# Large Constellations of Small Satellites: A Survey of Near Future Challenges and Missions
## 3 Future Challenges
### Constellation Management  
With such a number of active elements in orbit, their management is a fundamental point of interest. Since constellations have not been widely used in the past, not too much work has been done in this sense. However, some key experiences can certainly be located in projects such as Galileo (European GNSS) [18], Globalstar constellations [19] and other GNSS constellations (GPS, BeiDou, etc.). In [18; 19] different strategies have been explored, all driven by the common objective of enhancing the level of automation, so that more satellites will not translate into a proportional increase of managing effort. Proposed solutions fall into two main categories:  
1. Optimization of automatic satellite tracking (such as telemetry download). 2. Automatic failure detection, so that the operator does not need to manually check the satellite's status of health. Still, there are a great number of non-automatic operations left to do; [19; 20] pointed out some desired operational improvements, along with lessons learnt from operating constellations. Some points have been found to be very real and worth being analyzed here:  
1. Splitting between payload operations and spacecraft operations, possibly with dedicating ground segments to each of the two. 2. Increasing automation onboard the spacecraft, which is not always possible due to satellite size constraints. In the latter case, expert systems (intended as an ensemble of algorithms, and machines aiding the operator's decisions, usually associated with artificial intelligence, are applied for high level tasks--prediction, planning, diagnosis, repair, etc.) shall be deployed on ground. 3. Taking not only the ground segment into account but also the operations from the initial phases of the constellation design. 4. Expert systems shall be designed to assist operators and keep the workload constant during constellation operations, e.g., mitigating the heavier workload during launch and early orbit phase. As per point 2, the need for automation, a relevant example is the collision avoidance assessment and maneuver planning, which is now largely manual. This problem is addressed in [7], where the need for an increased accuracy in orbit determination is pushed forward as a means to reduce false alarms and implement automatic orbital corrections. In favor of point 2, the authors of [21] propose an onboard automated management system, based on artificial intelligence. In their implementation, failures are not only detected, but also handled automatically towards a resolution along with a re-scheduling of the original plan. The main drawback of the proposed approach is its need for an intersatellite-link, which is usually not affordable for low-cost strategies. Moreover, it implies intensive intersatellite communication contributing to RF spectrum crowding. Another approach is proposed in [22] for an Earth Observation constellation. It consists of an autonomous (re-)planning strategy for maximizing the total science return of observations over time. Despite not reaching the degree of automation as in [21], the automatic re-planning decreases the ground workload, allowing the operator to concentrate more on the goal, rather than the path to it. Thus, this is also in favor of point 4. Regarding point 3, a few aspects that shall be included at the beginning of the constellation design operation-wise will be now discussed. The automation logic given above is one example. In fact, it impacts greatly on the development complicating the space segment design, at least software-wise. The replacing strategy is another operational aspect that must be taken into account from the beginning: a replacing or spare strategy is the policy adopted by the operators to substitute failed or terminated satellites of the constellation. Most of the constellations need such a policy to guarantee a 24/7 service, affecting launching phases and satellite design (reliability).