Official U.S. government information about the Global Positioning System (GPS) | Hacker News Reader

Official U.S. government information about the Global Positioning System (GPS)

gps.gov

118 pointswaterheater131 comments

A bit of development history at "GPS Almost Ended Before it Began":

> The death struggle: NRL wanted a two-dimensional system, and Parkinson and the Air Force wanted three dimensions plus time. The NRL patent called for atomic clocks in the user equipment, which would have made them prohibitively expensive. Parkinson and the Air Force wanted atomic clocks on the satellites instead, which would greatly simplify the GPS user receiver if atomic clocks could be modified to fly in the high-radiation environment of space. It wasn’t going to be easy to do, but space-qualifying atomic clocks turned out to be possible. NRL had tried to orbit a laboratory atomic clock, but it had quickly failed under the intensity of space radiation.

> With these modified concepts all defined, Parkinson spent the next three months trying to, at least, convince the key players in DoD not to say no. In December 1973, the Defense System Acquisition Review Council reconvened and voted yes. It was not that everyone in the military liked what they saw. Many navigation officials in the services liked the systems they had such as TACAN, VOR, DME, LORAN, etc. The Air Force was particularly opposed to GPS and no doubt preferred more airplanes.

* Embedded PDF: http://www.brightcopy.net/allen/avne/60-9/index.php#/p/20

* https://en.wikipedia.org/wiki/Bradford_Parkinson

It makes you wonder what other proposals actually were killed at an early stage that would have succeeded brilliantly had they been funded.

I firmly believe that the GPS we got is the best option we could have picked.

I've read a lot of the design papers from the 60s and 70s that formed GPS. The initial design was a 3-satellite constellation that would only cover Viet Nam. Options considered [1] included a receiver broadcasting to the satellite (wouldn't scale infinitely) or having an atomic clock in each receiver (high cost and weight). The expectation would be that there were about 100 GPS receivers worldwide: those installed in USAF and USN planes, and possibly some in USN ships or US Army tanks.

One constraint they eventually settled on was that a receiver should be portable (less than 50kg) and inexpensive (less than $500k adjusted)[2]. It just goes to show how far we've come from what was at the time considered ambitious in its size and cost goals.

[1] https://www.xyht.com/wp-content/uploads/2016/07/3-5-redacted... page 57

[2] https://www.xyht.com/wp-content/uploads/2016/07/3-5-redacted... page 73

The original Woodford & Nakamura TOR has a really cool hand drawn table showing all of the alternatives and their pros/cons. It was definitely for the best that they went with the most technically challenging option (inventing atomic clocks that are small enough to go to space) since that choice is responsible for a lot of the neat aspects of GPS (users don't need to be near ground stations, they don't need their own synchronized clocks, and GPS receivers really are receive-only)

Edit: just scrolled down and saw that my comment is a bit redundant with syncsynchalt's :)

Relevant this week:

- GPS costs around $2B/yr to continue operating

- Most of this comes from the defense budget

- This year, because of shenanigans, the NDAA has _not_ yet been passed b/c there are significant and controversial amendments in the House version

- We're careening towards a government shutdown

I'm not saying this _will_ happen, or _should_ happen ... but if GPS went off with the rest of the government, would Congress reach a resolution faster?

https://www.gps.gov/policy/funding/

EU Galileo, RU GLONASS, and CN BeiDou constellations all continue to operate but GPS receivers are not allowed to receive their broadcasts within the United States for political reasons only.

It's been legal to use Galileo signals in the US for several years now. https://www.gps.gov/spectrum/foreign/

I never could find where the US authorization was for GLONASS signals. I imagine it was included in some kind of bilateral treaty between US and Russia. The program to authorize later constellations (which Galileo went through) was not in place yet. It's like everything they issued since then ignores that we were using those signals since the 2000's.

Signals from Galileo, GLONASS, and BeiDou (and other semi-regional navigation satellite constellations) can be received in the US. Any decent GNSS receiver you can purchase in the US will be able to receive all 4 major global constellations. I know this cause I work with them everyday.

Even the typical “premium” cellphone will receive all 4. (I know some Android manufacturers disable the used of BeiDou, but they can still receive it.) The iPhone 15 Pro’s tech specs mention “Precision dual-frequency GPS (GPS, GLONASS, Galileo, QZSS, BeiDou, and NavIC)”.

Plus, some receivers can work with signals coming from different cobstellations, e.g. one GPs, one GLoNAS and one Galileo, in order to provide a proper position.

I think technically they should only receive for Galileo in the US - unless any of the others have also received exceptions from the FCC.

While in other countries, presumably your phone turns on the additional sources.

My - few years old, mid-range - Android phone shows that it is using 5 of the 6 GLONASS satellites in view as part of its fix. (In California, USA.)

Edit: Using GPS Test: https://play.google.com/store/apps/details?id=com.chartcross...

Not sure about “technically” or what the legal situation is. In practice, I know consumer smartphones in the US use a combination of all 4 constellations for positioning. If you have Android, you can easily verify this using various apps. If you're on the west coast (California Bay Area, at least), you can even receive unreliable signals from one or two Japanese QZSS satellites, which are supposed to be regional over the Asia-Oceania region. You can even see a few SBAS satellites (mainly used in aviation.)

Have an American model Android phone. Used it in Mexico, the US and Ireland. In all places it used GLONASS, GPS and Galileo equally (assuming equal satellite visibility, of course).

Yea. Even if not officially approved by the FCC etc, the signals are still being broadcast. So there may be interference, but the signals are there and often receivable.

The problem is really just that while premiums consumer devices support basically all GNSS providers, that's not true of a lot of devices. There is a lot of cheap/old/specialized systems out there that are still GPS only.

In the case of a GPS shutdown, I'm not worried about my iPhone 15. However, what is currently in place and certified for flight in a 30 year old 757? Have they gotten around to generic GNSS support or is it just GPS? I honestly don't know and would rather we didn't have to find out the hard way.

The "interference" is actually how GNSS systems work and it makes things better.

> Any decent GNSS receiver you can purchase in the US will be able to receive all 4 major global constellations. I know this cause I work with them everyday.

All common handset devices used in CONUS (every iphone, android phone, etc) will ignore BeiDou signals. Would love if someone hacked the GNSS firmware to make it usable, but haven't seen it happen yet. If you travel outside the CONUS geofence these signals will show on the device, but FCC has some mandate requesting a block, so it's blocked.

>All common handset devices used in CONUS (every iphone, android phone, etc) will ignore BeiDou signals.

This is correct for smartphones and the likes (all handsets??), though not entirely correct for all GNSS receivers. For example, I regularly use receivers with uBlox F9P chipsets with BeiDou enabled from factory [0].

>Would love if someone hacked the GNSS firmware to make it usable, but haven't seen it happen yet.

If we ask the Broadcom or Qualcomm engineers nicely, they might tell us how to unblock them for their smartphone chipsets in the US. I might’ve seen it before :)

[0] https://www.digikey.com/en/products/detail/ardusimple/AS-RTK...

It will receive them, but it won't use them without FCC approval. (Or isn't supposed to.)

https://www.gps.gov/spectrum/foreign/

I'm currently in a basement in the US, and my cheap phone GPS has acquired 6 US, 4 Russian and 4 European satellites in view.

The 2018 shutdown did not appear to affect GPS. Is this looming shut down different in some way?

No, it's not.

I don't actually expect GPS to be impacted in the coming week, but I think the big difference is that in the last partial shutdown, DoD funding had already been separately approved in September [2,3], and the shutdown started months later [1]. This cycle, b/c the NDAA has _not_ been passed, the DoD will be meaningfully impacted [4]:

- service members will not be paid

- some post and base services, commissaries and medical procedures may be stopped

Note that legislation has been proposed but not adopted to make sure that military personnel are paid even during a government shutdown. Note also that key military promotions/confirmations have been blocked for months in the Senate. It really seems like Congress does is not currently taking the military and national security seriously.

[1] https://en.wikipedia.org/wiki/2018%E2%80%932019_United_State...

[2] https://www.congress.gov/bill/115th-congress/house-bill/5895

[3] https://www.congress.gov/bill/115th-congress/house-bill/6157

[4] https://www.npr.org/2023/09/27/1201956915/government-shutdow...

> Note also that key military promotions/confirmations have been blocked for months in the Senate.

One man in the Senate is responsible for this one.

Non-essential civil service (a substantial part of DOD operations) will not be working come Monday (unless an agreement is reached by then) unless they are on multi-year funds. Program offices and large portions of logistics are largely staffed by civil service at this point so this could have huge impacts on military readiness.

> One man in the Senate is responsible for this one.

One man bears special responsibility, but as the recent confirmation of 3 top appointments shows, there are mechanisms that could be brought to bear. My (limited) understanding is that Senate holds can be ended with a cloture vote, so any 60 Senators could agree to move the nominations to consideration.

There's a recent article I read (NPR, I think?) where Mick Mulvaney was interviewed about that shutdown. He made the point (and spun it to make him look especially noble, of course) that the effects of a shutdown are largely decided by the Office of Management and Budget. They can choose to soften the blow, or they can make it hurt.

He argued that in 2013 under Obama, the OMB really turned the screws and hired contractors to fence off even national monuments which were otherwise unmanned anyway. Complete with signs announcing why they were fenced off. They wanted the public to feel the pain and associate it with Republicans (which seems to work, historically the GOP gets the majority of blame for shutdowns).

I think it's unlikely anybody is going to turn off GPS because the gov't "shutdown". I think the larger risk is that they will stop paying air traffic controllers, while expecting them to come to work anyway. That'll work for a while, but eventually some of those controllers may opt to stay home.

Republicans don't get blamed for the government shutdowns because of signs or fences by national monuments. They get blamed because they are at fault, whether that's trying to cut the increasingly popular ACA purely out of spite to Obama in 2013, or party infighting like this year or 2018.

The "blame" is exactly what Republican Party wants, to keep up their kayfabe marketing of "fiscal responsibility". Put interest rates around 7% for the next several decades, end the inflationary treadmill, let the everything bubble deflate, let the Wall Street debt/financialization cancer recede - and then it would make sense to talk about fiscal responsibility as a real thing. Until then any peanuts thrown to the plebs by the Democrats are really just mitigating the economic damage from the trough of overwhelming corporate welfare.

There are a couple of german words which made it into mainstream english like angst, zeitgeist, abseil. Based on your comment I would suggest a new one: brandrede. Look it up, afaik there is no english equivalent.

From a quick look the literal translation is "fire speech" which doesn't seem that hard to understand. So it would seem the significance is its place as a cultural touchstone, and cultures can certainly differ.

As to your larger point, pointing out errors in the social consensus is intrinsically inflammatory, as it upsets most everyone's neat little boxes they conceptually pigeonhole things into. That's unfortunate, but I don't think I said anything unnecessarily inflammatory - rather just tying together concepts each claimed by the red and blue tribes that they mostly use to talk past each other.

TIL:

* https://en.wikipedia.org/wiki/Kayfabe

I'm sure there are _some_ discretionary choices made but ... what does a shutdown that doesn't hurt look like? At some level, the government is huge and expensive, and society is adapted to it continuing to operate. If we had a no-pain way to not spend all that money, we should do it. But I'm guessing that putting up some fences around a few monuments is pretty cheap (it's not like they put up a fence around the whole grand canyon or something) relative to the funds that are being stopped.

I think the political showmanship here is that it's easy to have an above-the-fold image of a "closed due to shutdown" sign in front of the Lincoln memorial that's immediately comprehensible, but there's not a great pic to show more substantive but dry government work that doesn't happen or gets delayed. It's hard to visually instantly convey "you're still paying all your taxes but you're getting a whole lot less government right now."

If everyone's turn-by-turn navigation stopped working, there would be riots in the streets (if they could figure out how to get there).

If you want to bring attention to the everyday person, turning off GPS would definitely get their immediate attention.

Don't a lot of modern systems combine GPS with GLONASS and Galileo?

Pretty much every smartphone younger than four years.

Also most receivers for super economically important things like a avionics or marine navigation, but shutting off WAAS specifically (augmented accuracy data for GPS broadcasted using a much smaller set of satellites) would affect air traffic that need it to land in low visibility at an increasing number of airports.

Though, to the OP's point about GPS, WAAS is operated by the FAA, so it isn't clear to me whether not passing the defense authorization would affect it in the near term.

If they stop paying the ATC controllers, there won't be any planes needing to land. So no problem! /s

> but if GPS went off with the rest of the government, would Congress reach a resolution faster?

Oh hell yes. GPS is the basis of trillions of dollars of business globally. Just look at sea shipping and aviation alone. Would a lack of GPS ground every commercial plane? Maybe.. maybe not... but it certainly would be a huge disruption.

The first time it happens everyone would just drive hard to making sure all "GPS" systems were actually multi-system GNSS systems. Which is already true of most consumer phones, but I imagine there are still plenty of GPS-Only stuff out there. After the first GPS shutdown and the time for retrofit, everyone would have support for Galileo etc... and the Europeans would be standing there going "SEE!!!! I told you we can't trust the Americans!". And they'd be right.

it wouldn't just disrupt planes, it would crash planes if this were not even planned as a contingency. as a senior operations manager for a global logistics company, I was actually not even aware of this being a possibility until TIL here

> it wouldn't just disrupt planes, it would crash planes if this were not even planned as a contingency.

It would not. Aircraft have other navigation systems they can use instead, such as inertial reference and ground-based VOR navigation. And where there's radar coverage, they can be vectored by controllers.

This probably couldn't be used to keep up with the volume of air traffic today, but it would be enough to get existing flights down safely.

For landing, ILS is still very common, and doesn't require GPS either. An aircraft planning to land at an airport without ILS will have no trouble finding a nearby diversion airport that does have it.

Even as more ground-based navigation beacons are decommissioned, retaining enough for such an emergency remains an active consideration.

To my knowledge planes don't use the gps for very much, and AFAIK they use the barometric altimeter for maintaining their flight level. A controllable, plane that can maintain altitude is difficult to crash. The GPS stuff is mostly supplemental and is relatively new.

And pilots can land with a visual approach and still have ILS.

You would still have ADSB with your altitude, and I beleive heading and airspeed.

Because of the existing requirement to be spotting traffic and being over 1000ft away from other planes, they should be fine.

ATC should still have radar vectoring for their airspace.

I assume planes use more then just GPS, and use galileo and the other networks like consumer GPS do.

GPS is used a lot to figure where planes are but I think they are still not allowed to rely in it as it's not considered reliable enough by the regulators. At least not for things like altitude settings and instrument landings.

For light aircraft getting lost in the old days one hack used to be to fly low till you came to a motorway and read the signs. I imagine most such pilots these days would pull out their phone and open the maps app. Commercial flights of course have more high tech stuff like radio beacons.

I'm pretty sure all GPS receivers now are multisystem. So GLONASS and Baidu will continue to serve you

Transportation is the backbone of global economy.

Planes, ships, cars, trucks, food delivery - it would be a humongous impact.

GPS is pure magic from the perspective of someone a few centuries ago.

GPS is far from the only GNSS now. This whole shutdown business justifies our EU decision to develop GALILEO despite the cost and the Americans not being too happy about it.

See also "Phasing Out GPS Reliance in U.S. Military Operations: An Imperative in the Face of Emerging Threats":

> Global Navigation Satellite Systems (GNSS), such as the American GPS, Europe's GALILEO, China's BEIDOU, and Russia's GLONASS, play an essential role in modern warfare. Despite their different technical specifications—like frequencies and orbits—these systems are designed to be compatible, allowing for greater positional accuracy. However, their signals are susceptible to various forms of interference, such as jamming and spoofing. While there are security measures like anti-spoofing in place, these are not foolproof.

* https://www.realcleardefense.com/articles/2023/09/26/phasing...

There are plenty of techniques the military could use to make jamming or spoofing almost impossible.

For example, use a signal encrypted with a key not known to the enemy. That prevents spoofing.

Jamming can be pretty much eliminated by using phased array antennas. They can reject signals coming from the wrong direction. To fully block a military unit receiving GPS, the enemy would need to have ~30 planes in the sky over the unit, between the unit and each GPS satellite, broadcasting the jamming signal. Pretty infeasible for them.

There are other ways to make jamming far far harder. For example, if you are happy for your GPS receiver to be always on and have an atomic clock, you can make a new signal that replaces the 1024 bit PRN codes with multi-gigabyte codes which are secret. That effectively means rather than the enemy needing to overpower the real signal, the enemy needs to be billions of times stronger than the real signal to prevent you decoding it.

> To fully block a military unit receiving GPS, the enemy would need to have ~30 planes in the sky over the unit, between the unit and each GPS satellite, broadcasting the jamming signal. Pretty infeasible for them.

How did you arrive at this number?

It's approximately the number of GPS satellites in the sky from a given location on the ground. One plane per satellite. Of course this varies due to geography, orbits, etc. but is a good rule of thumb.

Number is a bit high but may be counting GPS, and EU GPS satellites you would have to block. You can "see" 4-12 GPS satellites at a time for one system. Assuming the EU system has the same style layout.

You don't need to block line-of-sight to the GPS satellites to create a GPS jamming attack. You just need a more powerful emitter near enough your intended target. No need for 30 planes trying to interpose themselves between the target and satellites.

Such a powerful jamming emitter is also a wonderful automatic target for loitering munitions

If it's within a region that can be attacked and the opponent has those capabilities, yes.

If you ask "the internet", the answer is drones. Regardless of the question being asked...

Recently huge drone swarms have been launched recently to increase the chance of mission success.

And to hope your enemy doesn't just launch a couple of anti-radiation missiles at your most powerful radiation source nearby.

Assuming they possess that capability, that is a risk, yes.

"home on jam" capability is being rolled out into smaller drone systems right now (it's not complicated per se, just somewhat risky since it could target friendlies).

You do if the victim uses a phased array antenna.

And any decent military receiver will do that, precisely to prevent jamming.

> Number is a bit high but may be counting GPS, and EU GPS satellites you would have to block.

All of which transmit on the same 3-4 frequencies:

* https://gssc.esa.int/navipedia/index.php?title=GNSS_signal

so you'd only have to have high-power transmitters on those and you can block things.

* https://gpsjam.org

Do it from a plane, or drone, and directional antennas become less effective as the jamming is coming from the same direction as the real signal (the sky).

Phased arrays can have a beamwidth of 1 degree... Which means the enemy has to get their jamming signal within 1 degree of the victim and the satellite.

And now you need to do that 30 times when there are 30 visible satellites.

A phased array can transmit to with-in a small angle, but how does that help the receiver? It is still coming from "the sky", so if you have an airborne jammer (EW fighter, drone), how will the receiver what's signal and what's noise?

I am aware of the spot beam functionality, which has higher power, which could help with overcoming noise:

* https://en.wikipedia.org/wiki/GPS_Block_III#Military_(M-code...

The biggest recent help against jamming was the introduction of the L2C signal on a different frequency with the Block IIR-M satellites, and then the more powerful L5 signal on Block IIF.

A phased array antenna can both transmit and receive within a small beam angle.

Even with a key you can drown out the signal by blasting enough RF waves at the receiver or area. I’ve heard a Growler pilot describe what they do as shining a spot light in your face to blind you.

Yes but you would have to do that from top down. You can't shine a spot light on the back of a person's head to blind them. Not to mention the tricks they use to make RF signals directional to both send and receive.

> You can't shine a spot light on the back of a person's head to blind them.

You can, if the spot light is strong enough, and the person in question isn't floating in deep space. Just keep adding power until the light reflected by the person's surroundings is strong enough to blind them.

> Not to mention the tricks they use to make RF signals directional to both send and receive.

Makes me wonder if there's an RF equivalent of getting permanently blinded by laser light scattering off random objects in your line of sight.

that's called jamming and was addressed by the comment. if you wear very narrow blinders and point your face right at where you want to look, then the enemy needs to install their spotlight right ahead of you instead of just anywhere in front.

It is a mitigation that may help, but not foolproof: someone can jam or spoof from the sky.

Current areas where jamming is being detecting.

* https://gpsjam.org

This jamming isn’t from the sky though.

Incidentally, there has been a recent escalation in this: over Iraq commercial airline pilots have been reporting increasingly successful GPS spoofing, with GPS positions being shifted by tens or hundreds of nm, the clock being shifted, etc. In some cases this has exposed bugs in aircraft software where it has not successfully recovered the GPS position even after leaving the affected area, leaving them with degraded navigation all the way to the destination.

This is not as dire as it might sound, as there are plenty of other ways for an aircraft to determine its position, though it certainly reduces the margins for error in some cases. But it’s interesting because such spoofing should be extremely difficult to carry out successfully, especially on a moving target like an airliner.

> This jamming isn’t from the sky though.

In that particular instance, but:

* https://en.wikipedia.org/wiki/Electronic-warfare_aircraft

Waiting for drones:

* https://apps.dtic.mil/sti/citations/ADA508909

Encryption still won’t stop it. You will need many blinders to get a fix on enough satellites.

Or make a stronger spotlight and point it at whatever is directly in front of you, to blind you with reflected light.

Continuing that analogy, in the case where a sky facing directional antenna is being used: the pilot is crouched under a sewer grate, shining a light at the soles of your feet, as you stare up into the night sky. If you are pumping enough power to overcome a directional antenna, than you are more in the realm of a directed energy weapon - which requires an insane amount of power (see inverse-square law).

> There are plenty of techniques the military could use to make jamming or spoofing almost impossible.

The US DOD is not as confident as you are, as they looking at backups/alternatives:

> DOD primarily relies on GPS for accurate PNT data, which is essential to effective military operations. However, multiple threats can render GPS data unavailable or inaccurate. DOD recognizes the threats to GPS and is taking steps to address them by developing more robust GPS capabilities and alternative PNT technologies. GAO was asked to review DOD's acquisition of alternative PNT technologies.

* https://www.gao.gov/products/gao-22-106010

All GNSS signals live on a finite number of frequency bands: L5/E5, L2, E6, L1/E1:

* https://gssc.esa.int/navipedia/index.php?title=GNSS_signal

* https://www.geospatialworld.net/blogs/gnss-frequency-bands-f...

* https://www.tallysman.com/gnss-constellations-radio-frequenc...

* https://en.wikipedia.org/wiki/Satellite_navigation#Frequency...

> The ITU have allocated the following L-band frequencies to GNSS (bold are aeronautically used):

> *GPS, the centre frequencies are 1575.42 MHz (L1), 1227.6 MHz (L2) and 1176.45 MHz (L5).

> *GLONASS operates as frequency divisional multiple access (FDMA) and there are two operational centre frequencies 1602 MHz (L1) and 1246 MHz (L2) and at 1207.14 MHz (L3). GLONASS over this decade will also introduce Code Divisional Multiple Access (CDMA) similar to GPS.

> *GALILEO has a range of frequencies assigned in the L-band as follows:

> ** E2 – L1 – E1 – Centre frequency 1575.42 MHz (band from 1559MHz – 1591MHz).

> ** E5A – Centre frequency 1176.45 MHz (band from 1164 MHz – 1188 MHz).

> ** E5B - Centre frequency 1207.14 MHz (band from 1188 MHz – 1215 MHz).

> ** E6 – Centre frequency 1278,75 MHz (band from 1260 MHz – 1300 MHz).

* https://pbnportal.eu/epbn/main/Overview-of-PBN/PBN-Concept--...

A handful of planes (or drones) focussing on those few frequency ranges can do a lot of mischief.

And this is not theoretical, but observed events:

> Russia has been thwarting US-made mobile rocket systems in Ukraine more frequently in recent months, using electronic jammers to throw off its GPS guided targeting system to cause rockets to miss their targets, multiple people briefed on the matter told CNN.

* https://www.cnn.com/2023/05/05/politics/russia-jamming-himar...

GPS guided missiles are a unique case, because there is a good chance the enemy gets hold of the electronics. Therefore, you don't want to put your military GPS encryption keys in there, because if the enemy got hold of those keys, they could spoof your satellites.

> Therefore, you don't want to put your military GPS encryption keys in there, because if the enemy got hold of those keys, they could spoof your satellites.

IIRC, I remember reading that the NSA has cipher(s) where the decryption 'algorithm' is completely than the one for encryption, i.e., just because you have the decryption code doesn't mean you can encrypt.

So normal public key cryptography? ;-)

No, this is for symmetrical encryption.

So think something like AES, but where the 'distributor' encrypts something, gives you a key and an algorithm, and you can decrypt it. But you cannot use the same algorithm to encrypt to send back anything.

For the life of me I can't remember where I read it (which book), so have never been able to dig into it more.

PFS

How feasible would you estimate passive positioning systems?

For example consider solar radio noise at frequencies that are partially reflected, partially transmitted by the ionosphere.

Do you consider it possible for an SDR to geolocate by comparing multiple antenna signals say:

1: "direct signal" : sun -DOWN-> partial transmitted through ionosphere -DOWN-> antenna 1 aimed at sun

2: "geography / terrain fingerprinted signal" : sun -DOWN-> partially transmitted through ionosphere -DOWN-> reflection on Earth -UP-> partial reflection from ionosphere -DOWN-> antenna 2

Or any suggested variations? (reflection from moon for example).

https://commons.wikimedia.org/wiki/File:Ionospheric_layers_f...

i.e. can Western citizens prepare to still have geopositioning in case of an adversarial (successful) first strike against GPS and Galileo where the citizen refuses to rely on adversary positioning systems.

With a clock and a telescope, you can get super precise positioning the same as the 16th century way... Atomic clocks are now good enough to make those positions really good.

You can do the same with radio frequencies and a phased array antenna if you like.

I'm not sure if other approaches would perform better.

I mean without a clock, the reference being the direct path of noise from the sun.

Telescopes are impeded by clouds.

During night there is no line of sight to the sun, so I concede my rudimentary idea or question has similar drawbacks.

It would be interesting if precise time could be derived from imaging the night sky, so that the synchronized clock becomes unnecessary.

I've actually heard of research into putting celestial navigation in guided missiles (from people who've worked on it -- so the idea that this is possible seems to not be a secret). Cameras and optics have become good enough to make this plausible with those kind of constraints.

I don't know if it's been deployed, but I would not be surprised.

Some ballistic missiles have already been using celestial navigation operationally for decades. That is used primarily during the mid-course phase, and then they switch over to inertial (or maybe radar in a few cases) for the terminal phase.

https://en.wikipedia.org/wiki/UGM-133_Trident_II

check this out: https://en.wikipedia.org/wiki/Pulsar-based_navigation

this is interesting, thanks for the reference, but it doesn't seem affordable for average citizens:

"The main error sources that impact the instrument design are the number of collected photons (related to the detector size and the integration time) and the timing accuracy."

Detecting and filtering for appropriate X-rays would seem to require a large and thus expensive detector.

Being able to self-locate during aerobraking (as described in following URL) is a very neat application though...

https://web.archive.org/web/20170316044511/http://gsp.esa.in...

I wonder what materials withstand the heat and plasma during aerobraking, yet are transparent enough for X-rays?

> How feasible would you estimate passive positioning systems?

Totally feasible. DECCA and LORAN-C basically work on the same principle. have a number of transmitters at known locations and measure the phase/interference to get location.

However, there are other ways. Visual navigation is totally possible (think SLAM, but with a pre-computed map)

DECCA and LORAN-C are not passive but rely on second parties maintaining transmitters.

I may be wrong, but I would hazard SLAM would require an enormous amount of data

> rely on second parties maintaining transmitters.

yeah, this is true. Passive radar does the same thing. its passive because the system isn't transmitting, something else is.

SLAM data can be pretty sparse. The keypoints for a city the size of london is under 100gigs. (although that depends on your algorithm)

but if you think about the amount of effort required to make GPS work, keeping an active visual map of a country isn't actually that much in comparison.

I was reading the specs for the GPS satellites vs. the GLONASS ones a while back, and it seemed like they transmit on a similar frequency but with a different modulation scheme and protocol. Pure speculation, but you could imagine a Tom-Clancy-esque scenario where their secondary role is to jam GPS when they need to.

To jam something, you ideally jam the same frequency, PRN code, and direction.

If you don't know frequency, you need to jam a wide range of frequencies. Defences like frequency hopping make use of this. Bluetooth uses this for example, to prevent inadvertent jamming. In GPS, frequencies aren't secret.

If you don't know the code, you need to jam with as much power as the benefit of the code. For GPS, thats a 10x 1024 bit code, which provides 43 dB of processing gain. This can be increased almost without limit, as long as you don't plan to move much during the code period. There are clever methods to make encryption keys double-up as PRN codes that the enemy doesn't know.

For direction, it depends on the receiver. Your phone has an omnidirectional receiver, so the attacker is 'on par with' the real signal. A military directional phased array system might have 200 elements, and therefore 30db of gain.

Add that up, and the enemy needs to use far far more signal power than the real signal to 'outshine' it. Turns out that's pretty much impossible - and if they get near to doing that, your biggest problem is probably being cooked by their jammer.

> Jamming can be pretty much eliminated by using phased array antennas.

citation needed there. GPS typical signal strength is -125dBm.

For ground based stuff, its pretty simple to overpower the band so that nothing can hear it.

This has been a solved problem for a very long time. The nice thing about GPS satellites is that you know where they should be, and where they shouldn't, so long as your RTC is properly set and you've got a compass.

A GPS Digital Phased Array Antenna and Receiver (2000): https://apps.dtic.mil/sti/tr/pdf/ADA452307.pdf

yes, phased arrays, especially if you have a tight beam, can give you an extra 40db of SNR (see figure 9 https://www.analog.com/en/analog-dialogue/articles/phased-ar..., also in the charts on your link)

but thats still like -70dbm at best. That's about the level of being a few meters away from a wifi AP level of power.

a 1 watt(RF) jammer is going to cast a large cone of non-GPS even with a phased array antenna.

You are forgetting about the phased nulling. Military GPS has been including jammer detection in the firmware for a long time. I remember my handheld unit constantly annoying me anytime the IED jammer (which was waaaay more than 1 watt of rf) went active... 2005ish - it would simply mask the signal coming from that direction and continue to provide positioning data.

Add in coding gain over 1 second (a code 10 million chips long), and that gives you an extra 70dB. Now it's getting hard to jam.

Downside: This only works if your movement over the 1 second period is predictable, to a small part of a wavelength. If you're moving unpredictably, you'll have to give up a few db's of coding gain.

"The sky" covers 180°x180°, or more if you're in a plane. A jammer that can block out half of that can result in a significant reduction in accuracy

So, basically back to old school, "manual" stuff? Should be doable, if less convenient.

The US Navy brought back celestial navigation (with sextants) as a backup:

* https://www.usni.org/magazines/proceedings/2021/december/bri...

* https://skyandtelescope.org/astronomy-news/u-s-navy-resumes-...

And all the other, old school stuff is still available anyway. Still, without GNSS, things are complicated.

Cell phone towers depend on GPS for timing and frequency precision. A lot of processes depend on GPS availability.

Military navigation doesn't so. Sure, GPS makes things a ton easier but it is perfectly possible without it.

Fun fact: You know the "Hidden Figures" women who did the intense calculation for the military and space programs in the USA before computers were powerful enough?

GPS was one of those systems needing intensely calculated orbits, Dr. Gladys West was responsible

https://www.afspc.af.mil/News/Article-Display/Article/170746...

My second favorite part about GPS is that it's a general relativity experiment, for anybody still on the fence. GPS satellites run 38 microseconds faster/day (from our perspective) due to the difference in gravity which works out to being off by ~6 miles/day.

This is interesting

>The 2024 GPS Testing for Critical Infrastructure (GET-CI) event is set for the fall of 2024.

>This event provides an opportunity for CI O&O and manufacturers of commercial GPS receivers used in critical infrastructure to perform equipment evaluations in a rarely available live-sky spoofing and jamming environment. For the 2024 GET-CI event, S&T will create a live-sky GPS environment primarily for fixed infrastructure applications but will also support some ground-based mobile applications.

> perform equipment evaluations in a rarely available live-sky spoofing and jamming environment

I'm guessing there are aspects of this prepared interference that are unique.

We've had a number of planned & unexplained outages/jammings in the US. I recall a planned one shaped like an inverted cone, with the impact broader area at higher altitudes. I was able to math the cone-point, somewhere out west and at a low alt.

Diff sites track them.

ref: https://gpsjam.org/

ref: https://www.navcen.uscg.gov/gps-problem-report-status

I remember something like this from 2011: https://web.archive.org/web/20111015232120/http://navcen.usc...

“SOUTHEAST ATLANTIC COAST: GPS Testing Information THE GPS NAVIGATION SIGNALS MAY BE UNRELIABLE FROM 20 JAN 2011 - 22 FEB 2011 FROM 0000Z - 0245Z DUE TO TESTING ON GPS FREQUENCIES USED IN SHIPBOARD NAVIGATION AND HANDHELD SYSTEMS. GPS SYSTEMS THAT RELY ON GPS, SUCH AS E-911, AIS AND DSC, MAY BE AFFECTED WITHIN A 150 NM RADIUS OF POSITION 30 49.09N 80 28.18W. DURING THIS PERIOD GPS USERS ARE ENCOURAGED TO REPORT ANY GPS SERVICE OUTAGES THAT THEY MAY EXPERIENCE DURING THIS TESTING VIA THE NAVIGATION INFORMATION SERVICE (NIS) BY CALLING (703) 313-5900 OR BY USING THE NAVCEN WEB SITE'S GPS REPORT A PROBLEM WORKSHEET AT WWW.NAVCEN.USCG.GOV.”

I specifically remember it because I was trying to navigate my roommate and I to the Atlanta IKEA but my phone showed me as being, like, south of Macon. ~100mi of error.

Fun fact I learned recently: The GPS reference epoch was moved from 2000 to 2001 due to an Alaskan earthquake in 2002. It's crazy how an event like that can change something like GPS!

https://en.wikipedia.org/wiki/International_Terrestrial_Refe...

The article on Consumer grade GPS receivers and their associated maps not being good enough for truckers was a surprise. That's something I had never thought about.

Low bridges are a hazard for inexperienced truck drivers. https://www.youtube.com/@11foot8plus8

This SoftStop barrier looks promising.

https://www.laservision.com.au/portfolio/softstop/

> Is military GPS more accurate than civilian GPS?

> The user range error (URE) of the GPS signals in space is actually the same for the civilian and military GPS services. However, most of today's civilian devices use only one GPS frequency, while military receivers use two.

> Using two GPS frequencies improves accuracy by correcting signal distortions caused by Earth's atmosphere. Dual-frequency GPS equipment is commercially available for civilian use, but its cost and size has limited it to professional applications.

> With augmentation systems, civilian users can actually receive better GPS accuracy than the military.

Is it fully true ? So the only advantages of the military GPS is that the codes are unknown and that it is more jamming proof ?

they also have the ability to fuzz civilian GPS while not doing so for the military... so probably, until they don't want it to be true.

They only need to change the CDMA codes of the civilian GPS to render it unusable. Also at the beginning of GPS they purposely added inaccuracies into the timing to make it less precise for civilian use, but because of the FAA and other needs (and methods that have been developed to counter the injected inaccuracies) they finally removed it in 2000.

To respond to my question. With long term measurements and the help of the L5 band it is possible to obtain sub-millimeter accuracy [1]!

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

Civilian GPS signal can be selectively diluted in certain areas, which is useful for stopping most non-military and a large number of military navigation kit.

For a video form of this topic, Scott Manley did a review of how GPS actually performed better than initially designed:

https://www.youtube.com/watch?v=qJ7ZAUjsycY

Relevant occurrence in GPS history: https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007

Linked from this page: https://www.navcen.uscg.gov/gps-problem-report-status

About 500 reports from people saying "GPS on my phone isn't working" and the US Coast Guard and Department of Homeland Security replying that "perhaps your phone is broken".

I didn’t know that they used to degrade GPS signals in something called Selective Availability.[0]

[0] https://www.gps.gov/systems/gps/modernization/sa/

This is why GPS only seemed to appear in the 00s despite having been around since the late 70s.

GPS spoofing will be an excellent technique for false flag operations

Building giant space lasers will be an excellent technique for holding the world ransom for ONE BILLION DOLLARS.

(This is where we pitch one sentence villainous plots right?)

I make no recommendations! It is only observation of an imbalance of power that could be exploited by the dominant owners of satellite technology

Water is wet. This statement is as useful as "The GPS's continued existence benefits systems built on the GPS" but with more fear implied. It doesn't really feel like a novel thought, more like one of the first 5 things you might throw up in the "cons" column when discussing it vs alternatives.

Don't get me wrong, I have plenty of qualms with the US government, but walking into a discussion on say, foreign aid spending programs and saying "The Government will spend this money almost entirely in ways that advances its global agenda" is just sort of a Duh moment.

I'm sorry, this is a very long way of saying "so what, who cares, what are you proposing?"

I mean, that was the entire point of GPS in the first place. Allow the US to navigate quicker, more accurately so that they could get stuff where they needed to be quicker and more efficiently.

GPS spoofing, not jamming, is causing issues around Iran-Iraq such that the FAA put out a notice:

* https://www.ainonline.com/aviation-news/air-transport/2023-0...

There was/is speculation that spoofing may have been used in the capture of a US drone:

* https://en.wikipedia.org/wiki/Iran–U.S._RQ-170_incident

Don't forget the sharks! I want those!

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