CPUT planning a successor to Africa’s first nanosatellite

June 30th, 2015, Published in Articles: EngineerIT


Daniel de Villiers

Daniel de Villiers

The Cape Peninsula University of Technology (CPUT) is planning a successor to its first CubeSat and has set its vision on ship-tracking. In a paper presented at the recent SAAMSAT space Symposium in Pretoria, Daniel de Villiers, development engineer at French South African Institute of Technology (F’SATI) at CUPT, said that  ZACube-2 will be the second satellite in F’SATI’s ZACube-I nanosatellite mission series. These missions are developed at the French South African Institute of Technology (F’SATI) and the Africa Space Innovation Centre (ASIC) at CPUT with funding principally from the Department of Science and Technology (DST) and the National Research Foundation (NRF). Development of some subsystems has been ongoing for a number of years and has yielded a suite of commercial CubeSat parts that is ready for use in the satellite.

ZACube-2 will serve as a technology demonstrator for essential subsystems required in operational satellite missions for very specific applications that are deemed especially suitable for implementation with nanosatellites. The mission will further grow the core expertise of CPUT and its technology partners and validate the technology innovations that result from it.

“ZACube-2 will be a test bed for a ship-tracking payload and will be used to validate the use case of employing nanosatellites in ocean vessel detection through the automatic identification system (AIS) protocol. Additionally, the satellite will carry medium resolution imagers as a secondary payload to demonstrate the feasibility of using a nanosatellite for imaging applications, such as ocean colour monitoring and fire tracking”, he told delegates.

“The mission’s performance will be measured against the effective tracking of vessels within the borders of the South African continental shelf. There are an estimated 6000 ships in South African waters, a number that is small enough to sustain successful reception of AIS messages”, he said.

Although this mission is a single satellite mission it would be useful if a constellation of satellites similar to ZACube-2 could be launched. The revisit interval is the time delay between consecutive visits of satellites in the constellation over the same area. It is calculated by considering orbital height, antenna pattern, the number of satellites in the constellation, and the ability to control the orbit. Since orbit control is not within the scope of this project, the satellites will drift with respect to one another and revisit intervals will vary with time. Under precise placement conditions, preliminary simulations reveal a nominal revisit time of 30 minutes over Southern Africa with a 9-satellite constellation covering the globe in 3 separate sun-synchronous orbit planes consisting of 3 satellites evenly spread out per plane. Conceptually this configuration could enable vessel tracking with an enhanced update rate.

“Due to the practical launch logistics and cost implications, such a constellation of satellites will most likely be a secondary payload on another mission, thus control of precise constellation placement may be limited. This will have an impact on the actual revisit time,” he said.

For the sake of limiting orbital lifetime, it is preferable that the orbital height does not exceed 600 km, but the available deorbiting devices are very effective at reducing orbital life significantly and may allow a higher orbit than 600 km to be used. Limiting the orbit height to 600 km may limit the availability of launch opportunities.

An artist impression of a typical 3U CubeSat

An artist’s impression of a typical 3U CubeSat.

He said that preliminary design work shows that the entire SDR payload will fit into one nominal sized CubeSat module. Some additional RF front-ends and antenna switches are required for the SDR to interface with the antennas. Depending on the level of redundancy added to the system, either a 2U or a 3U CubeSat form factor could accommodate all the subsystems. A 2U structure may present more risk in that deployable solar panels may be required to satisfy the power budget. There is also less space for the deorbiting mechanism, which is regarded as mandatory. A 3U structure may have enough surface area so that fixed body mounted solar panels capture enough solar energy and the increased spacing of the internal modules will reduce the likelihood of thermal hot spots. It also allows for up to 0,8U of volume for the imager payload. The larger form factor will accommodate additional RF front-ends and allow the SDR payload to operate in more communication modes using the available antennas. It is proposed to use a nanosatellite structure already developed by Clyde Space as baseline for the concept design. The fully qualified structure was designed with adaptability and ease of integration in mind.

Being a responsible space citizen

“In order to comply with the UN policy on the sustainable use of outer space to deorbit satellites within 25 years, it is imperative that the spacecraft features a deorbiting device once end-of-mission life is reached. Various analytical tools and techniques can be used to estimate orbital life of a satellite. Further analysis will determine what deorbiting device would be a good fit and at what time during the mission it should be deployed. The orbital height has an exponential effect on the deorbiting lifetime of the satellite. Operating in orbits below 650 km should be sufficient to rely on atmospheric drag to deorbit a typical 3U satellite within 25 years; however, it is beneficial and encouraged to deorbit in the least amount of time post mission life.”

The full paper  can be downloaded from www.amsatsa.org.za

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