The ability to know where you are and where you are going is crucial to so many activities. Over the years, all kinds of technologies have tried to help us figure this out. To date, none have had more impact than the Global Positioning System (GPS).

What is GPS and How Does it Work?

You may have heard how the use of GPS is revolutionizing the agriculture industry—from finding your way to the middle of a corn field and accurate field guidance without foam markers, to critical row-crop driving and precise elevation mapping. But what is it? See the GPS tutorial to learn how GPS works and how people are using it.

Differential GPS, Satellite Differential & WAAS

Differential GPS is an advanced form of GPS that provides the accurate guidance data you need for precision agriculture operations. Differential corrections can be obtained from several sources: your own base station, the worldwide network of DGPS radiobeacons, or a satellite differential service provider.

The network of DGPS radiobeacons throughout the world is rapidly expanding and the signals that most radiobeacons transmit are free. There are also a number of satellite differential service provider options including subscription services you pay for, and now the Wide Area Augmentation System (WAAS) or EGNOS satellite systems for the US and Europe respectively that will improve integrity, accuracy, and availability of GPS for users.

Centimeter Accuracy & RTK

For certain precision agriculture applications such as row crop bed preparation and planting or topographic map generation, submeter DGPS accuracy is not enough. That's where centimeter-level solutions using Real-Time Kinematic (RTK) can give you even greater accuracy, as well as huge savings in time and money. The Trimble AgGPS® 214 receiver is an RTK receiver designed and built for agricultural applications.

Trimble pioneered the commercial use of RTK back in 1992, and since that time has been used for a wide range of projects all over the world spanning marine, survey, mining, and construction applications. 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—typically about one minute. Trimble's RTK receivers also output data at rates better than one second—in fact, a data rate of ten times per second is standard in the AgGPS receiver.

Difference between DGPS & 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.