Comment by Rebelgecko | Hacker News Reader

Comment by Rebelgecko | Hacker News Reader

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I'm not an RF person, but the way it was explained to me is that GNSS signals are in the neighborhood of 1560mhz-1610mhz. With the advent of GPSIII, even more of the important signals are getting pushed outwards towards the extremes of that range. Ligado/Lightsquared initially wanted permission to basically surround GPS (1526-1559mhz and 1610-1660mHz)

After lots of complaints, they amended their request to leave a buffer of 20mhz or so between their signals and GPS. However, Ligado wants to transmit at ~10watts, and the GPS signal is in the neighborhood of a femtowatt by the time it hits a receiver. That can apparently still cause interference. For the GPS chip in your phone it may not make a huge difference. However testing has shown degraded performance in aviation GPS receivers more than a mile away from a 10watt transmitter. For even higher precision receivers (think surveyor equipment or maybe an autonomous tractor) the radius of the degradation stretches more than 2 miles from the transmitter. Ligado wants to put their transmitters ¼ of a mile apart.

Source for some of those numbers here, and an entertaining talk if you're ever able to see it in person: https://rntfnd.org/wp-content/uploads/Brad-at-UAG-Users-Advi...

> the GPS signal is in the neighborhood of a femtowatt by the time it hits a receiver.

This sounds more dramatic than it is - my WiFi signal from less than 10 feet away is approximately 1 nanowatt, or approximately 10^8 times weaker than when it left the AP radio. Thats just how RF works.

It's still pretty dramatic when you compare them directly. A 500watt transmitter that's 30,000km away is going to be many orders of magnitude (11?) weaker than a 10watt transmitter that's only 1km away.

Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats?

>Why can't we just greatly improve the signal strength at source by an order of magnitude? Like I know the new satellites can use beam forming to increase power for some large area for military use but it looks like even so total power is about ~500W right now. What's stopping us from deploying 10KW sats?

Even assuming that the very smart and capable people who developed GPS haven't thought about that already, there is zero-point-zero percent chance that the government will (or should) turn 24 satellites that cost 500 million dollars each into very expensive flying paperweights, for the benefit of some tiny 5G startup.

Deploying a 10KW sat is not a problem. Replacing a whole fleet just to keep it operating the same way as before is.

Supplying the power to the satellite, and getting the heat out. The RF amps are at most 50 % efficient, so you are dissipating 1/2 the input power as heat.

> Like I know the new satellites can use beam forming to increase power for some large area for military use

No, what you think you 'know' is plain wrong. Usually when discussing satellites, we're talking about a directional antenna that focuses a spot or wide beam for broadcast. A spot beam allows you to target a particular region, DirecTV uses spot beams to provide local broadcasts to general regions of the US. The military uses M-code shit on GPS with a directional antenna for the same sort of use case.

Beam forming is a different concept where you get creative with multiple antennas broadcasting at once and exploit signal cancellation to improve the data rate by shifting the phase between them. It does increase your received signal but relies on the receiver transmitting information back to the satellite (which is not going to happen for GPS, at least not the vast majority of consumer equip). Beam forming discussed on HN just last month: https://news.ycombinator.com/item?id=22784132

> What's stopping us from deploying 10KW sats?

Sat power is around and above that already, it doesn't make sense to increase the spot transponder ERP too much or you end up with issues like interference with adjacent bands since RF gets trickier to filter the higher the power it gets. Keep in mind a comm satellite usually has lots of transponders using maybe 100-500W each, so it adds up to some geosynchronous ones being like 20kW total consumption. Smaller LEO ones obviously not going to be close to that.

GPS released a paper about interfering with itself: https://www.gps.gov/multimedia/presentations/2010/10/ICG/bet...

Communications history is littered with failure stories of people who wanted to throw power at the problem, when they only needed to find ways to listen better.

Thanks that's actually helpful. I did some more reading and it looks like it is as you say, the new military transmitters are directional and steerable, beam forming done on the receiving side.

But still it seems that power is still part of the solution given the increased power density of the new military antennas. I guess what I'm wondering is why they can't just reserve more band, move to regional antennas for whatever other technologies in addition to fighting polluters/jammers/spoofers and noisy neighbors. Or is that the plan?

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