Ha, I met somebody who was testing in 1991. The first gps satellites were up and running, already. A handful only. It was pretty special.

They were called pseudolites. [^_^}

SLGR then PLGR. Worked good enough.

😂That might be the grain of the truth in a stack of flat earth conspiracy theories 😯🤷‍♂️🤦‍♂️😂

Basically how aircraft navigation worked using radio beacons. Such as TACAN and VOR

Wi-Fi also plays a part in this. I recently moved to a new state where I've got no cell service inside my house. My phone sees my Wi-Fi, and thinks I'm back at my old address. This makes it impossible to get a precise location lock unless I can talk to a cell tower. I had to buy a new router. Once I did that, the phone would use it's last location to look for satellites, and wouldn't think it teleported to another state!

Good point. The key factor is that your phone will remember that AGPS data for a long time, even when out of cellular range. Removing the phone battery will force a cold start of the GPS receiver.

​@@PsRohrbaugh that issue is not from real GPS, google or some provider keep a database of WIFI acess points and their location. A phone will send a list of nearby acess points to this provider, wich will look for matches in this database and return a estimated position. This can then be used as a initial position until a real GPS fix is achieved. Then the reverse is done, a phone with a good GPS signal will send the list of visible AP to the provider to update its database.

I used to have a handheld GPS receiver in the 90's which took forever to lock but I never thought about how today it doesn’t take long because of the dropped selective availability thing, thanks for that little gem about cell towers.

A long time and a lot of battery, I presume?

That is using the carrier wave, that kind of equipment is very expensive though. When you used that with the coded data and a differential source you can get below 10 centimetres or 3 inches. I worked on a project in 2003 that did just that. We had things then, that made google map/earth today, look like a joke

@@nicholaswjamrocksmartphones of the past couple years have carrier tracking, on android there's an API for that so you can use custom software for sensor fusion or recording to later post-process or do differential RTK...

Using Kalman filters it is also possible to combine many different measurements (GPS, IMU, Odometry,...) into a really precise and also fast (high update rate) position estimate

that would be a good video

That’s cool! So is the velocity data similar accuracy to position data? Or is it better? I’ve noticed a gopro will give very responsive velocity information

There are drones for agricultural work that have RTK and can stay within 1/2 inch of their course. Really cool stuff

My brother uses RTK GPS and that has less than 1 inch accuracy. All he has to do is turn around at the end of the field.

Yup, from robotic lawn mowers to large scale ag. The green and yellow tractor company promises centimeter accuracy.

Yeah that is pretty fascinating. I have a cousin doing the same thing; the on board computer is given the soil conditions ahead of time and can turn individual spray heads on and off to match. Then he got certificated and got himself an Airtractor and does it all at 140 knots.

A lot of Ag GPS systems augment their data with ground stations too. At the sugar refinery where my dad used to work there's a base station on one of the silos that helps improve the accuracy for local farmers.

RTK if you want to google it.

I think SparkFun sells a complete kit for advanced embedded developers.

Can you confirm globe earth, as a surveyor?

@@alexlandherrThey do. I quite like their surveying pole. It measures the position at the top of the pole, but the pole may not be verticle. So it uses accelerometers to work out the tilt of the pole, and uses that to work out the position of the spike at the end of the pole.

@@alexlandherr In the early 2000's, they cost around $50k, now a RTK system runs about $8k.

Wow that must have been a discussion topic next day in the office! I'm curious, how precise was the tracking before that? Did you see the bread crumbs in general proximity of the road?

@@MatteoGalet Yes. Haul roads are made at around 20-30 m width. I called the guys at the office at the time. I must say that the supplier of the system had their own form of correction as well. Now they use GPS to steer autonomous trucks. The company is Modular Mining Systems from Tucson, Az.

What AVL system did you use then? Oh never mind I see it

How was radio direction finding done in WW2 without GPS?

Most WWII DF was HF, compared to VHF in the 1960s-2000s. Otherwise, it was basically done the same way, but it's difficult to get a good fix (where 2 or more lines of bearing from your direction finders to a transmitter cross) if you have an imprecise starting location. We needed a minimum of a 6-digit grid (10m) to start from, which frankly wasn't hard to get anywhere besides a featureless desert.

@@jarink1 thanks for the explanation. I remember a documentary about SOE agents dropped into France that had secret radio sets that they clandestinely used but the Germans were monitoring it and found their location

Better then the hand drawn maps we had in the rear. MSR, to MSR. And of course, the 7th, and 18th had different names for the same MSR. We had a lot of luck guiding us.

I vaguely remember those things. Now we have a gps in our watch that is 1000x better and uses a zillionth of the battery power 😂

Yes! I was going to mention that as well! 😀

As far as I know, it's the only system that requires both quantum mechanics (atomic clocks) and general relativity (frequency corrections) to work properly!

And even more interesting is the fact that the in the first gps satellites two clock system were onboard. One with relativity correction and another without the correction because the military where note « sure » that the Einstein relativity was true !

The satellites are gaining 45 microseconds per day as predicted by General Relativity. That's taken up by lowering the clock frequency in the satellites to synchronize with ground clocks. The satellites are moving sufficiently fast enough to lose up to 7 microseconds per day as predicted by Special Relativity. That's calculated and corrected by each receiver.

re: "it requires General Relativity corrections to obtain the accuracy we are used to. " Adjustments are made by ground control stations on a routine basis to correct for this. No magic involved here. Per the ops plan they use. This can be verified.

The Russians are working as crazy to jam the GPS signals in Ukraine

Good point. I just knew the basics of how gps works, in fact the math resembles using a sextant and the Reed Almanac tables, it's known orbits and the reveiver does observations, it's just slightly different observations, here it is distances. So, I did understand how hard it must have been these days, to get these calculations done in time. A cpu in 1991 was bloody expensive, or it was cheaper and lacking number crunch power. How did they do it? And WHO built these chips? It was a military project, after all, that precision restriction was a big deal. Well, your comment explains a lot, it was dedicated hardware, not just fast i/o and smart software.

Increasingly cheap computer power allowed the change from sequential tracking to simultaneous tracking. One of the freely available, but not open source, SDR packages available to day allows one set of hardware that samples at a high rate to feed a generally CPU capabilities or screen size allows number of simultaneous receivers. It's as if you had 16 receivers all sharing the same antenna, as might have happened in some military installations in the 1950s to surprisingly recently. Now they share one analog to digital converter. The rest is "a simple matter of software." {^_^}

The first GPS receiver I had was about 1995, a Magellan with a single channel correlator. The start-up time could be up to half an hour if the geometry of the satellite constellation was unfavorable at the time. I left the receiver in the car always on because it took so long to acquire the satellites. Back then, if you wanted maps, you had to run a program on a laptop computer. I had Delorme Automap which ran on Windows 3.1. At least the computer didn’t have to remain in the car and always on. Selective availability could easily put you on the wrong street when navigating in a city. The error was up to about 1/8 mile. You could usually tell when an SA glitch was being introduced and they were usually about 30 to 60 seconds. Clinton signed an executive order in 1999 to shut off SA by May 2000. I got a Garmin 2620 automobile receiver in 2003 which included built in maps with enough internal memory to cover all major and minor roads in continental US. The 2620 also includes WAAS reception. I was pretty skeptical that WAAS would work because it is something like -140 dBm due to the distance to geostationary orbit, and the car receiver has just a very small internal patch antenna. But work WAAS does. When it has lock, which usually only takes a few minutes, I’ve seen estimated positional errors reported by the receiver as small as 2.5 feet. That is probably good enough to land a plane, but not good enough for an autonomous driving car without other sources of position input. The other restriction that has been claimed was put on early receivers is that they were not supposed to work correctly at high speed. The intent was supposedly to prevent unfriendly powers from using commercial GPS units as guidance system building blocks. I took a Garmin Nuvi on a regional jet flight between Chicago and Denver and held the Nuvi next to the window. The Nuvi seemed to be accurately updating the the indicated position with the jet flying well over 400 MPH. I didn’t have any means to tell if the reported elevation data were correct.

@@wtmayhew The speed limit is somewhere in the supersonic range. And there is also a height limit. Both won't restrict any commercial planes, but are intended to make commercial GPS unuseable for ICBMs and other rockets.

So cool, I’ve heard that North America raise and fall about 10 meters when the Moon passes over it , is that right?

@@renesoucy3444 The crust is a lot stiffer than the water, you're looking for an amplitude of around 0.3m.

No one is mentioning WAAS. Wide Area Augmentation System.

No one except Scott, who went into it at least a little bit. Was the first AS he mentioned.

Most current GPS receivers have 12+ channels- you only *need* a fix on 4 satellites, but if you track more, the potential error goes down. Since aircraft tend to have a very good view of the sky, they can track all the SVs above the horizon, with some channels searching for fresh. Receivers track but only include SVs a threshold above the horizon in the fix, since their ionospheric variation may be worse.

So with multiple receivers in various locations they could get heading/pitch/roll from GPS? neat

​@@triffid0hunterheading (yaw) is common with multiple antenna assorted setups. Often called AHRS for attitude heading reference system. However I haven't seen anything on pitch/roll from the antennas. But I guess it should be just as possible.

Coast guard ships use bow and stern GPS RX in combo for precision station-keeping in all weather conditions.

If you move the second asterisk after the period, you will get the desired *bold* attribute.

We have the same issue on trains. a 220M long train with ant&reciever at each end. I was just using them to detect station arrival for selective door operations...fairly important not to open a train door where there isn't a platform.

That was called differential gps. It was used well into the 2000s. I did an integrated system for PANYNJ to track trucks on the GW bridge in 97-2000 that used DGPS

re: "They set up two fixed base stations spaced apart each with it’s own GPS receiver." The basis fir something called Differential GPS.

S\A.. could you really remove the error with some smart math?

@@michaelhompus2475 NOT w/o averaging a single receiver's position over ... say, hours.

Hahaha why did you write this as a clickbait article

​@@woutervanheusdenYou will never believe what happened next...

Lol! Yah I did something similar back in the day, I remember I was watching some tech nerd channel on the tv (basic coax cable mind you) and they were talking about cell phones and how one day we would be watching tv on them and also be able to go online and view websites. Well I laughed and said that’s bs and that the dude was smoking crack. But here we are and I’m watching this vid on my iPhone. But mind you back when he said it most cell phones had a tiny screen that mostly was green or grey like Nokia and we could play snake. 🐍

This is similar to what some British politician (I forget who it was) said back in the early days of the jet engine, that those engines were "a scientific curiosity and would never seriously challenge the 'airscrew' " (propeller).

We at TI (in 1978) had a contract to develop both a MANPACK and HDUE (aircraft platform) GPS receivers. We had field engineers at Yuma Proving Grounds working on that ground range testing equipment and also I recall the mention of three satellites "in service" at that time too.

The y1.999k bug. But really what did they expect to happen? Did people think those devices were not going to last that long?

There was another rollover in 2019 and according to Wikipedia it affected a lot more devices. Brace yourselves for 2038 when we're going to have both GPS and UNIX time rollovers in the same year.

There is this quirk in German regional trains. They often have a display with the current date and time, and the name of the next station. This gets its info from GPS, and lately there where a lot of sightings with a date 1024 weeks back from 2003.

That is truly unbelievable. One would have thought people would learn from the Y2K hysteria once and for all. I know you can’t use long integers for all variables just in case, but can’t we just all agree to come up with a more robust way to store date/time in general ? 😅

@@miroslavmilan Computers and calendars/clocks have never got on well!

good comment. Yes CDMA sites "breath" which can cause real problems for operators. Thats why 5G does not use CDMA.

@@rgbaal To my knowledge CMDA has only been used in 2G and 3G standard of mobile communication. 4G was designed completely independent of 3G. 4G uses OFDM signals with FDMA and TDMA instead of a single carrier signal with CDMA in 3G. One big advantage of OFDM in a multi-user scenario is which each user can be assigned to the part of the spectrum which is received the strongest by him. This type of resource allocation is called multi-user diversity. The same diversity concept can be nicely extended to the Multi Antenna transmission. The so-called Multi user MIMO or MU-MIMO.

re: "This effectively means that the range with CDMA based protocols reduces as congestion goes up," Proper RF coverage, as performed by the RF Engineering department, in core areas of coverage work to assure sufficient margin exists to keep this from happening. Engineering isn't a 'crap shoot' where things just happen, there were methods and plans in place to accommodate the technology, JUST as there was for 1G (analog) FDMA cell technology. Former RF cell eng here.

@@uploadJ I wasn't implying it was some wild uncontrolled factor; obviously it's accounted for when you design cell spacing, power levels etc. Designers know very well how various numbers of active users at various distances will affect the performance of their cells and ensure they're managed accordingly to provide consistent performance. It's simply a factual property of CDMA modulation, and an unusual one compared to other modulation schemes where range is not dependent on congestion (only throughput), so I thought it was worth mentioning. It takes a pretty exceptional scenario to even notice this (edges of coverage at unusually busy times) which is a testament to the exact fact you mention, which is that a lot of design effort goes in to ensuring you have a good experience with cell networks across a broad range of conditions and locations.

@@siberx4 Fair enough. You have a good reply there too.

The first successful tests of the Transit system were made in 1960, and the system entered Naval service in 1964. This system was first envisioned when two physicists (William Guier and George Weiffenbach) at Johns Hopkins Applied Physics Laboratories October 4, 1957 heard the Doppler shift of the Sputnik signal. Transit was in use until retired in 1991.

@@mdxtrains "Retired", yes --- but not shut down. From what I understand, it's still being used to monitor Earth's gravitational anomalies.

Thanks for bringing up “Transit”, as it was a “good” system that established a baseline for the future GPS specifications.

GPS not working in Moscow because of "wascally Ukrainian drones"? Thank you for that tidbit, good sir. My schadenfreude is immeasurable, and my day is perfect. Also, I'd heard about something like that with (1). Something like, you knew when some particular war started, because GPS suddenly got a whole lot more accurate. Didn't know that it was due to SA being turned off, neat!

There is a misnomer here though that the techniques used by SA were turned off. In fact the GPS system still skews time. UTC(USNO) (GPS time) "is kept within a close but unspecified tolerance of the international atomic timescale." The military knows the tolerance in real time, the timing data is released two weeks after the fact (so atomic clocks can be sycned up). SA was just a very large interruption in the clocks which could be corrected by simply having a clock station on the ground as the video notes.

5 is fake.

We were using the NAVSTAR SYSTEM(GPS) In Gulf War 1 in 1982 - it actually played a MAJOR Part in the Victory - for once the Allies KNEW Precisely where they and everyone else where..... The First GPS Satellite was launched in 1978....... The full 24 Satellite Constellation was operational in 1993........ Selective Availability was turned off for Gulf War 1(Kuwait) - NOT IRAQ 1 - basically as you have said there were not enough Military Compliant GPS Recievers so they bought a load of Civilian Grade ones - as SA Degraded thier Accuracy they turned it off temporarily.?.. SA got switched off permanently @ 24:00 1st May 2000

But the waskilly Ukrainians can do the same thing by building ground stations as in the WAAS system or the Coastguard did.

My point was they were all part of the US Government and they were wasting money and effort to circumvent SA

The DOT had the coast guard for years decades because the US Lighthouse Service was merged into the USCG aim 1939 and that is when the maintenance of marine navigational aids such as bouys, lighthouses and Lightships. That is why the USCG would also later take on the mission of maintaining electronic navigation systems. They operated the Loran A and Loran C systems and eventually took over the GPS and WAAS system operation from DOD

The USCG was one of the agencies that was merged into the Department Of Homeland Security as a result of the panic that created DHS post 9/11 that also resulted in their “law enforcement” mission. Keeping the USCG out of DOD also elevated posse comentatus (yeah I know my Latin sucks)issues because the military is prohibited from civilian law enforcement activities

@@scottmanley I one hundred percent agree. I was not trying to correct anything that you said. Just trying to explain why in the 90s the Coast Guard was not part of the military.

@@shawnmiller4781 USCG was part of the Dept of the Treasury until 1967. Presumably because it was involved with immigration and customs (which also used to be part of the Treasury). It then got moved to Transportation, until the Dept of Homeland Security was created. It gets transferred to the DOD during times of war. Used to require a Presidential executive order. But they passed a law in 2006 making it automatic after a declaration of war.

Part of the problem is that the geoid used by GPS is a theoretical model, while sea-level depends on gravitational fluctuations in the earth's crustal rocks.

@@gordonrichardson2972 I know about that, but it doesn't explain from 70 feet above sea level to 50 feet under water in one mile. When all your triangulation points are above you, accuracy suffers.

With more precision you run into the problem of how do you define what 'sea level' actually is. You can take an average of the sea's height over many tides, but that will vary according to location, some places have very large tides, in some places the difference between high and low tide is relatively small.

@@phuzz00 Well, my rule of thumb is that if fish are swimming in it and it's tidal, it's below sea level. I do have concerns about depth soundings, but as my boat draws 3 1/2 feet, if I run aground, I can jump over the side and push it off.

He showed a slide covering MON but didn't talk about it. MON stands for minimum operational network and is a network of VORs that the FAA will keep operational as it serves as a backup to GPS when it comes to aerial navigation.

Yes. A really big solar flare could throw the whole thing out of whack, I guess.

Expanding on this, ‘classic’ DGPS corrects the code phase of the ranging signal, whereas RTK/PPP etc use a variety of techniques to correct the carrier phase of the ranging signal for a higher degree of precision/ and ideally, accuracy. The limit to achievable accuracy in cases where range correction terms are used is that they assume that the range error for the reference station and remote are identical. The further we get from the reference station, the less this assumption holds true. Modern state space correction techniques synthesise a wide area correction model from multiple reference station observations within it, from which locally applicable correction terms can be calculated by the receiver.

If it is able to use the same satellites the plane can correct distances better because the signals pass through pretty much the same ionospheric effects as the reference location. You do not HAVE to use the same satellites. But, you can in principle get better results that way. {^_^}

They did turn it back on after the war, but that was one of the main reasons for turning it off permanently in 2000. One of my favourite pieces of historical irony.

Search terms might well include Magnavox. For increased accuracy look up Kalman filters. That is a trick for deriving better data by grading multiple sources for goodness and factoring that into the final solution. That was another lunch time discussion that resulted in table linen leaving with the customers. {^_-} {o.o}

I think another factor in disabling SA was the launch of the other systems such as Glonass and Galileo. The purpose of SA was so an adversary could not use GPS to guide a bomb to an accurate target. But if those adversaries have their own system, then there's no reason to degrade our own civilian usage. But they have tested a new type of SA which can be turned on or off for small regions and specific times in response to a specific threat.

SA was definitely turned on through about May of 2000 in the area where I live. It was easy to tell when the satellite clock was being messed with because it would suddenly make you appear to be up to 1/8 mile from where you thought you were a few seconds ago. SA made using GPS based road maps for navigating in cities annoying to say the least. Clinton signed the executive order to ‘permanently’ turn off SA in 1999, and it was done in 2000. It would be interesting to see what the economic boost has been to have a reasonably accurate civilian positioning system. I am sure the benefit is huge and outweighs the cost of the system many times over.

@@brianorca Do not confuse Selective Availability with Denial of Accuracy. They were two different animals. I touched on both. On SA I guessed which way NSA was going to jump for encrypting P-Code, disabling military precision. But, that was not enough. So they cooked up Denial of Accuracy, fuss with the base satellite frequency source in a manner that should be undetectable. They settled for "very hard to detect". The betraying signal artifacts were nominally buried in oscillator native noise by about 20 dB, a factor of 100. I designed that synthesizer concept. Another fellow did a bang up job designing the actual circuit more or less while I was designing and developing the RF half of the pre-launch test set with those 60-70 hour work weeks due to (dirty word alert) politics. We came in on time. ITT had their Nav Data Unit, NDU, ready about when the unpaid overtinme I had accrued proved the OT was not needed. SIGH. Basically the design theoretically permitted enabling either the SA or the DA only for partial orbits. I never heard anything about whether it could do that or some problem they''d missed popped up. And don't forget that the synthesizer allowed Phase 2B to run with an accuracy greater than the standards aboard, which is nice. Rb standards are wonderful for most casual uses but really not that good for something like GPS. And the GPS standard design flown (memory says 2 Rb and 1 Cs) was not as good as what exists on the ground. So the synthesizer gives a nice accuracy boost by fine tuning the onboard standards. Those birds are nice beasts. Um, and Rockwell International provided me with some nearly magical tools to work with. And Sam Costanza, our department manager, built the best group of people I ever worked with. Sadly he passed die to pancreatic cancer and the department slowly decayed. That's why I ended up "across town" at Magnavox in Torrance. {^_^}

Thanks Obama!

Redundancy, so that you still have one if the other fails. No doubt they eventually will have a single system that is a tenth the size, with both integrated into it, and with a quarter the power consumption. then will demand 2 as well.

Secondary radar does not require GPS - it will work just fine without so it will be kept (like ILS/VOR) do deal with situation of GPS failure. Some reason that the powers that be are restarting LORAN and eDME research.

CDMA is easy compared to QAM, that is still some kind of sorcery to me.

@@HappyBeezerStudios they didn't cover QAM in as much detail as CDMA in my uni subject. I learnt a lot more about it when I was a broadcast engineering trainee, since it's used in DVB-T transmission and satellite up/downlinks.

Ya gotta know your trig and complex math to get QAM! Digital trellis modulation works the same way :)

That's probably multipath! The signals from the satellites can bounce off things and take a longer path to the receiver, which makes the receiver think it's in a slightly different location than it actually is. That or there's someone spoofing signals nearby, though that's usually against the radio transmission rules in most countries. I'm not sure why they would put up a sign if they were illegally transmitting.

I suppose what must have been happening was that the mountains were not only blocking direct signals from satellites, but the detector could still pick up reflections bouncing off other mountainsides (and thus traveling a longer distance due to not following a straight line). This is very much analogous to echos.

@@Keldor314 Yes multipath reflections and diffraction off of the sharp edges of the mountain peak, which means the signal appears to be shifted in time with the reflection off the other side, and also takes longer with the diffraction, so the GPS receiver calculated position is wrong, but to the GPS it is still a valid position on the internal map it has of the area.

Where did you find this sign?

Lauterbrunnen Valley, if my memory is correct 46°35'18.8"N 7°54'29.5"E

We (at TI) had pretty much finished up the HDUE (High Dynamic User Equipment) and MANPACK (portable) GPS receivers at the end of 1978 ... I would have expected smaller units were being made by 1985, like the size of their LORAN receivers and the marine radios of that same era.

woah, what company/organization?

im talking about optical clocks based on doppler free spectroscopy, those havent launched yet but are being prepared to fly on the ISS @@mandellorian790

some national space agency in europe@@clayel1

interestingly enough, GPS time is not equivalent to UTC. for example, it doesn't include leap seconds, so it's like 17 seconds off of UTC at present

@@Octa9on Very true ! But IIRC the clock tick message sent by the GPS system has a "leap seconds" (as well as a century field just in case) field to indicate the offset ! It's probably very important for the thousands if not millions of events that occur between 23:59:59, 23:60:60 (the leap second) and 00:00:00 next day

Yes, I thought Scott might mention the derivative uses and the impact GPS has had in global cache coherence. The Spanner system is a brilliant example of this. For every OPS in the GPS constellation there are now m(/b)illions of correlated IOPS.

Biggest impovement was the patch antenna, which placed most of the receiver filtering at baseband into a single precise ceramic puck, that not only was the antenna with a wide acceptance angle, but also the front tuned circuits and selectivity, and with it being a very high Q filter to reject out of band noise as well. Got rid of half a rack of cavity filters and amplifiers, that you needed to keep at a constant temperature, and also a good chunk of power needed for all the amplifiers and temperature control.

Nowadays GPS antennas can be like 1/10th of your fingernail!

Normally we run with autotune on, so all 6 DME's are jumping around controlled by the FMS. And regardless of whether the first 2 are auto or manually tuned, they are used in the multilateration, if receiving a station. Also the GPS'es are running full time and mixed into the FMS position (as are the IRS'es, if installed). The FMS position uses all* of these sources simultaneously, but weighs the GPS info far more heavily. You can check all this stuff out by hitting MFD DATA, then MFD MENU and then line select NAV STATUS. It shows you on the MFD all the nav sources, each with yes/no being currently used, and the difference from the FMS position. GPS is always 0.0 or 0.1 NM, while the others are normally half a mile or more off. Since you're on the ground there's a good chance it won't be catching more than 1 (or even zero) DME's, so it might not be using that at all. For even more detail on that, line select VOR STATUS. * when I say "all," actually it only uses 1 GPS and 1 IRS at a time, I guess ready to switch to the other one if the first fails. Why not both simultaneously, I don't know.

I've often wondered how apps like Google maps know where I am so accurately. Is it GPS (and if so, have I got it for free?!), or is my phone just triangulating from the nearest masts? Turns out (if I understand your post correctly) that it's both - my phone triangulates, but the transmitters know where they are via GPS. Have I got that right?

that isn't true. Most base stations use an internal time base and will synchronise across the network to ensure clocks are synchronised.

That's not quite right. The cellular networks use GPS for time and frequency references, to help keep all the base stations in sync (which is necessary for modulations such as CDMA (3G) and OFDM (4G upwards). The phone itself takes its timing from the cellular network, and doesn't need GPS at all.

@@paulhaynes8045 The phone has GPS yes, but also the towers, because they have a fixed nearby location, also will send the ephemeris data for all the locally visible satellites to the phone on request, so the phone GPS can lock really fast, and it also uses the time difference between local towers to gat a very fast and somewhat accurate location if indoors, so that it can at least give a position to within 30m almost immediately, even if there is no sight of the sky for GPS signals. The GPS signal is used outside in your car, to track better, but rough location is done using the signal strength and timing of local towers. Did once turn on a phone GPS with no coverage from the network, and it took around 20 minutes to finally position me, within a 100m sphere, while another phone, with service and data, took under a minute to position, using data and the 2 visible towers to get a rough area to gain coarse position fast. GPS in your phone needs data, stand alone GPS can do without it, though your accuracy improves with time on, as it refines errors out long term, though you also get the map used providing some sort of sanity check, as there is an assumption if you are moving you are on a road, allowing it to remove parallel paths easily.

@@salerio61 Yes, but the local master stations will also use a Cesium or Rubidium clock, which is slaved with the GPS to provide an accurate local time and frequency, which will be shared by the local cells. But the local stations all will ultimately have a clock derived from a GPS receiver somewhere local, which is at a precisely mapped point, used as a reference marker for all timing in the area. Depends on cell density just how big an area is covered by each one, at least large enough that the coverage to the other GPS equipped stations is under the error budget, so that you can hand data traffic over to cells with different master clocks and not loose too many packets in the handshake.

I don't know how Caterpillar's system works, but if the two gps receivers can exchange phase data from the satellite signals they can calculate their relative orientation much more accurately than their absolute position

@@Octa9on I never worked with the system during my career there but I worked around people that did. I think you are probably right but I never learned the details.

If it was an old clunky Apollo GPS, I programmed them when I worked for II Morrow.

@@baboutheocelot778It was a Lowrance Airmap handheld unit. Literally about the size of a brick.

I was playing geostationary bingo too. Double fun.

I have been involved with CORS (Continually Operating Reference Station) and Network RTK for many years. With multiple constellations and improved processing algorithms, you don't need a base every few km's. Generally speaking, the accuracy of modern Survey RTK GNSS equipment is 8mm + 1ppm in the horizontal with vertical being 1.5 to 2 times that. So at 5km, the Hz accuracy is 13mm. Network RTK allows the modelling of Ionospheric, Tropospheric, Clock and Orbit errors over a wide area such that you don't need to be really close to the base to get the accuracy. Most CORS networks work on a 40km baseline between CORS. Improvements in algorithms has seen 80km, 100km baselines provide great results. Saying all of that, the GNSS Reciver and Antenna are critical to the accuracy. The Rover Antenna's ability to filter out unwanted signals, the ability to detect and reject multipath (reflected signals) and the algorithms used to compute Network RTK, are vital to accurate and reliable results.

​@@jasonspall2157 13mm accuracy... Based off signals coming from satellites in medium earth orbit. That's absolutely amazing, bonkers, wild, and a whole host of other superlatives. Mind = blown.

GPS signals are incredibly weak at ground level. So you either block them completely (can be done with a $200 box) or transmit more powerful signals with spoofed signals (can be done with a $20,000 box). This only works in a local rang of you transmitter, so hundreds of meters to 10s of kms depending on transmitter power. But it's very easy on a local level. Like say around an airport. Jamming on a large scale requires much more power or number of transmitters, since GPS signals are coming from space.

@@PsRohrbaugh Plus possession of such a jammer, or at least using one near an airport, will have a few government agencies find you, and have a very long and pointed talk to you. Ask the truck driver who did that near a US airport, because he wanted to jam the GPS tracker on his truck, so he could have some off time on the clock.

@@PsRohrbaugh Or put your jammer on an aircraft.

She was Austrian born. Her mother was Hungarian, but Hedy was born in Vienna and never lived in Hungary nor (as far as I know) spoke Hungarian (her mother, like most middle class Hungarians of the time, probably spoke German). Her father grew up in what is now Lviv, which was then in the Austrian part of the Austro-Hungarian Empire, but it was never part of Hungary.

@@paulhaynes8045 Yeah, but they didn't have GPS back then, so who knows? 😅

@@geirmyrvagnes8718 I'm not sure it would have helped! The political geography of that time was a nightmare. It's indicative that almost every city in that area (modern day Slovakia, Ukraine, Poland, Romania, Hungary, even parts of Austria) has at least three names in different languages, sometimes four! They might have known where they were, but they would all have had different opinions as to who or what they were!

@@paulhaynes8045 Yes, I was in the Balkans this summer talking to various people about what nationality Nikola Tesla was, and I figure the most correct is probably to call him Austrian, because that whole region is more of a mess than even the stereotype says... Beautiful area, lovely people, but... I wouldn't want to be a kid learning the last few hundred years of history. It is a lot to take in.

@@geirmyrvagnes8718 especially as everyone has their own version of that history - often nothing like that taught in the country next door! The current situation with Crimea is a classic example of this. My youngest is studying a poem at school and I was helping him with it the other day, when I suddenly realised that it was set in the Crimean War (as we in the UK call it). The Turks, the French and the Brits fighting the Russians over a place that didn't really belong to any of them. Try explaining that to a 14 year-old in 2023! And the lad in question (my son) is half Hungarian, so he is currently utterly confused as to why his mother's and father's views on the Russians invasion of Ukraine differ so much (his mother, although Hungarian, was actually born in what is now Ukraine but is pro-Russian!).

This ability is incredibly important given geopolitical issues. For example, I was using GPS + GLONAS for years but when Ukraine stuff started, my accuracy went WAY down. Like, hundreds of meters off. Turning off GLONAS fixed everything.

@@PsRohrbaugh Related to the Ukraine stuff starting. GPS was jammed in the Eastern Finland and couple airports had to cancel some passenger flights because instrument landings were impossible. VOR had been disabled years ago so the approach was GPS only. Weather and jammed gps ended up being a major problem.

This sounds like you were cheating!

Yeah, I can easily tell which side of a road I was on when logging runs on my Garmin Endura. Clever kit which seems to be getting more accurate as time goes on!

Yes, and this can be problematic in corner cases. I moved to another state, where I didn't have cell signal inside my house. Apparently Google had scanned my wifi at my old address, so when I went inside my phone would think it was at my old address. My phone could NOT get a GPS lock until it saw a cell tower, because it was trying to warm boot from another state. I ended up buying a new router the issue was so annoying.

This is what you get for the dumbification of all user interfaces. A proper GPS device allows the user to do a cold boot, but that would be too difficult to undestand, right? @@PsRohrbaugh

When you say "GPS controlled CNC milling machine", are you talking about the ITAR-controlled machines that lock out if they're moved? Or something else?

@@sentinel76 no, I am talking about a CNC milling machine that uses GPS receivers instead of positioning transducers or shaft encoders ;) Picture a GPS antenna on the base for the RTK signal, one on each end of the head, etc. Totally impractical, but it would be a fun project for Make Stuff Here or someone to make! Lol

hi laranja, what do you think about all gubments drawing a line around you(Antarctic Treaty) and saying you are not allowed to leave? If you don't know what I mean, read my about tab.

Do you know of a specific airport where that’s the case? I’ve yet to find one personally. I believe you lol I just haven’t seen it.

@@seanspoerhase3878LFPN has one for the 25R. 07L is 250ft above ground, which is also impressive. Both are DA. To be more specific, it's 200ft for cat A aircraft, otherwise, it's 220ft. For the 07L, it's 250ft for every aircraft.

@@seanspoerhase3878 Off the top of my head, KARG RNAV RWY 18 and 36 have 200ft mins. Same with several of the RNAV's into KOKC and KOUN

@@seanspoerhase3878very common in the US. KDLH RNAV 09 is the first example that comes to mind.