It's amazing how far we've come in a few decades - the article says they started testing the CDMA system on the ground in New Mexico less than 50 years ago. 20 years ago "Garmin" was a noun that meant "fancy mapping/navigation device". Today you have a global map in your pocket, you can buy $10 modules to use the constellation, and 1:24000 scale geospatial data is free for the asking from the USGS.
One of my favorite parts of the Woodford/Nakamura report mentioned in the article is that when they list the ideal requirements a new system should satisfy, they say that receivers should be cheap and portable.
Their threshold for cheap and portable was "Less than 100 pounds, less than $100,000". Definitely shows how far we've come.
> I wonder how we'll be navigating 50 years from now.
Hopefully using OpenStreetMap :D
To think you can download a 48.1 GB PBF-compressed file [1] and get a map of just about every trail and built structure that exists* for free and store it on an SD card the size of a fingernail - it just blows my mind.
* There is better coverage in Western countries unfortunately
Yes, OpenStreetMaps is an excellent resource. But again, at least in the United States, a lot of that information is actually sourced from the USGS which puts a lot of effort into providing accurate geographic information for free. You've been able to get "USGS quad sheets", PDFs, and database files with that information for quite some time.
Maybe that sort of extensive government support has something to do with the better coverage in some Western nations...and in practice, OSM doesn't provide a fundamentally different experience from other mapping/navigation devices/apps.
If you want to learn more about what makes GPS tick, here’s a home made receiver that was build from the ground up. A stunning project with an incredibly detail writeup.
One of the things that really caught me by surprise is that the GPS signal is way too weak to be detected out of the noise in isolation. It’s only possible to receiver it if you know what to expect:
“All GPS satellites transmit on the same frequency, 1575.42 MHz, using direct sequence spread spectrum (DSSS). The L1 carrier is spread over a 2 MHz bandwidth and its strength at the Earth's surface is -130 dBm. Thermal noise power in the same bandwidth is -111 dBm, so a GPS signal at the receiving antenna is ~ 20 dB below the noise floor. That any of the signals present, superimposed one on another and buried in noise, are recoverable after bi-level quantisation seems counter-intuitive! I wrote a simulation to convince myself.
GPS relies on the correlation properties of pseudo-random sequences called Gold Codes to separate signals from noise and each other.”
What's even more amazing, is that differential GPS works at all, given that it relies on measuring the timing of the underlying carrier signal. (As opposed to the pseudo code.)
It takes the advantages of digital signaling, and throws them out, instead measuring the analog carrier waves directly. That this can be done at such low signal levels is unbelievably impressive.
(The statistical methods involved in getting an initial signal lock are nearly as impressive.)
And the whole thing was a 1980s hack for land surveyors to get better than 100m accuracy while systematically eliminating uncertainty due to Selective Availability (SA).
It's a hack that improved the overall accuracy of the system from ~100m to sub ~1cm, and eventually sub ~1mm accuracy, when used with some other methods.
Fun fact: in 1992 a group of political activists broke into a cleanroom containing GPS satellites and started smashing them with axes. They called the event "The Harriet Tubman-Sarah Connor Brigade."
>They used wood-splitting axes to break into two clean rooms containing nine satellites being built for the U.S. government. Lumsdaine took his axe to one of the satellites, hitting it over 60 times.
>They were arrested and faced up to 10 years in prison for destroying federal government property, causing an estimated $2 million in damage.
That seems to be a pretty minimal amount of damage, considering the total cost per satellite.
Also, the article gets some of the technical details quite wrong. For example:
>In 2000, Selective Availability was disabled and from that point on, anyone with a GPS receiver could get location data as precise as the data used for military and missile navigation.
This isn't true.
1) A GPS receiver that works above 60000ft or at speeds beyond 1000mph is still considered a military munition. You can't buy one and you can't leave the country with one, even if it only uses the civilian signal.
2) Selective Availability essentially distorted the timing of the L1 (C/A) (coarse / aquisition) unencrypted civilian signal, resulting in location errors up to ~100m greater than the nominal accuracy of ~16ft. It was the government essentially spoofing it's own signal at varied amounts to introduce uncertainty.
Turning off SA had nothing to do with the encrypted military (P(Y)) signals.
The encrypted (P(Y)) signals (on both L1 & L2) are still very much encrypted, and clearance is still required to obtain a compatible receiver, at significant cost. In practice augmented (agps) and differential GPS technologies produce higher accuracy and precision than the P code alone would, at less cost and hassle.
The new block satellites coming online add some signals for greater accuracy, but the civilian / military signal segregation and encryption still exists.
Sad to learn that Dr. James Spilker has passed away this year. I worked with him at his company Stanford Telecommunications from 1983 until 1992. He was quite a character back then and was an avid body building enthusiast. Sometimes he'd come to work all buttered up with tanning lotion.
GPS is one of the most amazing technical onions I've ever dealt with. I've just been scratching the surface of it with understanding positioning for a robotics project. I kind of wish I had an opportunity to do more paid work with it. But it seems to be the realm of PHDs.
One of StarLink's biggest overlooked opportunities I think is radically re-engineering a GPS like system that can work indoors and outdoors as well as offering some pose estimation capability. It would make implementing robots way easier.
Well, this is embarrassing. MvB's title says 100 episodes but he has completed 18 as of this time. I had only gotten to Ep. 15 so I didn't twig to the work-to-go until just now.
I'll update my original and check out the Stanford course as well.
The technical side alone is unbelievably interesting, but when viewed in the context of military mapping/ navigation history and cold war geodetic / geospacial IBCM targeting, it goes all the way down the rabbit hole.
I took an undergrad course in GPS at Colorado School of Mines and enjoyed it immensely.
We didn't work on the radio level (other than learning about the 2-frequency ionosphere delay measurement as a calculation input), but did the rest of the stack above that including working out programs to derive position from given data input. I was a civil engineer but I think it's a great cross-discipline subject for a lot of fields.
You would need to get a facing direction. Most robots are doing that with a magnetometer, gyros and an extended Kalman filter. This has quite a few disadvantages though. First and foremost the magnetometer which is really your source of truth is unreliable. It wanders constantly and frequently your Kalman filter will start up in a bad state.
With 3 or 4 channels you could use the GPS almanac to get satellite direction and your heading. Though you could do it with two if you could get two antennas a couple of feet apart. But smaller robots don't have that kind of room.
Right now I think the biggest pain point with GPS is that if I want single redundancy GPS with 2 CM precision and true heading I need a whole bunch of stuff. 2x 200$ GPS receivers (ublox f9p) on my robot. 1x 200$ receiver with a decent antenna and a xbee radio to act as a RTK base station. And this was just for a simple outdoor rover. And the base station always had to go in the same place because you have to do everything relative to the base station. Then you need all the software to coordinate all that and manage everything.
I want 1 receiver unit, global absolute position within 2 CM with a true heading. Additionally, I want seamless transitioning/augmentation with a global network of terrestrial beacons for interiors and areas with poor surface coverage. And I want all that in a 100$ish module and some sort of subscription.
Have you looked into using gyros for north-finding? There's been a lot of progress in recent years with MEMS gyros (which normally have too much zero-drift to work with traditional gyro north-finding techniques).
That's a pretty neat technique. It seems superficially similar to how mems magnetometers work. The size and oscillation make me question how robust that is though.
One stop gap solution I has was just an IR beacon on my GPS base station. I got an initial alignment with the rover camera and my two gps points. But ended up adding a second gps to the rover.
It came as an unpleasant shock to Saddam's Iraq, because it allowed US armored forces to navigate accurately through the desert and drive him out of Kuwait.
The NAVSTAR GPS constellation was incomplete at the time, so accurate positioning was only available for 19 hours a day. There weren't enough mil-spec receivers to go around, so some units relied on commercial receivers that parents had bought their sons and mailed them to them.
I was out of the USAF by this time, so I have no direct knowledge. But I imagine they did a risk assessment over the possibility of the Iraqi forces gaining access to GPS, either because they bought commercial receivers, developed their own, or got loaned some from more-capable country who would have liked to have seen the US + coalition fail.
They didn't "turn it off" as much as set it very very low, maybe even to 0. But even then, military grade receivers still enjoyed 1-2 magnitudes of improved accuracy.
Clinton turned it off after surveyors figured out differential GPS, which essentially made it obsolete.
https://www.usgs.gov/core-science-systems/national-geospatia...
I wonder how we'll be navigating 50 years from now.