NavList:

Hi Frank

You mentioned: So even if GPS were turned off or placed in a mode equivalent to SA, the outcome for navigation would be minor --except for the US military itself!

You may not be aware of this but US military GPS receivers are very tightly controlled items.  If you happen to have one without permission, the government will seize it from you without recourse.  One of the reasons for this is that they are 100% immune to Selective Availability and use a different GPS channel.  The US military is not affected by SA, only users of the system that do not have military grade receivers are affected by SA.

Otherwise, I think we are in agreement regards the affect on marine navigation.  SA subtly smears your position around, but for marine navigation, this is relatively moot.  It was subtle.  It did not throw your position miles away.  SA may, should you follow the GPS fix blindly, put you out of channel.  But the AtoNs, should we bother to look out the window, will put us right.  When crossing an ocean, the affect is zilch.  

That was the beauty of SA.  It was just right enough to put you in the ballpark, but wrong enough to prevent your enemy from using it for a precise strike, whilst simultaneously permitting us to use it for a precise strike!  Diabolical!

Think of SA as a subtle spoof.

Spoofing

There are two types of spoofing to be considered.  Attempts to spoof and or jam the GPS signal during military conflict.  Attempts to spoof or jam to disrupt civilian use of the GPS system.

Military Spoofing 

I posit that any transmitter, powerful enough to send a spoofing signal over a wide area, will also send a signal strong enough to target. That power makes it a fairly easy target, just as any radar, AM or FM tower becomes.  Further, the military channel, to my understanding, is far more immune to spoofing in terms of encryption.  It potentially could be jammed, but again, that is a powerful, easily detected, easily targeted transmitter.  With every broadcast, it says "Here I am".  

The most likely country to try this, in my view, will be Iran, in the Gulf of Hormuz.

Civilian Disruption

Jeremy indicated recently that his chartplotter failed to get a signal, when off the coast of Israel.  He notes that he simply retrieved his battery powered unit and got a fix.  That indicates that the jamming signal was potentially short lived or that it wasn't a jam at all, but rather some ship board glitch.  Note that Jeremy did not reach for his sextant, even though I would certainly agree that he is a careful practitioner of CN. 

CN may provide a sanity check but it's only going to provide a check good to (the oft discussed) few miles that CN provides.  A good spoof will stay within the nominal CN error.  In other words, undetectable by CN.  In a crude or coarse spoof, your app will indeed detect the spoof.  The GPS position is so far out, even CN can detect it.

Jamming is an entirely different issue.  This essentially turns GPS off.  GPS systems alert you that the last reported position is old.  That will indeed disrupt marine traffic.  Arguably, this is the refuge for CN as a position fixing discipline: jamming.  There is no choice to use GPS, you are forced into a CN regime. 

Again, I think we are in general agreement.

Brad

On Mon, Sep 17, 2018, 6:39 PM Frank Reed <NoReply_FrankReed@fer3.com> wrote:

Brad, you wrote:
"I know the public statement is that the US will not use SA, but those satellites are under direct control of the US Air Force.  It would be very easy to command an equivalent to SA."

But again, how would this play out given that there is one fully-operational alternative GNSS available, and there are two others on the verge of being fully-operational. Suppose a DOD source calls up the GPS command at the Air Force and insists in a panic that they turn off the GPS satellites. What would the effect be? I would estimate that roughly 95% of users would see only a slight drop in accuracy in their position fixes since they would automatically, seamlessly, and without interruption continue to receive and process position information from the other GNSS constellations. The actual number may well be higher than 95% but that depends on how many systems are using chipsets manufactured before 2012 or so, and it depends on how many systems would not encounter software bugs when "Old Reliable" is missing, and it depends on how many US govt users are required to use devices that only access GPS signals. So even if GPS were turned off or placed in a mode equivalent to SA, the outcome for navigation would be minor --except for the US military itself!

What would happen if the system were hacked to produce highly inaccurate data? From my limited understanding of the system, standard software (hard-coded into the chips) will ignore outlier signals, just as any prudent celestial navigator would ignore a wildly out-of-line altitude observation, and once again most receivers would fall back on the remaining GNSS signals. Yet another hacking mode could produce an effect similar to GPS Spoofing yielding a fixed but otherwise consisent (and seemingly accurate) position offset at least in some part of the globe. That signal offset, combined with signals from other GNSS satellites could yield a real degradation of position fixes unless software is designed to detect such a thing (it could, but I highly doubt that this is currently available in off-the-shelf tools), but then we're into the territory of GPS Spoofing generally. And as I have said --and have emphasized in my apps-- this is a case where celestial can still have an impact. I don't mean that we should then fall back on 1970s celestial navigation with paper tables and plotting but simple comparison between expected altitudes and observed altitudes. And note, too, that well-designed GPS Spoofing can spoof the signals from all GNSS satellite constellations simultaneously, which is far more dangerous than anything that the USAF could do by manipulating the GPS signals only. 

Pirates with spoofing tech are a big risk. Degradation of GPS signals by the USAF simply wouldn't accomplish much. It's possible in panic-mode, but probably irrelevant. Does that make sense?

Frank Reed