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Laser Data Link
To quantify the performance expected of a micro spacecraft
One of the most critical functions to be performed by micro spacecraft is to return data and permit telescopic tracking
of the spacecraft.
Some of the available critical technologies include:
•high power diode lasers
•photon counting Geiger mode avalanche photo diodes
•astronomical grade CCDs with very low dark current
•very low mass motors, actuators
By performing a study and preliminary design of this and the other main systems within a spacecraft, the resulting total spacecraft
mass can be estimated.
Here, it is assumed that a typical near term mission will be to pass and photograph a NEO at 1/10 AU, or about 15 million km.
In fig 1 the downlink is chosen to be at 650 nanometer wavelength, a deep red that can be visually seen as about
a 10th magnitude star.
These diodes are used in writable CD drives and are widely available.
The uplink is proposed to be done with lasers of 780 nanometer wavelength which are available at several hundred mill watt
output levels and for which a silicon CCD is at near the maximum sensitivity.
Because the uplink data rate for the earliest generations of spacecraft need only be enough to give simple commands, the initial
design is to use the CCD itself as the data receiver.
The bit rate will then be at the frame rate, and might be one to 100 bits per second.
The beam width assumed for up and down link is to be 2 arc seconds, or 10 micro-radians. To accomplish this the optical aperture
required at the spacecraft would be about 7 centimeters.
A Pyrex or quartz mirror that is 2 millimeters thick will weigh about 15 grams.
The total power requirement can be kept below 200 mill watts, most of which is required by the 30 mill watt laser.
The wavelength of the laser can be calibrated prior to launch to determine its operating temperature to give a standard
wavelength can be maintained by parts of the thermal control system.
The CCD will be maintained at about minus 50 Centigrade for a low dark current, as is the usual practice for astronomy.
This gives a dark current less than one electron per second for each pixel.
The uplink image can be made to be the intersection of four pixels, making the dark current of insignificant value.
Frame rates higher than 100 per second can still reliably receive low noise data from Earth.
The other main function of the uplink camera is to keep an image of the crescent Earth aligned in such a way as to place the
150 km (100 mile) diameter aim point of the downlink laser at a selected location on the night side.
To accomplish this the downlink laser beam aim point will be calibrated to coincide with the intersection of four pixels near
the center of the CCD. An onboard processor will than steer the telescope to shift the Earth image so that the desired location
falls on the selected spot.
Amateur astronomers at organized "data parties" like their star parties, can gather at receiving locations.
They can use a 650 nm interference filter to speed up the hunt for the spacecraft signal and a demodulating device
can retrieve data.
With the use of a 20 inch reflector and with the efficiency of the available components, about 100 thousand photons
can be registered each second, enough to return data at a useful rate.
In case there are technical problems with using a Geiger APD on Earth, an older technology, photomultiplier tubes can be
used with the recently available blue 405 nanometer laser diodes.
Blair Gordon COO