RTK corrections explained: from base station to NTRIP service

This article explains two methods for obtaining RTK corrections for surveying workflows: using an NTRIP service or your own base station. We will describe how each method fits into a broader workflow, covering various use cases, key benefits, and limitations. This information will help you choose the best method for your application.

Breaking down an RTK system

To master the basics of surveying with RTK (real time kinematic positioning), you need to know the key elements of an RTK system and how they are connected.

GNSS (or GPS) receivers use GNSS signals from satellites to determine precise locations. Since satellite signals travel through different levels of the Earth’s atmosphere, this impacts the signals propagation, causing measurement errors.

An RTK technique improves the positional accuracy of a GNSS receiver from meters to centimeters by correcting satellite data errors. A typical RTK setup consists of two devices: a rover, a base station (base), and a communication link between the two.

• A rover is a mobile GNSS receiver used for data collection, capturing satellite signals to determine its precise location on Earth. 
• A base station is a static GNSS receiver that transmits both GNSS data from the satellites and its own position (correction data or corrections).  
• A communication link connects the base station and rover, typically via radio or the internet using NTRIP (Networked Transport of RTCM via Internet Protocol).

However, there is a way to get RTK corrections and achieve centimeter-level precision without having your own local base: you can configure RTK using only one GNSS receiver acting as a rover. This is possible via an NTRIP service, where you receive corrections from the nearest third-party base.

Let’s explore both setups and see which works best for your project.

How to get RTK corrections with your own base station?

Using a local base station is a reliable method in areas with limited internet access. You can go autonomous by getting corrections from your base over a LoRa (long-range) or a UHF radio. Another advantage is full control over a local base position, which might be handy for achieving higher-accuracy results.

The downside of this method is that it requires additional setup time and extra equipment: one more GNSS receiver to act as a base; tripod for the base; LoRa antenna for both base and rover. Another point to keep in mind is that LoRa has a limited baseline which highly depends on environmental conditions.

RTK workflow with a base station

You need to position a GNSS receiver at a fixed location as the base station. The base station will collect satellite data and transmit it together with its own position to the second GNSS receiver— the rover. Receiving satellite signals and corrections from the base in real time, the rover is able to fix measurement errors and calculate its position with centimeter-level accuracy. 

Your workflow will include:

1. Base setup. Place the base over a point with known coordinates. If you don’t need to tie your coordinates to a real geographical position, for example, when measuring a distance or building a track for a tractor, placing the base over a known point is not required. 
2. Initialization and transmission. The base station starts receiving satellite signals and transmitting them together with its own position. It can deliver data to the rover via radio or, in case you have Internet, over NTRIP, using an NTRIP caster.
3. Receiving and applying corrections. Then, you can set up your rover and start point collection or stake-out. 

Check out this video tutorial to learn how to set up two Emlid Reach receivers over LoRa radio.

How to get RTK corrections with an NTRIP service

There are state and commercial NTRIP services that provide corrections for GNSS receivers over the internet using NTRIP protocol, so you don’t need a local base station to work in the RTK mode. This application reduces equipment costs and setup time, but requires internet connection and a receiver with a working cellular or Wi-Fi modem. 

So, what’s an NTRIP service? It’s a network of ground-based reference stations with precisely known locations, which are basically GNSS receivers, just like Reach RS3 or RS2+ by Emlid.

To provide real-time correction data, an NTRIP service requires three components:

• NTRIP client: typically, this is a GNSS rover connected to the internet.
• NTRIP server: this is the source of the correction data from all reference stations in the network. It is responsible for choosing the best station for you, based on your location and other parameters.
• NTRIP caster: a server that acts as an intermediary between the NTRIP client and server. 

NTRIP caster transmits RTK data from base to rover.

RTK workflow with an NTRIP service

Before everything, you need to subscribe to a local NTRIP service for your area. Because there is no global NTRIP coverage, NTRIP services vary in availability, accuracy, and cost. Government agencies or public institutions typically provide free NTRIP services. 

Examples of free government NTRIP services are the Continuously Operating Reference Stations (CORS) network in the USA, and the EUREF Permanent GNSS Network in Europe. Other regions have commercial services that charge subscription fees.

Once subscribed, your workflow will include:

1. Connecting your rover to the Internet. To get NTRIP corrections, connect your receiver to the Internet directly via SIM-card or Wi-Fi. 
2. Rover setup. Configure RTK settings. This involves connecting to your receiver and introducing GNSS settings like selecting satellite systems and elevation mask and updating rate. 
3. Connecting to the NTRIP service. Choose NTRIP as correction input and add your NTRIP provider’s details to the profile.

One of the examples of commercial NTRIP services is Swift Navigation’s Skylark Precise Positioning Service, primarily covering North America, Europe, and parts of Asia. It offers extensive geographic coverage through a network of ground-based reference stations, with centimeter-accurate GNSS corrections delivered in real-time. 

Here is how you can set up your Reach device to get corrections from Skylark:

Which method of getting RTK corrections to choose? 

Here is a summary of the key benefits and limitations of both methods to help you choose the right one for your surveying project:

Access to the Internet and local NTRIP provider, accuracy, and costs (one-off and recurrent) are the major criteria you need to consider when choosing an RTK method.

Using a base station instead of an NTRIP service is beneficial for applications requiring the highest accuracy possible, complete control over the data collection process, and the ability to operate independently of external correction services, particularly in remote locations. 

NTRIP services are perfect for working in urban areas where internet access is guaranteed. If carrying around or displacing equipment over large areas is a problem, NTRIP is an excellent choice. The same goes for companies and/or projects where scalability is impossible. For example, when small companies don’t want to or cannot invest in a base station or when high investment costs do not compensate for the returns.

On the other hand, surveying with a base station with LoRa is the best solution in scenarios with limited internet access or remote locations without NTRIP coverage. 

Wrapping up

We’ve covered the two methods of getting RTK corrections: with and without using your own base station. Apart from one-off and recurrent costs, the biggest differentiator between both is internet and NTRIP coverage for the area you’re surveying. Another point to keep in mind is that having control over your base’s position and surveying on short baselines can bring higher accuracy levels and more overall reliability, as you’ll always know whether your base is working or not.

Now you know which method is best for your application. If you need help or just want to share your experience with Emlid RTK receivers, join us in Emlid community.