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Unmanned aerial systems: UAS to monitor crop health status

UAS is a flexible and agile way of collecting imagery to monitor crops and for mapping out differences in crop growth and health status

The uptake of geospatial technology creates many new opportunities for improving farming. One of these opportunities is the ‘variable rate application’ or VRA, where farmers can apply different rates (nutrients, water crop protection agent etc) depending on the local situation. In contrast to current practices, VRA responds to the spatially varied needs of the crop, or simply put, apply only what is needed. For instance, based on the mapped nitrogen deficiency in the crop, fertilisation can be done only on areas where it is required. VRA is an opportunity for farmers to reduce environmen- UAS to monitor crop health status UAS is a flexible and agile way of collecting imagery to monitor crops and for mapping out differences in crop growth and health status tal impact while improving on their own financial margin.

Sowing new seeds
Unmanned Aerial Systems or UAS are a flexible and agile way of collecting imagery, compared to alternatives like satellites, manned aerial photography or terrestrial sensors (e.g. vehicle mounted or handheld). UAS flies a pattern of parallel paths covering the whole field, taking multiple images that are later stitched together.

It uses multispectral camera payload to measure sunlight reflectance in the visible and near-infrared spectrum to determine specific vegetation indices. An image of a whole field provides insight in the subfield differences. Based on these differences, management zones could be determined, each with their own intensity for spraying. This could then be fed into agricultural machinery with GNSS location to apply the right dose at the right place.

The UAS used in agricultural monitoring are small, light-weight platforms. Around the world, different universities, extension organisations and service providers experiment with small multi-rotors, helicopters and fixed-wing aircrafts. Each has its own benefits and drawbacks. Depending on the type of usage and service, most important differentiators are the space needed for take-off and landing, the endurance of the platform and the payload capacity.

The use of UAS is subject to legislation as many countries are still defining the regulations for airspace use. Safety is the main issue for integrated use of manned and unmanned aircrafts. Use of commercial UAS flights are banned in the US till 2015 but the American ‘Association of Unmanned Vehicle Systems International’ (AUVSI), which in its latest economic report had identified the commercial agriculture market (for UAS) as “by far the largest segment, dwarfing all others”, expects a positive decision from the Federal Aviation Administration (FAA) on opening the US airspace for UAS use. AUVSI foresees that besides remote sensing, unmanned systems could also be used for precision application of crop protection agents or nutrients. This is already taking place in places like Japan where fields are sprayed using unmanned helicopters.

Reaping the benefits
In Europe, a consortium called Field- Copter is investigating the use of UAS for crop state monitoring and how to set up a reliable service. FieldCopter is a EU-funded project and a consortium of SMEs TerraSphere, Aurea Imaging, Flying Cam and AeroVision, and research institutes National Research Council (CSIC) and Centre for Advanced Aerospace Technologies CATEC of Spain.

FieldCopter is using an autopilot to guide the UAS on a pre-programmed flight path. It is using the European Geostationary Navigation Overlay System (EGNOS) for accurate positioning. EGNOS is only available in Europe, and similar augmentation systems could be deployed in other parts of the world. The consortium has developed a lightweight board computer that integrates the EGNOS receiver with gyroscopes and accellerometers for optimal autonomous flight control. The board computer also controls the on-board camera to make sure that the right image is taken at the right spot. FieldCopter uses two different camera systems — multispectral and thermal infrared. The multispectral camera is configurable, in which bands could be measured both in terms of the central wavelength and bandwidth. It could therefore mimic the bands from any satellite sensor currently available.

The multispectral imagery is used to determine crop vigour, biomass growth and crop nitrogen status. The thermal infrared camera measures the emitted long-wave radiation that is directly linked to the temperature of the emitting body. For crops, canopy or leaf temperature could be related to evapo-transpiration status. FieldCopter is using thermal camera to determine irrigation performance and crop water needs in irrigated agriculture.

Cloud cover is a major showstopper for many satellite applications in agriculture since the opportunity window for image acquisition could be as small as a 5- or 10-day period and farmers want to use the imagery in near real time. Field- Copter investigated the weather conditions in the Netherlands and concluded that the use of UAS (standalone, or in combination with satellite imagery) could increase the service performance to an acceptable level.

A vineyard in Spain monitored with FieldCopter using a thermal infrared camera shows differences in vine temperature. The red colour reflects higher temperature and thus higher crop water requirement

Besides multispectral and thermal cameras, hyperspectral cameras are also being made available for UAS platforms, thus increasing their performance. Given the increasing autonomy of the UAS platform, it may not be long before every farmer posseses his UAS. This will create a wealth of remote sensing data on crops and fields, paving the path for more resource-efficient farming. This is good news for the environment, the farmer and the consumer.