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| Startram is not interesting. It won't work, because it is based on misunderstandings of launch, orbits, coilguns, and superconductors. Enough of that ... | Startram is not interesting. It won't work, because it is based on misunderstandings of launch, orbits, coilguns, and superconductors. |
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| Most of this page is to respond to some thought-provoking questions raised about Server Sky around mid-April in the comments. You will find a lot more detail in published papers and the pages on this site. Use Text search, and Recent Changes, to look at the state of things. The site is not complete. This is a work in progress. It will become a book, and more papers, over time. I am sharing prior to completion tto show my work as I go along, and get feedback. | Most of this page is to respond to some thought-provoking questions raised about Server Sky around mid-April in the comments. You will find a lot more detail about Server Sky in the pages on this site, and in published papers it refers to. Use Text search, and Recent Changes, to look at the state of things. The site is not complete. This is a work in progress. It will become a book, and more papers, over time. I am sharing all this prior to completion to show my work as I go along, and get feedback. Here's some excerpts from comments, with links to fuller discussion of them: |
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| * [[ #Bandwidth | ... much lower bandwidth and huge antennas ... digital beamforming ... ]] | |
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| * [[ #Bandwidth | ... much lower bandwidth and huge antennas ... digital beamforming ... grating lobes ... ]] | |
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| Other inspirations include laptop screen thin films, [[ http://linearsignal.com/ | Linear Signal's ]] phased array TVRO antennas, rad hard hafnium oxide gate stacks, noise-tolerant computing, rad hard indium phosphide photovoltaics, and many other developments in solid state electronics. Moore's Law halves transistor prices every two years, and has transformed almost every industry. Relatively speaking, big iron satellites and space technology move at vacuum tube rates. You will be assimilated. | Other inspirations include laptop screen thin films, [[ http://linearsignal.com/ | Linear Signal's ]] phased array TVRO antennas, rad hard hafnium oxide gate stacks, noise-tolerant computing, rad hard indium phosphide photovoltaics, and many other developments in solid state electronics. Moore's Law halves transistor prices every two years, and has transformed almost every industry. Relatively speaking, big iron satellites and space technology move at vacuum tube rates, and that industry is ripe for a Moore's Law makeover. |
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| '''Crowding:''' Server sky puts many more objects in orbit, but they travel as large arrays - essentially, large satellites connected by maneuverability and active cooperation rather than by aluminum struts. The same satellite mass is distributed more widely and more productively. Don't think of them as a bunch of independent satellites - think of them as a multi-ton satellite divided into cooperating components. Ant colonies, not elephants. The arrays are deployed at 1 radii out, in the van Allen belt, where the non-constellation collider density is far smaller. | '''Crowding:''' Server sky puts many more objects in orbit, but they travel as large arrays - essentially, large satellites connected by maneuverability and active cooperation rather than by aluminum struts. The same satellite mass is distributed more widely and more productively. Don't think of them as a bunch of independent satellites - think of them as a multi-ton satellite divided into cooperating components in nearly identical orbits. Ant colonies, not elephants. The arrays are deployed at 1 radii out, in the van Allen belt, where the non-constellation collider density is far smaller, and few other satellites can survive. |
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| If a thinsat does get irreparably damaged, it is collected for ballast as mentioned above. | If a thinsat does get irreparably damaged, it may be collected for ballast as mentioned above. If it is completely derelict, another thinsat can be sacrificed to nudge it towards a collector, or lower the perigee for reentry. ---- <<Anchor(Timing)>> === ... Timing coordination and accuracy ... === Big iron satellites are made with lots of discretes, hand wiring, and physical connectors you attach with a wrench, using military production techniques perhaps 20 years behind those used for consumer products and precision industrial gear. My background is semiconductor design and test, and I helped write the IEEE 1149.4 Analog Boundary Scan standard. Ten years ago, I helped design timing generators for semiconductor testers, which must deliver a wave of thousands of digital signals through meters of wiring to a test fixture, with relative timing accuracies and jitter of about a picosecond, one sigma. I did the error budgeting and signal conditioning design that helped one subcomponent deliver less than 8 femtosecond jitter one sigma. These were single edges, and hundreds of thousands of edges on thousands of pins must all happen within very narrow time constrains. This is possible because of surface mount connections, shielded balanced differential lines on special circuit board materials, tunable drivers, and system calibration at multiple temperatures. Special test circuitry was built into all signal channels - we could "TDR" any critical signal path and collect waveforms showing discontinuities or mismatched termination (which we could also adjust). Timing delays were adjusted during operation to compensate for interference and crosstalk. Initial calibration of a system might take hours, but once it is stored and the system stabilized, these big testers can operate very reliably. They are taken out of service and recalibrated daily, and immanent failures noted, either to be bypassed by reconfiguration or replaced with scheduled downtime. This is the sort of thing you can do when you can put a billion transistors, consuming nanowatts each, on a chip. And Moore's law doubles capability every two years, so it is possible to do 30 times better now. I've seen some presentations on military radar hardware, and it is where consumer/commercial capabilities were 20 years ago. Timing for server sky is relaxed, compared to what I was doing a decade ago, because we do not have to measure single edges - we can signal average over many seconds, change temperatures and measure changes, and build calibration tables that will allow us to trim delays on the fly. The calibration tables will be built with software and CPUs, but the hardware that delivers the timing changes will use digital to analog converters fed by calibration registers fed by specialized DSP lookup engines. The CPUs will be involved at a higher level, looking at measurements, making complex decisions, and collecting anomalies for engineers to analyze. If our timing signal buses are resonant, with 100 ohm differential impedance, and a noise bandwidth of 1 GHz, with 350K terminators, the thermal noise is v^2^ = 4kTRB, 44 microvolts. On a 200mV peak-to-peak sine wave, that is an edge timing jitter of 70 ppm added to the outgoing signals from one thinsat. Averaged over an array of a million thinsats, that is a beam wander of less than a meter. Other effects will be much larger. The calibration will look at many cycles, of course; we will be mixing our I and Q primary reference timing clocks with the timing clocks of neighboring thinsats, measuring phase differences and adjusting frequencies. We can also measure the temperature of the resonators, the physical spacing drift, etc, and add adjustments (in hardware DSP, not software) for this drift. Error correction circuits should correct error only, not known offsets and drift. Besides relieving the CPU of computational burden and reducing the system power, doing the calculations with hardware DSP increases radiation resistance; a soft error can flip a calculation bit or tickle the phase of a resonator, but it can't rewire a DSP engine. The phase synthesizers do not need the flexibility (or the vulnerability) of software. It is possible that we will make errors in programming our DSP, and need to change the wiring masks on our chips, but that is much cheaper than launching programmable flexibility that we do not need. Server sky will not function using 20 year old microwave technology. Advances in DSP and software radio show that it does not need to. |
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MORE LATER |
=== ... much lower bandwidth and huge antennas ... digital beamforming ... grating lobes ... === |
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| <<Anchor(Timing)>> |
Brin2012Mar17
A response to comments to David Brin's Startram post
The Startram paper by Powell and Maise makes some "interesting" claims. The JHAPL folks assume repulsive magnetic levitation between a floating conductor and a surface conductor. If we help the authors by closing the loops to make coils (paging Dr. Kirchoff!), the result is a huge inductance, on the order of Henrys, and energy storage on the order of gigawatt-years because you are filling nearly a million cubic kilometers with high magnetic field energy. That's assuming that they don't quench their magic superconductors first. The conductors must be very large diameter to keep the B fields below critical. If there is a quench, there will be an electric arc with the power of a multimegaton nuclear bomb.
See also:
Startram is not interesting. It won't work, because it is based on misunderstandings of launch, orbits, coilguns, and superconductors.
Most of this page is to respond to some thought-provoking questions raised about Server Sky around mid-April in the comments. You will find a lot more detail about Server Sky in the pages on this site, and in published papers it refers to. Use Text search, and Recent Changes, to look at the state of things. The site is not complete. This is a work in progress. It will become a book, and more papers, over time. I am sharing all this prior to completion to show my work as I go along, and get feedback.
Here's some excerpts from comments, with links to fuller discussion of them:
For one of the inspirations for Server Sky (there are many), read Ivan Bekey's 2003 book Advanced Space System Concepts and Technologies. Also look for Bekey's book on Worldcat . Bekey, former head of NASA's Advanced Concepts office, tells us to use gossamer structures and connect them with information and maneuvering.
Another inspiration is the electrochromic windows on the 787 Dreamliner. electrochromic mirrors are appearing in buildings, they are cheap, durable, reliable, and thin.
Other inspirations include laptop screen thin films, Linear Signal's phased array TVRO antennas, rad hard hafnium oxide gate stacks, noise-tolerant computing, rad hard indium phosphide photovoltaics, and many other developments in solid state electronics. Moore's Law halves transistor prices every two years, and has transformed almost every industry. Relatively speaking, big iron satellites and space technology move at vacuum tube rates, and that industry is ripe for a Moore's Law makeover.
... a straight road to the Kessler syndrome ...
The Kessler syndrome occurs when derelict satellites collide, and the shrapnel disables other satellites, adding to the density of colliders. For a collision to occur, two quite different orbits must intersect, and the objects arrive at the intersection at the same time. Kessler syndrome is the potential result of:
- Crowded altitudes
- Fragile, large, easily disabled satellites
- Inaccurate tracking
- Inclined orbits
- Loss of manuevering fuel
- Satellites left in orbit after end of mission
- No systems for collecting derelict objects
- No economic incentives (positive or negative) to do so
So, don't do that!
Crowding: Server sky puts many more objects in orbit, but they travel as large arrays - essentially, large satellites connected by maneuverability and active cooperation rather than by aluminum struts. The same satellite mass is distributed more widely and more productively. Don't think of them as a bunch of independent satellites - think of them as a multi-ton satellite divided into cooperating components in nearly identical orbits. Ant colonies, not elephants. The arrays are deployed at 1 radii out, in the van Allen belt, where the non-constellation collider density is far smaller, and few other satellites can survive.
Fragility: The thinsats themselves are arrays of smaller subcomponents; except for memories and processor cores, the radio and maneuvering and optical thrusters are redundant, and can function independently even if debris punches a hole.
Tracking: See Active collision warning and avoidance below.
Inclined Orbits: All arrays are in near equatorial, shallow inclination orbits (less than 0.2 degrees) designed to never intersect with each other. Relative closing velocities will be centimeters per second. See ToroidalOrbits.
Fuel: Thinsats maneuver with light pressure, not expendable fuel. Rapid maneuver is not possible, but they can move tens of meters in an hour. With accurate tracking, that is sufficient to avoid colliders if they are all accurately tracked.
End of mission: Server sky thinsats will go obsolete at Moore's Law rates, at which time they could maneuver to reenter. Given their tiny ballistic parameter, they will come down in days if perigee drops below 1000 km altitude. They can also be nudged back into larger trackable stacks, or get cut apart and used as ballast for thinner next-generation thinsats.
Collecting derelicts: Solar powered electric thrusters take longer to maneuver, but with patience can create far more delta V with less fuel. Jerome Pearson ( author of first technical space elevator paper in Acta Astronautica, 1975 ) says it can be done with tethers. It may also be done with laser ablative thrusters, or VASIMR electric engines, or ... Which is to say, I don't know which option is best, but there are many. Small objects can be "caught and caged" through controlled collision in a capture structure, without matching delta V.
Economic incentives: Given a growing demand for ballast for ever thinner thinsats, the value of collected debris to be reused as ballast will approach that of earth-launched mass. Rather than trash, debris will become valuable. A kilogram in orbit is far easier and cheaper to deliver to the m288 server sky orbit than a kilogram on the ground. Debris passing near the server sky orbit (mostly spent upper stages for GEO satellites) will be the cheapest to maneuver, while removing the biggest collision threats).
So, thinsat arrays can be the solution to Kessler syndrome - a high paying market for the debris that is up there, and an element of the collection process.
... Active collision warning and avoidance ...
Thinsats will be functionally agile. When they are not sending intra-array or ground transmissions, the transmitters are reconfigured to transmit narrow beam, lookdown, time-coded radar chirps. At 6400km (1 earth radius out), they can see far more of the sky than ground radar, at shorter wavelengths, unhindered by the atmosphere. Thinsats are poor radar receivers, but other satellites in different orbits can pick up the signals. It should be possible to track much smaller objects for a much longer time, permitting orbit determination to meters, not kilometers. That makes avoidance much easier.
Lageos demonstrates that we can measure satellite position within fractions of a micrometer over tens of thousands of kilometers.
Thinsats maneuver at about 10 micrometers per second squared - a thinsat can move its 10 centimeter radius out of the way of a sub-centimeter collider in 2.5 minutes, a 7 meter object in 20 minutes. Assuming precision radar and a correspondingly large ephemeris, a thinsat is unlikely to encounter anything it can't avoid and can't survive.
If a thinsat does get irreparably damaged, it may be collected for ballast as mentioned above. If it is completely derelict, another thinsat can be sacrificed to nudge it towards a collector, or lower the perigee for reentry.
=== ... Timing coordination and accuracy ... ===
Big iron satellites are made with lots of discretes, hand wiring, and physical connectors you attach with a wrench, using military production techniques perhaps 20 years behind those used for consumer products and precision industrial gear. My background is semiconductor design and test, and I helped write the IEEE 1149.4 Analog Boundary Scan standard. Ten years ago, I helped design timing generators for semiconductor testers, which must deliver a wave of thousands of digital signals through meters of wiring to a test fixture, with relative timing accuracies and jitter of about a picosecond, one sigma. I did the error budgeting and signal conditioning design that helped one subcomponent deliver less than 8 femtosecond jitter one sigma. These were single edges, and hundreds of thousands of edges on thousands of pins must all happen within very narrow time constrains.
This is possible because of surface mount connections, shielded balanced differential lines on special circuit board materials, tunable drivers, and system calibration at multiple temperatures. Special test circuitry was built into all signal channels - we could "TDR" any critical signal path and collect waveforms showing discontinuities or mismatched termination (which we could also adjust). Timing delays were adjusted during operation to compensate for interference and crosstalk. Initial calibration of a system might take hours, but once it is stored and the system stabilized, these big testers can operate very reliably. They are taken out of service and recalibrated daily, and immanent failures noted, either to be bypassed by reconfiguration or replaced with scheduled downtime.
This is the sort of thing you can do when you can put a billion transistors, consuming nanowatts each, on a chip. And Moore's law doubles capability every two years, so it is possible to do 30 times better now. I've seen some presentations on military radar hardware, and it is where consumer/commercial capabilities were 20 years ago.
Timing for server sky is relaxed, compared to what I was doing a decade ago, because we do not have to measure single edges - we can signal average over many seconds, change temperatures and measure changes, and build calibration tables that will allow us to trim delays on the fly. The calibration tables will be built with software and CPUs, but the hardware that delivers the timing changes will use digital to analog converters fed by calibration registers fed by specialized DSP lookup engines. The CPUs will be involved at a higher level, looking at measurements, making complex decisions, and collecting anomalies for engineers to analyze. If our timing signal buses are resonant, with 100 ohm differential impedance, and a noise bandwidth of 1 GHz, with 350K terminators, the thermal noise is v2 = 4kTRB, 44 microvolts. On a 200mV peak-to-peak sine wave, that is an edge timing jitter of 70 ppm added to the outgoing signals from one thinsat. Averaged over an array of a million thinsats, that is a beam wander of less than a meter. Other effects will be much larger.
The calibration will look at many cycles, of course; we will be mixing our I and Q primary reference timing clocks with the timing clocks of neighboring thinsats, measuring phase differences and adjusting frequencies. We can also measure the temperature of the resonators, the physical spacing drift, etc, and add adjustments (in hardware DSP, not software) for this drift. Error correction circuits should correct error only, not known offsets and drift.
Besides relieving the CPU of computational burden and reducing the system power, doing the calculations with hardware DSP increases radiation resistance; a soft error can flip a calculation bit or tickle the phase of a resonator, but it can't rewire a DSP engine. The phase synthesizers do not need the flexibility (or the vulnerability) of software. It is possible that we will make errors in programming our DSP, and need to change the wiring masks on our chips, but that is much cheaper than launching programmable flexibility that we do not need.
Server sky will not function using 20 year old microwave technology. Advances in DSP and software radio show that it does not need to.
... much lower bandwidth and huge antennas ... digital beamforming ... grating lobes ...
MORE LATER