7 ms·
Note that this isn't a GPS failure, it's a failure with Inmarsat's Satellite-based Augmentation Systems (SBAS), which supplements GPS data with signal measureme
by blantonl 3y ago
Note that this isn't a GPS failure, it's a failure with Inmarsat's Satellite-based Augmentation Systems (SBAS), which supplements GPS data with signal measurement errors corrections. This allows for centimeter level positioning.
- causi 3y agoInteresting that whatever technology underlies the SBAS system isn't just integrated into GPS. It would be nice to be able to map property lines with just my phone.
- kylehotchkiss 3y agoWe don’t want other countries weapons guided by our gps satellites!
- LeifCarrotson 3y agoThis isn't 1993 anymore. Beidou and Glonass (and Galileo) are global networks that do the same things for those other countries, and the ITAR restrictions on GPS receiver altitude and velocity are increasingly able to be overcome by a hobbyist in a garage, much less a nation-state.
- bluGill 3y agoIt doesn't matter - the speed of light in air varies by enough that no satellite can provide the needed accuracy alone.
- fsh 3y agoNot quite. Geodetic GNSS systems use two different carrier frequencies to measure and correct for atmospheric dispersion. This reduces the error to sub-mm levels. The inaccuracy of real-time GNSS is due to unknown variations of the satellite orbits, satellite clocks, and earth rotation.
- MikeDelta 3y agoIt seems that GPS signals can be used for higher accuracy if the receiving equipment uses two frequencies instead of one. This can freely be done by civilians, but requires more expensive equipment. Perhaps this solution with SBAS requires a cheaper setup? https://www.gps.gov/systems/gps/performance/accuracy/ https://www.gps.gov/systems/gps/performance/accuracy/
- nradov 3y agoMulti-frequency GPS is already available in many inexpensive civilian devices such as smart watches and cell phones. It improves accuracy a little, primarily when the device doesn't have a clear view of the sky like when next to buildings. But it's still not as precise as SBAS.
- michaelt 3y agoThe highest precision GPS receivers require dual-frequency reception and perform "carrier differential GPS" - any time people are talking about GPS with 2cm accuracy, that's what they're talking about. The GPS signal has a digital signal (with a wavelength of ~300m) modulated with a sine wave (with a wavelength of ~20cm) - you can measure the 20cm signal much more precisely than the 300m signal, but it's 'ambiguous' because it repeats every 20cm and you don't know how many waves of the sine have passed. If you can receive two frequencies, giving you two slightly different sine waves, you can multiply them together and get a sine wave with a longer period, which (together with some clever maths) lets you bridge the gap between the unambiguous-but-imprecise digital signal and the precise-but-ambiguous sine wave. You also need a fixed base station at a known location - or access to a commercial network of such stations.
- AlotOfReading 3y agoThe US government does provide an SBAS called the WAAS, operated by the FAA. It's also had a hand in designing and operating a number of other augmentation systems [1], including CORS, GDGPS, and NDGPS. As an aside, don't you just love how many government acronyms there are? [1] https://www.gps.gov/systems/augmentations https://www.gps.gov/systems/augmentations
- tesseract 3y agoWAAS doesn't cover Australia.
- AlotOfReading 3y agoWasn't saying it did. MSAS (Japan) can be used in AU if you accept the performance degradation, but the lack of a public SBAS for the southern hemisphere is a big reason why SouthPAN is being brought online.
- ericpauley 3y agoFAA sure loves their TLAs. Some acronyms (or initialisms, for the pedants), like VOR, are TLAs inside TLAs!
- Waterluvian 3y agoTypically these systems are broadcasting data obtained from many private regional reference stations on the ground. Look up “VRS RTK” as an example.
- sumtechguy 3y agoYou can use the cell towers to assist in mapping it correctly too as they are fixed position and the location is well known. Pretty sure the QCOM patents on it are out of patent by now. Not sure if the android/ios api will expose the whole list of towers the chip can see though. The chip can do it though.
- bradknowles 3y agoThere are GPS RTK systems designed for use by surveyors. They can cost over $10k, but you can also get much cheaper kits that you can put together yourself from SparkFun -- like less than $1k, easy.
- outworlder 3y agoThank you, I was very confused as to why a single satellite failure would have had any impact.
- cjensen 3y agoSea ports used to broadcast a local correction signal[1] back when GPS was intentionally less accurate. Seems like an oversight that farms don't have their own correctors. [1] https://en.wikipedia.org/wiki/Differential_GPS https://en.wikipedia.org/wiki/Differential_GPS
- Analemma_ 3y agoPhones and other devices with cellular radios still use differential GPS by way of the cell towers (which have known, fixed positions) to do those corrections. You can notice this because, when you don't have a cell signal, your phone is considerably slower to get a GPS lock and less accurate when it does. Presumably these farms are out in the middle of nowhere and so that's why they can't do the same thing and need a satellite-based augmentation instead.
- fortysixdegrees 3y agoCell towers have no relation whatsoever to any of the different types of differential GNSS
- 747fulloftapes 3y agoMost modern cellphones use AGPS, assisted GPS, to reduce the time to get their first fix. Basically, AGPS primes the GPS receiver with an accurate time of day and the GPS almanac (satellite ephemeris data). That's rather different from Differential GPS, which is basically corrections used to compensate for fluctuating conditions in the atmosphere. I may be wrong, but I don't know of any system for cell-tower derived corrections being redistributed out as GNSS corrections. Could you clarify the specific system you described?
- fortysixdegrees 3y agoNot so much. Modern GPS will get a first fix pretty quickly regardless, you don't need to seed it. AGPS is used when satellite coverage is poor, like urban canyons in a city, also a situation where there are a lot of cell towers and triangulation is somewhat useful
- dingosity 3y agoLol. I clicked "discuss" to make this exact point. Glad to see HN readers are up on which satellites do what.
- giantg2 3y agoSeems one could use GPS for less granular positioning and then use visual landmarks like the soil/crop edge from the prior pass or row locations to align for the next pass.
- bluGill 3y agoThe point is to be accurate to within 2cm across the whole field, for the entire year. You cannot do that with landmarks and human driving.
- Groxx 3y agoIt's probably something that pays for itself reasonably quickly too. Minor human wobbling means more difficulty and damage when harvesting, probably a wasted row or three per field, etc. Add that up over a decade and many, MANY acres of fields and it's probably a lot of money. Precision enables a lot of savings.
- HeyLaughingBoy 3y agoOn /r/farming yesterday someone was showing how he uses John Deere's AIG (Active Implement Guidance) to come back after seeds have germinated and basically put "pockets" in the ground to allow water to pool around the seedlings. The accuracy that it requires to do that without damaging the plants is amazing.
- giantg2 3y agoI'm talking about computer vision using the visual landmarks for the fine tuning.
- TaylorAlexander 3y agoI am designing an autonomous farming robot. You're not wrong that this is possible, except in a large field, crop rows would move over time if you were using them as a visual reference, and it could be non-obvious which row you are in from visual features alone. The bigger problem is that computer vision is hard and expensive, while doing differential GPS is less expensive and much easier. I made our robot fully autonomous for development using only differential GPS, and we are working now on adding a vision system. A couple of GPS receivers and some very simple code is just easier to manage.
- 747fulloftapes 3y agoSpecifically, it is the AU/NZ SouthPAN SBAS that's delivered via Inmarsat. They use PRN 122 for the corrections. https://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-australia/about-the-program/southpan https://www.ga.gov.au/scientific-topics/positioning-navigati... For details: https://www.ga.gov.au/__data/assets/pdf_file/0011/123320/SBAS-STN-0001_02_SDD-OS.pdf https://www.ga.gov.au/__data/assets/pdf_file/0011/123320/SBA...