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Planned GPS outages in southern California (avweb.com)
389 points by eajecov on June 7, 2016 | hide | past | web | favorite | 217 comments

From the article: "Operators of Embraer Phenom 300 business jets are being urged to avoid the area entirely. “Due to GPS Interference impacts potentially affecting Embraer 300 aircraft flight stability controls, FAA recommends EMB Phenom pilots avoid the … testing area and closely monitor flight control systems,” the Notam reads."

That is beyond scary; how anyone can defend having critical aircraft control systems rely on an input which may be turned off at will is beyond me.

Let us at least hope the system fails gracefully and notifies the pilot that something odd just happened and you will have to do your own flying from this point on, rather than just going titsup and be done with it.

I'm pretty sure Iran captured that US drone with GPS spoofing. I have no idea how you could provide that data in a secure way. But, uh, i can envision some scenarios where the bad guys would want to remotely take over planes.

Also, i'm not so sure about the graceful failure. Hypothetically, the human takes over. But if the human hasn't actually flown in months or years the'll likely be kind of rusty. Now you're throwing them into a complex situation - the autopilot can handle the simple stuff. Coupling weak skills with difficult situations seems like a bad idea.

I kinda think autopilots and self driving cars should give a limited clock. Every, say hour you do it manually buys you a few hours of autopilot. Just to keep skills relatively fresh.

There's a recent econtalk episode that delves into transferring control from machine to human. Specifically, regarding the air france disaster:

"The Air France story is a story about a failed handoff, where the automation onboard an airplane found a relatively minor fault and handed control of the plane back to the human pilots, too suddenly and ungracefully surprised them. And they had lost some of their skills flying too much with the automated systems and lost control of the airplane. Which actually was a perfectly good airplane about a minute into the crisis. And so they went from tens of thousands of feet flying through the sky and ended up spiraling into the ocean, tragically losing all aboard."



That tragedy was probably the seed of my concern. Lately i've been thinking a lot about how automated systems become brittle. The systems don't change, people forget the dependencies and requirements and in something real time like an airplane, the feel of the system.

A configuration management system that incorporated spaced repetition would be cool. Every once in a while, go into an incremental mode where you actually type in the commands. It has the added bonus of getting new people aware of the system. Sure, you can always just go read the source and figure it out. Having the system force some human awareness from time to time would be handy.

Why hand control back to a human when you can do even better?

Have them both handling control at the same time, reconcile the inputs in a sane manner (or have a master/slave where one is providing phantom inputs). Added benefit of being able to error check each other.


This was exactly the problem in the Airbus crash shown above; reconciling the inputs is hard. One pilot kept holding one stick back, unbeknownst to the other. This plane averages them by default, and therefore the plane stalled. That couldn't happen on a Boeing because the yokes are yokes (inputs on one are easily visible to both pilots) and physically linked.

(Supposedly the plane is supposed to loudly complain if the inputs diverge. I'm not sure if this happened.)

The Airbus did make the expected warnings ("DUAL INPUT") when the control inputs conflicted, at least according to this Vanity Fair article: http://www.vanityfair.com/news/business/2014/10/air-france-f...

"DUAL INPUT" is only marginally better than "PC LOAD LETTER"

On the other hand, typical Airbus A330 operators probably have a lot better training about what diagnostic messages mean than typical HP LaserJet operators do.

On the other other hand, two pilots issuing opposite control inputs generally has a bit more impact than an empty printer tray. Boeing's relatively low-tech solution of making the two controls physically interlinked seems like a common-sense solution to a potentially immense issue.

But obviously the training did not suffice in the Air France case, because those were experienced Airbus pilots and they did not detect and resolve the contrary inputs.

Something easy to ignore when you're freaking out over a plane going straight into the ocean.

Yeah. In an emergency with a frazzled brain it's entirely possible someone'd read that and say "No shit, of course we're both trying to pull up..."

Pulling up being exactly the wrong action in this case!

As others point out, just because the warning is displayed as expected, does not make it a good warning.

In flight testing, whenever we encounter situations like this (the aircraft does not react as the pilot expects for a given input), we don't necessarily blame the pilot. Especially if multiple pilots get into the same mess. Then we know it's time to change how the aircraft behaves and bring it in line with what the pilots expect.

A little blinkenlight among the masses is easy to miss. Having your controls physically fight you because the other pilot is doing something is impossible to miss.

It was actually a voice alert. I'm not making any judgement about its appropriateness.

In fact by the time it arose, the aircraft may already have been in an unrecoverable dive (or whatever the word is for rapid descent while stalled), I don't remember. So it's not a primary cause of the accident.

A voice alert in a cockpit where people are probably frantically shouting at each other is just as ridiculous.

There's no substitute for physical feedback. It's instantaneous and impossible to miss. It's completely beyond my comprehension that Airbus eliminated it.

As I recall, the one pilot held full back stick all the way down, while the other pilot tried and failed to recover. Releasing the back stick and executing a normal stall recovery would have saved the airplane at just about any time.

I understand the Boeing approach of physical linkage to be the best idea, rather than any kind of indirect force feedback which is possible to misinterpret.

As for your last point, I'm not an expert, and I'm not as incredulous about this scenario as you seem to be. (As a teenager I did fly a lot in PC flight simulators, though.)

The non-technical Vanity Fair article I referred to claims that the accident investigators estimated the last point at which the aircraft could have recovered from the dive was around the time it passed 13,000 feet:

"Though precise modeling was never pursued, the investigators later estimated that this was the last moment, as the airplane dropped through 13,000 feet, when a recovery would theoretically have been possible. The maneuver would have required a perfect pilot to lower the nose at least 30 degrees below the horizon and dive into the descent, accepting a huge altitude loss in order to accelerate to a flying angle of attack, and then rounding out of the dive just above the waves, pulling up with sufficient vigor to keep from exceeding the airplane’s speed limit, yet not so violently as to cause a structural failure. There are perhaps a handful of pilots in the world who might have succeeded, but this Air France crew was not among them. There is an old truth in aviation that the reasons you get into trouble become the reasons you don’t get out of it."

If that's what they did then I believe it. I'm coming from a light aircraft background where stall recovery takes much less altitude. Still, they had about two thirds of their descent to initiate recovery before it became too late. If the controls had been linked, the other pilot would have discovered the problem instantly.

I don't understand why you'd want this averaging behavior at all.

When trying to get the plane onto the center of the runway, both pilots' actions are probably pretty correlated, and averaging could remove any noise (e.g., twitches).

But in many other circumstances, it seems like it would be downright dangerous. Something's in front of the plane. One pilot goes to the left, the other pulls to the right, and the system averages the commands and smashes straight into it.

The benefits of a marginally straighter landing don't seem like they would outweigh the potential of a massive catastrophe.

Perhaps all critical automated systems should have some level of Chaos Monkey testing built into them, though it might be best if it was good enough to not fail even if that test was failed.

Chaos Monkey is great. I'm suggesting something slightly different. Imagine you have a bunch of cent 6 machines that have been running great for years and years. upgrades go smoothly and deployment is a breeze. Now you need to upgrade to cent 7. Stuff that used to happen in init.d now happens in the systemd, for example.

Reviewing changes by hand every few months keeps you aware of what needs to happen. Automation doesn't get brittle, people forget how systems are flexible and inflexible. Spaced repetition would give people a chance to keep up with the current design, so when that crazy security vulnerability happens, you can jump in and change stuff knowing how it works, rather than having to figure it out on the fly.

I don't think it would take much. Configure a box today, tomorrow, next week, next month, 3 months, then perhaps every 6 months.

There are very smart people that sort of intuitively keep up with those kinds of changes. But those people change jobs. How do you get a mere mortal up to speed on 20-30 machine configurations? config management can configure hundreds of machines a day, no problem. but it's easy to forget what's really going into each of those boxes.

This is more about preserving organizational awareness, not so much robustness of a running system.

The Langewiesche article is a great read as well, with more details:


Thanks, that's a really interesting episode.

The military has never used GPS guidance systems, the Soviets could disable it at will when it was designed. Consequently, no amount of spoofing GPS will cause a serious navigation error. The assertion of GPS-guided systems in the US military is a widely repeated myth that never seems to die even though even cursory research confirms that it is a myth.

US military systems pervasively use inertial navigation (INS), many of which can accept micro-corrections from the GPS system. However, they only accept corrections within the intrinsic error bound of INS, which is quite small. Spoofing GPS can buy you a deflection of meters at most, not kilometers.

The US military is developing new types of inertial navigation systems that are so accurate that it obviates the use of GPS altogether. (And the media will probably still call those systems "GPS-guided".)

Are you sure about this? Why does the DoD SBIR mention GPS spoofing avoidance as something they will fund?

The INS system you mention has small errors, which aggregate/accumulate over time, this time being in minutes and tens of minutes.

These errors cannot be rectified inside the INS, from what I understand. They require external fixes, like GPS.

I am willing to be wrong however.

The US military has many use cases for GPS far beyond weapon guidance and navigation. Many of these use cases cannot be served by INS and are both high value and done primarily in peace time i.e. the loss of the GPS constellation in a war won't affect the military utility of having the GPS constellation generally. Spoofing these use cases is still a giant nuisance.

There are a couple things that people forget about US military INS:

US military INS actually operates as a giant swarm of INS computers with other sensor inputs that compare notes -- wisdom of crowds -- to correct accuracy. If one INS computer drifts too much, the other INS computers will notice and correct it. Even without the swarm, many larger military systems will have multiple INS computers distributed throughout the platform that can compare notes.

The US military has been at the forefront of inventing exotic INS technology since the 1960s, and the details of their INS capabilities are a closely guarded secret. It is used ubiquitously in almost every combat system they produce. What is known is that even in the 1990s the accuracy of INS significantly exceeded design requirements, and there are current military research programs that suggest they know how to build INS that exceeds current GPS accuracy over long temporal baselines.

Given that they already have a GPS constellation that they use for a wide variety of other cases, GPS-corrected INS guidance is a very cheap way to squeeze some additional precision out of weapons that were already pretty precise in the first place. It allows them to build a dirt cheap and reliable guidance package that is also highly precise in environments where GPS spoofing is not a credible problem. Remember, these particular guidance packages tend to be modular and swappable: use the cheap stuff that leans on GPS more for low tech enemies, use more sophisticated and expensive INS for the high tech enemies.


Certain systems do indeed operate as you say, with INS as the primary guidance/navigation mechanism. Submarines for instance, cannot rely on a GPS fix. Strategic missiles as well.

But ground forces do indeed use GPS systems. INS systems are very costly to build and maintain, go GPS is a go-to for missions and requirements where it's a good fit. You simply cannot outfit every ground-pounder with an INS system and then keep it properly operational in their task envelope.

Also, parts of the GPS system are quite jam-resistant. Not entirely, but when it was first developed, GPS was pretty secure for it's time, using a cryptographically derived spreading code for the carrier.

Here is an example of a system in use today:


Some of you might be interested in an article I wrote on the subject:


You are right. The military relies on INS and GPS. Most systems such as the navigation in an F/A-18 are INS with GPS to maintain accuracy like you described. I thought JDAM was entirely GPS, but apparently it is only an INS supplement to.[1] Cruise missiles use both plus terrain mapping.[2] Military could probably handle total GPS denial, but it would be ugly, especially since it may mean no SATCOM either. I ponder what WW3 looks like and total space denial seems like a strong possibility. Spoofing is probably worse than denial since it isn't just no information but bad information that could throw off the INS.

[1] https://en.wikipedia.org/wiki/Joint_Direct_Attack_Munition [2] https://en.wikipedia.org/wiki/Tomahawk_(missile)

> But if the human hasn't actually flown in months or years the'll likely be kind of rusty.

Along the same lines:

I work in manufacturing, and design systems to automate processes. "What is the current, manual process?" is the first thing I ask for when beginning a new project. I then try to design the automated system, so that the manual process can be used as a fallback should any part of the automated system fail. What happens when we have a network outage or something similar? The people involved suddenly forget the manual processes that they had used for years, and sit around twiddling their thumbs and calling IT every 5 minutes asking when the system will be back up.

I'm so sorry that you seem to do your job in an elegant way and that the other humans can't seem to respect that.

They respect me and I respect them, and I'm far from elegant, so I'm not really sure how you inferred any of that. I just find it funny how fast they forget processes that they did everyday for years, after relying on the automation - just like the pilots in the story I was relating to. I'm the one that has to remind them what their manual processes were, which is why I ask the question and document the response first and foremost.

I don't think it was spoofing as military probably uses encrypted GPS signals.

What Iran did probably simple jamming that caused drone to either crash or emergency land.

Ah. That's where my flawed memory came from

"... Iranian engineer's assertion that the drone was captured by jamming both satellite and land-originated control signals to the UAV, followed up by a GPS spoofing attack that fed the UAV false GPS data to make it land in Iran ..."

Wikipedia has more of the story https://en.wikipedia.org/wiki/Iran%E2%80%93U.S._RQ-170_incid...

That was disputed "... GPS is not the primary navigation sensor for the RQ-170..."

Anyway, thanks!

Can you jam the only non-encrypted part of GPS?

Kind of. The GPS signal portfolio is evolving. In the beginning, the encrypted code was broadcast on two frequencies, L1 and L2. The public code was only on L1. So you could in theory jam just L1 to block the un-encrypted portion, but why would you?

In recent years GPS modernization means satellites have been using new codes on L2 (L2C) which are un-encrypted. Only about 18 of the satellites carry that capability at this time.

How else are you going to land the drone safely? I think the idea is jam the encrypted data, spoof the unencrypted part to point to your location.

As far as I know military devices always use the anti-spoof (encrypted) military code, and never the commercial code.

Given the public knowledge of the old P(Y) code and the new M code, you can remove 'probably' from your first sentence.


I agree. At the time people were noting the drone the Iranians were showing off was the wrong color, which would imply they fixed some damage and repainted it.

> But if the human hasn't actually flown in months or years the'll likely be kind of rusty.

Cruising at altitude is not that hard. I'd be far more concerned if they were interfering with the various Instrument Landing Systems during inclement weather.

> Every, say hour you do it manually buys you a few hours of autopilot.

Most pilots already hand-fly in clear weather during landing and takeoff by default; somewhat for the reasons you suggest, but it's also the safer as it actively maintains pilot situational awareness.

You're generally only going to use the A/P during cruise, departure and approach or during instrument flight conditions.

> Cruising at altitude is not that hard.

Not in RVSM airspace above FL290 (29,000 ft). In RVSM [0] airspace its required to have a functioning autopilot coupled to an altimeter of a certified accuracy (+- 65 feet)

Most pilots couldn't fly the required accuracy, that high up, for any length of time, with manual hand-flying. Even in a FBW airliner, it would be hard.

[0] https://en.wikipedia.org/wiki/Reduced_vertical_separation_mi...

The idea for FL290-FL410 is more to have a designated "highway" of sorts, where all craft are flying within the same system. There is no requirement for auto-pilot above FL410, for example... There are plenty of jets that can easily get that high, it's more about aerodynamics and engineering than pilot skill.


No, "Cruising at altitude is not that hard." isn't correct. That is the coffin corner:


Being near the coffin corner is often desirable for performance reasons, but it sure isn't easy.

There is a separate encrypted GPS signal, which used to have better precision as the public signals had intentional noise added to them.

Of course, GPS is a rather old system and I'm unclear how you would add encryption in a way that it's not compromised when someone gets access to a decoder but also doesn't endanger "mission success" because a encryption key punch card is missing and the drone refuses to start. It's like the nuclear launch codes being all zeroes.

As I understand it, encrypted GPS (L2) does depend on key material being periodically loaded from a fill device for proper operation.

The keys are rotated periodically, so if somebody stole a decoder, it would only work until the key in memory expires. (Military GPS receivers also have a "zeroize" button to destroy the keys in case of imminent capture.)

I'm pretty sure Iran is full of crap. If they really had this technology, we'd either have drones raining from the sky, or they'd keep their mouths shut and save it for a special occasion. Also, this: http://www.popsci.com/technology/article/2013-04/6-most-absu...

For graceful failure, I imagine you would want something akin to radar and a topological map so you can match your position, probably combined with lower fidelity means of locating position (angle or location/angle of sun/moon/stars) to reduce the topological map search space if starting from scratch. I would be surprised if the military didn't already have ways to mitigate lack of GPS, considering it's an obvious information attack vector.

Terrain matching has been around for a long time, with the earliest systems dating from the 1950s, so that's definitely a possibility. This gets a lot of use on cruise missiles.

Another fancy way to navigate without GPS is to use automated celestial navigation. The SR-71 had one of these in the 1960s, and it's also good for submarine-launched nuclear missiles. The hardware is able to sight stars even in the middle of the day (and not just the Sun, funny guy).

For commercial aviation, the typical backups are radio beacons such as VOR and NDB, inertial navigation systems, and good old dead reckoning plus pilotage (i.e. looking out the window).

Old 747s had a window on the ceiling of the cockpit for using celestial navigation.[0]

Also with the reliance on GPS and its inherent fragility has caused the US Navy to restart training in celestial navigation. In order to reboot the training regimen, the Navy is relying on Coast Guard instructors, since the USCG never stopped.[1]

[0] http://www.airliners.net/aviation-forums/tech_ops/read.main/... [1] http://www.militarytimes.com/story/military/tech/2015/11/01/...

Military aircraft mostly use GPS and INS for navigation. Cruise missiles the same, as well as TERCOM [1], as you alluded to.

I recently spoke to old aircrew guys I worked with and apparently they stopped teaching manual celestial nav in the late 90s finally. Automated celestrial nav was, per my understanding, never too common although SR-71 aircrews commonly used its system.


It's still pretty common in space stuff. From Wikipedia ( https://en.m.wikipedia.org/wiki/Celestial_navigation):

A variation on terrestrial celestial navigation was used to help orient the Apollo spacecraft en route to and from the Moon. To this day, space missions, such as the Mars Exploration Rover use star trackers to determine the attitude of the spacecraft.

I figured it had existed for quite a while, but didn't imagine it traced back to the 1950's.

> Another fancy way to navigate without GPS is to use automated celestial navigation.

I figured as much. I'm just under-informed in this area, and try not to state things as fact that I don't know as such. I did mention the stars, but it didn't occur to me they are visible during the day with the right equipment. :)

> inertial navigation systems

I'm aware these exist (due to some military fiction I've read), but that's the extent of my knowledge. I'm not aware of how accurate they are.

> good old dead reckoning plus pilotage (i.e. looking out the window).

I was thinking of systems that replace pilots, even if for short whiles, not supplement them, so discounted human correction while in flight.

Inertial navigation is interesting because the error starts out at zero and then builds up with time. Let it run for long enough without recalibration and you'll have no clue where you are. Other techniques tend to have steady error bounds. (Aside from dead reckoning, of course, which is basically just inertial navigation done by hand.)

As far as quantifying that growing error, Wikipedia says it's typically less than 0.6 nautical miles per hour. An airliner after a long oceanic flight could know where it was to within a few miles, good enough to reorient and find the destination airport.

Early efforts in autonomous navigation came out of a strong desire to blow up the Soviet Union, so cruise missiles and ICBMs and such are a good place to look if you're interested in early examples.

Inertial navigation systems are used a lot by submarines, since there's no GPS down there. I think it's pretty accurate, particularly if you combine it with other sensor data like gravitational field strength maps.

Indeed, early cruise missiles used terrain-following radar before GPS was available:


Military aircraft utilize INS (Inertial Navigation Systems) which are extremely accurate for positioning.

As part of a larger system yes, but on their own they're only accurate for a while, as they constantly accumulate error. They require input from other sensors (generally GPS) to provide accurate location.

They need recalibration after every 1-2 sorties for best results but they don't require input from GPS or other sensors. GPS synchronization largely eliminates the need for support staff to recalibrate. INS is incredibly accurate on its own, assuming pre-flight calibration.

Source: Worked on said GPS/INS systems in the military.

And what do you use to tell it where exactly it is during pre-flight calibration?

They "require" GPS input in the sense that if I handed you an INS that was initialized/calibrated with some precision (Oministar/RTK/P-code GPS/ whatever) reference but had been running in the trunk of my car (or dangling from the collar of a feral cat) for N hours/days, there's no way you'd fly a plane with it without initializing it with some sort of position referene.

(If you came up with "sit it on some previously surveyed datum, and you don't need GPS" on your own, you get an A)

GPS already has an encrypted channel. It's a severe pain in the ass to work with computationally, so nobody does. Even the military.

It used to be that the clear channel was less accurate (even beyond the 'selective availability'), but signal theory got us to cm granularity without the computational expense of the military channel.

> I have no idea how you could provide that data in a secure way.

Just sign the signals with RSA. This depends on keeping the private key out of hostile hands, or allow key revocation and generation. But it's possible in theory.

I do not believe that drone crashing was am accident, wasn't there already a release of info showing that is how stuxnet was originally released?

Somebody else posted a link to r/aviation where a Phenom 300 pilot gives additional details on this issue. It has happened a couple of times to this pilot apparently.


"When the Phenom loses both GPS 1 and GPS 2 it also gives you an AHRS FAULT 1 and AHRS FAULT 2. For whatever reason this situation can cause the autopilot and yaw damper to disengage apparently, thus at high altitudes may cause the aircraft to dutch roll. I have had this exact situation 3-4 times, generally near the white sands missile range though and my autopilot worked great the whole time."

AHRS = Attitude and Heading Reference System

So basically, it seems it's not a big issue, especially since his aircraft kept the autopilot on. But it might require some attention from the manufacturer

Dutch roll may cause passenger discomfort though

If I was on an aircraft doing a Dutch Roll [0], I'd definitely feel discomfort!

(Note for readers: A "dutch roll" is when the plane rolls (when one wing dips down and the other rises up) and yaws (when the rear of the aircraft moves in one horizontal direction and the front of the aircraft moves in the opposite horizontal direction) at the same time.)

[0] https://en.wikipedia.org/wiki/Dutch_roll#/media/File:DutchRo...

Ah yes, I would be definitely feeling very uncomfortable as well in a Dutch Roll!

Some people seem to adapt easily to boats, planes and other types of activities that cause motion sickness. I'm not one of them.

In principle, the system should be capable of dropping down to a stabilizing mode that relies on high-pass filtered rate and acceleration inputs. Even without GPS, the autopilot should+ provide dutch roll damping, short period damping, coordinated turning, and a few other stability control loops.

+ "should" means that it is technically possible, not that this particular flight control system does

The Phenom uses a GFC700 autopilot by Garmin. Interesting tidpit from its webpage:

>enabling it to fly fully coupled GPS-only LPV approaches into runways not served by ILS or other ground-based electronic approach aids.

GPS-only only approach is probably the issue here. If the runway its using doesn't have ILS then it only has the GPS fallback. If there's no ILS and no GPS, then I would expect issues.

This last comment in this forum points to GPS interference issues with the GFC700 that sounds significant. Sounds like the GFC700 is susceptible to GPS issues and because this is a corporate jet and may be landing on a privately owned airstrip, it may be the case that there's isn't any ILS or unreliable ILS at those strips. Unreliable/non-existant ILS and a wonky GPS? Yeah, an autoland procedure could be fatal in those circumstances.


I think they also just exposed a vulnerability that others haven't thought of. Not sure how easy directed GPS jamming is, but pointing out a specific aircraft model is a bit scary.

FAA notification of the Embraer Phenom 300 issue, and an explanation:


Testing against GPS is an interesting challenge. It's technically a critical public service, so any disruption of it should also be broadcast. Any weapon therefore must be tested out in the open.

The development of these weapons probably has some influence on the Navy's decision to bring back celestial navigation[1].

Come to think of it, I don't think I even own a compass.

[1] http://www.npr.org/2016/02/22/467210492/u-s-navy-brings-back...

Seems like the risk/disruption involved in bringing down a critical public service would require them to do it somewhere else. The second largest city in the US is a poor choice.

Aren't there relatively isolated places where this could be tested with much less risk? The middle of the South Pacific? The Australian Outback? Or even Northern Canada?

Actually that is the best place to test it. LA might be right there, but almost all of Nevada is government owned. Imagine doing this test on the east coast and knocking out NYC, Philly, Chicago, DC, etc.

"not the worst" doesn't really make it the best.

There are still vastly less-densely populated areas of even the continental US.

> Come to think of it, I don't think I even own a compass.

If you have an iPhone or (nearly any) Android phone, you have a compass.

A noisy battery driven compass though. If you are planning for the end of GPS, it doesn't seem like a great idea to lean on your cell phone.

Easier, I think, to just make oneself slightly more autonomous with a solar-panel USB charger, than to acquire the sheer number of items that the smartphone phone has replaced. Just the number of physical maps you'd have to carry in place of one offline navigation app would make one regret the project.

Not to sound too crazy prepper, but getting a compass and a couple waterproof paper maps of the region around me wasn't a huge lift.

I tried to use my Android phone as a compass for aiming my roof antenna... it was not very accurate.

It's too sensitive. the ones attached to a physical pointer have inertia, and therefore resist oscillating or brief interference. Whereas the ones in your phone can actually give you an instantaneous reading, which is way more vulnerable to the odd magnetic fluctuation in the air.

If your android compass took a hundred readings a second and gave you a rolling average of the last two seconds, that would probably be better...

(note, just a theory, but one that seems consistent with my observations)

The earth's isn't the only magnetic field to which the phone is exposed, but unless one installs an antenna in a problematic location it should certainly be the dominant field. I have a hard time believing that the behavior described is "too good". It seems more likely that the phone is interfering with itself, and the circuitry required to avoid that simply isn't justified from a BOM perspective.

Also, any metallic object in the vicinity. Took me a while to realize why the compass goes haywire whenever I hold the phone with my left hand: a ferromagnetic-metal ring right beneath the device.

you're joking! That's awful.

I kinda like it. The ring, I mean. That specific phone is crappy for a host of reasons; this is just icing on the cake, so to speak.

If that was the problem then it would be easy enough to implement a damping filter in software.

does the hardware sensor report the strength of the magnetic field?

Can't average out your readings unless they have a magnitude...

AIUI the hardware reports X, Y and Z components of the magnetic field strength.

Okay, I looked one up, the HMC5883L-BB, and it seems you're right.

In that case somebody in the phone's hardware design department must just be dumb.

Apropos celestial navigation: its usage isn't unknown in aircraft, surprisingly:


Fancy automatic systems like that one are rare, but celestial navigation done by hand used to be quite common on board commercial aircraft. Making a long flight over the ocean in decades past, there really was no other way to keep track of where you were with any sort of accuracy. This lasted well into the jet age. Early 747s had a sextant port so that the navigator could use it to get a position fix.

There are rumors that the 747's sextant port fit a hose pretty well, and could be used as a nice vacuum cleaner while at altitude.

> The mounting location for the sextant remains in current 747 designs since it was originally designed and certified with an installed sextant. The sextant (and all associated hardware) has long since been removed with the advent of multiple, highly reliable long-range navigation systems. When installed, there was no requirement to depressurize the aircraft.


No requirement to depressurize, but it had the potential.

The "vacuum hose on the sextant port" thing sure sounds like an urban legend. On the other hand, it also sounds like the sort of thing some of my old pilot friends would try.

EEVBlog did a teardown part of one of these systems from a B-52 (https://www.youtube.com/watch?v=hzGaO4Kdz2Y). IIRC, it used a completely mechanical analog computer.

If you have a very good faraday cage / RF test chamber (such as a third party certification lab that does paperwork for FCC compliance has) you can test GPS jammers and their resultant effects on your gear...

ISTM a "good Faraday cage" would disable GPS with or without the presence of a GPS jammer.

yes, what I meant is it's only legal to operate an actual GPS jammer (a transmitter that shits noise on the 1.4 to 1.6 GHz band spectrum) inside a faraday cage, and test how your equipment responds to that (perhaps while simultaneously operating a GPS simulator RF source in the same faraday cage), because you definitely do not want your jammer signal to escape from the cage and interfere with nearby unrelated third parties' equipment. The cage in this case being to keep your experiment contained inside, not to block outside signals from getting in.

I only took a year of physics, but in my experience a Faraday cage that blocks electromagnetic signals in one direction also blocks them in the other direction. GPS functions by receiving signals from several satellites at once.

Yes a Faraday cage blocks both directions. What walrus is saying I think is that if you are a good citizen, you should do any radiating GPS jammer testing inside a Faraday cage. Doing it outside a cage is probably illegal and could create a public safety hazard (911 on cell phones for example).

You could have a test setup bringing GPS inside the cage with coax+circulator or radiate a GPS constellation simulator inside to test the jammer.

You own a compass, most high end smartphones have one that can work completely offline to gps and airplane mode.

At least my $50 smartphone has one

Luckily, the physics on that one doesn't mandate overly technical fabrication.


There are GPS simulators for testing, things don't have to be tested only in the open.

Clearly there is some value in large-scale testing, thus the link and announcement.

So, here is just a little bit of amateur desk research into some things we might be able to gather from the information:

The FAA flight advisory provides the coordinates and the nature of the GPS signal disruption, which is centered near China Lake, and has expanding rings of area, each of which rises in altitude. For the pilots out there, imagine the classic upside-down wedding cake shape. Or cone with its point at the ground.

This would seem to indicate some kind of broadcast or interference from a source that is located at the ground, propagating line of sight with larger radii with altitude. Rather than something to do with the satellite itself.

The center of the coordinates are 360822N, 1173846W, which is in a big empty desert area, just south (SSW of Darwin, California), see here: https://www.google.com/maps/place/36%C2%B008'24.0%22N+117%C2...

It could of course be some kind of antenna, or even a flight that is producing this signal. But there's also an interesting long V-shaped two-legged testing(?) facility just to the east of these coordinates, which you can see in the Google Earth image. I might be mistaken about what that facility is, because aeronautical sectional charts also show a mine in that area, but this doesn't look like a mine site. Also there are a bunch of vehicles that look like Humvees on the pad nearby. And there are three antenna looking structures at the north end of the paved line.

Anyway, it's interesting to speculate about.

My suspicion is that the prominently paved structure you're referring to is related to the publicly known Junction Ranch radar range located a half-dozen miles to the south, and indeed the eastern of the two long paved roads (which you can tell are elevated somewhat from the ground, as they have culverts under) appears to have antennas at the north end. There also appear to be some kind of portable towers set up around the area.

However, the farther west of the two seems to have a small structure made up of shipping containers or similar portables. This looks more like a setup for explosives testing, which you can see more of to the NNW. I'm not sure what to make of it besides to speculate that it could be for RF shielding.

Like most military ranges of its size, though, China Lake's back lot is full of so much miscellany that it's hard to know what's even from this decade, much less what anything is for. The structure you point out is fairly new, built since 2007, so it's a good bet it's still for something current.

That said I find it very unlikely they would have provided particularly accurate coordinates for the NOTAM. They probably used the geographical center of the range's northern part or something similarly unhelpful but justifiable.

> The center of the coordinates are 360822N, 1173846W

Noob question here: what kind of coordinate system is that? I know the degrees, minutes, seconds one (51°30', 005°09') and the nowadays more common decimal degrees (e.g. 51.5,5.15), but not this big number.

Edit: Also, for those who don't like Google Maps because it's slow or just because it's Google: http://www.openstreetmap.org/search?query=36%C2%B008%2724.0%... or http://binged.it/24xCR6D (I would use the long link but I can't link to satellite imagery -- the only thing bing maps is good at).

It's still Degrees, Minutes, Seconds, but with N/S/E/W postfixed (is usually prefixed)

36°08'22''N, 117°38'46''W

Ah okay, thanks!

> China Lake, California, home of the Navy’s China Lake Naval Weapons Center

From Reddit comments at r/aviation (https://www.reddit.com/r/aviation/comments/4msmh7/gps_interf...) it appears like this could affect civilian GPS usages such as geolocation apps. I wonder if Google Maps or any other GPS apps should be showing a warning that because those apps can just behave weirdly?

As a foursquare/swarm user myself I would be quite pissed off by my OCD if I cannot check in to places I go haha.

On a positive note, it might add some variety to Tinder.

Unless you're on a bridge it should be fine. The article says funkiness starts at 50 feet above ground level.

That cannot be significant. Bridges have ground, and it's also possible to be above 50 feet above Sea Level while still on the ground. I'm suspicious.

From the discussion in this thread it seems likely that the source of the disruption is something that will be broadcast from ground level. Your position relative to that transmitter (bearing in mind the curvature of the earth) will likely matter far more than your elevation relative to the ocean.

If you are on the ground you will presumably be sheltered by intervening obstacles (building, hills, the earth) while a plane in the air (or anyone else significantly above ground level) may have line of sight to the source of whatever is going on.

Interesting that the interference occurs 50' Above Ground Level, not sea level. I can't even imagine what technology that is that can somehow jam along the contours of the earth.

If it's ground-based and directional then I can easily see the system having the highest gain within, say, a 170 degree arc. Point that straight up and ground-level (and slightly above ground level) wouldn't be affected much.

Useful if you want to avoid disrupting ground travel. It probably gives significant cloaking benefits too if you can limit effective signal source discovery to airborne platforms with all the right equipment -- much more difficult to dig up than something ground-based.

The FAA has announced that GPS jamming will only affect aircraft above 5000 feet, not above 50 feet that was mistakenly reported.

But I'm still confused by your explanation about how GPS jamming can avoid affecting areas below 5000 feet.

I can imagine two ways that the jamming could work:

(1) You're jamming the GPS receiver. An analogy would be shining a laser pointer into a photographer's camera. You're overwhelming the receiver so he can't receive the signal, but you are not actually modifying the original signal. This seems to fit your explanation.

(2) You're blocking or modifying the actual GPS signal. The analogy here would be spreading out a huge curtain in the sky at an altitude of 100,000 feet to block out the sun. Aircraft flying at 50,000 feet won't get any sunlight. But neither will people on the ground. Your explanation does not seem to work in this case. You can't block sunlight at 100,000 feet without also blocking it for people at ground level.

What am I missing here?

Your #2 isn't possible without actually spreading something out in the sky, and blocking a lot more than just GPS. "Jamming" refers to #1. You broadcast a signal on the same frequency with a greater strength, preventing the receiver from being able to pick out the original.

For a ground-based jammer, if you're at some distance from it then the lower you are the less jamming you'll receive because stuff gets in the way. At some point the strength of the jamming signal isn't enough to prevent you from picking out the real signal.

some ideas:

The jamming sender(s) could be flying as well. If you only send upwards from a flying plane, you obviously won't jam receivers below it.

and/or the areas where you can receive it below 5000ft are inside restricted airspace. There is a lot of that around China Lake, but I don't know if they just can't tell you about issues in there. I'd expect so though, given that they do all kinds of exercises in there that all would need an extra warning otherwise. (EDIT: but the sizes/the angle of the cone don't look as they'd work out for a sender on the ground, unless they added massive safety margins)

Or they are doing something clever with multiple senders, and below 5000 you can't receive the signals overlapping in such a way that actual jamming happens. Not sure if that would work in GPS frequency ranges though.

>Plane at 5000ft obviously can send signals only forwards/upwards.

Why? Can't you put a sender on the base of a plane?

bad wording on my part, meant that if you put a sender that only sends upwards on a plane at 5000ft, it it obviously won't be received below it. Edited the comment.

> The FAA has announced that GPS jamming will only affect aircraft above 5000 feet

Where did they announce this? The NOTAM linked from the parent article is still on the FAA website and clearly says GPS may not be available between 50 Ft and 40,000 Ft. (NB I am not confusing FL 50 for 50 Ft)


My apologies, you are correct; the jamming will affect aircraft above 50 feet.

I was reading an article on Gizmodo[1] that originally said 50ft, then they "corrected" it to 5000ft[2], and then corrected it again to 50ft. Serves me right for relying on Gizmodo rather than reading the official FAA site.

[1] http://gizmodo.com/faa-warns-of-gps-outages-this-month-durin...

[2] http://www.govtslaves.info/faa-warns-of-gps-outages-this-mon... [this is someone's capture of the incorrect Gizmodo article where they said 5,000 ft]

I'm curious - If I fly a GPS-dependent drone above 50 feet within the jamming footprint, do you think it would degrade or fail? If I were near the area, I might try the experiment.

Perhaps they're just saying that any spot below 5000 feet AGL is subject to jamming, not that all points above that level will be jammed.

(I have no idea how any of this would work.)

GPS signals are somewhere around the 1.4-1.5GHz mark if memory serves; that is basically line of sight only and the curvature of the earth solves the rest.

As for 50ft and up, I think - mind, think - that NOTAMs are limited in scope to issues not on the ground; hence, 50ft and up is just a way to let pilots know the problem persists at any altitude; you will not see a NOTAM referring to something at sea level.

Again, I /think/ this is why it is worded the way it is, but I am not a pilot.

I can confirm that notams apply much more broadly:the faa black and white would routinely visit my rc airfield during notams in Low Angeles.

In the context of a NOTAM you should read this as "all the way to the ground".

A cone that increases in diameter with altitude is consistent with a jamming device located on the ground.

That might just be the point at which the FAA stops caring about it, so they don't bother listing the distances for lower altitudes.

The aviation community uses altitudes AGL (above ground level), not sea-level. It's the more interesting number.

I'm pretty sure it's just going to be any airspace that has line of sight to the jamming emitter.

Interstingly, it looks like this is a semi-regular thing in different parts of the US:


Last one looks to have been May 22-23 in Louisiana, with another one from June 1-30 in New Mexico.

It's interesting to see HN discover this routine test; the Phenom warning is boilerplate.


The Coast Guard is the middleman for the DoD, obtaining FAA approval for the tests and coordinating shut down of the tests in emergencies.


I wonder how many of the plane "GPS receivers" can pick up GLONASS as well?

Most consumer units can see both networks these days, however aviation tech is known to lag a lot in such matters.

fun fact: most consumer grade GPS stuff can pick up GLONASS, particularly the RF baseband chips found in all modern smartphones, because about 5 years ago Russia threatened to put a 250% import duty on all imported smartphones that didn't function with GLONASS. Therefore companies like Qualcomm which develop the RF chips in a smartphone were required by their customers (such as LG) to include GLONASS.

Do you know if it's supported in software; say iOS or Android?

On Android, check https://play.google.com/store/apps/details?id=com.chartcross... - it will actually show you which satellites from the constellation are whose: I can see US, Russian and Chinese units. On a higher level, it's integrated into the location provider - the apps don't need to care, they just get a location fix, regardless of the technology used.

As far as I'm aware, all current major android and iOS devices support GLONASS. You can check the phone's specs at gsmarena.com to confirm.

Russians got GLONASS support in so many devices so quickly by simply passing a law that any device sold in the country with GPS support also has to have GLONASS

I wonder whether this GPS jamming also affects GLONASS.

It definitely will affect GALILEO, the US threatened to embargo the EU if GALILEO would be impossible* to jam.

(Well, "impossible" – the original EU plan was to reuse the same frequencies the US and Russia use for military GPS and GLONASS for GALILEO’s civilian use, but the US threatened some embargo, and so GALILEO isn’t the awesome thing it could have been)

That's an interesting thing I didn't know about it!

How does the FAA think this is going to work after 2020 when air traffic control will run off of ADS-B positional telemetry from aircraft. It seems a GPS shutdown like this would basically shut down IFR flying and airport terminal control since ATC has no other way of knowing the position of airplanes.

Modern Inertial Reference Units [0] (IRU/IRS/INS) are fitted to most airliners and business jets, and are an alternative to GPS. IRU accuracy is less than GPS, and older units have issues with drift.

The FAA hasn't approved any IRU for ADS-B[1] backup.

More importantly, ATC can use the Mode-S[2] transponder without GPS to locate the aircraft. Secondary Surveillance Radar won't be phased out in 2020. ADS-B will be required by the FAA in 2020.

[0] https://en.wikipedia.org/wiki/Inertial_navigation_system

[1] https://en.wikipedia.org/wiki/Automatic_dependent_surveillan...

[2] https://en.wikipedia.org/wiki/Secondary_surveillance_radar#M...

I was under the impression that the explicit aim of the ADS-B directive is to phase out SSR, which would would save a massive amount of money.

> It seems a GPS shutdown like this would basically shut down IFR flying and airport terminal control since ATC has no other way of knowing the position of airplanes.

I highly doubt that good old-fashioned radar will be shut down. It has to be, anyway, for all the planes either that don't have the equipment for IFR, and the pilots which are not certified for IFR, only for VFR.

By 2020 any airplane that flies in controlled airspace is required to be equipped with ADS-B out. This applies to VFR as well as IFR flights.

Of course, VFR flights can continue flying VFR if GPS shuts down, but IFR flights wouldn't have that option.

Traditional transponders aren't going away and they would still work with secondary radar.

Why on earth does the military-industrial complex need to spend money on a duplicate, irrelevant technology? I worked at Trimble Nav in the radio group, and the POTUS has the ability to increase selective available (SA) to an enormous number (which can be defeated by differential/kinematic corrections, but was set to 0 by executive order under Clinton) or disable the unencrypted channel entirely for a particular region or the entire planet (which includes space). WTF!

Satellites designed after May 2000 no longer have SA capability.

because it's pointless and since areas can be and have been turned off for non-military receivers.

No, it has never been turned off since it first went to full availability [1].

[1] http://www.gps.gov/support/faq/#off

> Why on earth does the military-industrial complex need to spend money on a duplicate, irrelevant technology?

Why on earth do data centres need to spend money on duplicative, irrelevant RAID-1 disks?

Presumably whatever they are testing will work on other, similar systems over which the US military does not have control.

I know someone who was at sea when their GPS stopped working. They found out later a nuclear sub had come into port around the same time. Seems like it wasn't a coincidence as gps outages are rare.

I remember hearing about the 1990 BOC single handed Round The World Race - where the competitors all noticed the CEP on their GPSes dropping way down, indicating SA had been switched off - and being amongst the first civilians to have noticed the Gulf War had started...

Where was this?

He could tell you, but his GPS was down.

Moreton bay, look it up. Few years ago now, but the sub visit was big news.

GPS jamming is old hat.

A lot of weapons testing takes place at China Lake (where the disruption will originate from), including missiles and guided bombs that use combination GPS and inertial guidance systems.

They're probably testing various weapons systems' ability to continue to function in the face of GPS jamming.

I, for one, cannot believe that GPS doesn't have a pre-prod environment. I guess they just don't grok dev ops like us young hip developers.

No but seriously, the fact that we don't have a backup for when GPS inevitably shits the bed sometime in the future is a fundamental existential threat to mankind. We should probably do something about that.

The existing backup is the Russian GLONASS system, which many consumer chips support since the start of this decade or so, and there are two more constellations being deployed, by ESA and China.

I'm not sure how you can test such things not in the production environment. The early development was done with the transmitters on airplanes, but i would guess that the jammability can't really be tested that way.

Commercial vendor Iridium as part of their "Iridium NEXT" constellation currently being deployed will have a secure alternative to GPS as well, their STL service. [1]

[1] http://gpsworld.com/iridium-launches-alternative-gps-pnt-ser...

GPS uses a certain frequency. Use the high side band for prod, the low side band for staging. Simple :)

The nice thing about PRN codes is that you can just use a different set of gold codes in the same band, as long as they have low enough cross-correlation

Yup my GPS watch does GLONASS. Have never tried GLONASS only, but GPS + GLONASS is a fair bit faster to lock on to.

Most smartphones do both now as well. iPhones got it starting with the 4S, and various Android devices support it. I believe it's a requirement to be able to sell them in Russia, but it benefits us all.

I suspect this is because AWS hasn't yet added an API call for placing a fleet of satellites in semi-synchronous orbit

I can neither confirm nor deny that AWS is working on an API call for placing a fleet of satellites in orbit.

On a more serious note, I'm excited to see what happens to the NTP pools in the area, since GPS is often a timing source for them. It should be a non-event, but it could get interesting.

Oh, and what happens to CDMA cell towers that use GPS to sync? Do they go off-line?

Anywhere that GPS is used in a timing-critical application, they'll be using a timing receiver (like http://www.trimble.com/timing/thunderbolt-e.aspx) to handle their time reference; those use the GPS signal (when they have it) to discipline a (ovenized) quartz crystal, so that they have good performance during "holdover" (loss of GPS) especially for short-term outages. In its default configuration, the Thunderbolt-E linked above, for example, will be within +- 8us after being in holdover for 24 hours.

A Rubidium reference gets you down to the 1us/day range, cesium gets you down to a tenth of that. It all depends how much you're willing to pay.

±8µs can be rather bad, e.g. for financial applications.

There is certainly hardware available that can do better than this, but the parent asked about cell sites in specific (and cell sites are one of the top users of the Thunderbolt). You can get GPS hardware based on atomic references, for example, that will do better over these kinds of timespans.

(You really need GPS or some other precision time transfer mechanism for long-term accuracy, though ... there's no getting past that.)

Well...if you're in that position, you probably have money to spend on failover config. Pretty accurate hardware clock modules were available even before GPS. Not cheap, not simple, but reliable - if you have the need and the money.

Anything relying on GPS for timing is going to revert to an onboard crystal timer for the outage duration.

There already is a partial backups, and eventually there will be three: Galileo, Beidou, GLONASS. Many devices can access multiple systems.

I thought it was pretty obvious from the article that they're most likely testing GPS jammers. In the case of a war, the US probably doesn't want anyone else to be able to use their GPS network.

Um .. why do they need a weapon to "disrupt" GPS? It's their birds. They can turn it on/off selectively whenever and wherever they deem necessary. Or is this meant as a test of something to disrupt the Russian/Chinese systems?

And why southern california? Alaska, the pacific... northern Canada ... there are lots of lower-traffic areas. We aren't getting the full story.

According to the NOTAM [1], this extends to SF Bay area as well, even near ground level.

Tests may be repeated on June 7, 9, 21, 23, 28, and 30, between 9:30a-3:30p.

[1] https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_...

I have a hunch that some people are going to have difficulty withdrawing cash from ATMs[0] tomorrow. Although, perhaps they are being overly cautious here.

[0] https://www.newscientist.com/article/dn20202-gps-chaos-how-a...

I wonder what this will do to ground-based GPS users. Aviation doesn't really need GPS; aircraft have multiple other systems. But phones, cell towers, and other devices have no other position input. Car navigation systems may become lost. We now get to see which GPS units have enough smarts to detect inconsistent data.

Satellite warfare adaptability simulation?

Take that Vandenberg AFB. Surprised they didn't use the Nevada test site.

Something like that could wreck havoc in a world where all cars are self driving... !

Self-driving cars use GPS for route guidance; moment-to-moment driving decisions are supported by built-in all-around sensors. The worst that might happen in a GPS outage is that all the affected cars might safely and carefully pull over/become gridlock.

> the affected cars might safely and carefully pull over/become gridlock.

As a best case scenario I'd still file that under "wrecking havoc"

That's the worst case scenario. Best case is that the cars are smart enough to fall back to navigating by road signs and stored maps, and there's no real interruption apart from maybe a warning light on the dash. It's not even that improbable; it's not uncommon to lose GPS signal in e.g. deep valleys, so any fully autonomous self-driving car would want to have some kind of fall-back scenario.

Deep valleys, such as Manhattan offers ;) In other words, this is one of the most often encountered scenarios, and for momentary dropouts, even consumer-level navigation apps have dead reckoning algorithms (I was surprised by this when my smartphone kept the "you are here" dot moving inside a tunnel, fairly precisely)

Wreak havoc. Wrecking havoc may be seen as a good thing.

I'm definitely crossing off the Embraer Phenom 300 from my shopping list of personal jets !

Seriously though, who thought this was a good, or safe, idea?

Most likely the military because they need to test something... A GPS jammer perhaps?

NAWS China Lake [0] is home to the EA-18G [1] test fleet. They carry the ALQ-99[2] jammer which can disrupt GPS quite effectively.

[0] https://en.wikipedia.org/wiki/Naval_Air_Weapons_Station_Chin...

[1] https://en.wikipedia.org/wiki/Boeing_EA-18G_Growler

[2] https://en.wikipedia.org/wiki/AN/ALQ-99

You'd think they could test it in a less populated area.

Maybe population was a part of it. There are so many things they could be doing/testing we can't really make any assumptions other than that some government organization is doing something.

Does that have anything to do with voters suppression for today's election?

I struggle to see how aviation GPS jamming would impact voter turnout.

Aviation schedules are already very unreliable (75% on-time?)... I would assume that this could only make it worst.

Sorry, I guess I should clarify. Who would be behind this plan, and what votes would they be looking to suppress?

I don't know, I just thought that it was really odd that they decided to experiment with this on election day where an important election is being held...

Coincidence? Is this meant to disrupt the California primary? Voters getting lost on way to voting stations without reliable GPS. There are a lot of first time voters.

Unless these first time voters are flying their private aircraft to the polling station I don't think its related.

The article says it affects "GPS signals down to 50 feet AGL". (I believe that's Above Ground Level.) So it shouldn't affect surface navigation.

Does anyone have a good explanation for how that's possible? Aren't the same GPS signals used regardless of elevation? Coming from a satellite, wouldn't they have to traverse the area where they're supposedly "jammed" before they can make it to ground level?

I would guess that this is due to the jamming antenna's visibility over the horizon.

You can't jam a signal in the air, all you can do is transmit a jamming signal towards where the receiver should be. And if the receiver is on the ground, out of your line of sight, then you can't do anything about it.

Jamming doesn't block the signal, it shouts over the signal. So, if your receiver can't hear the jammer, it will be unaffected, even if receivers between you and the satellite can hear the jammer.

IIRC, there exist things for GPS called "null steering antennas" that allow a receiver point a "null" at a jamming source to block it. I believe they're considered controlled munitions by the US government.

Sure, but signal interference is about mixing two signals together. Once the GPS signal has passed the interference signal (below 50 ft) it shouldn't matter anymore

Turn on a GPS jammer, you'll jam everything up to a certain distance away that has line of sight to the antenna. Anything on the ground that is over the horizon won't be affected, but anything above ground has a much longer horizon distance.


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