In a student project such as SEDSAT-II there is always the need for an infusion of new recruits as existing members become swamped by other academic commitments or graduate, or even to simply supplement the existing knowledge base within the project. It is important the integrity of the project’s timeline not be compromised, and it is for this reason that we have, on several occasions in the past, initiated recruitment drives. We believe that the time has come again for a fresh influx of participants, both generally and particularly in the Attitude Determination and Control system (ADCS).

The ADCS team is responsible for designing and implementing the systems required to efftively control the orientation of satellite. Currenty the team is looking into attitude determination using magnetometers as well as earth sensors and attitude control using magnetorquers. In addition the possibility of a secondary attitude control system using a new type of cubesat thruster is being investigated. More information about the cubesat thrusters can be found here.

The minimum requirements for the applicant to the project includes completion of at least one year of a relevant undergraduate degree in science or engineering. For the ADCS team it would be an advantage, but not a requirement, to have completed the equivalent of an introductory course in Attitude Determination and Control Systems. Other relevant skills include embedded computing, electronic engineering, aerospace engineering, physics and mathematics.

Why apply? Participating in the SEDSAT-II project, or indeed, any CubeSat project, provides students with an  opportunity to apply knowledge gained in the classroom to a real-world project, an opportunity that should not be missed for the advantage it will provide upon graduation and initiation into the space industry. The SEDSAT-II project, in particular, aims to provide this opportunity to as many interested students as possible, from a diverse range of educational backgrounds and nationalities.

More information about the ADCS subsystem can be found here.

An application form can be found here.

A CubeSat communication subsystem needs to provide a method of transferring data between a control team on the earth and an orbiting satellite. The system can be divided into two overlapping blocks, the ground station subsystem, and the on-satellite subsystem. This article is from the perspective of the ground station subsystem.

The ground station team for SEDSAT II currently consists of engineering and science students from India, Canada and the United Kingdom; all working together to design and build at least one installation that will provide daily contact with the SEDSAT II cubesat. The installation will use a high gain antenna to receive payload data from the orbiting cubesat, and also to transmit commands that maintain the health and update the task list of the satellite.

The first installation is planned to be installed above the Electronic Engineering department at the University of Bath, UK (shown above) to be ready in time for testing with a SEDSAT II prototype communication subsystem. When both designs have been declared spaceworthy, other installations on different continents can be quickly built to increase the coverage before and after launch day.

The first difficulty with orbital communication is that a satellite can only be ’seen’ when above the horizon, and only then for a limited time known as a ‘window’. These short windows (approximately 20 minutes, as given by the comms team) require a fast tracking antenna that can follow the cubesat from horizon to horizon, which requires the use of either an electrically guided Yagi antenna or a mechanically steered parabolic dish.

The second difficulty is in picking up the weak signal from the cubesat, which is currently being designed to have a transmitter with a power less than 1 watt. The comms team are working on methods of error detection which is caused by signal noise, this however can only be implemented once the signal is received, which will then need to be cleaned. To do this, the highest possible signal to noise ratio (SNR) needs to be achieved by the ground antenna, to ease the pressure on the requirements of the tiny cubesat transmitter. The simplest way to get a high SNR is to build a bigger antenna; however there are subtler ways like carefully selecting impedance matched equipment, or more accurate pointing at the cubesat.

On the other hand, acting in our favor, the transmission power of the ground antenna (up link) can be anything up to 100 Watts with a full amateur radio license, and the antenna will be as large as wind loading and the university will allow. An advantage of the proposed location is that Bath University is located on top of a hill, making more of the horizon visible than would be possible in most urban areas.

During the mission phase of SEDSAT II the ground station will operate in a semi-autonomous mode, requiring it to be able to track the cubesat independently by use of orbital prediction software and data from previous orbits. Adjustments to the known orbit of the cubesat can be sent to the ground station via a web interface from anywhere around the world, and this same interface can be used to distribute payload data and reports on the cubesat health, as well as commands to be transmitted back up to the cubesat.

Right now we are still in the design stage, which makes it a perfect time to join before all of the building starts! We are looking for any computer/engineering/science students with basic knowledge in anything described above, or at least the willingness to learn the basic knowledge of anything described above. The intention is to create a ground station that will last the length of the SEDSAT II mission, and still be around for use by its inevitable successors.

To join SEDSAT II visit the application page:

http://wiki.seds.org/index.php/SEDSAT-2_Team_Application

To learn more about the ground system visit the wiki page:

http://wiki.seds.org/index.php/SEDSAT-2_Ground

The paper “SEDSAT-II: Design and Implementation of a High Resolution Imaging Payload with On-board Image Analysis” has been selected for presentation at the International Astronautical Congress 2008, to be held in Glasgow. This paper is based on the work carried out on the SEDSAT-II imaging payload and is co-written by payload team members Tom Nordheim and Brynjar Larssen from the University of Wales, Aberystwyth and communications team member Lavina Parwani from PhilSEDS.

The abstract can be read here.

A CubeSat communication subsystem needs to provide a method of transferring data between a control team on the earth and an orbiting satellite. The system can be divided into two overlapping blocks, the ground station subsystem, and the on-satellite subsystem. This article is from the perspective of the on-satellite communication subsystem.

The SEDSAT-2 “comms” team, who develop the on-satellite component of the communication subsystem for the SEDSAT-2 CubeSat, are a group of mainly electronic engineering/computer science students, with a representation from mechanical engineering too. Currently, we are seven people strong, with three located in Manila, three at separate locations in India, and one person in the UK. In a distributed environment such as this, it is quite a challenge for a team who are new to a technical topic such as communication system design, to work together to figure out how to attack the problem. The solution to this, as far as we have figured out, is careful planning, breaking down problems into independent work-packages, and regular team meetings where we can discuss our work and decide what we need to do next. Underpinning this are the technical tools which facilitate our process; wiki’s, for sharing and archiving research ideas, Instant Messaging software, for live team communication, and revision control software, to provide a shared filespace accessible on each team members computer.

So perhaps you are wondering what is involved in a satellite communication subsystem. The most obvious problem is the distance we have to transmit over. We expect our satellite to be in a low-earth orbit, so typically the distance from our ground station to our satellite will be around 2000km, although this can rise to 3000km and drop, on rare occasions, to 700km. A second, connected problem, is that the satellite is continually moving relative to the ground station. This in-deed provides the ground station team with some problems, as they need to keep adjusting where they point their antenna in order to find the satellite, but for the on-board satellite communication subsystem the problem is more fundamental. As we only expect to have one or perhaps two ground stations, and the ground station can only contact the satellite when the satellite is above the ground stations horizon, most of the time it turns out that the satellite is below the horizon at our ground stations, so there can be no communication! This gives us our first parameters: some orbital modelling with the Orbitron software shows that in a typical 24 hour period, a single ground station may only see the satellite once for about 20 minutes, and with perhaps two or three other “passes” of 1 minute of so. In this 20 minute per day “window”, all of the data we want to send from the satellite to the ground station must be transmitted, and any control information we want to send from the ground to the satellite must also be transmitted.

We expect the payload team to generate around 1MB of data each day, so in order to transmit all of this in our 20 minute window, our comms subsystem must be able to transmit at more than 6990bit/s. To allow for some overhead, we have decided to target a transmission rate of 9600bit/s. This is our first major specification!

Now, back to the distance problem from earlier. We are typically 2000km from the ground station but because we get all our power from solar panels on the satellite (which isn’t a huge amount), we can only afford an on-satellite transmit power which is similar to that of a mobile phone. But our distance is about 1000 times greater than what a typical mobile phone signal needs to travel, so we’ve got a problem. There are ways to overcome this problem, but they involve a big antenna on the satellite, and this doesn’t work on a CubeSat, as we only have a little cube (10cm by 10cm by 10cm), so no room for a big antenna. So we decided to turn this distance problem over to the ground station team – they don’t have any size limits (well, within reason), or any power limits so they are much better placed to design a huge directional antenna, or to commandeer the Arecibo radio telescope in South America in a black-op style raid, whichever they feel is most appropriate. This turns out to be a fairly effective strategy (shifting communication problems to Ground, not black-op raids) in general, for various communication problems, and is usually termed a “System Design” decision – if a problem can be better solved elsewhere in the system, then do it!

So far, we have defined our basic system performance specification, and we have decided not to worry about our limited transmitter power. Next, we must start thinking about how the communication subsystem will work.

A signal transmitted from our satellite has to travel a huge distance to get to the ground station, and on its way it has to go through the earths atmosphere. Various parts of the earths atmosphere have effects on our radio signal, causing distortion. Also, while the ground station should be able to pick up our satellite signal, it will also pick up lots of other interference from things such as lawn mowers, powerlines, lightning strikes and automotive engines. When all this is put together, we find that our received signal doesn’t look particularly like our transmitted signal, and that there is a chance that ground station receiver will not be able to properly decode the received signal. The result is that some of the data we transmit will not be received correctly.

We need a way to overcome this problem but luckily the answer is well established in the protocols which make up the internet; an automatic retransmission system. In this system, data that we want to transmit is broken into chunks or “packets”. Our satellite can start transmitting packets and after the ground station has received each packet correctly, it sends an acknowledgement to the satellite, to let it know that the packet was received. If however the ground station detects an error, it sends a “not-acknowledged” to the satellite, to let it know to retransmit the packet. We’ve taken this basic structure and added a few more features and have given it the name SEDSAT-2 Reliable Transmission Protocol. All companies and projects have their own acronyms and we at SEDSAT-2 don’t want to be left behind in this regard, so the acronym for our transmission protocol is: SRTP. A full technical protocol specification for SRTP will be made available on our wiki page soon, so if you’re interested in more information, that will be the place to look.

In summary, we have been developing a communication system which provides a reasonable data rate, works with intermittent communication windows, is tolerant of communication errors and emphasises a system level design approach where we simplify the satellite where possible by pushing as many problems to the ground station as possible. Besides making available the increased resources available to the ground, this latter approach also means that there is less to go wrong on the satellite – as once the satellite is launched, it can not be debugged or fixed, so it absolutely has to work. With the ground station however, if there is a problem after launch then it isn’t the end of the world as we can still debug and fix the problem.

I hope this article gave you an (very rough) idea of what the communications team is working on. If you’d like to find out more, take a look at our wiki page[1], or get in touch with the team (contact details are on the wiki!) with any questions you’d like answered. And if you would be interested in joining the SEDSAT-2 team to work on the communication or ground subsystems, take a look at the vacancies on the SEDSAT-2 recruitment page[2], we’re always on the look out for motivated people who have an interest in Space, who want to do something challenging, exciting and unique.

[1] http://wiki.seds.org/index.php/SEDSAT-2_Communications

[2] http://wiki.seds.org/index.php/SEDSAT-2_Team_Application

Up to date technology for Micro and Nano satellites include advanced attitude control and station keeping systems, especially with regard to soon scheduled missions including formation flying. Equipping even small satellites with fully operational thruster systems becomes particularly interesting with regard to the increasing focus on de-orbiting possibilities to avoid an increasing number of small satellites out of use in LEO. Miniaturized electric propulsion therefore seems to be a promising candidate for such future mission profiles.

To guarantee a safe integration, non-toxic - easy to handle - propellants are necessary, properties that make the Ablative Pulsed Plasma Thruster (PPT) become a promising candidate (Ref [1]). A PPT is a propulsion system which ionizes and then accelerates its propellant by means of electromagnetic forces to produce thrust. Due to its mechanical simplicity and its reliability, it became the first electric propulsion system ever flown in space on the Soviet Zond 2 mission which started on Nov. 30, 1964 toward Mars (Ref [2]). Since then, PPTs were used on various missions for orbital control such as drag make up and east-west stabilization (Ref [3], [4]).

The system feature high specific impulse compared to conventional chemical thrusters, mechanical simplicity, mainly due to the utilization of a solid propellant, and therefore high reliability (Ref [5]: 3f).

Schematic PPT (Ref [6]: 264)

The thrusters main components are a capacitor for power storage, the acceleration chamber consisting of propellants and electrode “rails” and an ignition system similar to a spark plug. Opposed to its fairly simple construction, the occurring processes within the PPT are highly complex: An external triggered, high current discharge ablates and then ionizes the solid propellant, in most cases Teflon^®, before both ions and electrons are accelerated by the induced magnetic fields to exit velocities beyond 30 km/sec depending on the degree of ionization. This way the energy stored in the capacitor is depleted within micro seconds, which makes the thrusters operate in a short pulsed mode, similar and well compatible to digital logic control (Ref [5]: 4). In addition to the electromagnetic acceleration, a gas dynamic process similar to the one in an arc jet accelerates existing neutral atoms. Both processes add up to impulse bits on the range of a few tens of micro Newtons which makes the system well fitted for very small satellites, despite its significantly poor performance with efficiencies below 10%.

Trying to implement such technology into satellites in the size of Cubesats of course brings a variety of new challenges due to its restrictive mass, volume and power budget, especially regarding the energy storage unit.

The testing of such a PPT, designed within the master thesis of a SEDSAT II member at the Austrian Research Institute in Seibersdorf (A), could be a good way to draw attention of the scientific community to our project for using technology with promising importance in the near future of small satellite missions.

References:

[1] Pottinger S. J., Scharlemann C. A. (2007): Micro Pulsed Plasma Thruster Development, IEPC-2007-125.

[2] Burton, R. L., Turchi, P. J. (1998): Pulsed Plasma Thruster. Journal of Propulsion and Power, Vol 14, No 5, p 716-734.

[3] Vondra R., Thomassen K., Solbes A. (1971): A Pulsed Electric Thruster for Satellite Control, IEEE 01450062 Vol. 59. No. 2.

[4] Ziemer J. K., Choueiri E. Y. (2001): Scaling laws for electromagnetic pulsed plasma thrusters, Plasma Sources Sci. Technol. 10. p. 395-405.

[5] Guman, W. J. (1968): Pulsed Plasma Technology in Microthrusters, Fairchild Hiller Corp., Technical Report AFAPL-TR-68-132.

[6] Jahn, R. G. (1968): Physics of Electric Propulsion, Dover Publications, Inc, Mineola, New York.

Set at the idyllic campus of the Charterhouse School in Godalming, Surrey it was time for the first UK Space Conference. The SEDSAT-II Payload team was represented by several members through genereous student sponsorship provided by UKSEDS, offering accommodation and conference attendance at greatly reduced fees. Serving as the venue for this year’s conference was the Charter House School; An exclusive boarding school founded in the 17th century with it’s sprawling campus of grassy fields and traditional buildings situated on the outskirts of the small English town of Godalming.

Gathered at the conference was an eclectic mix of UK Space pioneers, students, aerospace professionals and international guest speakers, most notably represented by NASA test pilot Joe Engle. The event proved an excellent opportunity to meet with the “who’s who” of the British Space community as well as getting to know students wishing to pursue careers within the AeroSpace industry.

With a strong focus on students, the final day of the conference was designated a “student day” and included inspiring talks by young professionals as well as a careers fair where many of the leading companies were represented.

We would like to thank UKSEDS for a providing the conference to us at a greatly reduced price as well as extending a thank you to all the people who worked for months to make this conference happen. Hopefully this marks the start of a great UK tradition.

SEDSAT II is an educational satellite project involving student members from over 16 different nations and 5 continents.

(Press releases can be found here: Press releases)

The project was started on the initiative of SEDS alumni who now have their work within the space industry, and who wished to contribute back to the organization, which had given them so many valuable experiences as students.

A picosatellite project was quickly identified as the option best suited for the follow up to the 1998 launch of the original SEDSat, a much larger satellite. This format presents students with big technical challenges in terms of miniaturization but also provides a hands on space engineering environment that can be achieved at very low cost, using commercially available components and technology.

Initiated in late 2006 the project is scheduled for integration at the 59th International Astronautical Congress to be held in Glasgow in Q4 2008. The project is currently in the beginnings of the design phase, evaluating different approaches and technical implementations.

Our vision is not only to design and launch a picosatellite but also to provide the participant with a valuable learning experience, bringing together people from different continents and different backgrounds to achieve a common goal.

For more information about each individual subsystem, click on the name of the desired subsystem in our menu on the right.

We are glad to welcome you to our new webpage. You will see that this page will update rapidly with the latest news from the SEDSAT II team. We would also like to remind everyone about our wiki-page at http://wiki.seds.org/index.php/SEDSAT-2.

SEDSAT II is an educational satellite project organized through SEDS Earth, involving student members from over 16 different nations and 5 different continents.

On Feb. 13th Alexandra Ruths and Paul Schmitzberger of SEDS Austria took advantage of the SGAC’s Permanent Observer Status with the UN COPUOS and made a technical presentation under the UNISPACE III agenda item at the Scientific and Technical Subcommittee.

Space Generation Advisory Council represents the global youth in front of the United Nations as well as other international institutions and governments. During this 45th session of the UN COPUOS Scientific & Technical Subcommittee Austrian SEDS students made a presentation titled: “Bridging the gap or why students of the 21st century no longer reach for the stars”.

The presetation can be found here: COPUOS SEDSATII presentation.

You can watch the presentation here:

Held in the technological centre of Hyderabad (also known as Cyberabad among the locals), this year’s International Astronautical Congress was a grand affair showcasing Indian culture and space technology alike.

Again greatly aided by our highly motivated marketing and management team, we proceeded to familiarize ourselves with the professionals of the space industry as well as fellow students from around the world. Our paper was presented in a packed technical session, and seemed to be well received by the attendees, who asked several good questions. We also further boosted our numbers by recruiting several new student members.