Küp uydular için tümleşik uçuş bilgisayarı ve haberleşme sistemi
A combined OBC and communication subsystem for cubesats
Yazar: MUSTAFA ERDEM BAŞ
Danışman: PROF. DR. ALİM RÜSTEM ASLAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Uçak ve Uzay Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Astronomi ve Uzay Bilimleri = Astronomy and Space Sciences ; Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering

Özet:
Son yıllarda küp uyduların artan yetenekleri ile uzay sektöründe büyük atılımlar meydana gelmiştir. Üretilen ve uzayda çalışan ilk küp uydularla uzay kalifiye ürünler de kullanmadan uzay sistemlerinin geliştirilebileceği ve farklı görevlerde kullanılabileceği gösterilmiştir. Günümüzde fırlatıcı firmalarının roketlerde küp uydulara ana uydu yanında yer vermesiyle fırlatma maliyetleri de karşılanabilir olmuştur. Daha önceleri büyük bütçeli orta/küçük uydular ile yapılabilen görevler artık çoklu küp uydular kullanılarak yapılabilir hale gelmektedir. Özellikle üniversitelerin ve küçük uzay firmalarının uzay yarısına katılması ile uzay sektörüne deneyimli iş gücünün sağlanmasının kapıları açılmıştır. Böylece uzay alanında çalışan mühendisler daha tecrübeli olmaktadır. Günümüzde birçok uzay alanı akademik kurumları üretilmiş ve uzayda test edilmiş sistemleri hazır alarak kendi uydularını yaparak yörüngeye yerleştirip görevlerini yerine getirebilme durumuna gelmektedir. Ancak görev hazır bir uydu yapmanın hazır sistemleri bir araya getirmenin çok ötesinde bilgi ve hazırlıklar gerektirdiği de unutulmamalıdır. Aksi takdirde uydu yörüngede istenen başarımı göstermeyebilecektir. Küp uydular ile temel uzay çalışmaları yapılabildiği gibi, gözetleme uyduları gibi ileri seviye çalışmalar da çoklu küp uydular kullanılarak belli ölçülerde yapılabilmektedir.
Bu tezde, önceki uydu çalışmalarında (ITUpSAT, TURKSAT3USAT) görülen sıkıntılar, hazır sistemlerin tam olarak uyduya göre üretilmiş olmamasının getirdiği eksiklikler ve birçok alt sistem barındıran uydulardaki yer ve güç sorunu nedeniyle geliştirilmesi planlanmış tümleşik uçuş bilgisayarı ve haberleşme sisteminin tasarım, üretim ve test aşamaları anlatılmıştır. Literatür araştırması, tasarım kriterlerinin belirlenmesi, PCB üretim aşamaları, test süreci bu tezin konusudur.
Tez boyunca bir küp uyduda yer alan alt sistemler araştırılmıştır. Bu alt sistemlerden özellikle UGB ve haberleşme sistemi ayrıntılı ele alınmıştır. Yapılan araştırmalar sonucunda COTS ürün özelliklerinden ve uydu görevi isterlerinden tümleşik uçuş bilgisayarı ve haberleşme sistemi (OBCOMMS)'nin sistem özellikleri belirlenmiştir. Tez süresince 4 farklı versiyon OBCOMMS üretilip denenmiştir ve bunlardan 2 versiyon 4 uyduda kullanılmıştır. Yapılan Termal Vakum Çevresel Testleri anlatılmış, sonuçları tezde incelenmiştir.
Sonuç olarak çok fonksiyonlu bir uçuş bilgisayarı tasarlanmıştır. Birçok giriş/çıkış birimi bulunan, haberleşme portlarına sahip ve gerekli tüm korumaları olan, aynı zamanda FSK modem barındıran bir OBCOMMS üretilmiştir. Gerekli tüm testleri yapılmış ve sistem bu testleri geçmiştir. Bu sistem sayesinde uydu içerisinde yerden, kütleden ve güçten tasarruf edilmiştir.

Abstract:
With the recent improvement on the CubeSats there have been big developments in space sector. With manufacture and delivery to its orbit of the first CubeSat, it can be seen that subsystems, without the space qualified elements, can be build and used is space missions. Launching a CubeSat as a piggyback significantly decreased the launch cost. Most of the missions which required enormous budget, now can be done by using multiple CubeSats. Especially when the universities and start-up companies being included in the space race, employment rate increased in the space sector. Therefore, the engineers in this sector became more experienced. Even an unexperienced research institute/country can integrate their CubeSat and place them in orbit easily by using COTS subsystems with space heritage. By using CubeSats not only basic space missions but also high level earth observation missions can be done.
In this thesis; design, manufacturing and test stages of a combined OBC and modem subsystem has been explained with respect to problems experienced in the previous CubeSat projects ( ITUpSAT, TURKSAT3USAT ), insufficiency of COTS products for the projects and power and volume problems caused by usage of individual subsystems in the CubeSats. Also; literature review, definition of designing criteria, PCB manufacturing steps and test procedures are the topic of this thesis.
For the first criteria of designing, it is important that the product needs to meet the international standards. All the subsystems which are being designed and developed need to fulfill the CubeSat standards and in addition to this they also need to fulfill the "European Cooperation for Space Standardization" (ECSS) standards. Especially these requirements become more crucial with respect to orbital insertion and rocket launch phases due to their critic operations.
The second design criteria is that the subsystems need to withstand the radiation effects in the space environment. Radiation effects are the charges which occurs due to highly energized particles when they hit the electronic equipment. There are five known radiation effects. The three of these are the most dangerous types. These are; total dose effect, single event effect and single event latch up effect. With respect to these events tests are being made for space flight readiness.
The last design criteria is to choose the correct component which are space compatible. Elements which are being used in the subsystems needs to be able to work in space and proven that they are capable of working properly in space. Outgassing values of these elements needs to meet the NASA standards. With the tests applied on the elements it has been shown that they are capable to work in space environment conditions. Priority is given to elements which have a high "Technology Readiness Level" (TRL). An element with a space heritage requires less testing and decreases the total risk as a result of this, they are the first choice while designing a new subsystem. If an element does not have enough TRL or does not have any space heritage, an element which has successfully passed the thermal vacuum chamber tests with a temperature range of $-45^{o}~+125^{o}$ are being used in the systems.
Throughout the thesis, subsystems which are being used in the CubeSats, reviewed. Especially OBC and communication subsystems are being researched in detail. With respect to researches of the COTS product's capabilities and mission requirements, specs of the OBCOMMS has been decided. In this thesis four different versions of OBCOMMS has been manufactured and tested. Two versions of this subsystem have been used in four CubeSats. 
OBCOMMS v1 is the first manufactured prototype. This prototype was developed in order to prove the concept of combination of the two different subsystems in to one. Combined tasks were the command and data handling (on board computer – OBC) and communication by using a single microcontroller MSP430 which is common in CubeSats. Design was made with 2-layer PCB. Cots modem modules were used to make the first iteration less complex. The aim of the first iteration was to develop and test the control software for a simple OBC system. This iteration was successful and it has been shown that two different subsystems can be combined.
OBCOMMS v2 was the second prototype. In this iteration main microcontroller was upgraded in to an ARM and a secondary microcontroller was added. The secondary microcontroller was implemented in order to control the modem unit and to reduce the work load of the main microcontroller. Since the RF design phase was still not in progress, external modem modules were used in the development boards. The design was a two-layer PCB which is also PC-104 applicable. It works via 3.3V supply voltage. The main microcontroller was STM32F4 and the secondary microcontroller was MSP430. In addition to these 2 SD card were added where one of them as a backup.
OBCOMMS v3 was the third version. This version is similar to OBCOMMS v2. With respect to the CubeSat which this subsystem was going to be used, a change was made in the modem unit. Receiver module of the modem was excluded from the system and transmitter module was changed in to a different module which was capable of delivering 1W RF power. This subsystem was capable of being used as an OBC and in addition to that it was capable of working as a modem transmitter and a beacon modem. This iteration was also PC-104 compatible. System was working via 3.3V and 5V supply voltages. Main microcontroller was STM32F4 and the secondary microcontroller was MSP430. It has 2 SD cards which can be up to 8 gigabytes.


OBCOMMS v4 was the last version. It was improved in multiple aspects. RF design phase was included and a receiver and a transmitter module have been design and implemented in to the OBCOMMS. By adding an external Ram, processing power was improved. OBCOMMS has been turned in to a product with an eight-layer class three PCB which is space compatible. An aluminum shield has been designed and manufactured for the modem unit in order to provide EMI protection for the system itself and the surrounding systems. In addition to these, impedance matching was made to provide a high speed signal busses without any disturbances. System is PC-104 compatible. System works via 3.3V and battery supply voltages. Main microcontroller is STM32F7 and the secondary microcontroller is STM32F4. It has 2 SD cards, 64 MBit SD Ram. In addition to these it has 2xSPI, 2xI2C, 3xUART and CAN communications units.
Thermal vacuum chamber is an equipment to simulate the space environment with respect to its pressure and temperature values. By decompressing the air inside the system, it is capable of creating a vacuum environment. In addition to this; by using the shroud inside the system, it is capable of heating and cooling the internal volume via radiation of heat. Tests are being made in order to prove that the system is able to work in space environment.
TVAC tests which were performed for this subsystem and its results has been given in this thesis. Final topic of this thesis is functional tests which were performed during and after the development/manufacturing stages.
As a result of the TVAC tests it has been proved that the OBCOMMS is capable of working in a temperature range of $-20^{o}~+60^{o}$. The system has been kept operational continuously during the tests without any problem. When it was operational there was not any overheating and the temperature of the system was close to the ambient temperature rates.
As a result, multi-functional OBC has been designed. The subsystem has multiple I/O interfaces, communication ports (SPI, I2C, UART) and an FSK modem with all necessary protections. The OBCOMMS has passed all the required tests. Power, mass and volume has been saved with the development of OBCOMMS.   
As a future work, radiation tests of the OBCOMMS v4 are going to made and the system is going to be used in a satellite in order to provide space heritage. The current modulation (FSK) requires a special and dedicated ground station setup in order to receive and send the commands and data. As a result of this, communication modulation for the modem is going to be changed into AFSK at OBCOMMS v5 and there will be a two modem options for OBCOMMS. By doing this change amateur radio community will be able to communicate with the satellites which are using OBCOMMS, by using their own setups without any upgrades. Also with the latest version, main microcontroller of the OBCOMMS is going to be reprogrammed while it is in the orbit. This is going to be achieved by sending the programming codes to the secondary microcontroller via OBCOMMS's modem and secondary microcontroller is going to upload the new software into the main controller.