## 7 Discussion  
In the first part of the article, a method to select a definite number of orbits to achieve the NGGM/MAGIC user requirements has been defined and used. The outcome coincides with a set of seven orbits, which need to be further investigated in the current feasibility phase of the mission. Priority is given to subcycles of 3, 5 and 7 days to achieve high temporal resolution and its relevance for emergency and near-real-time (i.e., daily to subweekly) products. A double pair of satellites has been assessed as the most promising solution, and the orbit candidates reflect this concept. In order to maximize the quality of the gravity solutions (and their processing), the inclined pair is recommended to orbit at an inclination around 70\({}^{\circ}\) and at a lower altitude with respect to the polar pair, which will be between 87 and 90\({}^{\circ}\) inclination. For selecting the candidate orbits, great importance is given to the ground track homogeneity and to the regular and common longitude shift. To find the optimal orbits, a rather strict homogeneity (\(h_{l}<1.5\)) is required at short time scales, i.e., when ground track density is inherently low, and at the same time a good homogeneity (\(h_{l}<2.0\)) is necessary for the monthly solutions as well. The proposed orbits satisfy both needs and offer multiple opportunities in terms of subcycles, inclinations and altitudes. Before providing the derivation of the engineering requirements, the article provides an overview of the orbit performances for altitude and inclination changes (Section 3). For the 3-, 5- and 7-day subcycles, the homogeneity is highly stable for shorter time scales. However, according to the altitude constraints discussed in the engineering requirements, the proposed sets of orbits (Table 3) guarantee an excellent homogeneity and sampling, including a matching ground-track longitude shift for the two pairs.  
The key instrument of NGGM as part of MAGIC is a laser interferometer with the resolution of few nanometers for measuring the relative distance variation between a satellite pair, where the two satellites are separated at low altitude by 100 km. Two interferometer schemes have been defined as suitable to meet the NGGM measurementrequirements and can be operated with the auxiliary metrology system (acquisition and pointing metrology system (APMS)) for optical link acquisition [51]. This is already tested with results and meets the performance requirements. Several breadboards of the building block technologies have been assembled and extensively tested: the test results will provide ESA with the necessary information to carry out the technology predevelopments in parallel to the foreseen NGGM Phase A.  
A discussion on the selection and possible improvement of state-of-the-art electrostatic accelerometers was presented, together with the design options for the drag compensation system. Alternative drag compensation scenarios are under investigation, where lateral and angular drag compensation requirements can be relaxed, accompanied with a progressive relaxation of the thruster requirements, while retaining the inline drag compensation and attitude control of the satellites--augmented or not by steering mirrors--for maintaining the intersatellite laser link. These scenarios will be further analyzed together with the orbit selection and optimization and assessed in-depth in the NGGM/MAGIC Phase A activities.