The principle of reconstructing space astrometric positions from one-dimensional observations carried out in the innovative Hipparcos sky scanning mode was fully original (and frequently questioned outside the project), and at the very limit of available computational power even by the end of the mission in 1997.

Already by the end of 1976, Lindegren had produced a set of definitive technical notes and simulations showing how to obtain a 'rigid sphere' with all astrometric parameters from a scanning satellite.

[2] In addition to the overall scientific coordinating responsibilities, he developed many of the approaches and algorithms related to the mission: his innovation, insight, and mathematical rigour impacted the optical and focal plane design, the instrument calibration, the scanning law, the attitude determination (and the associated 'dynamical smoothing'), the double star analysis (as observed via a signal modulated by a grid), the effects of chromaticity and thermal load fluctuations, the optimum combination of the NDAC and FAST catalogue solutions, and the link to an extragalactic reference frame.

He also led the detailed design of the overall scheme of the astrometric solution, being a block iterative adjustment determining the attitude, the calibration and the system directly from the CCD images.

[13] Crucially, the bulk of his contributions to space astrometry has been in the form of a series of unpublished technical notes for Hipparcos and Gaia, amounting to some 200 documents totalling around 3000 pages.

Amongst them are, for Hipparcos, the three-step astrometric reduction, optimization of the scanning law, notes on the imaging properties used for the multiple star analysis, assessment of chromatic effects, attitude developments, and many others.

For Gaia, his technical notes cover the mathematical and statistical aspects of the Gaia instrument and processing (including the attitude determination and its mathematical representation with quaternions and splines), the modelling of the point/line spread functions, the CCD geometric calibrations, broad band photometry design, maximum likelihood determination of the CCD image centroiding, differential equations and optimal properties of the scanning law, along with the subtle systematic effects in astrometry caused by instrumental misalignments.

All these important results that led to developments in the on-board metrology or to fundamental implementations in the processing, have appeared only in the form of technical notes, and therefore remain largely unknown outside of the Hipparcos and Gaia groups.

At a ceremony in Bern, Switzerland, the first four medals were presented to scientific consortia leaders of the Hipparcos mission: Catherine Turon and Jean Kovalevsky from France, Lennart Lindegren from Sweden, and Erik Høg from Denmark.