M. Lorraine Tighe, PhD
Dr. Tighe has a Ph.D. in Earth Sciences, a graduate degree in Remote Sensing and GIS, and a B.Sc. in Physics and Geology. Dr. Tighe has delivered lectures ranging from a half day workshop to a 4 week training program to over 2000 participants in USA, Canada, Jamaica, Brazil, Ecuador, Honduras, … More »
The importance of aircraft stability in Commercial Flying and Remote Sensing Data collection
September 24th, 2013 by M. Lorraine Tighe, PhD
I just traveled quite a distance to come to the Asian Geospatial Forum Conference in Malaysia on the new Dreamliner – 787. Six hours into the flight we had to back track to Anchorage to avoid an emergency landing in Russia, all because our flaps were not working. To accomplish a flapless landing the pilots had to speed up and drop down fast. In three or four minutes we descended about 5000 feet! They warned us, and I quote, “this is a serious but doable approach, remain calm and review the safety brochure.” The ability of the pilots to control or stabilize the aircraft to achieve a safe, albeit rough landing, got me thinking about our Lear jets used to collect interferometric synthetic aperture radar (IFSAR or InSAR) data. Ok, maybe I am a radar geek if that is what I am thinking about in an emergency landing?
In many cases, we fly in remote areas where there is little opportunity for us to deploy ground control, but still need to acquire high resolution data. How is it possible to achieve high resolution data collections without the deployment of in-scene ground control? The answer lies in the stability of our aircraft and our ability to precisely know where our two radar antennae are with respect to the ground we are mapping and our nominal flight trajectory (planned flight path).
Single pass aircraft systems, like our IFSAR sensors onboard Lear jets (e.g. Figure 1), are well suited for generating regional scale mapping information (image and elevation), anywhere on the globe. As with many surveying or mapping techniques, IFSAR determines the location of a point in three dimensions by solving for an unknown component (given by equation 1) of a triangle (Figure 2) associated to the observation geometry. We know precisely where our two antennae are during an entire data acquisition flight. This is made possible with solid knowledge of IFSAR sensor interferometric baseline (position of the two SAR antennae with respect to the ground), the imagery geometry, and the performance of the aircraft along the planned nominal trajectory, which allow for the generation of accurate elevation data without the need for in-scene ground control.
Our IFSAR systems have two X-band radar antennae mounted to a solid INVAR frame using a rigid boom construction to minimize baseline variation and variation relative to the INS. This rigid construction results in a 1.2 m physical separation (called the interferometric baseline) that provides a temperature and mechanically stable interferometric baseline (e.g. minimal baseline variation and variation relative to the INS). The frame is attached to the bottom of the aircraft on an azimuth steerable pedestal thus, allowing for mapping on both sides of the aircraft (Figure 1). The flight management system is based on an on-line kinematic DGPS system coupled with an INS. Motion measurement data is obtained using a combination of DGPS data collected using an Ashtech Z-12 receiver and a Honeywell 770 IRU. The Inertial Measurement Unit (IMU) is mounted to the same invar frame as the radar antennae to minimize lever arm errors between the IMU and antennae phase centers.
Intermap’s military-grade IMU, mounted on the INVAR frame (interferometric baseline) and coupled with the onboard GPS system, permits absolute angle determination with accuracy of approximately a few thousandths of a degree, significantly improving the critical determination of the interferometric baseline orientation angle. The design of these IFSAR systems, thus provides a stable system from a differential phase point-of-view. Therefore, it is possible to model the systematic phase errors between the two radar antennae and achieve highly accurate data collection without the use of ground control.