The big trend in Earth observation missions is to make use of the decreased cost and development time for satellites. This paper argues that recent EO missions are being limited in the planning phase by the optical and physical limits of communications links between node satellites and one hub satellite. The paper surveys previously published papers on swarm missions and proposes future mission configurations that allow commercially available radio frequency communication subsystems which can complete the mission.
Section 2 - the paper was very helpful in giving a concise section on the ‘Classification of Multi-Satellite missions’
The paper distinguishes between missions in swarm formations, trailing formations, satellite formation flying, and satellite constellations.
Single satellites can’t achieve the frequent images with desirable spatial/temporal resolution needed for the analysis or provide the necessary data for ‘‘Supporting the decision making” in as close to real-time as possible.
The current cost of putting up a satellite is between 15,000 and 30,000 $/Kg. Consequently, Nano- and Pico-satellite systems (1–10 kg) could be a motre cost effective method of mission configuration.
Section 3 - Satellite swarm missions
European space agency (ESA) swarm mission for measuring Earth's magnetic field - detecting Earth's magnetic field.
Starling swarm technology demonstration - testing inexpensive distributed systems configurations - 12 cube sats doing synchronized multipoint measurements
Swarms of Silicon Wafer Integrated Femto satellites - technologies for inter-satellite communication networking, and relative navigation,
Section 4 - Fractionated imaging satellite cluster (FISC)
The paper's fourth section goes into details about a mission proposal to target any part of the earth within 7 days using 21 identical 3-U satellites. The ground station can send commands, receive telemetry, and image data every orbital period (i.e. less than 100 min).
two configurations of satellite constellation, the first configuration is to use a satellite constellation which consists of 21 identical three-unit (3-U) standard CubeSats,
The papers analysis suggests a configuration of swarm satellites made up of 12 simple 3-U standard CubeSats as imaging nodes and one hub Microsatellite,
The analysis made by the authors recommends the use of satellite swarms over the satellites constellation.
The proposed satellite swarm has imaging nodes orbiting Earth in four sun-synchronous orbits with 490 Km altitude and 97.37 inclination angle, while the hub satellite orbiting the earth in circular equatorial orbit with 499 Km altitude.
The ground station has to be able to control and receive the payload data and is located near the equator. The 12 imaging nodes are distributed in an array shape within four sun-synchronous orbits
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The functions of hub satellites can be described as follows:- Communicate with ground satiation every orbital period to receive the commands.
Communicate with imaging nodes two times per orbit (2.5 min for each node per orbit) to send the received command from the earth. Collect the telemetry and imaging data from each imaging node. Transfer the huge imaging and telemetry data from the imaging node satellite to the hub satellite using the laser communication system. Send the telemetry and imaging data to the ground every orbital period.
The functions of imaging nodes The functions of Imaging Nodes can be described as follows:- Receive the imaging commands from hub satellite Execute the imaging commands Prepare the imaging and telemetry data and send it to hub satellite
Section 5 - The proposed new conceptual design of satellite swarm for Earth observation applications
Section five discusses the weaknesses in a FRACTIONATED IMAGING SATELLITE CONSTELLATION configuration.
The weaknesses identified were that using laser communication is still under development
And that lazer communications need accurate alignment and powerful actuators which is hard and effects the power budget.
The solution is to use a more up to date radio frequency systems and to equip the different component sataliets with x or s band receivers.
Sections 6 - Results and discussions
This section introduces how the proposed swarm design will be able to transfer all imaging data using overlapping but traditional (cheaper, lighter) and available RF communication subsystems instead of the optical communication approach.
So basically lasers are out
The paper suggests that Family swarms with different imaging modes will be the ideal configuration for EO missions going forward. Also, optimizing the communications based on the orbit and configuration of the swarm to get the most out of more available technology or components.
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The family swarm will image the Earth in two modes; swath width and high-resolution imaging modes then send data to the ground. These nodes can be described in the following subsection:
Swath width mode In this imaging mode, all C-Sats. and M-Sat. are commanded to image adjacent nearby target areas with minimal over labs. 10. Thus, the total imaged swath width equals the sum of all C-Sats and M-Sat swath widths (i.e. total swath width is about 240 Km).
High-Resolution mode In this imaging mode, all C-Sats. and M-Sat. are commanded to image the same target area (i.e. total swath width = 20 Km). In this case, the target area is imaged from different angles and at different orbital positions. Then, more features and a high-resolution image can be extracted using the image fusion software.
Family swarm data transfer modes The configuration of the family swarm supposes that the M-Sat is flying in the middle of the C-Sats array, so it can communicate with all C-Sats all over the orbital period (i.e. 95 min) in a sequential way. During this communication, M-Sat will receive imaging and telemetry data from C-Sats and send to them the ground commands. F-sat will communicate with M-Sat two times per orbital period (15 min for each session) to receive the imaging and telemetry data, and to send ground commands. Moreover, F-Sat communicates with the ground for 10 min per orbital period (Mahmoud and Farrag, 2015), during this communication F-Sat. will send/ receive imaging and telemetry data/commands to/from the ground. The management of the communication sessions between the family swarm members is planned by the ground station, these communication modes can be classified to
conclusions
The paper suggests the limitation of this configuration will be maintaining the relative distance between the imaging nodes and avoiding collision,
From my point of view I didn't realize the extra utility you could get from taking multiple pictures of the same spot from different satellites with slightly different viewpoints on whatever is being measured, the potential is huge.
and a potential next step would be developing a suitable algorithm for orbit control and increasing efficiency of reaction wheels and actuators.
Satellite swarm survey and new conceptual design for Earth observation applications
- Farraga, Saed Othman, Tarek Mahmoud, Ahmed Y. EL Rafei