The scientific and technical characteristics of GG are such that the spacecraft functions are completely standard, whereas the payload manufacturing, integration and test and the associated controls are new and complex. Accordingly, the spacecraft development programme can be matched to that of a standard bus (here, PRIMA is assumed), developed with a protoflight approach, whereas the payload is given a dedicated development model. The Work Breakdown Structure (reported on page 3 of annexed letter by Alenia) reflects this approach, with separate project offices at the system/SVM level and at the payload level.

The GG Satellite development and verification will be based on a simplified Protoflight approach in which only one complete satellite model is manufactured and assembled at flight standard. Furthermore, the subsystem verification is performed at system level in the frame of the integration and system tests. On the other hand, at payload level (PGB with test masses and active damping and drag-free control electronics) both a Structural-Thermal Model (STM) and Development Model (DM) are planned. The purpose of such models is to verify the mission specific functionalities separately, thus avoiding adverse impacts on the system level programme.

The STM will be subjected to a test and validation campaign whose main purpose is to validate and refine the mechanical and thermal mathematical models. As a result, simulations of the P/L performance will provide more accurate predictions. Moreover it will be possible to perform experiments on critical mechanisms, such as the lock/unlock devices. The Development Model will be a breadboard, functionally representative of the entire payload module, and will be subjected to a validation campaign consisting of functional and electrical tests. The main purpose of this model is to debug well in advance the design of critical items and verify the integration of the relevant software (active damping and drag-free control running in the main computer). The basic building blocks of the DM are already being assembled as part of the GGG laboratory prototype^{Sec. 5}. In this way the programme C/D phase can start with the most critical aspects of P/L design already verified.

The schedule is based on a Phase B of 9 months and Phase C/D of 27 months (see^{26, Ch. 8, Fig. 8.2}). An essential prerequisite to a 3-year programme is the advanced development of the payload critical items, already ongoing as part of the GGG laboratory experiment.

The proposed design presented herein does not rely on any completely new technologies. Although many requirements of the GG payload are very stringent, they can be met by technologies and processes already existing in the commercial market, which will have to be adapted for use in a space application. Field Emission Electric Propulsion (FEEP) is undergoing complete qualification under ESA contract, including an orbital test on the Space Shuttle. GG is likely to be the first satellite using FEEP for fine drag free control, a development that will be of great interest for future fundamental physics missions, such as LISA. A number of critical aspects of the payload design can be verified in the laboratory while testing the GGG prototype. Advanced breaboarding is planned on the lock/unlock mechanism, the elastic suspensions (prototype already manufactured and tested for losses) and other delicate mechanisms. A qualification campaign is required for the inch-worms. A first breadboard of the capacitance read-out circuit has been manufactured and is under testing with GGG, as is the active damper control electronics. These laboratory activities will continue and be concluded (for what concerns the space-qualification aspects) in the satellite DM programme. The complete drag free control system cannot be tested on ground. Verification will be performed by software simulation, incorporating results from laboratory tests of the key elements (capacitance sensors, active dampers, FEEP thrusters), so as to optimize the control laws with respect to real-world sensor and actuator characteristics.