SSETI Express Update - March 2005

Graham Shirville G3VZV

Background

SSETI Express is the first of a series of satellites being developed by the Education Office of ESA - The European Space Agency as part of the "Student Space Exploration & Technology Initiative"

The satellite is quite large - 600x600x700mm and will weigh in at more than 50kg.

It is being built by university students from a number of teams from all across Europe and being assembled at the ESA ESTEC facility in the Netherlands.

The satellite is set for launch on a Cosmos rocket later this year with a number of other satellites into a sun synchronous 98o 680km orbit from Plesetsk in Russia.

The payloads

There is an OBC, an attitude control system, a colour camera and cold gas thrusters on board as well as three cubesats which Express will "launch" soon after it separates from the launcher itself. All of these systems need to communicate with the ground both for tele-command uplink and for telemetry downlink purposes.

The Communications Suite

The main data transceiver is a UHF unit built by Holger Eckardt DF2FQ. It is based on his existing TF7 packet transceiver but the unit includes a 9k6 TNC and has it's own switch mode power supply.

Originally this was going to be the only communications device being flown but there is also a set of experiment S band patch antennas being flown and they needed a transmitter to power them. To start with this was also going be a full data transceiver but the costs for a commercial unit made this option "non viable".

This gave AMSAT-UK the opportunity to offer a 3 watt S band transmitter free of charge to the project - on the basis that it could be linked to the UHF receiver for operation as a single channel FM voice transponder when all the experiments have been completed. The unit also incorporates its own switch mode power supply and a 38k4 TNC to allow the rapid downlinking of data - especially necessary for the camera experiment.

The S band transmitter progress

The S band unit in its enclosure

The transmitter unit is built into a beautifully machined and finished aluminium box provided by the University of Wroclaw who have also developed the three way power splitter and the experimental patch antennas.

Except for the TNC, which is a commercial TNC 7, the whole unit has been designed and built from scratch by the team. There are five boards. The exciter from Sam G4DDK, the PSU from David G0MRF, the command & control board and the sensor board from Jason G7OCD and the 3W PA from Charlie G3WDG (an identical unit to that flying in AO51)

The unit was first "delivered" to ESTEC early in November 2004 but further work on the wiring was needed and it was then returned to them again at the end of the month. At this stage the first actual amateur call was put through the satellite in the clean-room from the "control room" next door.

"The Ground Station"

The unit then remained at ESTEC and was regularly exercised to download data via the temporary ground station that had been provided to them by Howard G6LVB.

During February it became clear that the data rate from the OBC to the TNC needed to be changed to accommodate other mission requirements. An AMSAT team went over to rewire and reconfigure the unit and success was quickly achieved.

The joy of a successful baud rate change

We then went on to apply the conformal coating to all the PCBs (except around the RF parts). At this stage disaster struck as the somehow some of the coating found its way right inside one of the filter assemblies - result no RF output!

The unit was then brought back to the UK for "repair" but quickly returned again during the first week of March by Sam and David.

They were able to demonstrate the unit working again and also helped the SSETI team solve a power limiting problem which caused the unit to be switched off for 150ms every time it was commanded on - not helpful when trying to transmit short packets.

Here is an extract from part of the SSETI Express Integration logbook for 2nd March 2005:

ESA_Neil, AMS_David and AMS_Sam power up the OBC, UHF and S-BAND for the purposes of testing.

- The initial power consumption of S-Band FM seems good
- The carrier is brought up successfully with the usual command
- The DTMF telemetry is turned on and received without issue
- The unit is switched to data config and data is transmitted without issue
- The carrier is brought back down
- The DTMF telemetry is turned on and received without issue
- The unit is switched to data config and data is transmitted without issue
- Transponding is tested without issue

MILESTONE 25: The S-Band sub-system is declared flight-ready.

Neil Melville - SSETI Express Project Manager captured on the Webcam on 8th March 2005 - admiring the satellite under construction in the clean room at ESTEC!

Actually the S band transmitter is the FIRST sub-system to be declared flight-ready!

It has been a great experience for the AMSAT-UK team to be working with both the ESA experts and also the enthusiastic students - a number of whom have expressed the intention to get an amateur licence for themselves!
It is a steep learning curve for them and for us - although we have been flattered in one presentation recently given by Neil Melville - the Project manager, which includes the text "Radio amateurs know what they are talking about"

What happens next?

A total of 150 solar cells, in ten strings of 15, are currently being laid onto the external panels of the satellite and we anticipate that these should be sufficient to enable the U/S transponder to work with the carrier up on a near 24/7 basis.

The flight model of the satellite will be completed over the next few weeks and will be subjected to the usual vibration tests and also thermal vacuum tests. Hopefully these tests will be completed without incident or problem.

The launch date is still not confirmed but should be late this summer.

The SSETI team will be providing full telemetry decoding information and will be encouraging us to provide downlinked telemetry data for them from around the world. They only have two ground stations of their own available and the software does not provide the "whole orbit data" that we are used to. Our worldwide network is certainly a unique facility and this data collection exercise will be good PR for the amateur satellite movement. ESA will be providing a prize for the amateur who provides them with the most. Full details will be available on the sseti website well before launch.

It is expected that the on board experiments should be completed within a matter of a couple of months from launch and after that the transponder can be placed into service. There is still a long way to go before that happy state becomes a reality and we have to remember that this is a high risk project - but if we don't try………

References:

Full information about the project is available at the regularly updated www.sseti.net website. This includes the complete integration logbook mentioned above. It is a big file but makes very good reading for satellite enthusiasts!

The webcam: http://sseti.gte.tuwien.ac.at/WSW4/express4.htm
Downloads: http://sseti.gte.tuwien.ac.at/WSW4/express_downloads.htm

Space Colloquium

Members of the AMSAT-UK team who produced the S Band transmitter will be giving a presentation on SSETI Express at the AMSAT-UK Space Colloquium. This will be held at the University of Surrey in Guildford from 29 - 31st July.

All Amateurs and SWL's are welcome to attend the event.

For further details contact the secretary Jim Heck G3WGM
Tel: 01258 453959
Email: g3wgm@amsat.org
Website: http://www.uk.amsat.org/