Radiometer measurements from a helicopter on Muragl block glacier

Transcription

1 Radiometer measurements from a helicopter on Muragl block glacier Daniel Vonder Müehll and Hansueli Gubler As a final test within Pace 1, but a preliminary test for air borne BTS measurements using a radiometer, radiometer measurements were performed from a low flying helicopter on March at the Muragl block glacier in the Engadin. On Muragl block glacier and its surroundings the permafrost distribution is well known from conventional BTS measurements, geophysical investigations and several bore holes. Setup The Mira 11.4Ghz radiometer was suspended on a special lever in front of a side door of the Ecureuil helicopter as can be seen from fig. 1 and 2. A special seat was mounted outside the door for the operator. In this way any influence of the helicopter on the radiometer could be eliminated. The operator could easily point the radiometer like a camera to any point on the ground sideward to the aircraft. The radiometer was connected in the usual way to the logging and control system placed inside the helicopter. A display / keyboard fixed to the radiometer indicated the actual apparent brightness temperature and other relevant data. An assistant inside the helicopter started stopped the measurements and stored the data. Daniel Vonder Müehll guided the pilot to the different measuring points. This setup was very preliminary and only allowed for a rough test of the systems suitability for air born BTS measurements. Figure 1. Installation of radiometer

2 Measuring procedure The radiometer was pointed from outside the helicopter toward the ground. The helicopter was flying 20m to 40m above ground for the measurements. 1. Positioning of the helicopter so that the sidelooking radiometer pointed roughly to the selected area on the ground. 2. Stationary flight of the helicopter. 3. Rough manual adjustment of the radiometer for the Brewster angle of 50 off nadir relative to the orientation of the foot print area of the radiometer. 4. Start of a measuring cycle. 5. Fine adjustment of the radiometer s elevation angle for maximum TB (fig. 3). 6. Stabilizing the radiometer in the position for 10 to 20 sec. 7. Termination of the measurement cycle and storing of data. 8. Flight to the next measuring point. Figure 2. Radiometer in flight TB TB( OFF NADIR), v, NADIR, Before and after the flight, the radiometer was calibrated in the usual way by measuring sky temperature and a known black body temperature ( at ambient temperature). Both calibration measurements were made at roughly the same height above sea lever as the measurements, about 1 km from the block glacier. The sky was slightly overcast by transparent Stratus that had only a very minor effect on the sky s TB. Figure 3 TB in function of off nadir angle, maximal value at the Brewster angle. Measurements Measurements were made at 23 points inside and outside the permafrost area and on the north slope on the right site of the block glacier. The 3db viewing angle of the

3 radiometer is 6 with a corresponding diameter of the foot print of less than 10m. All measurements were made at vertical polarisation. Effective emmissivity for snow covered ground was chosen from earlier measurements to be equal The ambient temperature during the measurements varied between 5.2 and 9.5 C. The Dicke reference temperature of the radiometer changed from +5.6 to 2.8 C during the measurements showing that the instrument was not completely in thermal equilibrium during the measurements. The drift of the calibration between the two instrument calibrations was less than 0.5 C. The ambient temperature increased from 2.5 to 0 C at the calibration site between the to calibrations. The snow height at the target sites is estimated to be between 1.5 and 2m. Measuring positions and results are given in fig 4 and 5. BTS Muragl BTS max BTS aver BTS min Reihe4 Reihe5 Reihe6 0.0 BTS, C location index Figure 4 Comparision of radiometer and conventional BTS measurements. BTS max, aver, min are radiometer measurements, Reihe 1..3 are conventional temperature measurements Possible sources of errors 1. The slope angle at the target areas varied from 0 to 45, therefore estimation of the initial elevation angle of the instrument to approximately meet the Brewster angle at the footprint before the measurement cycle was started was difficult. Large readjustments of the viewing direction during the measurement led to increased scatter of the data. 2. The exact footprint area may differ from the one.

4 3. The helicopter position was not always perfectly stable during the measurements. 4. The vertical viewing plane of the radiometer was not always exactly perpendicular to the slope.. Figure 5. Approx. position of the radiometer targets 5. The fine adjustment of the off nadir angle of the instrument during the measurement to maximum TB was not always easy and is responsible for the large scatter of data in some cases. The most reliable measurements have small scatter and a mean value of TB close to the maximum value of TB of the respective measurement cycle. 6. The ground surface below the snow cover especially on the rock glacier may be very rough. 7. An other problem for radiometer measurements in steep terrain and in valleys follows from the fact that the effective background temperature that is partly re-

5 flected into the radiometer may be significantly higher than the sky temperature because the sky around local nadir of the actual target may be partly hidden by much warmer terrain from nearby slopes. This effect may increase the effective TB significantly and modify the dependence of TB on local off-nadir angle. Measurement 15 is very typical for this problem. This measurement was made at the foot of the very steep lateral boundary slope of the rock glacier with the radiometer pointing toward the slope. Measurements for similar topography showed increasing TB between off nadir angles from 40 to 60 by more than 3 C com 8. pletely hiding the local maximum off TB at the Brewster angle ( fig. 6 ) So it can by assumed that this measurement may show a much too high TB. TB TB( OFF NADIR), v, NADIR, TB TB( OFF NADIR), v, NADIR, Figure 6. Left: typical example for a measurement at the base af the lateral boundary slope of a rock glacier with the radiometer pointing toward the slope. Right: TB in function of the off nadir angle may drop very quickly above the brewster angle 9. At point 18 I failed to correctly point the radiometer. Here the minimum value of the measurements is more likely to represent TB at the Brewster angle for this footprint. Here the footprint was on the steep lateral slope of the rock glacier, but most of the measurement taken corresponded to positions on the rock glacier s surface and were therefore at very high off-nadir angles. Similar measurements show (fig.6) that the apparent TB may drop as much as 7 C for an increase of the off nadir angle by 10. Discussion of the results The radiometer measurements were compared to conventional BTS measurements and temperature measurements in several bore holes. A comparison and a discussion of the results is given in table 1. The correspondence is surprisingly good. As for earlier measurements we may conclude that if snow conditions meet the following criteria areal BTS can be from an airborne platform. The snow cover has to meet the following criteria: Completely dry, no heavy crusts, mainly small grains. The measurement can undoubtedly be improved: Only readings taken at approximately the Brewster angle should be taken into account, The radiometer should be combined with a video camera to determine the exact target area.

6 No Site description BTS on ground air borne BTS Comment 1 Lowermost part of the rock glacier, on the tongue, central line. Permafrost -4.7, -5.5: these points are in fact some 30m apart. Other neighboring points are outside the rock glacier -8.4 Radio is quite cold 2 Above the lowermost steep ridge of the rock glacier, central part. Between borehole H2/1999 and the front. Permafrost 3 Borehole 2/1999, permafrost thickness is about 20m. Permafrost proven 4 Borehole 4/1999, permaforst thickness is ca 20m. Permafrost proven 5 On the orographic right part of the rock glacier. Permafrost 6 On the orographic right part of the rock glacier. Permafrost 7 On the orographic right part of the rock glacier, on the plain, where the rock glacier turns to the west. Permafrost 8 On the orographic left part of the rock glacier on the plain, where the rock glacier turns to the west. Permafrost 9 On the orographic left part of the rock glacier in the upper slope above the curve. Permafrost 10 North exposed steep scree slope southwest of the rock glacier. Permafrost 11 North exposed steep scree slope southwest of the rock glacier. Permafrost -5.5: another point more to the margin shows that towards the margin of the rock glacier, temperature decrease -5.1: Mean between several points which show similar values -5.0: several measurement points, mainly further down -4.8: nearest BTS-points are some 30m apart -3.4: this values is on the inactive lateral part of the rock glacier No BTS-values available, permafrost present No BTS-values available, permafrost present No BTS-values available, permafrost present -6.4: Two BTS measurements hint to permafrost -6.4: Two BTS measurements hint to permafrost -3.5 Good agreement -5.0 Good agreement -2.5 Radio quite warm -2.3 Radio quite warm -2.4 Radio quite warm -3.8 Good agreement with what is -5.2 Good agreement with what is -5.4 Good agreement with what is -7.1 Good agreement with what is Good agreement with what is and 12 Foot of the north exposed steep scree -2.7: Transition zone according tio several Radio quite cold slope southwest of the rock glacier BTS-values 13 Forefield of the rock glacier and the steep slope, no permafrost assumed -2.0: Points in the forefield indicated no permafrost -1.2 Good agreement with what is and 14 Forefield of the rock glacier and the -2.0: Points in the forefield indicated no -2.0 Good agreement with what is and

7 steep slope, no permafrost assumed permafrost 15 Lateral forefield of the rock glacier towards the creek, no permafrost assumed -0.5: Two measurements indicated no permafrost +4.2 ( more likely close to 0) Good agreement with what is and. BTS radiometer too high, see comment in 16 Lateral of the rock glacier towards the creek, no permafrost assumed 17 Lateral of the rock glacier towards the creek, no permafrost assumed 18 Between the large boulders and borehole 1/1999, transition zone between degraded permafrost and no permafrost text +0.5: The ground is certainly not frozen +0.2 Good agreement with what is and -0.9: The ground is certainly not frozen -1.7 Good agreement with what is and -3.5: Transition zone, one value indicate (-4.5 max) Hansueli reports about some problems. Pointing no, two say yes problem with the radiometer, see text. 19 Borehole 2/1999, permafrost thickness is about 20m. Permafrost proven 20 Orographic left part of the rock glacier (container was there). Permafrost 21 Left margin on the rock glacier. Permafrost 22 North exposed steep scree slope southwest of the rock glacier. Permafrost 23 Between the large boulders and borehole 1/1999, transition zone between degraded permafrost and no permafrost. -2.6: BTS are not too cold vue to the fact that the rock glacier indicate permafrost -2.7: One BTS value in the creek is warm, the other cold -4.0 Radio fits better to what is known from drilling! -4.9 Radio fits better to what is known from drilling! -6.9: Two values indicate permafrost Good agreement with what is, quite cold -6.2: Two values indicate permafrost Good agreement with what is, quite cold -3.5: Transition zone, one value indicate no, two say yes -2.1 Good agreement with what is and Table 1. Comments to the measuring points

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