Tuesday, 5 August 2014

Not all GPSs are equal


Research at Massey University has shown that spreaders fitted with DGPS systems (Accurate to 20cm) reduce in-field CV by 9-17% compared to no additional reduction for spreaders fitted with non-corrected GPS units.

Very few GPS units on the market in New Zealand today provide spreader drivers with the ability to guide the spreader and reposition it after it has left and returned to a paddock.

Ravensdown spreading trucks are fitted with this technology and it has been proven to reduce in-field CV by 10%. Remember in order to ensure you are getting the most accurate spread, check with your spreading operator that:

  • They have a Differential GPS system.
  • Their spreader has a repositioning and guidance system linked to DGPS.

What is in-field CV?

Professor Ian Yule (Massey University):

"The CV (co-efficient of variation) is a measure of the variation in application rate and is calculated by dividing the standard deviation by the mean value of the application rate.

The standard method of testing CV is to conduct a transverse test (Spreadmark) where the spreader travels over a line of catch trays. The fertiliser deposited in each tray is weighed and the spread pattern calculated. This can then be used to calculate the correct distance apart (bout width) that the spreader should be operated.

Standard deviation is the measure of variability around the mean, the greater the value the greater the variation. A CV of 15% is required for spreading fertiliser containing nitrogen; products such as Superphosphate can be spread with a CV of 25%. This test assumes perfect spreading, but there are a number of factors that will cause that variation to increase.

Field CV is a concept developed at Massey University and takes these other factors into account to model what is actually happening in the field. The “In-field” CV values (%) are much higher than the 15% suggested by the Spreadmark test, the main factors contributing to this are driving accuracy, starting and stopping positions as well as control flow rate according to change of vehicle speed.

The Ravtrak system addresses many of these issues by using differential GPS to ensure accurate driving as well as flow control to ensure a consistent application rate, automated starts and stops which eliminates driver error and the potential for double spreading and gaps. It also has the ability to match up spreading from another truck, so the guess work is removed from where to start and stop. Ravtrak is unique to Ravensdown and its joint ventures."

Use Ravensdown spreading technology and the productivity gains could equal the cost of the spreading.