when the opportunity presents itself on cloudy days, but the ability to fly more frequently allows for greater characterization of ETc

and SWC estimation.

The ability to vary flight characteristics of an airborne remote sensing platform is another benefit over satellite platforms. By ad- justing the flight parameters (e.g., flight speed, altitude, flight lines spacing, imagery overlapping, etc.), the spatial resolution of the imagery can be adjusted to meet the information requirements of precision agricultural and landscape irrigation systems. Addition- ally, based on the lower operating altitude of the aerial platforms, the amount of atmospheric correction is reduced.

airborne RS systems are adaptable; however, due to their high cost of operation and maintenance, it may not be feasible to keep this type of RS system idle waiting for a window of opportunity once the weather improves. Thus, sometimes it is very difficult to obtain RS data regularly (e.g., weekly) with these airborne systems.

The conventional manned airborne remote sensing platforms have their limitations as well. Like satellite platforms, manned airborne RS systems must collect data during clear sky conditions; a requirement for ETc

algorithms. The flight schedules of manned

In contrast, unmanned aerial vehicles are more flexible in terms of availability and low operation and maintenance costs. They also acquire RS data at high spatial resolutions (pixel size of 5 – 20 cen- timeters or 2 – 8 inches, also high temporal resolution of hourly- daily overpasses).

Regarding the terms used to refer to unmanned vehicles, the most common term used is drone. However, the term drone refers to

or is more appropriate for an autonomous system. This term has some military connotation. UAV is more appropriate for a platform that is flown or operated remotely with a pilot in control. Un- manned aerial systems are integrated systems that bring together the autonomous capability of flying a given unmanned aircraft with specialized multispectral, hyperspectral, thermal, lidar and/or radar systems. It requires a ground-based controller, base station, and predetermined RS cameras and flight parameters settings.

With new pending FAA rules governing the operations of UAS we expect that the use of this technology will become mainstream, and multiple applications will be developed or adapted, drawing from decades of research using satellite and airborne systems.

José L. Chávez, PhD, is the associate professor and irrigation engi- neering extension specialist for irrigation water management at the department of civil and environmental engineering at Colorado State University.

Christopher M.U. Neale, PhD, is the

director of research for the Robert B. Daugherty Water for Food Global Institute at the University of Nebraska, Nebraska Innovation Campus.

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