Why is RTK necessary for accurate measurements?

The simple explanation is that a lot can happen to the signals coming from GNSS satellites before they reach ground level. Remember, the GPS satellites are very far away - about 20,000km or 12,000 miles away. 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. View receiver boards
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. If you want a deeper dive into learning about RTK, see our What is GPS RTK? tutorial to learn more.

Base Station + Rover + Radio/Phone

If you don't mind setting up your own base station and need maximum portability then this may be the solution for you.

With this setup you would build your own base station or use the SparkFun RTK Reference Station and let it "survey in" for around 24 hours. As long as 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 very useful for very accurate relative distance (the distance between the base and the rover) with a much quicker "survey-in" time of ~60 seconds.

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

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. Currently, SparkFun uses the ublox NEO-D9S module for this method. This includes the RTK Facet L-Band which is an all-in-one, enclosed device that can receive normal GNSS reception as well as the the L-Band reception needed to receive correction data. You can learn more about how all this works in our RTK Facet Hookup Guide.