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Centimeter Precision GPS/GNSS - RTK Explained
RTK in 5 minutes
To achieve centimeter accuracy with GPS, you will need a GPS unit with “RTK” functionality: your phone or your pet tracker GPS will not work.
Once you have a RTK GPS receiver, there are 3 ways to make it measure points with centimeter precision:
Most countries have RTK networks for surveyors, sometimes free of charge. Some users also share their own base stations online for free. Alternatively, you can subscribe to a paid NTRIP service. If an RTK network is available in your area, this is usually the easiest option.
If you want to be completely independent from services, you can decide to install your own base station. This will allow you to travel with the rover within a 35km range from it.
Can you leave your device static for 1 hour? Then you don’t need correction data. Record your GPS data in the field and improve its accuracy later. Centimeter-level accuracy is calculated later, after you return to the office and process your data with a PPK service.
RTK in detail: understanding centimeter precision GPS
Think about the GPS receiver in your mobile phone or in your car: these are what is called Standard GPS (also called GNSS*) receiver. Standard GNSS receivers available in the market today, have an error between 1 and 10 meters, and only work in outdoor conditions. This error depends on the quality of the producer of the GPS receiver, and how much is the view of the sky blocked by any object or building. Under perfect conditions, the best you can achieve is an absolute accuracy of around 1 meter.
*GNSS stands for Global Navigation Satellite Systems, and it’s a word to include not only GPS but also other satellite based positioning systems like GLONASS, BeiDou, Galileo, QZSS…
Interestingly, if we place 2 GNSS receivers right next to each other in an open field, both will have the same 1 meter of error, and in the same direction! This error is known as the standard GNSS error.
Imagine now that we know with centimeter accuracy in which location is one of the GNSS receivers. We can then calculate the position of the second unit, with only 1 centimeter of error!
But this technique is not new. This is a very old technology called Differential GNSS or RTK GNSS (Real-Time Kinematic). For more than 20 years it has allowed civil applications to have 1 centimeter accuracy (RTK position). In RTK wording, the static unit is called “Base Station” and the moving unit is called “Rover”.
So what is new? I also heard it’s a very expensive technology, and that you need to receive a training to use it right?”
“Not anymore :)”
At ArduSimple we try to make GNSS RTK technology simple, affordable and accessible to everyone. With a series of starter kits, tutorials and OEM products, we allow our customers to:
- Test the technology and believe it is possible to achieve centimeter-level precision at affordable cost.
- Accelerate their projects and products, thanks to our plug and play development boards and starter kits.
- Create products and solutions, thanks to our support, turnkey services and pre-configured OEM products.
“So how can I start?”
The RTK receiver (aka rover)
simpleRTK2B – Basic Starter Kit
The most popular multiband RTK system in the market since 2018. Ideal for your first GPS project with reliable centimeter precision. With u-blox ZED-F9P and ANN-MB antenna.
The RTK base
RTK corrections are the information sent by the “Base Station” to the “Rover”, so the “Rover” can calculate its position with centimeter level. The language/protocol used to send this information is called RTCM (Radio Technical Commission for Maritime Services). Sometimes you will read about RTK corrections and RTCM corrections, it’s basically the same. In order to calculate this RTK position, both receivers need to see more or less the same satellites. For this reason the RTCM corrections are only valid up to a distance of 35km from the Base Station (some people say 25km, some people 50km, the exact number will depend on the antenna and receiver quality). It is also useful to know, that the corrections from one base station can be used by as many rovers as possible, as long as they are within this 35km range from the Base Station. This would be a multirover setup.
What happens when you go beyond this 35km from the base station? That it’s not possible to have 1 centimeter accuracy anymore, and accuracy degrades very fast to standard 2 meters.
Sending RTK corrections
RTCM/RTK corrections are sent usually one time per second. And can be sent via a wired serial communication, a wireless link or an IP network. Let’s check the differences more in detail.
Wired
For units which are very close to each other, we can use cables to connect them. A serial port (TTL, RS232) is enough.
Wireless
- Up to 10 meters between base and rover, you can do with 2x Basic Starter Kits.The GNSS antennas included in the kit have 5 meter cables. Additionally you can extend this distance to up to 20 meters (10 meters each side) by adding our low loss RF cables.
- Up to few hundred meters, 1x Medium Range (MR) kit. This kit only allows one rover for each base station.
- Up to few kilometers, 1x Long Range (LR) kit with multirover capability.
- Up to 50 kilometers, 1x eXtra Long Range (XLR) kit also with multirover capability.
Medium Range Radio – RTK Starter Kit
Preconfigured and ready to use RTK Base and RTK Rover kit including radio link up to 1 kilometers.
Long Range Radio – RTK Starter Kit
Preconfigured and ready to use RTK Base and RTK Rover kit including radio link up to 10 kilometers.
Extra Long Range Radio – RTK Starter Kit
Preconfigured and ready to use RTK Base and RTK Rover kit including radio link up to 50 kilometers.
Through an IP network
In many occasions, an IP network is available (WiFi, Ethernet, 4G). For this cases, a protocol has been developed distribute RTK corrections, called NTRIP (Networked Transport of RTCM via Internet Protocol). We will have a look at this into more detail in the next section.
What is NTRIP?
- NTRIP caster. It’s a server that collects RTK corrections from one or several base stations, and distributes them among Rovers.
- NTRIP server. It’s a single physical RTK base station. It’s role is to send RTK corrections to a NTRIP Caster, so the Caster can distribute them to other clients. In networks with only one base station, it’s quite common to have an NTRIP server+caster together. In this case only the caster function is configured.
- NTRIP client. An RTK receiver that wants to receive RTK corrections. RTK Rovers are NTRIP clients.
Our simpleRTK2B boards don’t have IP interfaces (ethernet, wifi, etc) and don’t understand NTRIP protocol. They need someone outside to convert from NTRIP to RTCM and viceversa. For this reason we developed a series of accessories to enable NTRIP in our boards:
- Bluetooth: connect your simpleRTK2B board to your mobile phone via bluetooth. Your phone can now act as NTRIP client (but also as server or caster).
Bluetooth module
Add Bluetooth to your board. Compatible with all ArduSimple boards with onboard XBee socket.
BT+BLE Bridge
Add Bluetooth to your board. Compatible with all ArduSimple boards with onboard XBee socket.
- WiFi: with a WiFi network available in your area, plug the WiFi NTRIP Master accessory and use your board as NTRIP client (but also as server or even server+caster).
WiFi NTRIP Master
Add WiFi functionality without programming, just with a few configuration clicks. Compatible with all ArduSimple boards with onboard XBee socket.
- 4G NTRIP Client: let your simpleRTK2B board become an NTRIP client anywhere where there is cell coverage.
4G NTRIP Master
Add 4G/3G/2G functionality without programming, just with a few configuration clicks. Compatible with all ArduSimple boards with onboard XBee socket.
RTK correction services
Post-processing (PPK)
Can you leave your device static for 1 hour? Then you don’t need correction data, base station or internet connection in the field. Your receiver records the raw positioning data while you work, but the final high-accuracy position is calculated later. Once you are back in the office, your recorded GPS data is processed together with correction data from a PPK service (like Postprocessing Master), giving you centimeter-level accuracy. Unlike RTK, PPK does not provide centimeter-level accuracy in real time.
simpleRTK 4 Optimum – Basic Starter Kit
Experience allband GPS/GNSS technology from u-blox with this kit including the latest u-blox ZED-X20P chip together with high ANN-MB2 antenna.
simpleRTK3B – Basic Starter Kit
Experience millimeter GPS precision with Septentrio Mosaic-X5 high precision RTK module. Get 6 millimeter position resolution on your PC, phone or favorite development platform.
Other positioning technologies
Now that you know how RTK and PPK can provide centimeter-level accuracy, you may also come across other GNSS positioning technologies. These can provide lower accuracy, but are often simpler.
Before looking at them, one useful term to understand is CEP (Circular Error Probable). CEP is a statistical way of describing positioning accuracy. For example, 1 cm CEP means that at least 50% of the calculated positions are expected to be within 1 centimeter of the true position. It does not mean that every measurement will be accurate to exactly 1 centimeter.
The table below compares RTK and PPK with other common positioning technologies, from centimeter-level to meter-level accuracy.
| Accuracy Level | Technologies & Beginner Explanation | Recommended kits |
|---|---|---|
| 1 centimeter | RTK (Real-Time Kinematic): Gives centimeter-level accuracy in real time using a local base station or internet corrections (NTRIP). The base and rover should be within 35 km of each other. | All ArduSimple RTK Development Kits Starting from 211,00€ |
| PPK (Post-Processing): Similar accuracy to RTK, but the precise position is calculated after the survey on a computer. No internet or real-time connection is needed in the field. The rover can be farther from the base (up to 100 km), but accuracy generally decreases as the distance increases. | All ArduSimple RTK Development Kits (using raw data recording) Starting from 211,00€ | |
| <30 centimeters | Galileo HAS: Free service that delivers correction data from Galileo satellites to your receiver via internet or satellite signal. May require enabling a specific band on your receiver. | Not all GNSS receivers support Galileo HAS. We recommend simpleRTK 4 Optimum – Basic Starter Kit Starting from 299,00€ |
| <90 centimeters | SBAS: Free service that delivers correction data over standard GNSS, completely out of the box, without requiring subscriptions or internet access. | Not all receivers support SBAS. We recommend simpleRTK2B – Basic Starter Kit Starting from 211,00€ |
| <1.5 meters | Advanced Multi-Band Standard GNSS: Achieved using multi-band technology without any base station or internet correction service. | simpleGNSS – Basic Starter Kit , as well as all ArduSimple RTK Development kits Starting from Original price was: 99,00€.90,00€Current price is: 90,00€. |
| 1 to 10 meters | Standard GPS / GNSS: What's commonly used in your phone or car. |