1. A method for functionally and physically integrating a new navigation service to be integrated into an avionics onboard system, the avionics onboard system comprising a digital core computer DAL+, having a first criticality level DAL+, integrated into an initial architecture of peripheral computers and of databases having second safety criticality levels DAL−, lower than or equal to the first criticality level DAL+, and serving as server by hosting a first plurality of generic open services Serv_DAL+(j), and a peripheral computer DAL− for managing the new service to be integrated, having a second criticality level DAL−, lower than or equal to the first criticality level DAL+, by dispatching service requests to the digital core computer DAL+ and/or to the peripheral computers and databases of the initial architecture through a communications network; wherein the method for functionally and physically integrating the new service comprises the steps consisting in: functionally decomposing the new service into a second plurality of elementary functions FU(i), determining, on the basis of the second plurality of the elementary functions FU(i), a first list of the elementary functions that can be executed in part or entirely by at least one generic open service, and for each elementary function a first sub-list of generic open service(s); determining an optimal functional and physical distribution of the elementary functions FU(i) within the onboard avionics system 2 over the set of possible distributions which minimizes a global cost criterion CG, dependent on several parameters, including at least the additional development cost of the elementary functions integrated within the digital DAL+ core computer and which guarantees the DAL level of the aircraft as a whole; and carrying out the integration of the new navigation service by actually implementing the elementary functions and their scheduling according to the optimal functional and physical distribution determined within the onboard avionics system.