Thinsat Details

Silicon circuits, solar cells, interconnect, are essentially two-dimensional systems. The horizontal dimensions of an integrated circuit die may be measured in millimeters, but all the important action occurs within a few microns of the top surface. Indeed, modern IC die are thinned to increase thermal conductivity and reduce package height. The target thickness for this version 0.4 design is 75μm, mostly aluminum, thicker than previous versions to insure orbit stability against light pressure perturbations.

If the 180cm² of solar cell at end-of-life is 16% efficient, then the solar cell will produce 3.9 watts peak. This will vary with night side maneuvering (for light pollution control) and eclipse, producing 2.6 watts average at EOL. The much-smaller backside solar cell chips will produce only milliwatts of power, but if the thinsat is somehow turned away from the sun, these provide enough power to gradually operate the backside thrusters and turn the thinsat face forward again.

The resonators (probably SAW resonators) calibrate the oscillator of the processor chip they are near. All the chips on a thinsat will send timing calibration signals to each other, so they can mutually average to a common frequency, with calibration for temperature and aging used to adjust the oscillators and frequencies. Thinsats will calibrate against each other, and whole arrays will calibrate from timing signals from the ground. While each chip oscillator (based on LC resonators) may be quite imprecise pre-calibration, timing precision of a whole array should be accurate to within picoseconds of jitter and femtoseconds of long term average drift.

Characteristics

Electronics

Electronics: A thinsat used for database and web service may need as much as a terabit or more of flash memory (note - databases will be distributed over many thinsats). That will be about 4 by 4 centimeters of silicon area, distributed in hundreds of pieces. Computational thinsats will probably need less. While some high-performance processors and chipsets use hundreds of watts, Giga-instruction-per-second level machines can get by with far less. Here is a complete 4 watt system (including IO and power conversion losses) with 990 bogomips performance. Optimized thinsats should be able to do far better.

Because the thinsat is extremely thin, some common electronic components cannot be used: foil-wound electrolytic capacitors, cored inductors, etc. Bypass capacitors can be made thin, but it is better to keep peak impulse currents low. Some components such as crystal frequency standards may be replaced by surface acoustic wave (SAW) devices. However, other devices such as radios can be operated at low voltages and low impedances, and if some devices need higher voltages at trickles of current they can be powered with capacitive charge pumps. At microwave frequencies, resonators can also be made with striplines and other components. The potential of solid state integrated phased array lasers needs to be investigated for communications, small debris location and control, power transmission and active optical propulsion.

The thinsat is patterned with slot antennas and transmitters to communicate with neighbors in the array, with other arrays, and with the ground. Besides communication, thinsats will measure radio propagation time to neighbors to accurately compute spacing and orientation, with additional location information provided by other arrays, ground stations, and possibly GPS. Multiple bands will be used, with frequencies that can penetrate atmosphere and clouds used for the downlinks, and other atmosphere-opaque bands used for thinsat to thinsat communication. The thinsats will not have dishes, but will act together as a phased array antenna. Thinsats are widely spaced in a rotating geodesic array, although the vast majority of the radio energy is scattered over a 100km ground spot, only the main lobe is high power.

By superposition, the array can send different high bandwidth signals to many closely or widely spaced ground stations simultaneously, far more than a traditional dish-and-transponder comsat. While each thinsat may only dedicate a few milliwatts to the ground transmitters, the sum of hundreds of thousands of transmitters will produce high power, high bandwidth beams. With the thinsat in a 4 hour orbit (repeating overhead 5 times a day), it will be 7 times closer than a geosynchronous comsat, so there will be a 50x advantage in beam power and ground spot area. Round trip ping time will be 70 milliseconds, less than U.S. transcontinental ping time through optical fiber.

Three optical thrusters perform pitch and yaw control for the thinsat. Normally, the thinsat is pointed straight at the sun, and the thrusters are electrically stimulated between black and reflective. The difference between the states is about half a nanoNewton of thrust. This is enough to slowly pitch or roll the disk. If all the thrusters are stimulated, this increases centerline thrust. Again, the thrust difference is small, but it is enough to maintain relative spacing of thinsats in an array to sub-micrometer accuracy.

There is no direct way to control roll. However, the thinsat can be rolled by combinations of pitch and yaw.

In orbit, the stresses are very small; the disk is relatively rigid by comparison. Maneuvers such as rotation and acceleration will take hours to days; microgee accelerations are involved. The main stresses on the disk will be from thermal contraction. When exposed to sunlight, the disk will be at 300K; as it passes into earth shadow, it will quickly cool to 150K or less, heated only by the infrared radiation from the night side of the earth. The thinsat is mostly aluminum. Interconnect metal layers should be designed with strain relief, and the whole metal and insulator stack should be designed for the same average thermal coefficient.

Thinsats aare stacked in cylinders for launch. Some kind of very thin separator may be needed, or the thinsats may stick together with vacuum welding or vander Waals forces. If a thinsat plus separators is 100 microns thick, then a stack of 10,000 5 gram thinsats will be 1 meter (39 inches) tall and weigh 50 kilograms (110 pounds).

A typical modern geosynchronous communication satellite such as HotBird 9 weighs 4880 kilograms (10,800 pounds) and has a 14kW array, launched by an Ariane 5 ECA, which can put 10,500 kg of satellite and apogee kick motor into a geosynchronous transfer orbit. The planned m288 thinsat orbit is lower; a larger payload is possible. 10,000 kilograms is 2 million thinsats, producing an average of 5 megawatts of electricity. Thus, a thinsat array can produce more than 300 times the power (and communication capacity) of typical comsats.

Thinsat geometry.