The article discusses and presents the formulation of problems of inter-project unification of on-board systems in the development of modifications of spacecraft that are part of space systems for remote sensing of the Earth. It is shown that when developing a complex of advanced space systems, it is possible to partially combine unified on-board systems and finished products, which, under given constraints, provides a minimum of total costs. The formulation of the main task of inter-project unification of spacecraft for remote sensing of the Earth using finished products and partially unified on-board systems and a special case of the problem — conducting an economically justified inter-project unification from completely unified on-board systems (aggregates) of promising modifications of spacecraft is given. The initial data and limitations for solving the main and particular problems are determined. The tasks are presented in a deterministic setting. The concept of optimality of the choice of areas of unification of each on-board system is formulated, which is characterized by the minimum of a criterion having an additive structure, this is the total economic effect for areas of unification. It is believed that the analysis of the results of solving the problems of inter-project unification in the development of promising modifications of spacecraft will reveal the directions of inter-project unification of those on-board systems for which it is most appropriate; to formulate the fundamental principles of modernization of space systems and creation of modifications of spacecraft for remote sensing of the Earth in the planned period.