Penny Strait

 

Day 13. Pond Inlet, Byam Martin Channel, Penny Strait. Successful field work and airborne survey execution requires a significant amount of preparation and an ability to bring the right tool for the job at hand. And even then, one has to be prepared for the situations that arise unexpectedly and can create instant problems that need to be solved. Here are three examples from the day's activities that can perhaps illustrate this.

 1. Ability to read the weather forecast. Originally, we were hoping to survey the sea ice near Pond Inlet, a community across the channel from Bylot Island and on the north coast of Baffin Island. This is where there is a significant SmartICE presence. However, Windy, our trusty weather app that uses several models for its weather forecasting was predicting high winds that would have made our surveys too difficult, especially for the EM bird which is slung below the Polar 5 at about 50 feet above the sea ice. So Plan B was to go to Byam Martin Channel where Sentinel 1 shows an ice island present in the channel just above the Northwest Passage. This would have been an excellent opportunity for the instrument suite to be tested on a variety of sea ice types. Unfortunately, however, with an oncoming system moving into the region, and high winds also in this area, we decided to aim for Penny Strait, off the northeast coast of Bathurst Island where weather conditions were forecast to be survey-favourable. At this site there is typically some open water and some rough ice that will provide some interesting survey lines.

 

Survey lines for Penny Strait. The western box covers the Byam Martin Channel region of interest (ROI) and the eastern box Pond Inlet

 

2. Ability to bring math tools to flight planning given the changes that were made during the day. This is a fun, superficially easy one but which actually can be surprisingly complicated to get right. Survey waypoints are used by the pilot to fly the survey lines along which our instruments operate and capture data. Most of the scientific instruments aboard record data from directly below (nadir) or from within a region plus and minus an angle off nadir. For example, the lidar records data from +/-30 degrees from nadir (a 60 degree angle swath width). From simple trigonometry, this covers 577 m across the surface below assuming we fly at 500 m above the surface below (note above sea level and above ground level are different).

 

Example instrument observation geometry for ground coverage from the aircraft. The swath width is 577m.

 

If you know the target area that is required, survey lines should be set to ensure that there is enough of them to cover the area that is required. In the example below, it is possible to have 2 flight lines coverghing the target. But to be safe, we would probably want a 20% overlap so three flight lines will be needed.

 

Example flight line planimetric view of the coverage. In this example, three lines would be needed and this flight line would straddle the southern boundary.

 

Simple-ish! The complexity comes when the area of coverage is rotated, the survey anchor is a fixed location with a heading (typically a centroid for the area of coverage), and it is not clear how many flight lines are needed to completely cover the required area. Further complexity comes with the CryoSAR imaging radar which looks to the right side. In this case the direction of flight matters and the offset has to be factored in. Then the number of swaths needed with the overlap factored in too. The figure below illustrates the radar coverage for a single imaging swath. The red lines are the nadir tracks and the blue box is the swath coverage that would be imaged by the tracks that look to the right (so NE -> SW track and SW-NE track).

CryoSAR tracks (red lines) to image the blue outlined rectangle. This configuration would give two passes but the incidence angles would be different for each pass.

 

 

the number of CryoSAR tracks needed to observe the blue box (required area of coverage) from both sides. Three swaths would be acquired from each direction. 

In the example above, the direction matters, so complete coverage is attained from both directions. However, if imaging from both sides is not required, then the configuration is simpler requiring imaging from one side only (and one direction).

 

In the case of Penny Strait, the goal was to fly on the outward track at 58m above sea level with the bird deployed. Then on the way back the aircraft climbed to 1000 m asl and the CryoSAR was used to image the same line that the EM bird took but in the opposite direction. In essence the return flight line needed to be 1300m to the left of the outgoing flight line. 

For Pond Inlet, we have and even more complex trig problem. Now we have two adjacent boxes that are about 2 swath widths of CryoSARfor coverage. SmartICE measurements are being made on the ice surface. So we need three passes from each direction. But the boxes are not aligned with one heading different than the other (see the light blue rectangles below). And for the pilot, we need three waypoints per complete line with the second waypoint of each triplet being the inflection point (or turning point). So we have to do the math and calculate the place where the two lines for each full survey line intersect. These intersection coordinates becomes the second waypoint in each line. 

 

Non-aligned areas of coverage and the nadir flight tracks needed for 3 point lines to image the region from north and south.

3. Agility to problem solve on the fly, literally. In this case, the question is one of being able to monitor the systems and see when they are not functioning correctly. While the instruments are all electro-magnetic, electro-optical or electro-microwave in design, these are complex systems that interface with other systems such as GPS and IMU devices and with RF communication and mechanical devices. One would have thought iut is all plain sailing and that they will funciton as built. But not so! For example, the CryoSAR was happily operating on the way out but the GPS navigation on the L-band was lost about three quarters of the way along the outward flight. After some light testing, the system was shutdown and rebooted and so we ended up with the "have you tried turning it off and on again" approach (see IT Crowd for the full context and habitual joke). Of course, this worked and were back on track. It pays to pay attention...

 

Photos from the flight are included below. The first set are from the aircraft hanger as we waited for fuelling - other people's workshops are always interesting places.

 

  

 

Various photos from an aviation workshop

 

Views from the flight and the midnight sky (all from Jeff Welch)

 

Despite the changes and many adjustmenst in the day, the flight survey was a success. We even spotted a walrus below the flight which we have imaged acrosss the VIS/IR and microwave sepctrum.

Tomorrow looks like a weather day.