The final observing programme was selected from some 214000 proposed candidates following a series of comprehensive numerical simulations of the satellite observations. Each iterative step of these simulations was followed by an in-depth analysis of the resulting provisional selection, its qualities and drawbacks with respect to the selection of stars from high-priority proposed programmes, to their observability by the satellite, and to the optimum use of the satellite's specific observing capabilities. The 48 minor planets and the three other solar system objects finally retained for observation were selected purely on the basis of their magnitudes and their observability by the satellite.
A variety of constraints on the contents of the Input Catalogue arise from the measurement principle of the Hipparcos satellite, which is designed to scan the celestial sphere systematically at a constant rate. With a field of view of about degrees, a modulating grid of period 1.2 arcsec, and observations conducted by switching the `instantaneous field of view' of the detector to those programme stars visible within the total field of view, the following broad conditions influence the global observing programme: (i) the number of stars per square degree contained in the observing programme is limited; (ii) there is, at the same time, a requirement on the minimum density of bright stars and their uniformity across the sky required for satellite attitude determination and the subsequent data reductions; (iii) the magnitude distribution of the retained stars must be consistent with the available observing time (the minimum acceptable, and target, observing times increase with the star magnitude, and this places limits on the density of faint stars acceptable within the programme); (iv) to point to each target object, and to observe it with an optimum distribution of the available observing time, requires adequate a priori knowledge of the candidate's position and magnitude; (v) the detector's instantaneous field of view profile places stringent constraints on the separation and magnitude difference of double and multiple systems that can be usefully observed by the satellite, as well as on the selection of stars in dense regions where the signal from nearby, bright stars could influence the measurements.
In addition to the selection of the precise satellite observing targets, the INCA Consortium also had the responsibility of compiling, and where appropriate improving, the data available for the proposed stars and minor planets in order to satisfy the conditions necessary for their observation with the Hipparcos satellite. Based upon the characteristics of the satellite detection system and measurement principle, specifications were set at arcsec on the a priori positional knowledge of each object at epoch 1990, and mag on the a priori knowledge of the or magnitudes for all programme stars. As a consequence, extensive compilations of astrometric and photometric data were undertaken by the INCA Consortium (Turon 1988), along with systematic tests of reliability and consistency of the collected data. Large preparatory ground-based observational programmes were subsequently organised in order to supplement the available information, and the resulting data finally included in the present Input Catalogue therefore comprise a substantial body of previously unpublished data: measurements were made of about 100000 star positions from photographic plates, some 10000 additional positions from automatic meridian circle observations, 10000 star magnitudes and colours obtained from multi-colour photoelectric photometry, and more than 100000 star colours computed from spectral types combined with a new model of galactic extinction. In addition, special programmes were undertaken to deal with particular objects, such as double and multiple stellar systems, variable stars, stars in galactic open clusters, and minor planets.
For the minor planets, a substantial number of new observations with the two automatic meridian circles of Bordeaux and La Palma, and with the astrographs of San Fernando and Barcelona, were necessary for improving the knowledge of the orbital elements to a standard suitable for the satellite observations. For the three satellites of the major planets, Europa, Iapetus and Titan, for which studies indicated that valuable astrometric information could be acquired from Hipparcos, the available ephemerides already satisfied the specified accuracy requirements.