1 provides an overview of the basic angle, its meaning importance and measurement. Sect. 5.2 describes the current knowledge of the BAM fringe phase and period variations and discontinuities, Selected work of the ESA-Airbus DS basic angle and straylight working group are presented in Sect. 5.3. Finally, a detailed fringe by fringe analysis and its application to the search for the white fringe are shown in Sect. 5.4.
The basic principle of Gaia has been described several times.6 See also Prusti et al. (2016 A&A, in press). To obtain absolute parallaxes, the differential positions in stars observed in fields of view separated by a large angle are determined by simultaneous observations with two telescopes whose line of sight is separated by a fixed and large basic angle. When a significantly large number of (pairs of) measurements is acquired, with different orientations on the sky, the individual proper motions and parallaxes can be determined for each star in an iterative way.
However, deriving absolute parallaxes relies in the basic angle either being stable, or at least known, at levels below the Gaia accuracy floor, which is a fraction of µas. Low frequency variations, slower than the six hours rotation period, are easily accounted for by self-calibration. Very short period variations are averaged during all transits. Heliotropic angle rotation-synchronised systematic variations provide the greatest challenge. They can create systematic errors in the astrometry (e.g. a shift in the parallax zero point) and it is unclear whether they can be fully self-calibrated (there are ongoing efforts in this direction, though).
An on-board metrology system, the Basic Angle Monitor (BAM), was included in the payload as a means to ensure the mission goals can be achieved even in the presence of such Sun synchronous perturbations. The BAM works by differential measurements. An artificial star is projected through each telescope onto a common dedicated detector in the focal plane. If the relative AL distance between the artificial stars change, this means the basic angle has varied. In order to achieve high precision in a short time, the artificial stars are interference patterns generated from a single laser source divided into four beams, two per telescope. An overview of the whole system is presented in Fig. 7 The design rules guaranteeing the BAM measurements are related to real basic angle variations and not to instabilities in the BAM itself are described by.3 It also presents the precision requirements, which are a differential measurement in the along scan direction better than 0.5 µas each 10 minutes. The on-board behaviour indeed fulfils those requirements, which are absolutely unprecedented. Note that if such a tiny angular shift is interpreted in terms of primary mirror rotations, we are considering pm level displacements, equivalent to microfringe shifts or, in terms of the SiC crystal structure, subatomic movements (∼0.02 atoms).

[Figure: Figure 8. BAM measurements during commissioning. Left: telescope 1, Right: telescope 2. Top: fringe phase. Bottom: fringe period. Reproduced from.3]

In terms of noise requirements, the system provides 12.6 µas Hz−1/2 ∼34 pm Hz−1/2 in a frequency range starting at 0.043 Hz, and well below the sub-mHz regime, ideally up to 2.3e-5 Hz. This is even better than the eLISA gravitational waves mission mHz requirements.7
Typical commissioning BAM fringe phase and period measurements were presented by,3 and are reproduced in Fig. 8. Four types of variation were present at that time 1. Fringe phase Sun-synchronous periodic variations with ∼mas amplitude. See above for their potential impact in astrometry.
2. Fringe phase discontinuities up to mas in size and several of them per day.
3. Fringe period variability, pseudo-periodic and with different magnitude per field of view.
4. Fringe phase mid- to long-term evolution, mas+ in amplitude, but over time ranges of days and longer.
Of them, the long term evolution was quickly found both wrong, the basic angle evolves but at a slower pace; and is anyway irrelevant because self-calibration handles it. However, the periodic variations of mas amplitude constitute one of the most challenging scenarios. A lot of effort was put first into determining whether the variations are real and in correcting the astrometry using the BAM measurements.
Lindegren et al. (2016, A&A in press) explains how the basic angle variations can be expanded in terms of a Fourier series of the rotation period. Most harmonic terms have been independently derived using stellar data within the astrometric solution and ad-hoc software. Table 1 shows the BAM values for the different cosine (C) and sine (S) terms together with the astrometric solution. The agreement between them is remarkable, at the level of 10-50 µas, much below the accuracy limit for Gaia Data Release 1. In addition, this demonstrates that the BAM is indeed working well and providing real basic angle measurements with unprecedented precision.
Some large BAM fringe phase discontinuities have been found real, when comparing the stellar residuals before and after the event. Routines for discontinuity automatic detection and correction have been put in place and phase jumps have been systematically identified.

[Table: Table 1. Fourier coefficients Ck,0, Sk,0 determined from the BAM data compared to those obtained in a special validation run of the Gaia primary astrometric solution. Taken from Lindegren et al. (2016, A&A in press).
BAM Solution BAM Solution [µas] [µas] [µas] [µas]
C1,0
+865.07 (fixed)
S1,0
+659.83 +605.66
C2,0
−111.76 −134.66
S2,0
−85.26 −77.34
C3,0
−67.84 −76.14
S3,0
−65.91 −63.34
C4,0
+18.26 +24.98
S4,0
+17.79 +19.40
C5,0
+3.20 +7.42
S5,0
−0.20 −6.44
C6,0
+3.51 +6.31
S6,0
+0.68 +1.02
C7,0
+0.03 +1.45
S7,0
+0.34 −0.31
C8,0
−0.62 −2.87
S8,0
−0.59 −6.56]

[Figure: Figure 9. BAM fringe phase discontinuities. Left: distribution as a function of the amplitude. Right: correlation between the discontinuity amplitude in both telescopes. Telescope 1: PFoV, Telescope 2: FFoV.]
+6.31
S6,0
+0.68 +1.02
C7,0
+0.03 +1.45
S7,0
+0.34 −0.31
C8,0
−0.62 −2.87
S8,0
−0.59 −6.56]

[Figure: Figure 9. BAM fringe phase discontinuities. Left: distribution as a function of the amplitude. Right: correlation between the discontinuity amplitude in both telescopes. Telescope 1: PFoV, Telescope 2: FFoV.]

The average rate is around one per day during the nominal mission, and mostly concentrated after big perturbations, such as payload decontamination, refocus or safe modes. They seem to follow a power lay on the amplitude with an exponent of ∼−0, 8 (the smaller the jump, the most probable it is). Discontinuities typically happen at the same time in both telescopes with a similar amplitude, which means that in many cases, the disturbance alters the payload geometry, but not the basic angle, at least at first order. See Fig. 9 for further details.
The current sensitivity limit is around 10 µas, which means that more discontinuities can be currently hidden in the noise. Better sensitivity is expected for future BAM data reduction algorithms. Once detected, they are included in the astrometric solution. In any case, the rate of discontinuities for the payload is orders of magnitude milder than for the service module, which is consistent with the payload being much better thermo-mechanically isolated.
Fringe period variations can be relevant if the white light fringe is not exactly located in the centre of the interference pattern. In this way, fringe period variations can mimic false fringe phase variations for simple algorithms. During early commissioning, pseudo-periodic variations were identified and correlated to changes in the laser temperature.3 The typical period stability was around ∼4 ppm for laser temperature changes at the level of ±5 mK. However, significant improvements were found after the radial velocity spectrometer CCDs were 5.497824

FringePeriod
5.497822 5.49782 5.497818 5.497816 5.485126 5.485124
FringePeriod
5.485122 5.48512 5.485118
OSEA_LASER_TEMP_1HOUR
5.485116 23.307
23.306
23.305
DOY_OSEA_LASER_TEMP 314.5 314.75 315 315.25 315.5 315.75 316 316.25 316.5 316.75 317 317.25

[Figure: Figure 10. BAM laser temperature (bottom) and fringe period (middle/top telescopes 1/2) evolution in a typical interval during the nominal mission. The variations are clearly correlated, although of different magnitude for each field of view.]
origin of the straylight and basic angle variations. Very soon after its creation, the conclusions of the ESA-
ESTEC members on the solar straylight, and of Airbus DS on the galactic component put an ending to the
straylight investigations. The efforts were then concentrated on the study of the basic angle variations. During
more than a year, many tasks were undertaken, including the analysis of big amounts of data without a-priori
filtering of explanatory hypotheses, integrating the results from the Gaia astrometric data analysis, driving
the refinement of the finite element thermo-elasto-optical model and finally carrying out several on-board tests
around a decontamination campaign in June 2015. The final report, issued on February 2016, is a comprehensive
report of all activities and results. In this subsection, only a few hints on selected aspects is presented.
suggests their origin are thermoelastic perturbations originated in the service module. As the spacecraft rotates,
the temperatures in the sun-illuminated side evolve by a few degrees, inducing thermoelastic deformations. They
are postulated to propagate to the payload via a yet unknown mechanism (payload and service module were
effectively decoupled by design). One experimental evidence supporting this behaviour is the spin restart data,
obtained in July 2014 after a safe mode was experienced. As a result, Gaia stopped spinning for some days, which
halted the basic angle variations and stabilised the temperature. When scientific operations resumed, the basic
angle variations restarted very soon after the rotation was initiated (within a few minutes). See Fig. 11. Only
the service module could react so quickly to changes in the incoming radiation, the payload module needing more
than a day to stabilise. Purely thermal or payload based hypothesis are hard to confront with this observational
result.
.
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).

During the tests, a combination of two basic actions was defined and carried out: switching on and off the laser thermo-electric cooling unit and changing the current intensity over all possible pre-defined values. Noticeable changes were found in the interferogram shape. However, they were not the simple homogeneous addition of a fixed quantity to the fringe period, even if its intrinsic 2D nature was considered.
In this way, the individual fringe analysis described above was applied to the white fringe tests. For each perturbation (laser temperature or current intensity), the fringe experiencing the smaller shift in average was identified as the white light fringe. Fig. 17 shows an example of the results obtained for one particular test. Two things are evident. First, the white light fringes are indeed well centred (between fringe 75 and 110, for a total of around 180 fringes) within the interferograms. Second, there is no single fringe static against all perturbations, pointing to the changes in the laser source also having a thermal impact on the payload. Impact in the Gaia mission, if any, would be very small (µas level) and is still under investigation. The addition of white light sources should also be considered for future missions requiring Young-like interferometers.
Three issues related to the Gaia in-orbit stability have been reviewed in this work: the focus evolution through the mission, the additional straylight and the basic angle variations.
Regarding the telescope focus, it has been verified that Gaia provides almost diffraction limited along scan performance over a very large field of view. However, the image sharpness is not static, but evolves with time with a typical time scale of several months. The typical mitigation strategy is payload decontamination (normally driven by throughput loss), followed by a small adjustment of the telescopes, if needed. Optimum quality has always been regained afterwards. This sequence of events: increasingly long quiet interval, decontamination and refocus is expected to repeat until the end of the mission.
The sources of the excess straylight have been unambiguously identified as a stellar and solar component. Stellar parasitic light, mainly coming from the Galaxy, integrated over previously identified straylight paths is responsible for the stellar contribution. The solar straylight is the unfortunate combination of three effects: sticking out fibres in some sunshield edges, the sunshield not blocking all diffracted light onto the windows in the thermal tent, and rogue paths transporting it to the focal plane. Software mitigation measures have adopted, both on-board and on-ground.
The basic angle periodic variations and phase discontinuities identified during commissioning have been verified after a careful comparison to the stellar astrometry. The mitigation measures are the BAM measurements themselves and self-calibration within the astrometric reduction. An ESA-Airbus DS working group was created to study the origin of the basic angle variations and straylight, discovered during commissioning by the Gaia scientists (payload experts). It revealed the detailed origins of the straylight and collected evidence suggesting the basic angle variations are thermoelastically driven from perturbations coming from the service module. The importance of having no moving parts and keeping a power load consumption profile as stable as possible has been revealed.
Some general advice for future missions requiring extreme stability has also been collected:
• Design for stability, but plan for instability
• The state of the art in stability is mas, nm and mK
• The future is µas, pm and µK. Modelling at this level (e.g. FEM) is very complex and might need additional research and new tools
• Some requirements are very difficult (and expensive) to verify before launch. Saving money here is typically a wrong decision
• Keep the spacecraft as stable and boring as possible. Routine is important
• House keeping is key. Appropriate precision, resolution and spatial and temporal frequency needs to be ensured, which means specialised equipment and additional downlink bandwidth

• High precision metrology is essential and (never too) expensive Finally, note that Gaia is providing (and will keep doing it during the whole mission) unprecedented quality astronomical data, with the Data Release 1 scheduled for 14 September 2016.
The authors wish to thank Airbus DS for their support and access to internal documents on the wavefront sensor and Gaia optical design. Some concepts and ideas presented here come from those sources.
Material used in this work has been provided by the Coordination Units 3 (CU3) of the Gaia Data Processing and Analysis Consortium (DPAC) and ESA-Airbus DS BAM and straylight working group. They are gratefully acknowledged for their contribution.
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