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Comment by blantonl | Hacker News Reader

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.

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Sea 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

Phones 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.

Cell towers have no relation whatsoever to any of the different types of differential GNSS

Most 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?

Not 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

AGPS refers to a specific set of technologies, MSB where the almanac and time seed is provided over the cellular network, and MSA where the unit sends GPS data to the AGPS server and the server solves for location using whatever data it would like (which can include a multitude of corrections).

I think what you're referring to with cell tower triangulation (and also WiFi location, etc.) is generally referred to as "hybrid location service" and is usually done at a higher level (ie - in software) using AGPS as one input to a sensor fusion algorithm.

The time to first fix is mainly impacted by whether the receiver knows which satellites to listen to. This requires two things: having the ephemeris for the constellation (either 10min of listening to GPS signal, or getting that through AGPS), and it's approximate own position/time (done through cell antenna locations, and NTP) to know which satellites should be visible.

Once this is available, the receiver can then focus on the signals that are expected and will then be able to provide a 3D fix as soon as it receives something from 4 satellites, which would be in a few seconds.

A-GPS can run in Mobile Station Assisted (MSA) mode, where the device sends raw GPS data to the A-GPS network and the assistance server processes it. In this situation, the assistance server can use whatever precise cell site location and ionospheric correction data it wants in its processing, although this is more of an "exercise left to the implementer" and I'm not sure sure MSA mode is even very common anymore.

I think was only a brief window where MSA was used, when phone processors were weak and cheaper to have limited GPS chip. Now, GPS chip do everything onboard and are included on radio processor or main processor. Plus, phone processors are powerful enough to do GPS themselves.

Not quite. Phones use assisted GPS, which means that they download GPS ephemeris data over the internet instead of getting it from the the satellite signal (because the satellites transmit data at 50 bps). So the time to first position is faster.

Differential GPS, by contrast, transmits the difference between the surveyed location of a base station and the GPS-derived position of the same station. That offset can be used locally to correct other nearby GPS receivers.

Phones also use trilateration on the cell tower signals to get an approximate solution before trying to solve 3d+time on the GPS signal.

It makes me unreasonably happy to see someone call it trilateration. Then again, you appear to know what you’re talking about.

They also use databases of know wifi hotspots as a way to bootstrap location when in difficult GPS environments like urban canyons.

And yet my uber driver still can't find my house.

> 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.

Easily maybe.

I can’t see any reason someone can’t take a stationary GPS enabled device and use that to broadcast the differential between received location and actual location. Probably could do it for less than $20 in equipment. Add multiple units and get triangulation for free.

Australia has a reasonable amount of yearly drift. It currently moves at about 6.8cm (2.7in) per year.

That is enough movement that it can throw off purely GPS solutions.

https://www.australiangeographic.com.au/topics/science-envir...

> ...

> But two important things have happened since then. Australia has moved about 1.6 metres northeast, effectively moving the location of mapped features and their associated GDA94 coordinates.

> At the same time, positioning technology has evolved considerably. By 2020, Australia will have moved by 1.8 metres and many of us will own devices that could pinpoint locations with accuracy as small as, well, a smartphone. With real-time access to precise satellite positioning at our fingertips, we’ll notice discrepancies with GDA94-mapped features.

> Imagine the confusion if a driverless car can determine its coordinates with (say) decimetre accuracy, in the GNSS datum, but the stored map features were referenced to our old datum GDA94 and and were nearly two metres different. That’s probably not even in the same lane.

As I understand it, RTK nulls that out. If your RTK base is fixed to the landmass that's moving, your rover positions will move right along with it. So a farmer with an RTK base stuck atop their silo or whatever should be fine.

Farms easily could. You can get GPS RTK kits that cost less than $1k and you could set that up at a central location, with wired power systems, etc.... it could calculate its own position down to the centimeter, and then you could use that to help provide correctional information to other clients.

SparkFun has some kits in this space, but you can easily pay up to $10k or more for a more commercialized system that is designed for surveying, or other related purposes.

Aren't there public RTK ground stations already covering most landmasses? I've used a few RTK receiver modules and always got centimetre level accuracy without having to buy an actual station. Was that actually SBAS data or what? I'd always assumed that SBAS was slow and only used by those geodesic receivers that get planted for an hour to refine their position down as much as possible.

If that's really true then what the was even the point of building Galileo HAS then? I swear these things could not be more confusing.

Yeah, my state's Department of Transportation runs a GBAS/CORS network and anyone can have access to it just by asking.

Trouble is, that (and Galileo HAS) assumes you have internet access. Maybe you're so far out in the middle of nowhere (This is Australia we're talking about in TFA) that the internet isn't available, or reliable.

So maybe you think satellite-based corrections are the way to go, but I concur, a local RTK reference would also work. (But 99.9% of the time, the satellite service is gonna be less hassle.)

Australia does have a public RTK network, which is free to access - you only have to ask. But you need internet access, and RTK only works is you are within 20km or so of the base station.

As other have said can do a local install. A pair of RTK receivers is only $500. But you need a connection between the base station and the tractor. These farms are pretty flat so perhaps perhaps technically it's not so difficult with LORA. But these are farmers, not hackers, and perhaps it has to talk to the tractor, and perhaps the tractor is made by John Deer. It all gets very hard very quickly.

In the mean time SBAS was a off the shelf solution the dealer could sell to everyone because it worked everywhere in the country. Until it didn't.

Provide local solution and let go of the sweet sweet ARR?

Specifically, 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-navigati...

For details: https://www.ga.gov.au/__data/assets/pdf_file/0011/123320/SBA...

Interesting 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.

We don’t want other countries weapons guided by our gps satellites!

This 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.

It doesn't matter - the speed of light in air varies by enough that no satellite can provide the needed accuracy alone.

Not 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.

It 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/

Multi-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.

The 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.

The 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

WAAS doesn't cover Australia.

Wasn'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.

FAA sure loves their TLAs. Some acronyms (or initialisms, for the pedants), like VOR, are TLAs inside TLAs!

You 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.

Typically these systems are broadcasting data obtained from many private regional reference stations on the ground. Look up “VRS RTK” as an example.

There 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.

Thank you, I was very confused as to why a single satellite failure would have had any impact.

Lol. I clicked "discuss" to make this exact point. Glad to see HN readers are up on which satellites do what.

Seems 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.

The 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.

It'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.

On /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.

I'm talking about computer vision using the visual landmarks for the fine tuning.

I 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.

Hey, I just got some land in N. California and I was wondering about building a robot like yours to work it. It looks like I might not be able to get up there personally for a bit (family obligations) so a tele-operated robot might be the way to go. (Also, I'm a hobbyist robot maker, so... yeah. Robots!)

- - - -

edit to add: I just saw your comment https://news.ycombinator.com/item?id=35637541 and yeah we are on the same page. Let's do this thing.

The land I just got is for a mutual-benefit non-profit corp I started (Ariadne Systems) to do pretty much exactly what you describe!

> ...you can ensure that everyone gets fed etc without paying for it by building an economy where everyone is part owner of the productive machinery they depend upon. Then no one is poor. Creating an economy where a preponderance of the firms are cooperatives is a good start. So this can be done in a traditionally libertarian way, without high taxes or strong government intervention.

The corp I started is just such a cooperative firm (well, it will be once I find some people to cooperate with, eh?) to co-own the productive machinery and ecologically-harmoniously agriculturally-productive farm land, and provide goods and services (and eventually homes) to the members.

That is so wonderful to hear! We’re working steadily towards a system that would be perfect for you! I suppose you have found this?

https://community.twistedfields.com/t/join-the-solar-farming...

Oh wow. We are soooooo on the same page. I'll be in touch. Probably be one of your first customers, eh?

Awesome! We would love to have your support on Open Collective in the meantime!

You got it. We just canceled Netflix and Hulu and I don't see why that money shouldn't be redirected to your project.

My desire is to get productive on the land (as in make some money) ASAP. Ideally it can pay for itself starting now.

Before I go off on a big ol' stream of consciousness brain dump, let me ask you, what about a scenario where you provide me with one of your kits (free or at cost maybe?) and I agree to give you, say, 15% of gross sales of produce raised with your machine? You would retain ownership of the machine, I would be responsible for maintenance and repair. Something like that? It just occurred to me so I haven't thought it through yet. (I can raise the question on your forums if you like? I just joined.)

Anyway, like I said, I want to make money on this land. I very much want it to pay for itself, I really want to avoid getting another programming job (now that the machines can talk and program themselves my heart isn't in it anymore.) And it would be fantastic if I can make enough money to fund all my grand ideas (including robots.)

I'm developing a plan (the purchase was kind of sudden) in roughly three phases:

I) collect and sell stuff laying around. Rocks, wood, etc. I am jealous of that matériel but it's a quick and easy way to make a few grand to fund the next stage.

II) SPIN (Small Plot INtensive) market gardens. ( https://spinfarming.com/ ) If i give over an acre or two to SPIN plots and figure out how to reach markets I should be able to make $50k-$100k per year easily.

Neither of the above are regenerative. The whole point is to fund:

III) Food forest. I figure you're hip to that already, but I want to share a video I just saw last night "From a Dry Field to a Lush Food Forest in 18 Months" https://www.youtube.com/watch?v=icJx01VuV6I So there it is: if you have the resources you can do that in 18 months. (I have many of the species he mentions already in my seed box! Now I have room to germinate them! Que mad scientist laugh. I have about four hundred species of seeds ready to go.)

Food forests not only produce, uh, produce, they also grow the seeds and cuttings etc. to make more food forest. (I've had people ask if regenerative ag is scalable. Yes, it is scalable. Life scales. That's like it's whole deal. That and being gorgeous!)

Long-term I want to have a botanical garden, a tropical conservatory, ponds and water systems etc... It's gonna be great!

> You got it. We just canceled Netflix and Hulu and I don't see why that money shouldn't be redirected to your project.

I would love that thank you so much!

This is all great stuff to post on the forum! It is hard for me to engage with everyone about the project because I am doing so much stuff, but I also want other people to see the enthusiasm our project generates.

> what about a scenario where you provide me with one of your kits (free or at cost maybe?) and I agree to give you, say, 15% of gross sales of produce raised with your machine?

Thank you for the suggestion. Our project is moving slowly. It will be some time before we are able to ship kits, and even when we do the plan is to ship them without tools. Its going to be a long but worthwhile road to profitable farming with these machines.

I love everything you're saying tho. Welcome to the community!

Alrighty, I'm a "lentil" now! Sweet!

Wonderful, thank you so much!!

Cheers! Frankly, I see it as extremely cheap R&D, eh? A very good deal. ;)

Hey Taylor, I joined the community and posted an introductory, uh, post, but I seem to be in a "pending moderator approval" limbo. I know you're busy but maybe you could clear the flag, or whatever, to let me participate?

( s/que/cue/ It bother me.)

s/bother/bothers/ Bloddy hell.

s/Bloddy/Bloody/

It's been that kind of a wekk.

Visual surveying within and over Australia is hard (source: a few decades of Australian geophysical surveying practice) .. wide area ground shots from 80m altitude (low altitude aircraft) look extremely similar across large areas making feature picking for location improvement difficult.

In an agricultural setting Western Australian grain fields are hundreds of acres in size across relatively flat landscapes .. it's a sea of waving grain to the horizon in all directions from tractor height.

While there are locations that have distinct visual features it's no universal panecea and won't offer the cm accuracy desired.

How will you handle this case (which while not common exists). Farmer goes in and buries irrigation pipes with underground emitters - these pipes are expected to last for 5 year or so (I don't know what the actual lifespan is - I know they are replaced every few years, but I don't know the actual schedule). Each season the farmer smooths the grounds, and plants each row exactly above the pipes in the ground.

Can your visual navigation system figure out where those pipes are each year to within 2 cm? visual geographic features are going to change a lot over the years - the local ground is smoothed out every year, trees (if any) will grow or die, fence posts will be replaced...

Assuming equal spacing, you only need landmarks (survey spikes) on the first row. Then use the implement impressions from the first pass. But I see your point.

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