3 ms·

Besides "access" to the PRS/ non-public keys, there are operational considerations as well. Which are generally more aligned with the reasons "redundant" GNSS s

Besides "access" to the PRS/ non-public keys, there are operational considerations as well. Which are generally more aligned with the reasons "redundant" GNSS systems were invested in by other countries(/EU), even though NAVSTAR GPS (US) has nominally free, worldwide civilian access, in addition to the open nature of Galileo mentioned by GP. Because the civilian aspects of it has become so critically important and relied upon, we tend to forget that GNSS is primarily a military program/tool/weapon, which for NAVSTAR, is still under US Airforce custody and control. We, as John Q Public, just aren't generally aware that military capabilities go far beyond a more precise location fix , since the other non-public considerations: 1)are highly technical "how the sausage gets made" decisions and details, including how the [geodetic] network adjustments are made, dictating things like leap seconds, the internal coordinate system's x,y,z origin point & orientation, and grid movement with relation to plate tectonics, etc), 2) are back-end, non public facing, or secondary capabilities (ie emergency beacon/ EPIRB monitoring, signals intelligence gathering, ballistic missile launch/ ordinance explosion/ nuclear blast detection, or measuring deviation the sat vehicles' orbital parameters [and atomic clock relativistic shifts, and suspected absolute gravity measurements] to develop a precise geoid model- for aiming icbms, modeling missile defense, and navigating and/or detecting nuclear submarines) or 3) are wartime capabilities that are turned off normally, but require "admin" access to implement, like selective availability (SA) type degradation/ spoofing, turning off civilian bands/signals (either locally or worldwide), controlling signal strength and anti-jamming capabilities, etc. This would also include things like controlling spare sat vehicles, and determining what areas get 100% coverage in the event of a less than 100% coverage sat network. All of these aspects affect various military operations, and any country(/EU) that aspires to be a world superpower will not be able to rely on third parties to get all the military benefits of operating such a system. Also worth mentioning that civilian NAVSTAR GPS recievers have both an altitude and speed limit imposed on them client-side, so (over simplifying things here) foreign parties looking for space or missile guidance based on GPS have always -theoretically- needed to re-engineer/re-invent a client receiver if they wanted such capability. Somewhat ironically, or at least counterintuitively, the US military specifically avoids relying on GNSS for navigating ICBMs, smart-bombs, cruise missiles, or any other systems that must operate in contested airspace, specifically because they are aware both of how fragile the system can be (signal strength is already well below the noise levels), and how easy it is to purposefully turn off at will. Consider that in the event of a major war, NAVSTAR GPS is more likely to be turned off over the US than over enemy territory, as we wouldn't want to help the enemy navigate or locate targets.

coredog64 6y ago
> Somewhat ironically, or at least counterintuitively, the US military specifically avoids relying on GNSS for navigating ICBMs, smart-bombs, cruise missiles, or any other systems that must operate in contested airspace The DoD has been purchasing GPS guided munitions for years because of the economics. They’re significantly cheaper than laser guidance kits and don’t require a persistent designation paint. The downside (besides jamming) is that they’re more subject to user error: I can recall at least one incident where users entered incorrect coordinates and essentially bombed themselves.
- beerandt 6y ago
  They use GPS, sure, otherwise what's the point of having it? But they can't rely on it being available, especially during a major conflict. So most weapon delivery systems require different forms of navigation to be available, even if GPS is cheaper and preferred.
tialaramex 6y ago
> dictating things like leap seconds Leap seconds have nothing to do with GPS. They're an artefact caused by a desire to reconcile two things humans sort of intuitively assumed were tied together but aren't. We set our day (24 hours, 86400 seconds) based on roughly how long the planet Earth takes to rotate. If it was a little slower or a little faster we could just have adjusted the second to fix that back in the 20th century. Not very palatable to modern physicists but it could have been done. But it isn't just a little slower, or a little faster. It's both. Varying from one day to the next, unlike a good clock, the Earth doesn't keep time very well from one day to the next. This adds up over time. It might gain half a second one year, then lose 0.1 seconds the next year. One approach would be to decide we don't care. Clock just tick forward monotonically, the Earth is a little faster or slower, nobody needs to care. Unfortunately astronomers care. They really want one calendar day to match one Earth rotation for their convenience. And so now instead of the monitoring of the rotation being just something a handful of specialists care about, it drives the creation of "Leap seconds" to make up for our planet being slower or faster over a period of years by adjusting UTC by one second at notified intervals compared to the TAI.

coredog64 6y ago

> Somewhat ironically, or at least counterintuitively, the US military specifically avoids relying on GNSS for navigating ICBMs, smart-bombs, cruise missiles, or any other systems that must operate in contested airspace The DoD has been purchasing GPS guided munitions for years because of the economics. They’re significantly cheaper than laser guidance kits and don’t require a persistent designation paint. The downside (besides jamming) is that they’re more subject to user error: I can recall at least one incident where users entered incorrect coordinates and essentially bombed themselves.

beerandt 6y ago
They use GPS, sure, otherwise what's the point of having it? But they can't rely on it being available, especially during a major conflict. So most weapon delivery systems require different forms of navigation to be available, even if GPS is cheaper and preferred.

tialaramex 6y ago

> dictating things like leap seconds Leap seconds have nothing to do with GPS. They're an artefact caused by a desire to reconcile two things humans sort of intuitively assumed were tied together but aren't. We set our day (24 hours, 86400 seconds) based on roughly how long the planet Earth takes to rotate. If it was a little slower or a little faster we could just have adjusted the second to fix that back in the 20th century. Not very palatable to modern physicists but it could have been done. But it isn't just a little slower, or a little faster. It's both. Varying from one day to the next, unlike a good clock, the Earth doesn't keep time very well from one day to the next. This adds up over time. It might gain half a second one year, then lose 0.1 seconds the next year. One approach would be to decide we don't care. Clock just tick forward monotonically, the Earth is a little faster or slower, nobody needs to care. Unfortunately astronomers care. They really want one calendar day to match one Earth rotation for their convenience. And so now instead of the monitoring of the rotation being just something a handful of specialists care about, it drives the creation of "Leap seconds" to make up for our planet being slower or faster over a period of years by adjusting UTC by one second at notified intervals compared to the TAI.