Helicopter Flights

The radar wasown over Line A six times - three times each at the two altitudes. Flights were repeated in order to minimise the possibility of the radar data being corrupted by biases, due to systematic and/or random errors as well as errors in any post processing algorithms - the use of which was anticipated.

In the validation analysis that follows, the data presented consists of three ights at nominal altitudes of 100 m (referred to as Pass 1, 2, and 3), and threeights at nominal altitudes of 50 m (referred to as Pass 4, 5, and 6). The comparison between the two altitudes was intended to gauge the relative contribution of the errors in the helicopter operations (such as off-nadir pointing), as well as the effect of different scales of surface roughness sampled by a changing active area of the radar.

Localisation of the Passes

In order to locate the start and end of the Passes it is necessary to locate the coordinates of the

ags between which the helicopter ew for Line A (ags 2 and 7 ingure 6.9(a)). Since GPS coordinates of theags were not recorded, the absolute localisation of theags andight lines in space and time is not possible. However, relative localisation is possible and is sufcient for our analysis due to the small distances involved.

It was expected that the radar data would contain a clear signature of the corner reectors, however this signal is not identiable in the radar data. Fortunately, examination of the laser data revealed spikes in the laser returns that corresponded to the corner reectors. Figure 6.14 shows an example of the elevation prole for Passes 1, 2 and 3, with a dip indicating a lack of return for this point i.e. the presence of a corner reector. This method allowed only the startag to be located. The absence of a corner reector spike for the Pass 2 may be a product of the data post-processing, whereby only points originating from nadir were kept.

The GPS location of theag at the end of the 200 m Line A transect could not be derived in this manner, as there was no record of the corner reectors in the laser data. However, since the transect is known to be 200 m long, it was possible to approximate the location of the endag using INS data, which provided the total forward distance travelled by the helicopter and its heading.

6.3. VOYAGE 1, 2008 119 0 50 100 150 200 250 0.2 0.4 0.6 0.8 1 1.2

Distance along transect (m)

Relative elevation (INS altitude

ï

laser range) (m)

Pass 1 Pass 2 Pass 3

Figure 6.14: A plot of the relative altitude derived from the difference between the laser range and INS altitude, demonstrating the ’’dropout’’ in laser signal when the helicopter ies over the corner reector placed at the startag. The absence of the signal in the second Pass may be due to the post-processing of the laser data which picked out only those returns at nadir.

A summary of the start and end ag GPS locations for the six Passes are provided in table 6.3. The GPS coordinates are specied to six decimal places as this is the required relative accuracy if localisation to meter is required. Figures 6.15 and 6.16 plot the distribution on a Cartesian plane of the start and end ag coordinates with the origin dened as their mutual mean location. The

gures demonstrate that the start and end of the six Passes are located to within 2-3 m of one another.

Pass # Start Flag Stop Flag

1 -68.575713, 77.686651 -68.573888, 77.689787 2 n/a -68.573883, 77.689865 3 -68.575695, 77.687783 -68.573872, 77.689832 4 -68.575703, 77.687686 -68.573879, 77.689745 5 n/a -68.573885, 77.689812 6 -68.575703, 77.687658 -68.573883, 77.689731 Mean -68.575704, 77.687744 -68.573882, 77.689785 Table 6.3: Coordinates of the start and endags of Line A.

Figure 6.15: Distribution of the estimated coordinates of the startag of Line A using the corner reector position.

ï1 ï0.8 ï0.6 ï0.4 ï0.2 0 0.2 0.4 0.6 0.8 1 ï1 0 1 2 3 4 5

Distance, EastïWest (m)

Distance, North

ï

South (m)

End flag: pass 8

End flag: pass 1

End flag: pass 2

End flag: pass 3

End flag: pass 7

End flag: pass 9

6.3. VOYAGE 1, 2008 121 ï68.5758 ï68.5756 ï68.5754 ï68.5752 ï68.575 ï68.5748 ï68.5746 ï68.5744 ï68.5742 ï68.574 ï68.5738 77.6875 77.688 77.6885 77.689 77.6895 77.69 Latitude (degrees) Longitude (degrees) Passes: 1, 2 and 3 Passes: 4, 5 and 6

Figure 6.17: The ight tracks of the six Passes. The maximum separation between them is 2 - 3 m, as shown ingure 6.18.

Data gathered between theseag locations were extracted from the INS, laser and radar logs. The localisation of the six Passes is shown in gure 6.17, which plots the latitude and longitude of theight tracks, and highlights the location of the six Passes. The separation seen between them is calculated to be approximately 2 m. Figure 6.18 plots the tracks of the helicopter for the six Passes on a Cartesian plane. The origin of this coordinate system is dened as the startingag, the y-axis points to the end ag. This gure demonstrates that for the purposes of the current examination, the helicopterew a direct path between the start and end-ags. The small deviation from a direct line-of-sight path may need to be re-considered during comparison with thein-situ

data since the measured snow thickness spatial distribution varied between the sampling groups (spaced 4 m apart).

In summary, considering that the start and end of the six Passes was localised, and the ight path between them retrieved, it was thus meaningful to look for a correlation between thein-situ

ï4 ï3 ï2 ï1 0 1 2 3 4 0 20 40 60 80 100 120 140 160 180 200 Starting point (m) relative to start flag, located at (0,0)

Distance along transect (m) Pass 1

Pass 2 Pass 3 Pass 4 Pass 5 Pass 6

Figure 6.18: Theight tracks of the six Passes plotted in a Cartesian plane with the startingag at the origin. The red lines show the assumed tracks along which

in-situsnow thickness measurements were recorded.