Levels of Satellite Signal Accuracy

Levels of Satellite Signal Accuracy 

By Kurtis Shipp

Many growers across the Corn Belt fully understand the concept of automated steering but many become confused when they try to decipher all the different levels of satellite accuracy available.  With terms like WAAS, SF1, SF2, XP, HP, CORS, RTK, and RTX it can be very hard to determine what is out there in the marketplace.  In this discussion I would like to help clarify and simplify things for the consumer.  When you really break things down there are 3 classes of accuracy.  These three classes are satellite based (free), satellite based (subscription), and ground based correction.  We will take a closer look at each of these 3 classes and discuss what makes them different from each other.

Satellite Based Free Correction Signal

The first category of signal accuracy is the satellite based free correction.  With this level of accuracy you are getting a free signal (no annual subscription) that gives you a typical accuracy of 9-13” pass to pass accuracy.  When we use the term pass to pass accuracy we really mean over the course of 15 minutes.  So with this type of signal you can expect your line to drift no more than 9-13” over the course of 15 minutes.  In many cases your line with only drift a few inches pass to pass but should be no more than 9-13”.  These free corrections are the least accurate signal but they are also the most economical.    In applications that do not need higher accuracy this is a great solution.  Typically field operations like tillage and harvesting are completed with a free satellite based signal.  John Deere’s SF1 signal is an example of this category of signal.

Satellite Based Subscription Correction Signal

The second category of signal accuracy is the satellite based subscription correction signal.  With this level of accuracy you are still getting your correction from a satellite but you are getting additional corrections that give you better accuracy and less pass to pass drift.  With a subscription based correction signal you will improve your pass to pass accuracy to about 2-4” over the course of 15 minutes.  With this added accuracy comes the additional cost of the yearly subscription.  Most subscriptions range from $400-$1200 per year, per GPS receiver.  This type of signal is commonly used for applications that need a high level accuracy from pass to pass, but do not need repeatability for later in the season.  The most common field application for this type of signal is planting where we need better than 9-13” accuracy, especially for planting corn.  John Deere’s SF2 signal is an example of a subscription based satellite signal.

Ground Based Correction Signal

The final category of accuracy is the ground based correction signal.  With this signal we are combining satellite correction with a local ground based reference that has a permanent location.  This permanent location can be commonly referred to as a base station.  With the base station in a permanent location we can use that reference to send a correction signal to the vehicle in the field to eliminate drift and give us sub inch repeatable accuracy.  A common name for this type of signal is RTK.  Typically with this signal you must subscribe to a dealer owned network ranging from $500-$1500 per year.  Two forms of ground based correction are radio and cellular, with the only difference being how the correction is sent from the base station to the vehicle or tractor in the field.  With radio the base station uses a 900 MHz or 450 MHz radio to send the information to the tractor and with cellular the correction is sent via cellular network similar to a text message.  Applications that use this highest level of accuracy typically need the repeatability for applications like strip till or controlled traffic where we need to drive in the same tracks throughout the growing season as well as year to year.  Another common place to use this type of signal is when growers have sectional control on their equipment and want to eliminate drift which causes the section control to shut off incorrectly over the course of several hours of drift when planting a field.  If you spent thousands of dollars for sectional control you want it to work as well as possible and ground based correction, or RTK, gives growers that capability. 

Hopefully you now have a better understanding of the different levels of signal accuracy available to growers and where each of those levels can be the best solution for a farming operation.