This weeks’ EYE on NPI is more like EYE to the SKY, as we feature u-blox’s low cost GNSS RTK eval/application boards. Digi-Key has plenty of stock of u-blox ZED-F9P eval boards, which are a fully-featured RTK application boards with a GNSS RTK module, WiFi module, Arduino-compatible headers, multiple power options including LiPoly charging, and easy-to-use configuration software. The price is under $250, bringing a multi-node RTK setup within many engineer’s budgets
RTK is a pretty interesting technology that has developed over the last few years, but even we had to do some research to understand it. GNSS stands for Global Navigation Satelite System, which for most people is synonymous with GPS – the American GNSS. There are some other, newer, GNSS’s like GLONASS (Russian) and Galileo (EU).
Now, GPS and GNSS receivers are super common and inexpensive, you can pick up a fully-integrated GPS module like the SAM-M8Q for about $13, it even comes with an tuned path antenna. These little receivers work very well for outdoor location identification, and they work anywhere in the world. There is one drawback that folks bump into when using GNSS – and that is the precision of location. Even if you are in the middle of a field, and you’ve got a perfect view of the clear sky, GNSS accuracy is around 3 meters / 10 feet. Which is pretty good for a $13 module, and for general navigation and asset tracking, 10 feet is a fair tradeoff.
For some applications, like autonomous cars or agricultural equipment, surveying, or drones – 10 feet won’t cut it. If the location is jumping around, it could cause the machinery to veer off course, smashing into a building or cliff or human! For these high-tech use cases, we need to boost the GPS accuracy and precision, which we can do by adding RTK.
RTK stands for Real Time Kinematics, a way of precisely determining location and orientation … in real time! How it works is pretty amazing. We already know how GNSS works – by getting timecoded signals from 3 or more satellites that circle the globe and comparing the time code deltas, we can triangulate where were are approximately on earth. Now say we have two GNSS receivers somewhat close to each other, say within a mile. Each one is going to receive pretty similar signals, and will see the same satellites. The receivers can then perform calculations on the phase of the same signal they received, and compare the differences between them. The differences will be incredibly slight, and the measurement requires good quality sensing, because we’re measuring the signal of a 1.5GHz transmission from literally space. That precision sensing is why the price of RTK has historically been in the thousands of $ to get a setup, especially since you need one base station and one+ rovers. Thanks to u-blox’s engineering and market skill, they’ve gotten a full development kit that can be used as a standalone base station or rover for under $250!
There’s some things to keep in mind when using RTK GNSS. First up, like GNSS in general, does not work indoors so this is for outdoor use, with good sky visibility. Second, you need to send that precision timing data from the base to all rovers, and you have to do it pretty fast: 38.4 kbaud minimum and 460.8k recommended for high update rates. So you have to consider how you are going to send that much data, in the local area. u-blox has app notes on the topic:
2.4 GHz technologies such Classic Bluetooth® and Wi-Fi are good for relatively short range applications1. They have the RF throughput and low latency capacity as long as any user data does not load the data link to the detriment of the RTCM stream requirements.
Wi-Fi at 2.4 GHz offers good range with a 2.4 GHz single-band antenna due to better gain than a dual band 2.4/5 GHz Wi-Fi antenna. If the UDP protocol is used over the link, it offers the lowest latency and reduced data flow over the link.
Legacy low power RF devices are usually not suited due to their limited TX buffer size and real time link data transfer capacity.
Longer ranges will require RF links with greater RF transmit power/ lower frequency, but will need to have the same throughput and latency requirement if running the ZED-F9P at 5 Hz navigation rate. In most countries longer communication ranges will require the use of RF links in licensed bands, however other countries do allow much higher TX power on 2.4 GHz and 900 MHz bands than Europe for example.
Considering that the minimum baud rate is 38400 baud for a 1 Hz navigation rate application and 460800 baud is recommended for a 5 Hz navigation rate application this will usually limit what radio technology can be used as the gross data rate will need to be considerably higher to transfer data forwards and backwards.
Bluetooth LE and Bluetooth 5 are not ideal technologies for the RF link between base and rover considering required throughput and range. Consider these very carefully along with practical testing before implementing these as the RF solution.
But, if you’re aware of how to best use RTK, there’s some amazing use cases that have been waiting for this kind of precision orientation and tracking capability. So, if you’ve ever wanted to play with RTK, there’s no better application board to start with, and you can pick one up today at Digi-Key, just search for 672-1110-ND on digikey.com
Don’t forget you’ll need two boards (unless there happens to be a public RTK base station somewhere nearby you). Then, once you’re ready to integrate the ZED-F9P into your development platform, you can pick modules up at Digi-Key as well!