Tech Briefs

New technology could give helicopters the ability to overcome zero-visibility brownout conditions during landing.

The development of sensors to assist helicopter landing in degraded visual environments (DVEs) is currently an important US Army requirement addressing the Survivability of Future Vertical Lift Platforms program, one of the Army's modernization priorities.

Schematic representation of the helicopter-borne radar system operating as FLSAR, showing the relevant sensing geometry from a) top view and b) side view. The small antenna diagrams mark the aperture sample positions. (Drawing not to scale.)

Over the past three decades, dozens of rotary-wing aircraft crashes have been responsible for a large number of casualties to US and coalition forces in different parts of the world. Out of these crashes, at least 75% have occurred in brownout conditions, where dirt or dust is stirred up and recirculated by the rotor blades, creating low- or zero-visibility environments for the pilots. Research and development efforts to mitigate this issue starting in the early 2000s recommended several possible solutions based on optical, IR, and radar sensors. Unfortunately, most of these solutions have proven to be either ineffective or they involved unacceptable size, weight, power, and/or cost (SWAP-C), leaving the Army with a capability gap to be filled.

The US Army Research Laboratory (ARL) is currently working on a sensor solution to this problem based on millimeter-wave (MMW) imaging radar technology. The main idea behind this sensor is to combine a linear antenna array with the radar platform motion to obtain a high-resolution 3-D terrain map of the landing zone. This information would be passed to the pilot via a helmet-mounted display to assist in deciding whether the landing zone is safe. Several previous efforts in developing similar sensors, based on passive or active MMW technology, have focused heavily on 2-D antenna arrays working in scanning mode to obtain a terrain map. These efforts generally produced devices that proved either too expensive, unreliable, and/or inaccurate for the required task. The ARL-proposed solution leverages advanced radar imaging methodology, together with the current boom in commercial MMW RF technology (driven by developments in autonomous car navigation and 5-G wireless communications), to produce a reliable, low-SWAP-C sensor prototype addressing this requirement.

The proposed radar system will use a linear antenna array and the forward-looking synthetic aperture radar (FLSAR) concept to achieve the stated goals. A linear antenna array mounted on the rotorcraft's front end will provide the required cross-range resolution, while the transmitted signal bandwidth (up to 1 GHz) will provide downrange resolution. To achieve resolution in the vertical dimension, the radar will exploit small elevation angle deviations in the helicopter flight path, which naturally occur when the pilot prepares for landing. Overall, this new radar sensor concept represents a significant shift in implementation from a hardware-heavy solution to an emphasis on signal processing and computational power, with large potential cost savings and performance improvements.

As part of this research, a detailed analysis of the 3-D imaging performance of the proposed radar system was performed by investigating the point spread function (PSF). The emphasis was on synthetic aperture radar (SAR) and antenna array processing, which are key to this sensor's implementation.

This work was done by Traian Dogaru for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Electronics & Computers category. ARL-0217