Commercial Space Flight General / Re: Communications Services Project: commercial successor to TDRSS« Last post by DanClemmensen on Today at 03:15 pm »
(This is an abstract description that may not be accurate in detail for Starlink)Concerning LEO users of Starlink and Kuiper, I donít think the responders here are considering the actual physical issues of a LEO constellation servicing LEO users. If the constellation is only 150 km higher than the user, there will be gaps in coverage, unless the constellation is specifically designed for coverage of LEO users.ISL links are point-to-point. Each LEO user is a single specific additional "satellite" in the constellation as far as the rest of the constellation is concerned, and there are very few of these orbiting users. At any given time, some particular ISL link on some particular comms satellite is programmed to point to the orbiting user and it dedicated to that user. this responsibility is handed off to some other other satellite as the orbits cause the relative positions to change. This is completely different than the way a satellite's radio beams to the Earth's surface work. It is identical to the way ISL creates point-to-point links within the constellation.
Does the satellite rotate to point the ISL laser at the user?
You cannot have a true ISL network unless multiple satellites have at least three active links. With only two links, you can have a chain or a loop but not a network.
To serve users on the ground, the satellite's main antenna array must point toward Earth and its orientation is highly constrained. The satellite is not free to rotate arbitrarily. In general, this means the satellite has a "front" end that points along its orbit. This in turn means a satellite can keep one ISL pointed at the satellite ahead of it in its orbit and another ISL pointed at the one behind it, with only a small amount of pointing gimbal range of motion since these satellites do not move much at all with respect to each other. Thus, each orbit forms an ISL ring. At least some satellites (realistically, all of them) will have two or more additional ISLs that have a much larger range of motion. These can point to satellites in other orbits. Those satellites do move with respect to each other, a lot. Thus, the network topology is continuously changing, but in a highly predictable way.
With this hardware, a few satellites in the each ring must provide network ISL to other rings (more is better) but there are spare ISLs that can point to "user" ISL spacecraft.
This entire elaborate dance of ISLs requires tight coordination, down to the millisecond, as the links are made and broken. A user spacecraft will need at least two gimballed ISL lasers to maintain continuous communication in a make-before-break manner.
The laser ISL hardware is basically a small telescope, probably about 4 cm in diameter.