AITscan division operates fixed-wing and rotor-wing aircraft to perform qualitative aerial infrared thermographic surveys of high voltage transmission lines for utilities.

SEE SLIDESHOW OF THE PROCESS

General
The infrared cameras that we use for aerial infrared thermographic surveys have 5122 focal plane array detectors–a total of 262,144 pixels, which is four times the resolution of most modern infrared cameras (2562 -a total of 65,536 pixels) of today. Because of the high-resolution, this type of camera can be mounted in a fixed-wing aircraft allowing the infrared thermographer to quickly fly over transmission lines. The problem areas are found and the exact coordinates are marked with an integrated video signal processor through a video encoder-decoder (VED) which displays on the screen and is digitally recorded along with the infrared image, displaying the date, time (to the millisecond), number a satellites acquired, altitude, mission code, Lat/Lon (differential) and speed over ground.

Qualitative v. Quantitative
Detecting electrical faults on high voltage electrical transmission lines is fairly easy and can be accomplished rapidly from a light aircraft. However, even from short distances, accurate temperatures of electrical faults are impossible to measure [quantify]. There are several problems associated with temperature measurement from the air. These include spot size to target distance ratios, reflection of the objects surveyed, having a sufficient load on the line at the time of the survey, among others. The spot size to target distance ratio is the number one problem with respect to temperature measurement. Specification writers have not yet realized the seriousness of this problem and continue to ask for quantitative data on fault areas. The fact is that infrared cameras that are in general commercial use today cannot measure accurate temperatures on small objects from distances of 50 feet...much less from reliably safe flying distances. A one-inch (relative size of a transmission line splice) target cannot be measured from that distance, plain and simple, although it can be detected. These spot sizes are unmanageable and inaccurate on any target that does not have a large homogeneous heat signature. The ground resolution element (GRE) is critical to the measure of spatial resolution in aerial infrared thermography. Nyquist's frequency theorem states that an object less than two times the size of a sensor's GRE cannot be resolved for measurement, so a 3x3 pixel or GRE spot is needed for reliably obtaining measurements.

This shortcoming may be addressed by using a more powerful lens to reduce the GRE for a given distance, but then the sensor's FOV is reduced, limiting the area covered over a given period of time. Using a more powerful lens also has limitations because vibration is more evident in the form of image 'shaking'. Image 'smearing' may also occur due to an increase in the apparent speed of the sensor's view across the ground. In the air, there are few substitutes for a large pixel array, but even using large format detectors, we cannot and should not profess to measure temperatures on very small objects. These anomalies can be seen, and by comparing them to similarly loaded phases or equipment, potential problem areas can be identified, saved and marked on a map. For ‘good’ measurements, a ground verification team should be used to inspect suspect hot spots from the ground (cloudy nights are best) and verify the findings of the aerial IR survey. They will be closer to the target and with a powerful lens on a stable surface, much more accurate.