RTK Network FAQ's

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What is RTK?

Certain row crop applications require precise machine operations with pass-to-pass and repeat accuracy of +/-1 inch (2.5 cm). Accuracy this fine requires GPS signal correction using Real Time Kinematic, or RTK, technology. Trimble pioneered the commercial use of RTK in 1992, and owns more than 30 U.S. patents pertaining to RTK systems and applications. Trimble RTK innovations are now used for a wide range of applications all over the world in the marine, survey, mining, construction, and agriculture industries.

To learn more about the RTK coverage GPS AgSystems provides for Northwest Iowa visit our RTK Network page.

How Does it Work?

With RTK, you need a base station placed on a known, surveyed point, and one or more mobile receivers within a ten kilometer range of your base station. The base station transmits corrections via radio to the mobile receivers in the field. A typical radio link required for RTK is in the UHF, VHF, or spread spectrum radio band. Radios operate best within line of sight or with a repeater.

Trimble RTK base stations transmit data once per second. Using radios like the TRIMCOMM 900M, the data is transmitted in a format called CMR (Compact Measurement Record). It's a dual frequency data format invented by Trimble that transmits data in a more compact and robust way than other formats. This makes data transmission to the roving receivers more reliable.

The GPS receivers used in Trimble RTK systems are dual-frequency receivers. Dual-frequency receivers are a must in high accuracy applications, such as precise guidance along crop rows or collecting GPS elevation data for topography mapping. As well as using the standard code and carrier phase GPS signals like all other GPS receivers, Trimble's dual-frequency receivers also track the second carrier phase signal. It is this signal that enable Trimble RTK receivers to lock into the higher accuracy, and initialize more quickly. Initialization is the process a receiver performs to get the information it needs from the satellites to generate high accuracy positions. Trimble's RTK receivers also output data at rates better than one secondin fact, a data rate of ten times per second is standard in the AgGPS receiver.

What are the differences between Differential GPS(DGPS) and RTK?

It can be difficult to distinguish between RTK and DGPS. Here's a quick review the differences:

• To get initialized, RTK needs a minimum of five satellites. After that it can operate with four. DGPS needs a minimum of three, though at least four are required for submeter accuracy.
• For RTK, you need a dual frequency GPS receiver. Single frequency receivers are sufficient for DGPS.
• For RTK, your GPS receiver must be capable of On-the-Fly initialization (obtaining centimeter accuracy while moving). For DGPS, this isn't necessary.
• With RTK, it takes one minute to initialize. DGPS receivers initialize immediately.
• You can expect accuracy of a few centimeters in all three dimensions using RTK. With DGPS, you can achieve submeter accuracy in horizontal position only.
• To obtain GPS corrections for RTK, you need your own base station that is no more than ten kilometers from the field you are working in. For DGPS, you can use your own base station, a correction service provider, or make use of the free radio beacon broadcasts in many regions.