The number of satellites launched in the context of civil space programs has increased in recent years. And this trend is set to continue. The most recent examples are what are known as mega constellations consisting of hundreds or even thousands of satellites, such as SpaceX’s Starlink or Amazon’s Project Kuiper. Simultaneous communication with these and all other orbiting satellites requires a range of different radio frequencies. The problem is, however, that the radio spectrum is a limited resource.
This is where the SALSAT (Spectrum AnaLysis SATellite) nanosatellite mission comes in. As part of a research project conducted at TU Berlin’s Chair of Space Technology, this mission will examine the utilization of the radio spectrum directly in orbit. The project aims to help achieve an efficient and sustainable use of the radio spectrum. After a two-year period of intensive development, a Soyuz rocket carrying SALSAT was successfully launched from the Plesetsk spaceport in north-west Russia on 28 September 2020 at 13:20 Central European Time.
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In order to be able to gauge the actual utilization of the radio spectrum, researchers have developed the SALSA spectrum analyzer using software-defined radio (SDR). This will be placed on board the nanosatellite to analyze the global utilization of VHF, UHF and S band frequencies. These frequencies are mainly used for amateur radio and scientific satellite missions. The project will look to explore the possibilities for the multiple use of existing frequencies as well as identify their location within the spectrum. A further goal is to detect and locate interference and disruptions within the bands.
SALSAT will also be equipped with a camera and a Linux-based processing system. This will enable the on-board processing of spectral data, demonstrated by means of a compact and experimental neural network. The satellite will also feature a new type of triaxial position control system with fluid-dynamic actuators, originally developed in another research project. This unique, patented system is intended to demonstrate that a more durable and wear-free position control of satellites is possible using a magnetic fluid than with reaction wheels.
The scientific spectral data collected during the operating phase of the mission will be made freely accessible via an internet platform. This will enable international scientists, students and amateur radio operators as well as any other interested parties to research and improve the analyzed frequency bands. SALSAT will thus make an important contribution to the future of satellite communication.
SALSAT’s launch on 28 September 2020 marked the end of a two-year development period. The idea for SALSAT was originally conceived during previous projects on frequency coordination, including the SALSA and S-Net projects. The S-Net project provided a flight spare model of the TUBiX 10 nanosatellite, while the SALSA project developed a spectrum analyzer as a satellite payload. Parallel to this, a fluid-dynamic actuator was also developed at the Chair of Space Technology. Taken together, these core components led to the development of the idea for a space mission and the SALSAT research project, which got underway on 1 July 2018.
Space travel projects are usually developed in different project phases (A to F). Each individual phase is critically reviewed using the milestone principle. In addition to the scientific goals of the mission, a particular challenge faced in the management of this project was the need to adapt an existing satellite bus to a new mission while limiting the development period to just two years.
The construction of the first prototypes and software solutions followed the preliminary design review (PDR) in October 2018. The design was then verified and a model of the overall system tested on the basis of the expected maximum environmental impacts. This process included powerful vibration and shock tests to simulate the rocket launch, and thermal, vacuum and radiation tests to simulate variations of temperature (such as the transition from day to night) and the natural radiation encountered in the Earth’s orbit. All tests were conducted according to the specifications of the launcher for the respective carrier system. In the case of SALSAT, this is the Russian Soyuz rocket with an unmanned Fregat upper stage.
After the successful completion of the acceptance tests for the final version of the satellites, clearance was given for the launch of SALSAT on 22 July 2020 – almost exactly two years after the start of the project. Once all the paperwork had been completed and the export license and customs clearance granted, the satellite and the necessary equipment for the launch were shipped to Plesetsk Cosmodrome. Members of the project team carried out the final functional tests at the launch site and integrated the nanosatellite into the upper stage of the rocket. The launch and the start of the operational phase took place at 28 September 2020.
SALSAT took up a sunsynchronous orbit (SSO) at an altitude of 575 kilometers and with a period of revolution of approximately 90 minutes. The satellite will pass over mission control center at TU Berlin at approximately 1 pm and 1 am each day. The operational phase will end after about two years. Thereafter, the satellite can be used for experiments and training purposes. According to current estimates, the satellite should reenter the Earth’s atmosphere (depending on orbit, alignment and solar activity) some time from 2032 on.
Acceptance tests were used to verify the overall system and eliminate any manufacturing defects. These tests were conducted simulating the environmental conditions experienced at the satellite’s intended orbit. Prior to and after these tests, a range of operational tests were conducted in labs to test the functioning of specific features of SALSAT. These included the position control and communication systems. The position control system was tested using a floating, low-friction air bearing table fitted with a special laboratory light (serving as an artificial sun). This test successfully demonstrated that the satellite is able to align itself during orbit and actively counteract any rotational or tumbling motion and that its solar panels can provide it with sufficient energy. SALSAT's communication system, which uses amateur radio frequencies in UHF and scientific bands within the S-band to command the satellite, update software and download spectral data, was also successfully verified.
The functional tests of all important subsystems and payloads was followed by a vibration test. During this test, the satellite is switched off and placed in its ejection container, which is tested several times in all axes on a vibrating table for different amplitudes and vibration profiles (sinusoidal and random vibration). It is particularly important that the satellite’s resonance frequency does not change or only experiences minimal changes. Larger shifts indicate changes in the hardware (such as components falling off). The process concluded with thermal tests to simulate the major changes in temperature experienced during periods of sunlight and shadow. The thermal cycles used in these tests are intended to detect electrical defects such as poorly soldered joints.
As SALSAT’s main payload is a spectrum analyzer, the electromagnetic properties of the overall system were also tested. The purpose here was to rule out any internal problems (electromagnetic interference) caused by, for example, the charging regulator as well as to check the system’s ability to deal with extraneous radiation (strong signals, other satellites). New functional and operational tests were then carried out to ensure that the satellite was still in good working order and that the ground staff would be able to command it properly. SALSAT passed all the acceptance tests. After its transport to the launch site, additional (scaled down) functional tests were conducted to rule out any damage caused in transport. These tests were successful, SALSAT commenced its journey into the Earth’s orbit. Just a few short hours later, TU Berlin mission control received the first radio signals.
On the basis of a resolution of the German Federal Bundestag, funding for the mission is provided by the Federal Ministry for Economic Affairs and Energy via the department of satellite communication at the German Aerospace Center (DLR). The German Center for Satellite Communications (DeSK) is also involved as an external project partner, providing support with the operation of the satellite as well as with public relations work.