Achieve Precision Positioning with RTK

RTK is short for real-time kinematics. A GNSS receiver capable of RTK takes in the normal signals from the Global Navigation Satellite Systems along with a correction stream to achieve extremely positional accuracy. How extreme, you ask? As SparkFun’s founder Nathan Seidle has said, “With normal GPS, you can tell what street you are on; with high precision GNSS, you can see what gum you are standing on.”

Over the last few years, SparkFun has put a lot of effort into unlocking the potential of RTK by releasing many products and resources to help our customers achieve millimeter-level accuracy in their GIS applications. If you’ve never utilized RTK in a GIS project it may be a bit confusing but we’re here to help shed some light on utilizing RTK.

A (Very) Quick Lesson on How GPS/GNSS Works

In a nutshell, GNSS location comes down to timing. Location is found by calculating the distance between a receiver and that satellite. This is done by multiplying the rate of signal (speed of light) by the atomic clock time on the satellite. You can locate your location from three satellites, but it takes at least four satellites to determine your location in three dimensions. Three satellites are needed for x, y and z coordinates (triangulation), and one satellite to determine the time it took the signal to travel from the satellites to the receiver. Typical GNSS receivers can get accuracy down to a meter or two. Utilizing RTK is what allows us to be much more accurate.

Why is RTK necessary foraccurate measurements?

The simple explanation is that a lot can happen to the signals coming from GNSS satellites before they reach ground level. GNSS satellites are very far away - about 20,000km or 12,000 miles from earth. Since a GNSS receiver pinpoints location by calculating the distance between you and satellites, everything comes down to timing. Geomagnetic storms cause slight timing delays which can cause location errors. Also, relativistic effects cause orbiting clocks to tick slightly faster than they would on earth. The mere microsecond differences between orbiting clocks and clocks on earth can add up to inaccuracies. RTK corrections help fix these inaccuracies by sending correction data from "surveyed-in" base stations or even geosynchronous satellites!

What is needed to utilize the accuracy of RTK corrections?

There are two main things that you'll need in order to use RTK:

1. A GNSS receiver capable of receiving and incorporating RTCM correction data.

2. A source of RTCM correction data

Similar to most any GNSS receiver, a receiver capable of receiving RTK corrections takes GNSS location data from GPS (USA), GLONASS (Russia), Beidou (China), and Galileo (Europe) satellites. On top of these signals, an RTK receiver takes in an RTCM correction stream to calculate your location with 1cm accuracy in real time. RTCM is technically just a government-created protocol that is now used to signify the bytes of correction data related to GNSS timing anomalies. These bytes of data are what allow us to calculate down to millimeter-level accuracy.

Read on if you would like to see the different ways to utilize RTK corrections in the real world.

RTK Positioning Use Cases

There's certainly no right or wrong way to utilize RTK corrections to achieve extreme accuracy, but the three listed below are some of the more common setups used out in the real world.

Base Station + Rover + Radio/Phone

This method involves setting up your own base station. If you need maximum portability then this may be the solution for you.

With this setup you would build your own base station and let it "survey in" for around 24 hours for fixed positioning. Surveying-in involves gathering 24ish hours worth of raw GNSS data from satellites in view using a method called Precise Point Positioning (PPP).

The PPP process works like this:

• Install an antenna in a fixed location
• Gather 24 hours worth of raw GNSS data from that antenna
• Pass the raw data to a processing center such as CSRS or OPUS
• Obtain a highly accurate position of the antenna we use to set a ‘Fixed Mode’ on a receiver

Once your base station has been surveyed-in and your rover is within ~20km of your base station, then your rover will receive absolute accuracy. Corrections are sent from the base to the rover via a radio connection or cell phone. This method is also a great choice for very accurate relative distance (the distance between the base and the rover) with a much quicker "survey-in" time of ~60 seconds.

LEARN HOW TO BUILD YOUR OWN BASE STATION

Pros:

• Very accurate (down to 10mm horizontal positioning)
• Super fast (less than 2 seconds for RTK fix)
• Relatively inexpensive

Cons:

• Must be within ~20km of the base
• Setup and maintain of the base
• Wifi signal must stay on if using cell phone

Paid Permanent Base + Rover + Cellphone

If you don't have a base station, or don't want to maintain one, you can use correction services like u-blox's PointPerfect,Skylark,UNAVCO or other services to get correction data over the Internet and deliver it to your rover using your cell phone's internet connection.

Based on the location of your rover, your cell phone can receive correction data from an existing nearby base station where corrections are sent to the Internet (NTRIP Network) and then passed on to your phone over the cellular network. Your cell phone then communicates the correction data (via Bluetooth or WiFi) to your rover.

LEARN MORE ABOUT CORRECTION SERVICES

Pros:

• Absolute accuracy
• Fast fix times
• Large network of base stations

Cons:

• Base stations may be further away from rover
• Not quite as accurate as base station
• Usually monthly subscription based

“L-Band” Rover Using Corrections From Geosynchronous Satellite

This method is really great for all uses, but it especially comes in handy when you need highly-accurate positioning in more remote locations without cell service and/or you don't have a base station close enough to receive corrections data.

Rovers with an L-Band antenna (and a monthly subscription) can receive RTK correction data from a geosynchronous Inmarsat Satellite without the need for a base station. Since the orbital period of a geosynchronous satellite matches Earth’s rotational period, the satellite always keeps its position in space. Companies, such as u-blox are able to use their large network of base stations to gather correction data and aggregate it into a series of correction feeds. Those feeds are then uplinked to, and broadcast from, a geosynchronous satellite using the L-Band frequency. L-Band is any frequency from 1 to 2 GHz. These frequencies have the ability to penetrate clouds, fog, and other natural weather phenomena making them particularly useful for location applications.

Currently, SparkFun offers the Facet RTK L-Band which is a single device that can receive normal GNSS reception as well as the L-Band reception needed to get correction data from a geosynchronous Inmarsat satellite. The RTK Facet L-Band does not require creating an account anywhere, does not require copy and pasting of keys, certificates, or any other materials. Everything is built into one unit and made to be as easy as possible to use. The only setup required is a WiFi SSID and password.

Pros:

• No need for base station
• Absolute accuracy

Cons:

• Only works in the 48 contiguous states and Europe
• Subscription required for L-Band use
• Slower fix times (~45 seconds)

Ultimately, the particular hardware and method you use will depend on available time and budget, but also requirements like necessary accuracy and size. Luckily, SparkFun has a plethora of prototyping options as well as enclosed and ready to use surveying products with documentation to match your vision. Thanks for reading! If your curiosity extends beyond the reach of this article, go to our RTK Solutions page for more resources and information on RTK products, projects, accessories and use cases.