ZACUBE-2 (South African CubeSat-2)Overview Spacecraft Launch Payload References
ZACUBE-2 is a follow-on mission of the ZACUBE-1 of F'SATI (French South African Institute of Technology) at CPUT (Cape Peninsula University of Technology), Cape Town, South Africa (in collaboration with SANSA and Stellenbosch University). ZACUBE-2 is a 3U CubeSat whose payloads include a medium resolution matrix imager and a number of communication subsystems. Imager data will be downlinked using a high data rate S-band transmitter and high gain patch antenna. As the main payload requires nominal nadir pointing for both imaging and data transmission, three-axis stabilization is implemented. The ADCS is provided by the ESL (Electronic Systems Laboratory) of Stellenbosch University and comprises one unit (1U) of the satellite. 1) 2) 3) 4)
Background: In December 2013, African Heads of States and Governments adopted the 2050 Africa's Integrated Maritime Strategy that desires to address Africa's maritime challenges for sustainable development and competitiveness. This consists of concerted and coherent long-term plans of actions to enhance maritime viability for a prosperous Africa by leveraging the use of novel technologies. 5)
South Africa is dedicated to support this strategy through national initiatives, notably its Operation Phakisa (meaning "hurry up" in Sesotho). Through Phakisa, the South African Government aims to fast track the implementation of its National Development Plan in two key sectors; the oceans economy and e-health. Monitoring maritime activity within the continental shelf forms a critical part of Phakisa.
F'SATI at CPUT and collaborators propose the development of a South African constellation of nanosatellites (MDASat-1) to facilitate South African Marine Domain Awareness (MDA), as required by Operation Phakisa. The MDASat-1 (Marine Domain Awareness Satellite) constellation will support international maritime communications, ranging from the current AIS (Automated Information Service) standard to the new and evolving VDES (VHF Data Exchange Service) standard. The constellation will provide South Africa with security and control of its AIS and VDES maritime data with associated improved control over data cost and access. AIS- and VDES-based value-add services to the international market will, furthermore, translate into significant foreign direct investment levels. This will position South Africa on the international forefront with regards to maritime space communication services, and translate into the socioeconomic development of the region.
CPUT pioneered nanosatellite technology in South Africa and the region, having developed Africa's first nanosatellite in space – ZACUBE-1 ("TshepisoSAT"). The satellite was launched on 21 November 2013 and is still being operated daily. The satellite is a 1-unit CubeSat that was primarily developed to train students, and to serve as a technology demonstrator of subsystems developed at CPUT. The mission also carries an HF beacon for ionospheric studies, and a low-resolution imager. ZACUBE-2 is the second CubeSat being developed at CPUT. The 3-unit CubeSat will be ready for launch in 2017.
The ZACUBE-i missions are developed within a university environment with its primary mandate of human capacity development. On the other hand, missions such as the proposed MDASat-1 have a distinctly more operational and commercial focus. The broad approach towards implementing the MDASat-1 constellation is to synergize its development with that of the ZACUBE-i missions, specifically ZACUBE-2.
In practical terms, the ZACUBE-2 mission objectives have been aligned with those of MDASat-1 to allow the mission to function as a precursor to the constellation. Building on the existing technology and resource legacy of ZACUBE-1, and leveraging the sustained investment in the ZACUBE-2 program, lower the cost and risk of the technology development of MDASat-1. In return, MDASat-1 will serve as a catalyst for establishing a nanosatellite manufacturing sector in the region, creating jobs and driving further innovation. This synergistic co-existence of research, innovation and commerce, within a tri-helix collaborative framework involving academic institutions, the public and private sectors, is critical to the industrialization of the regional satellite industry where such investments must be weighed against pressing societal needs.
This interplay among the various programmatic entities is illustrated in Figure 1. The CPUT/F'SATI nanosatellite program is a source of skills and innovation for future MDASat-i missions, which reciprocate by providing CPUT/F'SATI with innovation validation and commercialization of its intellectual property.
Figure 1: Synergy among research, innovation and commercial dimensions of the MDASat-i implementation framework (image credit: CPUT)
As of 2016, an opportunity is emerging whereby the abundant supply of miniaturized low-power processing components, combined with the flexibility achieved using SDR (Software Defined Radio) concepts, makes it possible to produce nanosatellites that are generic enough to benefit from economies of scale in production, but are also flexible enough to satisfy a broad range of applications. This enhances the ability of adapting in time to new opportunities in the public and commercial markets as they arise, which is a substantial benefit in an industry where existing deployments are rigid in their application. The adaptability of the SDR payload adds benefit to satellites already in service, allowing in-flight reconfiguration, as well as to satellites being developed for future constellation replenishment missions where changes to the payload can be implemented rapidly without any hardware changes.
These capabilities are especially relevant to implementing current and emerging vessel tracking services where evolving standards and specifications, such as the VDES(VHF Data Exchange Service) standard, necessitates in-flight reconfiguration. 6)
ZACUBE-2 and MDASat-1 will carry an innovative SDR-based payload that will provide AIS and VDES services.
Maritime vessel tracking services (Ref. 5):
AIS continues to be the primary method of collision avoidance for water transport by implementing an automatic timeslot-based broadcast and listen technology used on vessels to communicate to surrounding ships. The AIS signature contains a unique ID, position, course and speed of the vessel.
AIS only makes use of two channels and although it was initially designed for safety and collision warning, it is now also used for other applications with a wide range of custom messages. In congested areas the two AIS channels are overloaded, resulting in a risk to safety. This is especially true for space-based AIS (S-AIS) services where a larger number of vessels are visible to the satellites.
The future VDES standard, which can be described as ‘next generation' AIS, is currently under development by IALA (International Association of Lighthouse Authorities). The International Telecommunications Union (ITU) released the draft VDES specification after the World Radio Conference in November 2015. However, this excluded the final specification for the space segment; expected to be finalized by 2018 only.
VDES is being designed to extend these two channels to at least eight and will, specifically, include support for space communications for improved coverage. It includes the separation of position reporting and safety applications from other maritime data applications. Such data applications will be supported by wider bandwidth channels to enable more data-intensive applications; for example, supporting more detailed navigational, weather or radar data. Messages can also be sent via VDES satellites for issuing storm or tsunami warnings.
The SDR platform developed for ZACUBE-2 makes it possible to launch the MDASat-1 constellation even before the final VDES specification is released, and to update protocols and functionality via software over-the-air as the specification evolves.
Technology: Nanosatellite constellation
Mission objectives of ZACUBE-2: To align with the objectives set out in Operation Phakisa, the primary purpose for ZACUBE-2 is to demonstrate AIS message reception using its SDR-based payload. The geographic focus areas of the mission are the coastal waters off Southern Africa. The ZACUBE-2 mission objectives are summarized as follows (Ref. 5):
• Primary mission objectives
- Technology demonstration of AIS message reception using the primary SDR payload
- Human resource development
- Flight heritage for in-house developed hardware.
• Secondary mission objectives
- Technology demonstration of VDES message reception using the primary SDR payload
- Technology demonstration of primary payload participation in full VDES
- Technology demonstration of a low-resolution NIR (Near Infrared) imager payload
- Over-the-air update of primary payload software.
Mission objectives of the MDASat-1 constellation: MDASat-i will be operational nanosatellite constellations to provide VDES maritime domain awareness services, primarily, within the South African coastal waters.
Whereas the focus of both ZACUBE-2 and MDASat-1 will be to collect data and make it available to South African users, the constellation will allow for scalability to a global context capable of accommodating global vessel traffic of up to 88,000 ships, receiving and processing 45,000 active broadcasts a day.
The proposed constellation is required to have a revisit time that will allow vessels in the South African continental shelf to be tracked on an hourly basis. — Due to the emerging nature of the full VDES standard, in-orbit firmware upgrades of the SDR payload will be enabled.
The geographic region of interest where the AIS/VDES services will be provided by both ZACUBE-2 and MDASat-1 is limited to the Southern African coastal regions. Future MDASat-i missions will expand to global monitoring, in which case of the order 60,000 cargo vessels will be tracked. This then poses the challenge of signal collision and congestion for the AIS receiver. These are not expected to be of a concern for ZACUBE-2 where AIS signals from an anticipated 6000 ships will be received from the much smaller geographic region. Quoting the group from Aalborg University: "it has been found that continuous reception rates of above 13,000 AIS messages per hour is possible and that the main challenge is not to receive all the data, but to get it transmitted down to the ground stations".
Considering the worst-case scenario of global coverage, a first order estimate of the amount of data captured per satellite per orbit is made under the assumption that each AIS channel will be sampled at 38,400 sample/s (twice the Nyquist sampling rate of the 9600 bit/s AIS data stream). At 12-bit resolution, this results in a 460.8 kbit/s data stream per AIS channel, or 921.6 kbit/s for full AIS. If it is further assumed that actual AIS signals are received for 50% of the time, the effective data stream bitrate is 460.8 kbit/s. This translates to 311MB of AIS data per typical 90 minute orbit.
To estimate the required transmitter data rate to download all the data captured per orbit per satellite to a single ground station, it is assumed that the access time to a ground station is expected to be of the order 10 minutes. This access time translates to a data rate requirement of just over 4 Mbit/s. Although the in-house developed 2 Mbit/s S-band transmitter, which will fly on ZACUBE-2, does not meet this requirement, an upgraded 10 Mbit/s transmitter is being developed and will be deployed on MDASat-1.
Utilizing a QPSK (Quadra-Phase Shift Keying) modulation scheme, a bandwidth efficiency of about 1.4 bit/s/Hz may be assumed if baseband pulse shaping with a roll-off of 0.35 is being used. This requires an estimated maximum bandwidth of 5.6 MHz for the case of global AIS reception.
The number of frequency bands that the SDR payload can operate in is primarily constrained by the number of antennas on the satellite. The concept layout of the satellite allows for a total of three antennas. Transmit / receive RF front-ends will cover the extended VDES bands from about 156.75 MHz to 162.05 MHz. An additional SDR uplink / downlink pair will be implemented in the UHF band so that it can be used as a backup telecommand receiver / telemetry transmitter.
Orbital analyses and operations: The proposed constellation is required to give a nominal revisit time over the South African continental shelf of 1 hour. Initial calculations were performed on a circular orbit with a height of 600 km. 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 orbital planes, each consisting of 3 satellites, evenly spread out per plane. Conceptually, this configuration will enable vessel tracking with an enhanced refresh rate. Due to the practical launch logistics and cost implications, however, the satellites will most likely be secondary payloads on other missions; thus, control of precise constellation placement may be limited. This will have an impact on the actual revisit time.
A ground infrastructure is vital to the responsiveness of the system. With the initial focus on tracking within South African waters, local ground stations should be adequate for satellite data collection and processing. In order to realize a global tracking solution, international partners would be required to assist with data capturing. This will further facilitate pan-African and international collaboration.
The layout of the ZACUBE-2 nanosatellite, which also serves as precursor concept for the MDASat-1 constellation, is illustrated in Figure 2. The satellite bus subsystems that are particularly relevant to achieving the ZACUBE-2 mission objectives, are listed as (Ref. 5):
• UTRX (UHF Transceiver): The UTRX is an in-house developed product, similar to the telemetry transceiver on ZACUBE-1, operating a telemetry downlink and telecommand uplink in half-duplex mode in the UHF band.
• STX (S-band high data rate Transmitter): A high data rate downlink is required to transmit all the gathered AIS and collected image data back to the ground station. The current in-house developed STX, featuring a 2 Mbit/s downlink rate, will be feasible in terms of payload data rate.
• Antennas: The STX will interface with an in-house developed circular patch antenna mounted on the nadir facing side of the satellite. Two sets of deployable dipole antennas cover VHF (SDR payload and AIS reception) and UHF (Telemetry).
• ADCS (Attitude Determination and Control Subsystem): ZACUBE-2 incorporates 3-axis stabilization with a control accuracy of < 0.5º. Such attitude knowledge is necessary to achieve an image offset better than 10 to 13% due to pointing accuracies at an estimated 600 km orbital altitude required for the secondary imaging payload. The ADCS module, developed at ESL, comprises a 1U sized subsystem. This ADCS is not essential on MDASat-1.
The features of ADCS are:
- Unique control method using aerodynamic drag
- Deployable UHF / S-band antennas also serve as stabilizing tail feathers
- Deployable side panels control roll angle
- Full redundant backup with magnetorquers and reaction wheels
- CubeSense ADCS sensor module.
Figure 2: Illustration of the 3U ZACUBE-2 nanosatellite layout (image credit: CPUT)
Launch: The ZACUBE-2 nanosatellite will be ready for launch in 2017.
Orbit: A sun-synchronous circular orbit of 600 km is desired.
The various components of the payload can be defined as follows, and as shown in Figure 3:
• RF front-end: The RF receiver front-end interfaces with an antenna to receive analog signals. It contains various RF modules to amplify, filter and isolate the specific signals of importance. After isolating the required signal, it is digitized using an ADC (Analog-to-Digital Converter). From here all processing is done digitally. The RF front-end is also designed for a specific frequency range and performance; thus, there will be an AIS RF front-end used for the primary objective and an alternative pair of RF receiver and transmitter front-ends to interface with the UHF antenna. This will enable the SDR to operate in the UHF band as a redundant telemetry and control radio.
• Payload processor: The payload processor performs all processing related to the specific mission payload. In the case of ZACUBE-2, it performs SDR processing for AIS communication, as well as image processing. Its functionality will be augmented to full VDES for MDASat-1. For SDR processing, it receives a digital stream of sampled analog data from multiple RF front-ends and performs various operations on this data stream. It can both demodulate and extract radio messages, or it can store the raw signal for post-processing. A separate payload processor will also interface with the imagers on-board. In most cases, it would just store images to be downloaded at a later stage, but if needed, it can also perform limited image processing on-board the spacecraft.
A suitable payload processor was identified, and features both an FPGA and dual-core processors; thus combining high performance processing with flexibility. The functionality of the payload processor can be upgraded in flight.
Figure 3: ZACUBE-2 payload layout and interface (image credit: CPUT)
• Data storage: The payload will contain sufficient storage to store both AIS and imaging data of importance. The processor payload alone features 64 GB flash storage. When ZACUBE-2 is in range of a ground station, it will download payload data to preserve on-board storage. Additional solid-state storage will be investigated should the physical space permit it.
AIS and VDES services towards maritime domain awareness:Although there are commercial AIS satellite constellations in operation, there are various direct benefits for sourcing AIS or VDES data from a South African MDA (Marine Domain Awareness) constellation. These are briefly discussed here.
1) Data independence: The MDASat-i constellations will enable South Africa to monitoring its coastal regions without depending on foreign third parties, who may have undisclosed interests to withhold data. This ensures that the country can maintain critical operations of national importance.
2) Cost reduction: Current commercial satellite AIS offerings are costly and do not allow data sharing between various Government entities; thus, each entity needs to have a separate commercial contract and license. This results in a substantial total cost of AIS data to meet all governmental needs. Offering the AIS data collected by the MDASat-i constellations to all Government entities will therefore result in direct savings. It will also improve the operations of the various departments that do not have sufficient funding to make use of current satellite AIS data. The same applies to neighboring countries, enabling them to improve their own operations and relieving the amount of direct support required from South Africa.
3) Removing restrictions on data sharing: There are currently very strict restrictions in place on sharing commercial satellite AIS data. As stated earlier, this has resulted in various governmental entities obtaining individual satellite AIS contracts, requiring independent operational systems to manage this data. Collaboration and data sharing among departments are, consequently, negligible.
Removing the restrictions on data sharing will promote collaboration. It will also allow for the reuse and sharing of operational systems and procedures. This enables one entity to add additional information to the data, allowing the next entity to use this information appropriately. For example, if a ship has an emergency, additional information coming from other sources could be linked to the ship, allowing the rescue services to use this information and to take appropriate actions.
4) Enhanced AIS/VDES intelligence: With control over the both data sources and the data itself, additional intelligence can be derived from studying the nature of AIS/VDES signals received, as well as the tracking behavior of the vessels monitored.
Job creation: This project will result in two general areas of activity; first, in the manufacture of nanosatellites and second, in the operation and management of the MDASat-i constellations to generate AIS/VDES data and to enable related value-added services. The vast majority of these jobs require a highly skilled, well-paid workforce.
The Advanced Manufacturing sector has been shown to have a particularly strong spill-over effect into other sectors due to the rich value chain that settles around advanced manufacturing activities. A recent study conducted in South Africa determined that the Manufacturing sector has the second highest "multiplier effect" of the various sectors present in the country (Agriculture has the highest). The multiplier is defined as the knock-on effect into other sectors when money is invested in Manufacturing. The study estimated that for every one South African Rand (ZAR) invested in the South African Manufacturing sector, an additional value-add of ZAR1.13 is created in the rest of the economy.
The authors argue that the Advanced Manufacturing sector, specifically, yields far higher multiplier effects due to the fact that this sector invests so heavily in research, innovation and human capacity development. This in turn increases the productivity of the general manufacturing sector, which leads to an increase in demand for higher skills in that sector. This argument holds true for the MDASat-i program, will synergize the collaboration among universities, and the public and private sectors.
Foreign Direct Investment: Attracting foreign direct investment is vital to stimulate the South African economy. Developing the Advanced Manufacturing sector is an important strategy to do so. Furthermore, AIS- and VDES-based value-added services to the international market will also translate into significant foreign direct investment levels.
1) Francois Visser, Robert van Zyl, Herman Steyn, Pieter Botma, "CubeSat activities in South Africa," Proceedings of the UN/Japan Workshop and The 4th Nanosatellite Symposium (NSS), Nagoya, Japan, Oct. 10-13, 2012, paper: NSS-04-0316
2) W.H. Steyn, R. van Zyl, M. Inggs, P. J. Cilliers, "Current and future small satellite projects in South Africa," Proceedings of IGARSS (IEEE International Geoscience and Remote Sensing Symposium), Melbourne, Australia, July 21-26, 2013
3) Francois Visser, "A Technical Background of the ZACUBE-i Satellite Mission Series," Proceedings of the 11th Annual CubeSat Developers' Workshop - The Edge of Exploration," San Luis Obispo, CA, USA, April 23-25, 2014, URL: http://www.cubesat.org/images/cubesat/presentations/DevelopersWorkshop2014/Visser_ZACUBE-i.pdf
4) Daniel de Villiers, Robert van Zyl, "ZACube-2: The successor to Africa's first nanosatellite," URL: http://www.amsatsa.org.za/ZACube-2%20%20The%20successor%20to%20Africa%E2%80%99s%20first%20nanosatellite.pdf
5) Robert van Zyl, Daniel de Villiers, Eugene Jansen, Mark Silberbauer, Attie Labuschagne, "Nanosatellites as catalyst toward sustainable Maritime Domain Awareness for the African Continent," Proceedings of the 4S (Small Satellites, System & Services) Symposium, Valletta, Malta, May 30-June 3, 2016, URL: http://congrexprojects.com/docs/default-source/16a02_docs/4s2016_final_proceedings.zip?sfvrsn=2
6) "Technical characteristics for a VHF data exchange system in the VHF maritime mobile band — M Series Mobile, radiodetermination, amateur and related satellite services," Recommendation ITU-R M.2092-0 (10/2015), URL: https://www.itu.int/dms_pubrec/itu-r/rec/m/R-REC-M.2092-0-201510-I!!PDF-E.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).Overview Spacecraft Launch Payload References Back to top