What is RTK? (Centimeter-Level Accuracy)
For Beginners: The Moving Target Problem
Your smartphone's GPS is accurate to about 3 to 5 meters. That's fine for driving, but terrible for flying a drone, planting seeds with a tractor, or surveying property lines. RTK (Real-Time Kinematic) fixes this, giving you accuracy down to a single centimeter.
- GPS signals get distorted as they travel through the Earth's atmosphere.
- RTK solves this by using two antennas: A Base Station that stays perfectly still, and a Rover that moves around.
- The Base Station knows exactly where it is. It watches the GPS error, figures out the math to fix it, and shouts that "correction" over a radio to the Rover.
Visual: The Base Station and the Rover experience the exact same atmospheric distortion. The Base tells the Rover how to fix it.
Without RTK, your drone might land in a tree, your autonomous tractor might crush the crops, and a surveyor's property line might be off by several feet.
The "Carrier Phase" Magic
Standard GPS relies on the "code" signal broadcast by satellites. The "bits" in this code are relatively long—about 300 meters wide. Your phone tries to align these bits to figure out the distance, but the margin of error is high.
RTK doesn't just look at the code; it looks at the carrier wave carrying the code. The wavelength of the GPS L1 carrier signal is exactly 19 centimeters. By aligning these tiny 19cm waves (and using advanced math called integer ambiguity resolution), RTK can pinpoint location to within 1 or 2 centimeters.
NTRIP: RTK Without a Base Station
Buying two expensive GPS antennas (a Base and a Rover) is costly. Today, many industries use Network RTK (NTRIP).
How NTRIP Works
Instead of setting up your own Base Station on a tripod, you pay a subscription to a service (like a State Department of Transportation or a private company). They have permanent Base Stations installed all over the state. Your Rover connects to the internet via a cellular modem, sends its approximate location to the server, and the server streams the exact corrections over the internet (using a protocol called NTRIP) right back to your tractor or drone.
PPK vs RTK
RTK stands for Real-Time Kinematic. If your drone loses its radio link to the Base Station, it loses its centimeter accuracy instantly.
The alternative is PPK (Post-Processed Kinematic). In PPK, no radio link is needed. The Base Station records data on a memory card. The Drone records data on a memory card. After the flight, you put both memory cards into a computer, and software crunches the numbers to apply the corrections after the fact. This is highly reliable for mapping, but useless for autonomous driving where you need to avoid hitting a wall right now.