The Autocal equipment is capable of working in both dry and liquid applications and represents a further enhancement of the liquid flow control designed by Jim Graves some six years ago.

Louisiana operator Louis Broussard has successfully been operating dry flow control in rice applications for two seasons. Peter Mackay from Field Air in Australia travelled to Louisiana to meet with Jim Graves at Lewis Flying Service and observe the success of the constant flow rate technology in rice. Subsequently, the Autocal equipment was brought to Australia by Field Air for use in rice and fertilizer applications. Following discussion with Paul McFayden of SST Development Group Australia P/L, development was initiated to extend the constant flow control capacity to also provide variable rate. SST checked the compatibility of the Autocal and Trimble systems. Trimble Navigation engineers in New Zealand worked with Autocal for a number of months to establish communication between the Autocal solids/liquid flow control and the AgGPS Trimflight 3 navigation system.

The most important consideration with this system is that the Trimflight 3 is capable of accepting variable rate "prescriptions". A prescription is an electronic set of instructions that tells the marking system what rate of product is to be applied at any particular part of a field. As the aircraft traverses the field, the Trimflight 3 instructs the Autocal controller to adjust the hopper door opening to achieve the desired application rate. This achievement is a unique combination of geography, electronics, engineering and agronomy. Trimflight 3 is designed with precision agriculture in mind; it accepts industry standard "shape files". Most precision agriculture service providers use software of this data format to manage spatial data and analyze field activities. Trimble has recognized the importance of accepting these agronomic prescriptions to facilitate aerial and ground-borne variable rate fertilizer applications.

The science behind the prescriptions is also very important and potentially technical - but do not be afraid of this. In the USA, variable rate prescriptions have been created and loaded into ground spreaders for many years and there are many agronomists who understand how to build the prescriptions. In Australia this knowledge base is developing rapidly.

Agronomists are using spatial date types such as yield maps, GPS soil testing, GPS plant tissue analysis, infrared imaging and electromagnetic conductivity soil analysis to variable manage crops. This data can only be managed and analyzed using GIS software and it will be the agronomist's responsibility to process this data to create prescriptions. In Australia infrared imagery has been adopted widely in conjunction with plant tissue analysis to create variable rate prescriptions. In the Murrumbidgee Irrigation area of New South Wales there is potentially 2,500 hectares (6,000 acres) of rice that will be top dressed variably with this technology this year.

The ability to load a variable rate prescription and apply fertilizer accordingly is a major advancement for aerial agriculture. Finally, it delivers functionality to agricultural aircraft that has only been available to ground-borne operators and maintains the unique efficiencies of aerial application. This technology will provide incredible advancements to the application of fertilizer to forests, rice and grazing land that is inaccessible to ground machinery.

SST Australia (a subsidiary of SST Development Group Inc., Stillwater, Oklahoma) is an authorized Trimble AgGPS dealer and beta-test platform, and has been using the MAS Agwork facility to test and demonstrate Trimble equipment. In October, SST Australia created a GIS-based variable rate prescription using SSToolbox precision agriculture software. This was loaded into the Trimflight 3 accompanied by the Autocal solid flow system. The aircraft simulated the topdressing event unloaded and not fitted with a spreader. A review of the data logging revealed that the requested rate changes were being achieved within approximately 7-10 meters of the desired polygon boundary. The equipment was fitted to an Air Tractor AT-502B and the average speed was 230 km/hr (140mph).

This achievement convinced all involved that it was now appropriate to perform final rate calibrations and demonstrate the accuracy of this system over a number of aircraft loaded with granular fertilizer. On December 12th and 13th, Field Air, Thomson Aviation and MAS Agwork participated in field calibrations of this system in Jerilderie, NSW. With Jim Graves on-hand to monitor, adjust and analyze the variable rate application, all of the technologies performed perfectly to provide aerial agriculture with one of the most exciting new tools is has seen for many years.

On December 14th, MAS Agwork performed a world first aerial variable rate fertilizer application. Owner/Operator Paul McFayden flew an AT-502B to variably apply urea on 28 hectors (69 acres) of Amaroo rice using prescription determined by SST Australia.