MITSUBISHI PAVES THE WAY FOR MICROWAVE
FOR IMMEDIATE RELEASE
MITSUBISHI PAVES THE WAY
FOR MICROWAVE
AMATEUR SATELLITE RADIO
March 1998 – With a
distinctive record of over 30 successful satellites launched into space, AMSAT (The Radio
Amateur Satellite Corporation), the worldwide group of amateur satellite enthusiasts and
pioneers of modular satellite systems, is now set to launch its Phase 3-D spacecraft into
orbit.
Phase 3-D will provide amongst other radio-amateur microwave bands, 2.4
and 10GHz downlink frequencies. Although seen perhaps as exotic for amateur radio, this
will open up the radio market further.
Directional control for satellite antennae in space has always posed a
problem. AMSATs Phase 3-D satellite spacecraft features magnetic bearing to
stabilise the satellite, and thus keep the antennas pointed towards earth, put in place
with more than a little help from Mitsubishi Electric Europe and its specialist
distributor European Microwave Components (EMC). Mitsubishi and EMC have donated critical
internally matched GaAs devices to help provide the highest quality signal quality to
receivers on earth.
AMSAT, although an amateur organisation and truly international with
its heart in amateur band radio, has achieved remarkable success in building small
satellites for Amateur Radio. Compared to other space work, AMSAT is an extremely low cost
organisation, interestingly carrying out its own design, development and manufacturing
work including device and materials qualification primarily using non-space grades.
Notably, since AMSATs first satellite project, Oscar 1, launched in 1961, just a few
months after the Russian Sputnik, there have been few device failures. Activities since
1961 include Space Shuttle, Mir and Freedom.
The Phase 3-D spacecraft puts to the test AMSATs magnetic bearing
system for accurate directional control for the on-board solar generators and antennae.
Prior space configurations for this mechanical control have centred on momentum wheels
with integral ball bearings. Lubrication in the vacuum of space has always been a problem
and for the past 20 years, magnetic bearings involving no physical contact and hence no
wear or lubrication problems, have been under development.
This new system, developed by Dr Karl Meinzer of Marburgs
Philipps University, Is being tested in the Ariane 5 launch and makes available high
reliability three axes control for highly accurate spacecraft positioning.
Phase 3-D is a hexagonally shaped satellite and it will be mounted for
launch on Ariane 5. With a successful launch objective of a large ellipse orbit, AMSAT is
expecting 15 years satellite life. The elliptical orbit will range from 47000km at the
highest point to 4000km at the lowest. Only two components in the project are fully space
qualified and these are used in the fuel and cooling assemblies.
The satellite weighs 400kg and with its six sided configuration
provides locations to stack and fit modules. The spacecraft is equipped with a 400 N motor
and a smaller ammonia motor. The larger motor is used to position the satellite in its
correct orbit and the smaller one, which has been developed by Dr Messerschmidt, takes
over for positioning. This ammonia motor uses very small amounts of fuel and large amounts
of electric energy so that a long lifespan is expected. The electronic modules fitted to
the Phase 3-D are stacked outwards from the structure with stand-offs to avoid problems of
heat and static on the satellite. Cooling is only by radiation and all assemblies on-board
are thermo-coupled to stabilise satellite temperature.
AMSAT has incorporated a whole variety of radio frequencies on-board,
with a range of transmitters and receivers, provided by a matrix switchboard arrangement
to allow any combination of up- and downlink frequencies. This not only provides signal
handling flexibility but also reduces any effect of sub-system failure. The X- and S-band
downlinks are typically being used with 1.3GHz or 5.7GHz uplinks. The Rudak DSP system
enabling complex telemetry and a GPS receiver for independent positioning in real time are
also on-board. This not only provides signal handling flexibility but also reduces any
effect of sub-system failure.
On X- Band 60W TWT and 7W solid state amplifiers, each have their own
antennae with different polarisation, providing the opportunity for simultaneous use as
well as redundancy in case of failure.
Mitsubishis involvement in the Phase 3-D spacecraft resulted from
previous successes with MGF 0900 Series GaAs FETs around three years ago and MGF 1302 FETs
used in a Beacon project for the past five years. Notably AMSATs Daniel Orban
maintains that Mitsubishis MGF1302 devices are the most reliable he has ever seen.
With a maximum rating of 5V, the devices have never failed in a Beacon in the timespan and
have been operating at 8V.
Mitsubishi devices – MGF 1302 (low noise GaAs FET), MGF 1601B,
MGF0904A, MGF0905A, MGF0907B, High Power GaAs FETs and internally matched MGFX35V0005 GaAs
FETs – are incorporated into various payloads. Single ended line-ups of three devices
provide two way voice SSB transmission capabilities. MGFX35V0005 is used in conjunction
with MGF1601B which is ruggedly housed for easy heat dissipation, and the MGF2415A which
provides a +10dBm drive for the MGF1601B.
The GaAs FETs will ensure good signal reception on earth from the
35,000 mile orbit. The high reliability devices are ideal for 2.4GHz and 10.4GHz
operation. For the project, high reliability really comes to the fore in this ground
breaking project since high efficiency and heat dissipation capabilities of the devices
are essential. Notably prior to launching, AMSAT only carries out a standard quality
functional test on the devices. “Statistics are on our side,” said Daniel Orban.
“If you pick a component out of commercial volume production, the chances of picking
a bad device are really small.”
The Helaps (High Efficiency Linear Amplifier by Parametric Synthesis)
transponder system achieves efficiencies of over 30% where a classical transponder would
achieve 14% overall efficiency. Mitsubishis MGF 0904, 0905 and 0907 devices are
incorporated into the S-band transponder, providing superior performance to bi-polar to up
to 10V with the actual operation voltage being modulated between 1 and 10 V.
Helaps achieves its efficiency in the output transponder, in the case
of 10.7MHz IF, where modulation is separated into phase and amplitude components. The
phase component is up-converted and amplified up to the output frequency. Thus the entire
transponder can be non-linear and therefore reasonably efficient. The amplitude component
is used to modulate the power supply of the power amplifier and in so doing, it
regenerates the original signal.
According to Daniel Orban, there are some problems however in achieving
efficiency as the phase component is up-converted. Some work is required to resynchronise
the amplitude component. The power supply used is a switched type and needed to be able to
handle the amplitude component in a very linear way. This also poses problems, he added,
for example in decoupling the power the power devices as very little capacitance will
distort modulation. The Mitsubishi devices, it turned out, had a very linear gain versus
Vdd curve.
AMSAT operates with a concept of using the best possible technology
available in commercial grade components where possible, with qualification to its own
specifications. Its achievements are its metier. Oscar 10 for example has been operational
for more than ten years. For the future, AMSAT is planning a project to orbit Mars in
around 2005/2006 as part of the ESA Mars project.
The AMSAT project will be a repeater system enabling earth to
communicate with the satellite in Martian orbit and the orbitting satellite to communicate
to earth. The project will operate a 10GHz downlink based on technology from the current
Phase 3-D project featuring 60~100W TWTA backed up by 50W solid state amplifiers.
# # #
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daniel_orban@compuserve.com
Mitsubishi Electric Europe BV, Semiconductors
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