Originally Posted by
rlsbutler
We hear a lot of the Russians and Chinese developing anti-satellite weaponry.
We do? Sorry I didn't realise it was 1969 again. Russian ASAT program dates back at least that far. The Americans have one which launches off a F-15 following a zoom climb, the Russian one launches from a site like an ICBM.
This thing about these cubesats replacing Galileo or dedicated satellites is just BS.
(Listen in now sis 'cos I like you, we've got a lot in common.)
To pick up a signal on the ground using a nondirectional antenna mandates a directional antenna on the satellite _or_ a very very porky transmitter. The current GPS signals go out at 25w and a directional antenna is used. The antenna gain gives a directed power equivalence of about 300w. (That means, if you had no directional antenna you'd need to transmit with 300w to achieve the same signal strength at any given receiver.) A cubesat can generate about 20w max. So the
generated power budget doesn't even cover the required transmit power, let alone the much greater input power needed into the transmitter and then the power for all the other electronics, computer, clock, stabilisation etc.
There's also a very real question as to whether all the required components would physically fit into a cubesat.
The altitude of a cubesat is also very low which results in an orbital period of only a couple of hours. Assuming a 45 degree orbital inclination and an observer on the equator any cubesat will be visible for approximately 10 minutes per orbit. The current system results in a Doppler shift at the receiver of +/- 10KHz changing over a ~6h satellite visibility window. Cubesat altitudes make that +/- 80KHz over 8 minutes. So compared with 'standard GPS' the signal must be found in a frequency range 8x the size and within 1.25% of the time (= 80x faster). For a modern receiver that's trivial but it's just one example of how changing the satellite configuration can make a problem 8x80 = 640x harder.
In addition, cubesat LEO orbits are unstable - air drag is significant. Altitude losses of up to 10m per orbit have been measured. A 2m satellite altitude loss between entering and leaving a ground receiver's field of view is feasible. The ephemeris would have a usable lifespan of minutes. You would need a ground tracking/control facility in each sector where people would be using the system - because ephemeris data for a satellite that's just come over the horizon would almost certainly be outdated. With the satellites being so low the ground visibility radius would be ~1700km - so your ground station would have to be at least that close to your forces in order to be able to update at least 50% of the satellites they'd have in sight with one good ephemeris value.
The only thing that makes it objectively impossible is the power requirement. If we handwave that away - nuclear cubesats! - okay so then our GPScubes dont need orientation (a prereq for directional antennas) as they'll just belt out 10kw in all directions. The satellite's working parts then have to fit... And not melt... And the receivers have to be full of Xilinx's FPGA wonderfulness and able to apply a 500hz bin FFT to a 160khz range in five microseconds... And it's still all for nothing because the satellites themselves have unstable orbits. And our best case location is the sum of the best case orbital variance for each satellite we're using, factored for receiver aspect.... So each satellite could lose up to 10m alt per period, which of course means a reduced orbital speed... Starting at 450.01km altitude a 10m drop to 450.00 would mean the orbital speed would decrease by 0.559 cm/sec... Final orbital period would be 01h33m35.2s. Assume the ephemeris was updated one period before we gain sight of it, then during that period say it'll lose 10m alt and 0.559cm/sec orbital speed... so our average speed variance during that period will be half that so it'll have not-travelled 5615.2s x 0.2795cm/sec = 15.7m retrograde mean anomaly position error... then 10m altitude position error... So worst case pseudorange error...Satellite rises coming basically straight at you fudge the cosine error to zero for both terms so 15.7m error. You need 4 for a location so worst case = 4 satellites coming straight at you = position uncertainty a circle radius 15.7m = 31.4m. Then add all the errors that affect GPS. Splendid system. Solid gold.
Dunno why you can't figure this stuff out for yourself really it's not rocket science, just a bit of trig and some light reading.