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the esa gaia mission will provide astrometry of a billion objects in the galaxy with unprecedent precision and accuracy .
more details on the mission goals can be found in e.g. mignard et al .
( @xcite ) . for
the current status see prusti ( @xcite , this volume ) .
the payload is composed of two off - axis telescopes sharing a common focal plane .
the spacecraft has been designed with extreme stability as a key feature .
however , it can not ensure a passively stable basic angle ( chief ray angular difference ) between both telescopes within 0.5 @xmath0as ( 2.4 prad ) during a spacecraft revolution .    the basic angle monitoring ( bam ) laser interferometer is thus in charge to actively track those changes for subsequent on - ground processing .
the working principle is as follows : one artificial star ( interference pattern ) is created for each field of view .
the relative difference in fringe phase between both patterns corresponds to a change in the basic angle .
see gielesen et al .
( @xcite ) for further details .
early commissioning bam results were presented by mora et al .
( @xcite ) , a selection of those findings and new results are presented here .
several things were apparent after the bam was switched - on .
the most important one was that the system produced good quality high signal - to - noise fringes for both telescopes , similar to those obtained during on - ground testing .
however , some unexpected issues were also discovered ( see fig .  [
fig : bamfov ] ) .
first , the amplitude of the periodic sun - synchronous component is much larger than expected ( @xmath11 mas ptv ) .
second , this signal includes discontinuities ( up to several per day ) .
third , the bam phase show a mid- to long term trend evolution .
fourth , the fringe period exhibited variations at the level of a few @xmath0pix ( hundreds of fm in wavelength ) .
significant effort has been devoted to determine whether the bam measures real basic angle variations or just instrumental effects .    regarding the fringe period variability , the effect was verified and associated with small ( mk ) changes in the laser temperature .
the main contributors were the transitions between low and high resolution mode for the spectrometer ccds .
the situation has improved significantly ( around an order of magnitude less ) after those transitions were removed ( high resolution only ) to reduce the effect of straylight .
the bam measurements have been extensively compared to the one day astrometric solution ( odas ) , which is a routine diagnostic used to check the spacecraft health .
the odas confirmed that ( at least some of ) the discontinuities are real , while the long term evolution is different , this feature being an artifact .
regarding the 6 hour periodic variations , the odas uses a fixed value for the basic angle during each run and is in principle incapable of measuring them .
however , a footprint is left in the data , which can be revealed if the same ring is observed some months apart , so the sun has moved in the ecliptic .
additional modeling of the stellar parallaxes and galactic dynamics is required to subtract this effect from the bam signal .
such a configuration happened during commissioning , and two pairs of rings could be compared ( see fig .
[ fig : bamvsstars ] ) .
it was verified that basic angle variations of the same order of magnitude and shape as those measured by the bam were present in the data .
more than one year of bam data have already been collected and processed , comprising more than a million interference patterns .
the pipeline is dependable and robust .
however , several features have been identified that require future refinement .
two interesting cases are presented in fig .
[ fig : additionaleffects ] .
first , the bam signal contains other features in addition to the main periodic signal .
most notably , the fringe phase changes when gaia observes high density regions .
this could be a real effect due to e.g. additional heating produced by the on - board computers or just a new instrumental effect .
second , the fringes are not pure plane parallel lines , but the period exhibits a complex 2d local behaviour .
this is an expected consequence of the accumulated aberrations in the optical path , but will require additional modeling effort .
the esa gaia mission uses two telescopes to create the most ambitious survey of the galaxy . 
 the angle between them must be known with exquisite precision and accuracy . 
 an interferometer : the basic angle monitoring system measures its variations . 
 high quality data have been retrieved and analysed for more than a year . 
 a summary of the in - orbit performance and some early results are presented .