GLONASS – Deep South Robotics

Rover 2 Photo Essay

Roby — Tue, 02 May 2017 13:05:06 +0000

Welcome back guys!

I think the last post touched a nerve — it seems that many of you guys have had difficulty breaking into the precision GPS market. If you’re not yet inspired to take the precision RTK GPS plunge, just wait — we’re going to have some fun in upcoming posts pitting the ComNav K501G in an Epic Showdown with some of the competitors we mentioned last time. I don’t know about you, but I can hardly wait.

Rover 2

We now present the Rover 2 build as a photo essay:

Notice that we’ve labelled the main components to give you a high-level overview of what’s where. Let’s start digging into the build by working our way from the base up to the top…

Two on/off switches (like these) — one dedicated to the drivetrain (Sabertooth 2×32), the other dedicated to everything else.

Right up front, my eager sons, we need to have an honest discussion about on/off switches. So go ahead and strike up the fireplace, grab a cup of coffee, come over here and jump up close while ‘ole Dad Roby has a heart-to-heart discussion with ya’ about the temptations you will soon face. If you’ve built a robot before, you’ve likely experienced the brutal temptation that I must now warn you about…

Of course we’re talking about the temptation to mount an on/off switch in a non-standard posture.

Young sons, heed my words: OFF MUST BE DOWN. I really don’t care about which way you make ON, but you MUST make the down position of the switch correspond to OFF.

The only exception to this whatsoever is if what you’re turning ON/OFF doesn’t matter. For instance, if the switch controls your secret Justin Bieber loudspeaker, then you may get away with it. Otherwise, please dear sons, don’t attempt to retrain an entire lifetime’s worth of muscle memory just because you think a sideways switch will look cool, or because it will fit more cleanly in your enclosure.

If you refuse to make OFF correspond to down, and if one day things go wrong and you’re not able to instantly turn off your robot via muscle memory, then you’ll deserve the bruises you’ll receive. I pray it’s only bruises, my son.

Notice that’s a Raspberry Pi 3 in the black case sitting on top of the center case. Pixhawk autopilot is in the enclosure below the black case.

Cover pulled off of Pixhawk case reveals Pixhawk sitting all alone in center enclosure.

Power box contains Sabertooth 2×32 controller, terminal blocks, 24v-to-12v DC-DC converter, 12v-to-5v DC-DC converter, relays, and USB power supply. I know what you’re thinking — those wires look out of control. Agreed.

That long ribbon cable on the left snaking out of the Raspberry Pi is going up to the Pi camera.

Confession time: I can’t stop buying these waterproof enclosures or these waterproof cable glands. An orange ear plug fills in the air gap on a big cable gland.

Astute readers will note that a Raspberry Pi 3 has WiFi built in. So what’s up with the ethernet cable? My experience is that a single onboard router with devices connected to it via ethernet is a preferable solution.

Notice the small slot cut in the side of the Pi case. This allows us to easily remove the case without removing the video ribbon (and fiddling with the Pi’s regretfully brittle plastic camera ribbon lock).

Another picture to show the little slot cut in the side of the Pi case, enabling us to remove the case without pulling the video ribbon. Little tweaks like this will make you much happier after removing the case 100x.

View of the Raspberry Pi 3 with lid removed.

Notice that the Pixhawk is mounted on a vibration reducing mount within the enclosure. The Pixhawk’s gyros and accels would like to say “thank you”.

Can you guess why the Pi has a distinctive sheen?

Did you guess? It’s conformal coating! I like to leave the Pi exposed for testing — the reason for this is that it seems you’re always plugging in another cable or pin to the Pi, and enclosing it inside a case creates future time drains. Here we have a problem, though: what happens when the unexpected April shower pops up in the middle of your mission? If your Pi’s unprotected, you may have just lost $35. Our answer to this situation is a good coating of MG Chemicals conformal coating.

The coating glows when you illuminate it with a UV light, revealing how much coverage you achieved. Man, stuff that glows is timelessly cool.

Tip: insert the video ribbon cable, the HDMI cable, and the USB cable BEFORE applying conformal coating. Otherwise the coating has a sneaky tendency to get inside those connectors. You will have nightmares getting a good connection if you let this happen. Yes, I learned the hard way.

They put warnings all over this stuff like it’s pure Uranium. Probably best not to consume.

Conformal coating coverage test in process.

If Industrial Velcro ever ceases production I will quit robotics. Velcro, please don’t make me back that statement up. Seriously though, guys, think about how much it hurt to let go of that fav pair of Superman shoes at age thirteen eight with the Velcro straps. And here you’ve spent all these years thinking that you couldn’t look forward to Velcro again ’till you hit 75 and went shoe shopping at Wal-Mart. Velcro, we missed you. Welcome back.

Well, who do we have here? I see a Pixhawk up there at the top and that’s the power box over there on the left. Now, what is that little card at the bottom….

It’s the much hyped ComNav K501G L1/L2 GPS/GLONASS 10hz RTK receiver card!! You know, the card I talked so much about last time that Mrs. Roby started acting weird. You’ll observe that I make notes on the card about baud rates and port assignments — this method proves more reliable than my memory.

Robot building pop quiz: what in the world is that red arrow pointing at?

Do you know the answer?

It’s a Gore vent! Technically, this vent is a knock-off that you can buy here or here. If you’ve never read field scientists rave about Gore vents, you’re in for a treat. You’re likely familiar with GORE-TEX waterproof jackets, shoes, gloves, etc. It turns out that the company behind GORE-TEX is GORE Industries, and they also make GORE vents. These guys fired the Product Name Consultant 30 years ago and never looked back. GORE products are based off this fascinating synthetic material that’s somehow impermeable to water molecules, but it allows air to pass through. So, for the vents, the result is that the enclosure can breath, but it’s waterproof. Breathability is important because pressure inside the enclosure is equalized, greatly reducing stress on the waterproof seal and increasing the seal’s longevity.

Notice the dual nuts anchoring the center enclosure underneath the platform. This dual-nut trick allows us to level up the Pixhawk by adjusting the nuts. You’ll observe that both plastic enclosures pictured are floating on anti-vibration bushings. You’ll also note the blue ethernet cable winding out of the GNSS receiver’s enclosure on the left. If you’re not a network person, you may not have known that ethernet cables are just 4 pair of copper/aluminum wires — they’re often the perfect hack for low amperage power and signal wires.

Moving up from the base to the center section you see, starting at the top/back, the wireless router (running dd-wrt as a repeater bridge), the Odroid-C2 (barely visible inside the center case), and the GoPro camera case (like this) modified to hold a Pi Camera.

That mass of wires in the enclosure above is mostly related to the 6 ultrasonic sensors on the Rover. A little Arduino (impossible to see in this picture) is sitting in the upper right corner, processing all the readings from the sensors and feeding obstacle information back down to the Raspberry Pi. We also found room in that box for a 3DR telemetry radio that’s dedicated to receiving RTK corrections.

Here we see an Odroid C2 sitting all alone in the center enclosure. Note that the Odroid doesn’t have WiFi built in (in contrast the Pi 3 has WiFi) so having the onboard router is already paying off. The Odroid is an amazingly powerful little computer that we’ll have much more to say about in upcoming posts. We’re currently running an armbian distro of linux (highly recommended) on the oDroid. The oDroid is there to take care of heavy lifting; for instance, the code generating missions is a bit of resource-heavy java that the oDroid smoothly runs in a Tomcat Servlet container.

Here you can see the front ultrasonic sensor array mounted to a sheet of Lexan polycarbonate. Notice that the sensors on the left and right are not intended for outdoor use while the sensor in the middle is like the sensors you’ve seen on the rear bumper of vehicles for the past 15 years. The indoor ultrasonic sensors have always outperformed the waterproof ones in my experience.

If you look closely at the camera’s mount over on the right hand side, you’ll notice that we thread nylon cable ties through the rubber bushings, under the metal platform, and back up through a tiny hole beside each rubber bushing. This little trick serves the dual purpose of securing our camera and enabling us to make precision leveling adjustments by tightening the cable ties. If you’re looking to spend much time building robots, you won’t regret the decision to purchase a variety pack of cable ties.

I’ve got to give the TP-LINK router credit, it’s been a reliable wireless bridge and I’m quite sure the manufacturer didn’t intend for it to run around on a robot.

If you look closely at the camera case, you’ll see my hot glue crush springing up all around. The truth is, I could let go of Velcro before letting go of hot glue — but the thought of losing either knots up a strong man’s gut.

All of the enclosures are floating on some form of vibration dampening bushing. If you run a rover long enough off-road, you’ll likely develop similar habits.

Here’s a view of the rear of the Pi camera enclosure. There may be a good, standard weather-resistant pi camera enclosure nowadays, but I couldn’t find one when building this rover. These GoPro enclosures are readily available, their tilt is easily adjustable, and they will only set you back ~ $6USD.

Here you see the compass enclosure (holding a HMC5883L compass) mounted on top, as far away as possible from magnetic friends who would have a bad influence on her. If you look over at the left side of the compass enclosure, you can barely make out an arrow pointing North. I’ve never regretted making any little note like that which mitigated future confusion when the inevitable day comes that you will remove the component.

Every sensor will eventually be removed, replaced, changed, upgraded, etc. We need to push on this a little more: EVERY COMPONENT WILL BE REMOVED. You will not be eternally happy with the location you chose for any component. You will eventually add more sensors, add more servos, add more radios, add more antennas, add another computer, then wake up at 2 a.m. and realize your whole arrangement is sub-optimal.

When you are building your robot, always think about making your future self happy when you rearrange every component. The Good Lord, it seems, did not grant unto us roboticists the ability to perfectly place any part.

We’ve identified these components on top before, but let’s repeat: L1/L2 GNSS antenna (upper left), compass (lower left), 915mhz waterproof antenna (lower right) for 3DR radio. Here are links for purchasing the 900 and 433 version of that antenna (different frequencies are allowed in different countries, you likely know) — I think you’ll be impressed with the antenna’s build quality, especially for ~$8USD.

Wait a second, what kind of potting compound are we using to insulate the compass/magnetometer?

Hot glue, of course! Now, let’s be clear: hot glue is not an appropriate long-term electronic potting compound. If you’re wanting a permanent potting compound, you’ll want to check out something like the MG Chemicals offering, or even the random China stuff.

FAIR WARNING: the entire field of potting and potting compounds is wonderfully fascinating! If you’re not careful, your family won’t see you again tonight while you’re lost somewhere on the internet in a geek-binge of potting compound reviews and information.

If you look at the magnetometer’s clear lid above, you can almost make out another hack we pulled: Aluminum foil inside the lid (secured in place by a huge glob of hot glue, of course) to reflect sunlight. The objective of all this insulating is to shield the compass from sudden changes in temperature which could seriously degrade it’s performance.

One more thing: notice that we use our nylon cable tie threading trick to secure and level the compass enclosure. It’s very important both to mount a compass level relative to earth on your rover, and, later on when calibrating your autopilot, it’s important to calibrate the “level” reading on your autopilot to be true relative to earth. Look, I know if you’re new to this stuff that the last part of that sentence doesn’t mean much, but later on we’ll talk a lot more about this.

It’s worth keeping in mind, if you’re the kind of person who likes to keep random useful robot knowledge floating around in your mind, that a magnetometer’s performance, if used alone, severely degrades when you introduce much roll or pitch. Fortunately for us, however, we’ll be using the ArduPilot software which uses the Pixhawk’s accels and gyros, along with slick and impressive math, to give good heading readings even in steep roll/pitch situations.

One last note: rubber grommets will keep sharp metal corners from cutting into your sensitive wires. As an added bonus, I think the black grommets contrast favorably on the silver aluminum body

Wrapping Up

Man that was a big set of pictures! Hey, if anything wasn’t clear, just shoot a question in the comments and we’ll try to clarify.

I have a feeling that next time we’re going to stir up a little more GPS excitement.

Until we meet again,

Sincerely yours,

Roby

Spilling the Beans on RTK

Roby — Thu, 20 Apr 2017 12:57:41 +0000

June 29, 2019 Update — The RTK GNSS space has improved massively in the past 2 years. Not surprisingly, u-blox is leading the charge with outstanding products that are finally bringing robust centimeter-level GNSS to a vast untapped set of new applications, industries and possibilities. Please see The Taming of the u-blox ZED-F9P or Piecing Together an Auto Mowing Rig for current (as of mid 2019) info.

If you’re unfamiliar with GNSS technology terms, you’ll want to read Tuesday’s post to bring you up to speed for today’s discussion. A key goal at the outset of this blog was to cut through sales jargon and technical jargon and give the ambitious roboticist the knowledge needed to build high quality autopilot systems.

Let’s jump right into the details and begin drawing some lines in the sand. Line in the sand #1:

There are 2 kinds of RTK GNSS Systems¹:

L1/L2 GPS/GLONASS
Everything else

Question: if you’re a farmer/surveyor, do you agree with the above statement? If you’re not a farmer or surveyor, or if you haven’t devoted hundreds of hours to RTK GNSS experimentation, you likely didn’t know it was that simple.

How Many Satellites Can You See?

Now, you likely know that the number of GNSS satellites visible varies wildly primarily on your longitude/latitude and secondarily on time.

Consider these screenshots (taken from this slick site) of the satellites (with an elevation mask of 10°) visible on April 19, 2017 at 8 representative cities across our planet:

GNSS Satellite Coverage Beijing China

GNSS Satellite Coverage Melbourne Australia

GNSS Satellite Coverage Madrid Spain

GNSS Satellite Coverage Berlin Germany

GNSS Satellite Coverage Natal Brazil

GNSS Satellite Coverage Surgut Russia

GNSS Satellite Coverage Wales Alaska

GNSS Satellite Coverage Robertsdale Alabama

If you don’t care to click on all 8 of the pictures above, here’s a chart summarizing the results:

Everyone gets GPS & GLONASS love: BeiDou is nice in the East.

The numbers above are fairly typical of what I’ve observed historically. While the number of satellites visible is the single best vector available to give you an idea of your chances of achieving a robust RTK solution, you have to know there are other significant caveats. Let’s illustrate with a couple of highly technical graphs:

GPS and GLONASS satellites do not perform identically

Individual satellites within a constellation do not perform identically

If you doubt the graphs above, just review the wikipedia list of GPS satellites and compare the specs of the oldest operational GPS satellite (USA-132 launched 1997) with the most recent GPS satellite (USA-266 launched 2016).

Some satellites are more reliable than others, sometimes a satellite gives an invalid reading. A good GPS receiver makes it’s money by quietly taking care of all this (and a lot more) and kindly giving you ultra-precise location readings several times a second.

For 20 years the big boys (Trimble, Novatel, Leica, etc) have been shooting fish in a barrel

Please Get To The Point

We’ve barely scratched the surface of an introduction to RTK GPS technology. It’s wonderfully fascinating stuff that’s quite complicated. It turns out that over the past 20 or so years, a handful of manufacturers (Trimble, Novatel and Leica are the big ones) have figured out RTK L1/L2 and have managed to keep prices of the components (primarily the receiver boards and the antennas) incredibly high for the small quantity customer. When I say “incredibly high”, common prices I’ve heard for an L1/L2 GPS/GLONASS board are ~$3000 and L1/L2 antenna prices are typically ~$1000. Yeah, you read that right, and consider this: for an RTK setup, you’ll need 2 receivers and 2 antennas. Good luck building that setup with components from the big boys for much less than $10k.

Here’s another frustrating part: it’s like pulling teeth to get one of these guys to give you a price for their stuff — it’s almost like they’re uninterested in doing business with anyone who is not an S&P 500 manufacturer. Do you think I’m overstating this odd lack of price information? Here’s a challenge: find 1 URL where either Trimble or Novatel or Leica lists the price ANY L1/L2 GPS/GLONASS receiver or antenna and allows you to purchase said component for said price. Go ahead, find their prices and post them to the comment section.

Eventually it becomes apparent who graduated magnu cum laude from the De Beers School of Diamond Pricing. Fortunately, in the last few years, with the robotics revolution taking off and taking over, increased demand for high-precision & robust RTK has wrought wonders in the market. Let us now cover the important players since I’ve never seen one article covering hardly any of the information we’re now disclosing:

Swift Navigation

My Piksi set: for sale, make offer (don’t do it!)

Please see Swift Navigation update below (May 3, 2017)

If you google “rtk gps” and you don’t have AdBlock (does anyone actually not use Chrome with AdBlock?), you’ll likely see Swift Navigation sponsored near the top of results. Of course, what you’ve likely never seen is any kind of independent analysis of their RTK performance compared to a reputable system.

Back in 2015 I knew very little about GPS and made a huge blunder by purchasing their Piksi system. I should have heeded the warnings from users in their official forum, but I was itching for really accurate GPS, and no-one had written this guide we’re now presenting. Back then Swift employees would occasionally respond to complaints in the forum with some notion of wonderfully robust GPS upcoming in a software update. Of course, this was before I realized the near gospel truth that all future GNSS technological promises are maddeningly fickle.

For a long time I was bitter about wasting $1000 on a totally inept RTK system. But I have to give the Swift guys credit: they were one of the first companies trying to bring high precision GPS to individuals. I assume they employ smart people who were tasked with re-inventing the wheel. Trying to make L1 GPS RTK robust (on really limited hardware, at that) is probably a fool’s errand. Recently they’ve released a new L1/L2 GPS-only receiver that they market as “Hardware-ready for GLONASS…” Interpretation: GPS-only.

RTKLIB / Emlid

RTKLIB is a set of several open-source applications written by Tomoji Takasu that perform several RTK functions including generating an RTK position from 2 sets of raw GNSS observations (rover and base/corrections).

Emlid is a impressive Russian startup with really smart, focused people who have a history of delivering. Emlid, not having millions of VC funding to burn like Swift Navigation, realized that they could take a strong cheap processor (Intel Edison), couple it with a proven L1 GPS/GLONASS receiver capable of omitting raw data (uBlox M8T) and run RTKLIB to provide reasonable RTK results (I haven’t personally tested their product, but their community seems pleased) at a previously unheard-for-RTK price ($570 base/rover kit with antennas).

u-blox

u-blox is a super-solid big Swiss company that makes the de facto standard L1 non-RTK GNSS receiver for drones. For ~$20USD you get an absurdly robust GPS with accuracy around 2 meters. When I say robust, I mean this: I can take a u-blox M8N receiver with a cheap antenna/ground plane inside my house (single story asphalt shingle roof) and if you overlay the results with google maps, it will generally approximate your location WITHIN THE HOUSE! If you guys are inclined to doubt this (I would be if I hadn’t tried it), I may have to recreate this test in a future write-up.

Recently, as u-blox saw an RTK market spring up using their M8T/M8N products with RTKLIB, they decided to jump in on the action. So, u-blox has rolled out their version of an L1 RTK receiver called the M8P. I have not yet used an M8P, but considering u-blox’s history of delivering unquestionably robust products, I assume the M8P is the most solid L1 RTK receiver selling in the hundred-dollar range.

If you think about u-blox and EMLID next to each-other, it’s hard to escape this idea: u-blox should acquire EMLID and dominate the L1 RTK market. u-blox is a reliable company with tons of cred and EMLID is a rags-to-riches startup with a quickly-increasing resume of accomplishments. When the average drone builder thinks of u-blox, they think of a great faceless product. EMLID is similar, but with a twist: in the drone community, EMLID has a face and they have serious goodwill. If the big guys at u-blox can iron out the geo-political nuances, an acquisition seems like a wonderful idea.

BDStar / Harxon / Unicorecomm

If you’re heavily involved with RTK GNSS, you likely know about BDStar; otherwise, you’ve never heard of them. BDStar is a Chinese technology conglomerate that owns several companies that make proven products. Their two companies that matter for this discussion are Harxon (L1/L2 antennas) and Unicore (L1/L2 receivers).

Let’s start with what I haven’t personally used: Unicore L1/L2 receivers. A little story may be in order: back in my early days of RTK GNSS, I had trouble getting quotes for L1/L2 GNSS receivers from Unicore — considering that this mirrored my experience with the North American guys, I shelved communication with them and moved on. In the last few months, however, they’ve been more up front and I’ve got actual quotes for L1/L2 receivers. Specifically, the price I’ve seen for their UB352 (GPS/GLONASS L1/L2 RTK) board is between $450 and $600, depending on order quantity. All anecdotal evidence leads me to believe that Unicore makes a solid board on the level of Comnav/Novatel/Trimble. If you’re new to RTK, that price may seem expensive. If you’ve been on the RTK market for a few years, you realize that this price is an order of magnitude below where it was just a few years ago. The times they are a-changin.

Moving back to products I’ve personally used: Harxon antennas. Harxon has a well-earned solid reputation in the L1/L2 GNSS antenna community. They make a great antenna and they stand behind their product. You should be able to pick up a L1/L2 GPS/GLONASS antenna for around $200 — bump up the order quantity and you’ll get that price lower, of course.

Perhaps you’ll spare me the time to tell you a personal story about Harxon. I use one of their antennas (specifically the HX-GS481A) on one of our GNSS RTK base stations. At one point the antenna quit working: I’m inclined to believe it was a lightning strike because we get a lot of lightning hits, and the location of the antenna wasn’t shielded from a strike. I contacted Tilen, a sales rep at Harxon, and was really up front — explained the antenna quit working and that it was likely my fault. I really just hoped they would offer to do an inspection on the antenna and give me feedback so I could make future improvements. He promptly replied to my email, said to ship the antenna back to them, and they’d take a look. I shipped the antenna to China, and within 2 weeks they had delivered a brand new replacement to my front door. To wrap up the story, I haven’t heard if they’ve confirmed that it was a lightning strike, but the fact that they, at considerable expense, quickly shipped a replacement to me on an issue that was likely my fault was eye opening. I have never heard anyone have any other problem with a Harxon antenna, and I’ve only heard good reviews about doing business with them. If you’re looking to source proven L1/L2 GPS/GLONASS antennas from a company who’ll stand behind their products, you should seriously consider contacting Tilen at Harxon (sales@harxon.com).

In our little foray into bringing that which is hidden into the light, we’ve saved the best for last: ComNav Technology / SinoGNSS. ComNav is a proven Chinese L1/L2 GNSS receiver manufacturer that operates under the SinoGNSS brand in the North American market. I’ll start by giving you the Cliffs Notes: ComNav’s K501g receiver is an absurdly good board at a market-changing price ($600-$900 last year in small quantity, unsure of current prices) that effectively solves RTK GNSS for off-road precision navigation applications.

Another personal story is in order. Regular readers know that I spent a decade writing software at NASA before venturing back home to the deep south and starting up a robotics and software shop. When I began working with outdoor navigation (you know, in the real world where trees, buildings, and powerlines exist), it quickly became clear that the L1 RTK systems marketed at that time were a joke. So, in a quest to get a reliable system, I started with the North American guys, with limited success mentioned above. Upon hearing about ComNav, I contacted their sales department and a guy named Andy quickly responded and without much wrangling, gave me a quote on L1/L2 GPS/GLONASS sample base/rover system for development (K501g receivers, antennas, wires, dev boards, USB adapters, etc). We’re talking about over $2k worth of components. With much trepidation I wired the money to China, and to my great relief, within a few days the system showed up at my front door. Not long after that, I had configured the base & rover feeding into a Pixhawk autopilot, and the RTK results were seriously solid and reliable. If you’ve never seen one of these systems in action, it’s precision and resilience, even in tough conditions (trees, obstructions, etc), is something to behold. ComNav/SinoGNSS backed up every promise they made. You don’t have to be a S&P 500 company to get a fair quote from them — and they won’t harass you or pressure you to buy their products. If you’re looking to build the kind of system we’re building, you should consider dropping a line to Andy at ComNav (sales@comnavtech.com).

May 24, 2017 update: Check out the new post describing K501G configuration

Wrapping Up

I knew when I started this blog that I’d have to write this article. I don’t like criticizing anyone and would have preferred to write without mentioning any companies in anything less than a flattering light. But here’s the rub with that approach: if this blog motivates you to get really involved in path-following outdoor robotics, you’ll soon want a strong performing RTK GPS. For whatever reason, I have not found ONE ARTICLE ANYWHERE ON THE ENTIRE INTERNET explaining what is presented above. Without this knowledge, you’re at the mercy of advertisers and salesmen — of course this is not the position you want to be in.

Our goal in this humble blog is to shine light on every important piece of knowledge you need to build a high quality, large-scale navigation robot. Today we took an important step: we began pulling back the veil of pricing secrecy and marketing jargon that has kept individual roboticists and small robotics companies from using world-class GPS technology.

Next time let’s get back to the Rover 2 build.

Sincerely yours,

Roby

Notes

1. Roby can’t overemphasize that he has no personal experience testing RTK systems outside the southeast USA — BDS looks very promising, Galileo may be good. The future of Galileo, as a European Union project, feels like a proxy bet on the future of the European Union — considering the events of the last few years (Greece/Brexit), it’s success may not be a foregone conclusion.

Swift Navigation Update (May 3, 2017)

Swift Navigation contacted me the day this article was published and offered a complete refund for the Piksi purchase. Additionally they apologized for my experience with the Piksi. This was a super classy move, especially considering that we were really tough on them in this article. I want to state again: the engineers at Swift are attempting to solve a huge problem (of course, precision positioning is already solved by Novatel, Trimble, etc — but Swift seems unique in acknowledging that small-time robot builders exist). If Swift is able to build a product that can hang with ComNav or Novatel, I will likely write so many articles extolling their praises that you guys will get sick of reading about them.

RTK Autopilot Upgrade (part 1)

Roby — Tue, 18 Apr 2017 18:56:23 +0000

Welcome back fellow roboticists! I trust you guys had a great weekend. Over the weekend I decided to supercharge the roll-out speed for this project. So, without further delay, we present Rover 2 RTK Autopilot Upgrade:

Rover 2 RTK

Whoa, last time we talked we just had a wheelchair base with a humble metal plate as a platform! Hey, this upgrade is my idea of an exciting weekend. Let’s look into it a little more…

Rover 2 RTK Top Front

Rover 2 RTK Top Right

Rover 2 RTK Top Back

Rover 2 RTK Top Left

So now that you’ve seen all 4 side of the top, let’s break down what we’re looking at.

Dual Frequency GNSS

Prominently featured in the pictures above is a big beautiful L1/L2 GPS/GLONASS antenna. If you’re familiar with this technology, you realize that this project just got a lot more expensive. If you’re not familiar with this technology, you’re probably wondering why I just starting dropping acronym bombs on you. Let’s try to clarify these technologies concisely:

GNSS — Global Navigation Satellite System

Generic term for satellites orbiting earth that can provide ridiculously good location data. You get street cred for saying “GNSS” instead of “GPS”.

GPS — Global Positioning System

Satellite “constellation” owned by USA. While people often refer to all GNSS constellations generically as “GPS”, technically GPS refers to the American satellites. Other major constellations include GLONASS (Russia — very solid in USA), BDS (China — limited coverage in USA), and Galileo (EU — some coverage in USA).

RTK — Real Time Kinematic

Technology that enables our GNSS receiver to provide sub-inch (centimeter) accuracy. Yes, you read that right, the GPS knows where it’s at ON THE FACE OF PLANET EARTH TO WITHIN LESS THAN 1 INCH! Since we’re going to build a big rover capable of running missions like mowing grass or applying fertilizer, this technology is a pivotal enabling technology for what we will accomplish. Technically, RTK is accomplished by a software algorithm that uses the raw data from the GNSS receiver and another fixed source (known as a “Base Station” or a “Correction Source”). RTK is an entirely fascinating technology and we’ll have much more to say about it in upcoming posts.

L1/L2/L5 — GNSS Signal Frequencies

Different frequencies that GNSS codes are transmitted on. Click the L1/L2/L5 link for a technical explanation. Here’s a nontechnical explanation for what L1/L2 GNSS means for us:

Much more robust RTK
Very expensive (think orders of magnitude — but becoming less expensive daily — more on this next post)

So, if I were to summarize some useful GNSS knowledge for the novice, it would be this:

GPS: AWESOME
GLONASS: AWESOME
RTK (via L1/L2 GPS/GLONASS): AWESOME
Galileo: often promises to be awesome in a few years — I can’t speak to their current status
BDS is promising if you live under their satellites’ orbits (China, Australia, NOT USA)
Promises about future GNSS features are curiously fickle
GNSS SMEs are farmers and surveyors. If you want to know the gospel on GNSS, just hit a precision agriculture forum or surveying forum. Ignore this advice and buy an L1-only GPS-only RTK system you see advertised and you may regret it.

Wrapping Up

Perhaps the most important (and least answered) question about L1/L2 RTK GNSS equipment is this: how much does it cost? I’ll leave you in suspense…’till next time!

Sincerely,

Roby