The Centre would manage a portfolio of research projects within the
identified key technology areas. A speculative list of representative
projects is given here.
- System Configuration Modelling
Various system configurations are easily conceived - for example the
receiver could be surface-based or in Earth orbit. Similarly, different
laser wavelengths have various advantages and disadvantages, both for
transmission and reception, and for their scattering and absorption
characteristics within the atmosphere. Detailed models will be needed
to assess such major system parameters. Differing technologies will be
applicable to different approaches, and it is likely that no one
solution will be a clear favourite until more research is undertaken
in the key technologies.
- Computer Simulation
Computer simulation of a Mars link based on photon counting detectors.
The simulation would work by putting a sequence of bits through a
simulated link modeling the photon noise from signal and background, and
dark count. The model would include the modulation and demodulation
process (PPM), and Forward error correction codes (e.g. Convolutional,
Reed Solomon) and the dead time set by the detectors. The output of the
simulation would be compared with the original bit sequence to determine
the bit error rate (BER) as a function of various parameters.
Such modeling is needed to give a clearer idea of the performance of such
a link in terms of photons per bit, and data rates; to determine the
effect of background and dark count on performance; to determine the
number of photon counters needed to support a given data rate; and to
determine the most effective error correction schemes.
- Laser transmitter development
Much work needs to be done to realise a laser technology with suitable
characteristics. The high power, high beam quality problem has been
solved for certain specific lasers, e.g. for LIGO by the University of
Adelaide. This work needs to be extended to other possible wavelengths,
to maximise the efficiency of the lasers for space-based operation,
and to allow scalable communications-suitable technology, possibly with
multiple frequencies carrying parallel data.
- Daylight Sky Brightness Monitoring
Reducing the background to a minimal level is essential to getting photon
noise limited performance in daylight. We will most easily get sky
brightness data in a useful form if we measure it ourselves. A relatively
simple system could be set up using a small automated
telescope (such as a Celestron or Meade amateur telecope) controlled by a
PC, with a filter wheel and visible and IR photodiode detectors. This
could easily be programmed to make a set of measurements of the wavelength
and position dependence of sky brightness. A monitoring program needs to
be carried out over a sufficent period to determine the full range of
varaibility. Considerable variations can be expected depending on the
quantity of scattering particles in the atmosphere.
- Daytime Seeing Monitoring
The other important factor determining the background level is the
aperture size and this is determined by the daytime seeing. NASA studies
have suggested that daytime seeing is 5 to 10 times worse than nighttime
seeing (of 1 to 2 arcsec), and so a 20 arcsec aperture is needed. This
seems rather pessimistic. We need to look at what has already been done
on daytime seeing (e.g. by solar astronomers), but again it may be useful
to start our own monitoring program to get a good set of statistics. The AAO
can measure daytime seeing with the AAT and its acquisition camera on bright stars.
Alternatively an instrument could be built specifically for daytime seeing
measurements (e.g. an IR DIMM) though this would be a more expensive
- Tunable Filter Selection
To cope with the background levels we will need very narrow band filtering
to isolate the laser wavelength. The results of steps 1,2 and 3 will
enable the requirement to be fully specified but it is likely that we will
need filters narrower than 10-5 and tunable over a small range to allow
for the spacecraft velocity. We need to determine the most approporiate
- Design Study for the Multi-Telescope Telescope
Develop the optical and mechanical design for a multi telescope
system along the lines Peter Gillingham has suggested. Initially we
would aim at a prototype system using say 4 1m or smaller telescopes.
However, the design would need to be scalable to a final system of
about 10m effective aperture composed of 2m telescope tube assemblies.
The prototype telescope could perhaps make use of a standard telescope
mounting and control system from EOS, with just the tube assembly
being non standard.
- Laboratory Demonstration of a Photon Counting Data Link
This step would demonstrate a working data link in the laboratory,
operating at photon rates of a few photons per bit, and would enable us
to test the performance figures determined by the computer simulation.
The transmitter could use a standard fibre-optic communication light
source. It could use hardware error correction coding, and a PPM modulator
which we may have to build. The receiver would use a small number
of Silicon APD photon counters (commercially available modules, e.g. from
Perkin Elmer). We would have to develop the high speed electronics to
perform the photon timing, and software for demodulation and decoding
which has to be able to handle high data rates (though for initial
testing the data processing may not have to keep up with the data if
sufficient buffering memory is available). The software development
could build on that already used in step 1.
- Prototype Telescope
Build a prototype telescope on the basis of the outcomes of the earlier
modelling work and design study.
- Detector System
Build a detector system for the prototype telescope based on
experience from the laboratory system demonstrated previously.
- Tunable Filter System
Build a tunable filter system (one filter for each sub-telescope, or
perhaps one for each detector) based on results of the research into
sky brightness and wavelength selection criteria.
- Integrate Prototype Telescope System
Integrate the telescope with its detector system and tunable filter
system, autoguiding system etc.
- Telescope Tests
Various tests not involving data transmission. For example test ability
to work close to the Sun in daylight without excessive background. Test
ability to track and guide on a moving satellite.
- Ground Communication Tests
This is probably most easily done if the telescope is designed allowing it
to point horizontally, for ground-to-ground communications testing.
The transmitter need only be low
power, similar to that used for the laboratory tests.
- Ground-Space-Ground Communication Tests
This would use a laser transmitter on the ground and bounce a signal off
a laser ranging satellite to return a weak signal to the receiver. This
sort of test should test pretty well all aspects of the system. For
example, the ability to track a moving object, to operate in daylight
and to communicate at low signal levels.
- Further Development
The next step would be to demonstrate Space-Ground communication
from Earth orbit. This would involve a transmitter in orbit.
To accurately simulate the Mars communication link, this
transmitter would have much lower power and a wider beam than the proposed
Mars transmitter. It is likely that such a test would be beyond the
initial five-year ARC funding of the Centre, and would certainly
require substantial collaboration with NASA, but it is hoped that
further funding sources will allow the Centre to continue its research.
From there we would move on to developing a full scale prototype
receiving telescope and demonstrating deep space communication to
Originally created by
on July 10, 2002