Sense and Avoid

Introduction

    The most reliable presently developed sensor to meet the Federal Aviation Administration's requirement for sense and avoid for Unmanned Aerospace System (UAS) is radar. Radar systems are ideal in situations where normal optical vision is occluded, such as inclement weather (Barnhart, Shappee, & Mashall, 2011). Radar also offers advantages during night time when there is less visible light to illuminate hazards. The airspace below 18,000 feet MSL is the altitude where a sense and avoid system are necessary, however the majority of aircraft that operate under see and avoid operate below 10,000 feet. The majority of midair collisions occur within 3 miles of the airport, with 50% being below 1000 feet in altitude (Narinder and Wiegmann, 2001).

Capability

Haze is a common visibility restrictor typically encountered by aircraft that operate below 10,000 feet. Haze can significantly reduce visible light and obstruct hazards from timely optical acquisition. However, radar can penetrate hazy environments, giving radar equipped sense and avoid aircraft a hazard acquisition advantage over unaided aircraft which must visually acquire flight hazards.

Radar can also look through dust. Radar sensors offers a capability of transmission through dust, snow, fog, and spindrift (Pagels, Hagelen, Briese, & Tessmann, 2009 ). Hovering aircraft like helicopters and tiltrotor aircraft typically brownout when transitioning to an in ground effect hover while operating in dry unimproved desert environments. Sometimes these brownouts can occur at altitudes as high as 125 feet during a final approach of an aircraft's landing. Hazards quickly become occluded by the brownout. Pilots must rely on their drift indicators, groundspeed, radar altitude, experience and estimation in order to avoid these obstacles once they are inside a dust cloud. However, radar can penetrate through the dirt and dust encountered in these dry dusty environments. Radar could provide a look through capability, making obstacle avoidance more of a science and less of a calculated risk.

Capabilities

The limiting factor with radar in the past has been its size. Radar systems are typically too heavy for practical use on small and medium sized UAS. However, Northrop Grumman has developed a small lightweight radar the AN/ZPY-1 STARLite Small Tactical Radar – Lightweight which is light enough, 39-65lbs, to be carried on small and medium UAS (AN/ZPY-1 STARLite Small Tactical Radar – Lightweight). STARLite is designed as a tactical sensor for Intelligence, Surveillance and Reconnaissance. Although STARLite was not designed specifically as a sense and avoid system, its small size and Synthetic Aperture Radar (SAR) capability is an ideal sensor for sense and avoid integration.

The radar is mounted on a rotating mechanical gimbal with a 360 degree field of regard, the antenna itself has a 110 degree field of view. In addition to SAR, it has a dismount moving target indicator mode that can track a person walking on the ground from a range of 4.3 nm. STARLite occupies 1.2 cubic feet, and requires less than 750W of power (AN/ZPY-1 STARLite Small Tactical Radar – Lightweight).

Conclusion

    Although STARLite was designed as an Intelligence, Surveillance and Reconnaissance sensor, it has the potential to provide a sense and avoid capability requirement for unmanned systems. STARLite can provide a look through hazard identification capability when environmental factors would limit optical acquisition of those hazards. This capability does not come cheap. STARLite costs about $400,000 per unit (ER/MP Gray Eagle: Enhanced MQ-1C Predators for the ArmIy, 2014). Synthetic Aperture Radar is located onboard the UAS. It is a sensor that would enable a UAS to maintain a sense and avoid capability even if a signal with a ground based station or like system is lost. The fact that a UAS would still be able to provide a sense and avoid capability independent of external systems makes STARLite a present day viable solution to the sense and avoid capability requirements.

 

References

AN/ZPY-1 STARLite Small Tactical Radar – Lightweight. (n.d). Retrieved from http://www.northropgrumman.com/Capabilities/starlite/Pages/default.aspx

Barnhart, R. K., Shappee, E., & Marshall, D. M. (2011). Introduction to Unmanned Aircraft Systems. London, GBR: CRC Press. Retrieved from http://www.ebrary.com

ER/MP Gray Eagle: Enhanced MQ-1C Predators for the Army. (2014). Defense Industry Daily.

Narinder, T., and D. Wiegmann. 2001. Analysis of mid-air collisions in civil aviation. Proceedings of the 45th annual meeting of the Human Factors and Ergonomics Society.

Pagels, A., Hagelen, M., Briese, G., & Tessmann, A. (2009). Helicopter assisted landing system – millimeter-wave against brown-out. Paper presented at the 1-3. doi:10.1109/GEMIC.2009.4815878