## 2 Satellite Constellations: ANSER and ANSER-AT  
ANSER is an ad hoc constellation of clusters, where each cluster is formed by several CubeSat nanosatellites flying in formation. The higher the number of clusters in the constellation, the shorter the mission revisit time, and the higher the number of elements in any cluster, the wider the swath on the ground [6; 7; 16]. This concept of mission, when applied to Earth observation, shall be supported by five key technologies: stable and efficient formation flying control, highly accurate pointing control, precise guiding and navigation, inter-satellite communications, and high-performance payloads compatible with very small platforms.  
Each cluster will consist of one Leader and two Followers (Figure 1a) placed in a cross-track configuration with respect to the original orbital plane. The three observation nanosatellites will work together as if they were a single platform, monitoring the water quality of reservoirs and dams on the peninsula (Figure 1b). To this end, the Leader controls the cluster operation, distributing the tele-commands received from the Mission Control Centre placed in INTA. Each follower carries a miniature spectrometer with a spectral resolution better than 8 nm and a spatial resolution of at least 50 m. The whole cluster acts as a distributed EO instrument, with scientific and housekeeping data being collected by the Leader from the observing Followers via an inter-satellite link (ISL). The ideal distance between the followers, about 10 km, is estimated as a function of the swath capability of each individual observer.  
The three satellites, devoid of an onboard propulsion system, are required to maneuver in order to establish and maintain a formation utilizing the aerodynamic drag generated by the Earth's atmosphere. The capacity to generate differential aerodynamic drag between the satellites by altering their attitude forms the foundation for controlling the formation in the absence of a propulsion system. The operations plan stipulates that each satellite should be commissioned in sequence and then maneuvered to establish a distant formation that cancels any drift. Thereafter, the satellites would gradually approach each other to enable inter-satellite communications and even explore more advanced formations. The constellation was defined using one of the satellites (Leader) as a reference, with the other two (Followers) working with its coordinates.  
At this moment, the two ANSER Followers are maneuvering to approach a distance of 100 km and wait together for the reintroduction of Leader-S. Its launch, initially scheduled for 1 November, has been postponed by SpaceX to 14 January 2025. It has, therefore, been decided that both Followers will wait for the third element at an altitude of around 520 km [6; 7].  
In addition to the ANSER mission, the ANSER-AT (6U CubeSats) mission enhances Earth observation by focusing on formation control and resilience within a satellite constellation. This additional mission employs aerodynamic forces, such as lift and drag, for maintaining formation in low Earth orbit, instead of traditional propulsion. This innovative technique, employed by ANSER-AT, supports the broader objectives of the ANSER mission, which include the monitoring of environmental factors such as water quality and atmospheric conditions. Additionally, it aims to enhance cost efficiency and mitigate the risks associated with propulsion-based control [16].  
Figure 1: ANSER constellation: (**a**) ANSER Leader + Followers; (**b**) formation flight simulation.