GPS locations over CAN

[Aaron Ault]

I talked with the precision ag field guy at our dealership today about Jesse's question on whether GPS receivers send their data over the CAN bus. He installs and services all the precision ag systems for two of the stores in the company. Our dealership is Case, but he works with customers who have multiple brands.

I asked if the GPS receivers he deals with send their GPS data via the CAN bus or an alternate route. He said that all the ones he works with send their GPS locations via the CAN bus, although some can choose between the tractor or implement bus. I have a 15-year-old light bar that does not send things over the CAN bus, but everything I've bought since then uses the CAN bus. One item of note is that most of the GPS receivers do not get their RTK correction data via CAN, but they do send the final corrected locations via CAN.

As a side note, we've sold 8 Android tablets this year for the express purpose of using the tablet's data connection to send RTK correction data to the GPS receiver over Bluetooth. This is the first year such a setup has been available, and they were only expecting to sell 2. It looks like there is likely going to be a lot of mobile devices in tractor cabs in the coming few years precisely to eliminate the expensive modems.

Aaron

[dkhackney]

Here are the geolocation messages on the ISO 11783 CAN bus. 

There are a couple of old SAE J1939 messages that are defined for location information. These may or may not be populated, depending on the what device is creating the messages. 

For ISO 11783, both SAE and ISO punted the geolocation data to the National Marine Electronics Association (NMEA), so you must also purchase the NMEA standards documents to get the handful of lat/lon messages needed. The relevant standard is NMEA 2000.

Note that the NMEA 129029 message is the high-precision location data used in precision guidance and required if any GPS/GLONASS posiston augmentation is used, e.g. SBAS, RTK, etc.

As the NMEA 129029 message is > 8 bytes, it requires NMEA 2000 FastPacket protocol support.

****************************************************************************

J1939 – source: J1939 – 71 Vehicle Application Layer

*********************************************************************************************************

NMEA 2000 – source: Appendix B.1 Subset Report - Navigation PGNs NMEA Network Messages

Note: COG = Course Over Ground; SOG = Speed Over Ground; both are terms used in maritime navigation related to the course and speed of the boat over the bottom of the body of water, e.g. lake, river, ocean.

[Aaron Ault]

As for on-board GPS as per Jesse's suggestion, I think it makes sense to have an option for the Grower Gary's of the world which may be using this for telematics on all their tractors, including small ones which don't have GPS already, or on any existing setups which don't send their GPS over the ISOBUS.  I see 3 routes we could go here: 

- a low-quality GPS board with an integrated antenna and optional external antenna:  Pros: inexpensive ($20-$50), easy to find, can eliminate need for RF design, makes ISOBlue "Plug-and-play."  Cons: not much better than cell phone GPS.  I use the uBlox GPS board from DIYDrones ($75: http://store.diydrones.com/3DR_GPS_LEA_6_p/br-3drlea-6.htm) for my aerial drones and it seems to work reasonably well for flying a plane.

- a high-quality GPS chip with external antenna: Pros: high-quality positions, possibility for free RTK if someone wants to implement it.  Cons: expensive (~$1,500), harder to find, could involve RF design.  Last year we looked into finding a chip that reports phase measurements so we could do some inexpensive RTK by programming up the algorithm ourselves and using INDOT's free RTK network.  Some cursory searching led to a few solutions that were all over $1,500, which kind of defeated the purpose of cheap RTK.  Maybe a renewed search could find something more reasonable.

- a serial/USB interface to any standard serial/USB GPS device: Pros: quality can be determined when purchasing the other device, much faster development time for ISOBlue, no RF design or custom hardware interfacing, no added cost for ISOBlue.  Cons: extra cables that can jiggle loose, less "Plug-and-play" than an integrated solution, requires support for wide array of possible devices.

Given the three options, I like the third one best (serial/USB to standard external GPS), at least for version 1 of ISOBlue.  It has a high probability of success and takes zero development time.  I'd like to hear how Jesse thinks that would work for his business, however.

Aaron

[dkhackney]

Aaron, 

Where did you find a chip that supported RTK for $1,500? 

I haven't found much in that price range with those capabilities. 

Doug

[Jesse Vollmar]

I'd be fine with option 3. The bigger drawback there would be reading the data off of whatever gps gets plugged in. 

I think there are certainly middle of the road GPS chips around $200 that would work great. 

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[Aaron Ault]

Doug,

I looked back through my emails from a few years ago to find exact numbers on GPS chips.  Turns out we found some things less than $1,500.  I guess my memory isn't what it used to be.  At the time, I found the NovAtel 628 with a quick Google search, and priced it at around $1,000 in quantity 1  from autonomoustuff.com. The guy at autonomoustuff suggested a cheaper solution, the OEMSTAR-1Hz ($180) or the OEMSTAR-10Hz ($430) that only come in quantity 25.  That was over a year ago, however, so many things have probably happened on that front in the mean time, and we didn't spend too long looking, either.

We were looking at the time to build a nice little pocket-sized box with bluetooth that could give your phone RTK-level accuracy for under $100 because I would love to have something like that.  I thought it would be fun to auto steer a lawnmower or ATV with my phone for under $500.  If I had a $100 RTK box, a phone, a controller capable of filtering and inertial navigation (like the APM2.5 from DIYDrones for $175) and about $200 for servos and any custom boards, I'd have auto steer for around $500 plus the phone.  We have a free RTK network in Indiana from the Department of Transportation that works great for RTK on my farm, so there wouldn't be any cost for RTK corrections.  Since just the uninterfaced, antenna-less GPS receiver was going to start at $180 for only 1Hz, we had to by 25 of them to get that price, and we didn't have any funding at the time, we ended up moving on to other things.

Our plan was to figure out how to code up our own RTK implementation starting with this open source project: http://www.rtklib.com/.  The theory at the time was that all we'd need was a chip that would report raw pseudoranges and carrier phase measurements.  An L1/L2 receiver would be an added bonus because then it would be simpler to solve the unknown integer problem as well.  Since we never actually built the box, it remains a theory.

Aaron

[dkhackney]

Aaron, 

That low-cost RTK scenario is very intriguing. 

I may be able to recruit some volunteer resources to do some research on this. 

I'll let you know if I am successful in my efforts. 

Doug