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Precision Q&A – for Mar. 2, 2009


If I want autosteer for spraying and seeding, how accurate a GPS signal do I need? Is the free WAAS good enough for a typical grain producer in Western Canada? Or should I pay for a corrected GPS signal? Or should I invest in RTK?


For the answer, Grainews turned to Terry Aberhart. Terry is an Agri-Coach with Agri-Trend Agrology and he farms near Langenburg, Sask. Here is Terry’s answer:

Matching the type of signal you would need to the application is important. WAAS signals are defined as sub-metre (3.3 feet, or less) accuracy. The corrected signals (Omnistar, Starfire) are sub-foot accuracy and RTK signals are sub-inch.

WAAS systems are obviously the cheapest and once you get up to RTK, the cost can be fairly high. A mobile base station alone can cost up to $10,000. For agricultural applications, we are mainly looking at pass-to-pass accuracy and signal drift. Most WAAS systems are rated just over one foot for pass to pass. Corrected signals are four to 10 inches (depending on the system) and RTK would be one to two inches.

For spraying operations, as well as harrowing, fall tillage, and yield mapping, WAAS signals will work fine for the most part. For seeding operations, I would prefer a corrected signal. This is where the signal drift comes into play.

As a satellite moves through space, the angle the signal has to travel through the Earth’s atmosphere changes. And since the atmosphere refracts — or bends — the signal, as the angle changes, the degree of bending changes. Your onboard GPS receiver is programmed to correct this, but it can only do so much. Movement of the satellite and changing angles can influence a signal up to 15 metres (or 50 feet) so, even with the best programmed receivers, you will see some signal drift. Over a period of time, the signal will drift, taking you slowly off line. (This only relates to WAAS and corrected signals, as RTK systems do not have signal drift issues.)

Travel speed is a factor in signal drift. If you are using a WAAS signal on a high clearance sprayer, driving 15 to 20 mph, your pass to pass time is only a few minutes. Therefore the signal drift is not a big issue. If we look at seeding operations, driving five mph on a one-mile stretch, your pass to pass time would be about 20 minutes. You may run into drift issues with a WAAS signal. Then again, if you have small fields — and quicker pass to pass times — then a WAAS signal may be enough.

For most applications on the farm where you are using one system for many different applications, I would recommend a corrected or even a RTK system. If you are looking to run multiple spray applications and want to run down the same wheel tracks, RTK or a high-end corrected signal will be useful. The same is true if you want to use your GPS system to record elevation, line up drainage work, or seed in-between the rows of last year’s crop.

For RTK, you will need to set up a portable base station in the field that you are working in. The other option for those looking at RTK signals is to put the base station on a tower that will extend the range. Many equipment dealers are looking at setting up RTK base stations so their clients can access a RTK signal without having to set up a base station in the field they are working in. That makes the upfront cost for RTK much cheaper.

As precision farming evolves the shift to more accurate signals will go with it. On our farm, we started out with WAAS on our sprayer. Then we shifted to a corrected signal (subscription to a corrected signal costs $500 to $800 per year), running systems on our sprayer, drill, swather and combines. Now we are looking at moving to RTK. With one to two inch pass to pass accuracy, we will be able to use GPS to control drainage work, run the sprayer down the same wheel tracks and seed in-between the rows of last year crop.

The biggest thing is to balance the cost, and decide how many different applications you will use it for. When purchasing a system also keep in mind what you may want to use it for in the future. Purchasing a system that is upgradable would most likely be the best bet over the long term.

You can reach Terry Aberhart at [email protected]or 1-306-743-7657.



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