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When speed is all you need

When you think of global positioning systems (GPS), you probably think of hands-free steering and precision operations such as swath control. But sometimes you don't need to know where you are — you just need to know how fast you are going. For years, radar units met that need. But GPS speed sensors, which are typically less expensive and arguably more versatile than radar, can also give you true ground speed. And, like radar, these speed sensors can send a speed signal to planter monitors and spray controllers.


Tom White of Ag Express in Des Moines, Iowa, says, “They can be used almost any place radar is used. The monitor cannot tell if it is hooked up to a GPS sensor or a radar unit — the signals look the same.”

On the other end of the spectrum, the more expensive GPS receivers used for guidance also provide speed signals for monitors and controllers. But the cables you'd need could cost almost as much as an inexpensive GPS speed sensor, and that assembly couldn't be moved to other equipment as easily.

The market for GPS speed sensors is a vibrant one. Many of them are bought as replacements for defective radar units, while others are installed on tractors that never had radar.

Price, ease of installation, and accuracy are among the commonly cited advantages provided by GPS speed sensors. The most expensive GPS speed sensors are priced about the same as the least expensive radar units.

The GPS units are typically attached to the roof of a cab with a magnet. (If the cab is plastic, a small steel plate is attached to the roof of the cab first.) Consequently, the speed sensor can be moved quickly to another tractor or implement.

Under ideal conditions, radar units are quite accurate. However, tall crops can diminish their accuracy, especially if the crop is swaying in the wind. Vehicle vibration and wet surfaces also interfere with radar signals.

Disadvantages associated with GPS speed sensors have diminished over the past few years.

“Previously, signal strength could be an issue when operating in deep valleys or close to tree lines. But with the increase in the number of satellites in orbit and improvements in receivers, this has all but disappeared,” says White.

Rob Hoehn, sales manager for Micro-Trak Systems, says, “There are some instances in orchard chemical applications where tree canopies are dense enough that GPS signals do not penetrate.”

With radar, you are not dependent upon GPS signals.

In field crops, signal loss is infrequent and usually of short duration. Some monitors and controllers will warn you that the signal has been lost. Depending on the instrument receiving input, you may be able to run at a default speed.

There are now at least seven companies in the GPS speed sensor market, and some of them offer more than one model. List prices run from around $300 for 1-Hz models to $500 for a 10-Hz model to $575 for a 20-Hz model. (See table on p. 28.)

Hertz (Hz) indicates how fast the GPS speed sensor updates the speed reading. It's equivalent to cycles per second. A 1-Hz receiver updates one time per second; a 5-Hz receiver updates five times per second.

Which hertz value you need depends on how you are going to use the speed sensor. “The 1-Hz will work just fine with any monitor that needs to know its speed. But if you want to do mapping, the higher the better,” says Ralph Cassone with Sensor-1. His company has sensors ranging from 1 Hz to 10 Hz. None of the sensors discussed here is fast enough for guidance.

“There is a noticeable difference between 1 Hz and 5 Hz, but anything above 5 Hz is pretty negligible for ground-based applications,” says TeeJet's Marty Heyen.

Faster updates are more critical if you are running a controller as opposed to a monitor. And the faster you go or the more you vary speed, the more important it is to update more rapidly.

Before you decide which GPS speed sensor to buy, you need to determine exactly which monitors and controllers you'll want to connect it to so you can order the right cables. Another thing to consider is that some GPS speed sensors can accommodate more instruments than others.

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