Ice, Cloud and land Elevation Satellite (ICESat)

The Ice, Cloud and land Elevation Satellite (ICESat) was launched in January 2003 for monitoring inter-annual and long-term changes in polar ice masses of the Greenland and Antarctic ice sheets among other objectives. The ICESat is part of NASA’s Earth Observing System (EOS). Since February 20^th, 2003 the Geoscience Laser Altimeter System (GLAS), on-board ICESat, provides a multiplicity of data worldwide. Altogether the GLAS offers 15 different data sets, which can be downloaded at the home-page of the National Snow and Ice Data Center (NSIDC - http://nsidc.org/data/icesat). The altimetry measurements are carried out with a near infrared laser (1064 nm) with a pulse repetition rate of 40 Hz.

The laser illuminates a footprint of about 70 m at the Earth’s surface and the along-track spacing of data points is about 170 m. The height of the surface topography is calculated from the mean of the elevation in-between the footprint and the coordinates (longitude and latitude) showing the center point of the illumination spot at the surface. Zwally and others (2002), Brenner and others (2003) and Schutz (2002) described the concept of GLAS in more detail.

In this investigation the ice sheet elevation data of the GLA12 release 28 data set were used (Zwally and others, 2007). At the time of writing 11 laser periods were available (Table 3.2).

Figure 3.9.: A schematic illustration of the basic concept of ICESat laser altimetry.

Table 3.2.: Overview of the GLA12 release 28 laser measurement periods available at the time of writing.

Laser identifier Days in operation Start date End date

1 38 2003-02-20 2003-03-29

2a 55 2003-09-24 2003-11-18

2b 34 2004-02-17 2004-03-21

3a 37 2004-10-03 2004-11-08

3b 36 2005-02-17 2005-03-24

3c 35 2005-05-20 2005-06-23

3d 35 2005-10-21 2005-11-24

3e 34 2006-02-22 2006-03-27

3f 33 2006-05-24 2006-06-26

3g 34 2006-10-25 2006-11-27

3h 34 2007-03-12 2007-04-14

For the final GLA12 data, the IDLreadGLAS tool offered by the NSIDC was used to convert the binary raw file to an ascii file. Afterwards, a simple shell script extracts all necessary information (longitude, latitude, elevation, time of measuring, ocean tide, ocean load tide and saturation correction factor). The saturation correction factor has to be applied to the elevation data, if the return energy is higher than predicted. The elevation error caused by detector saturation is shown in Fricker and others (2005).

After adding the saturation correction factor to the elevation data, the ocean tide and ocean load tide correction (component of ocean tides, which is propagated a few kilometer inland on the grounded ice masses (Riedel, 2003)) is removed from the elevation data. Based on the laser shot time, the global tide model of TPXO7.1, recommended by King and Padmann (2005), was applied by using OTPS (http://www.coas.oregonstate.edu/research/po/research/tide/global.html), replacing the routinely

Figure 3.10.: A 2D-profile of a L2a GLA12 release 24 ground track. The black dots represent the elevation above the WGS84 ellipsoid, the red dots are the cloud top and the blue dots are the cloud bottom derived by simultaneous atmospheric measurements with the green laser (532 nm).

Figure 3.11.: A 2D profile of the Release28 ground track of GLA12. Plotted is the longitude against the elevation of the descending ground track. The red ellipse shows the effect of a cloud.

used tide correction which is based on a less accurate model for Antarctica. A serious error source of the ICESat altimetry data is clouds. The laser is able to penetrate optically thin clouds, but depending on the particle size and height of the cloud, the infrared laser (1064 nm) is reflected from the top of the near surface cloud (Figure 3.10). In release 28 of the GLA12 data, which was used for this investiga-tion, nearly all clouds were removed by the NSIDC. The few remaining data points affected by clouds (Figure 3.11) can still distort interpolation of the GLA12 data. To avoid interpolation errors induced by residuals of clouds, such residuals were removed.

According to different landscapes, the GLA12 data were separated into three main areas: (i) The coastal region north of the (ii) mountain region and (iii) the plateau region. The smaller the area of investigation,

the more precise are statements on accuracies. For the estimation of the GLA12 accuracies, crossover-point analyses within the laser periods and for every single region were performed. The results can be seen in Table 3.3.

Table 3.3.: Absolute mean elevation differences at crossover points in meter of GLA12 data. Numbers in brackets are the counts of crossover points contributing the mean value. The mean presented in the last row is the mean of presented mean elevation differences of the laser periods.

Laser ID Measurement period Coastal Mountainous Plateau L1 2003-02-20 to 2003-03-29 0.58 m (10270) 0.79 m (5780) 0.59 m (116006) L2a 2003-09-24 to 2003-11-18 0.53 m (576) 0.80 m (278) 0.17 m (42310) L2b 2004-02-17 to 2004-03-21 0.81 m (172) 0.99 m (97) 0.16 m (19837) L3a 2004-10-03 to 2004-11-08 0.68 m (384) 0.79 m (179) 0.16 m (27999) L3b 2005-02-17 to 2005-03-24 0.57 m (358) 0.92 m (148) 0.20 m (24312) L3c 2005-05-20 to 2005-06-23 0.57 m (385) 0.83 m (144) 0.16 m (21518) L3d 2005-10-21 to 2005-11-24 0.07 m (2935) 0.07 m (2132) 0.14 m (24288) L3e 2006-02-22 to 2006-03-27 0.70 m (305) 0.65 m (154) 0.15 m (23738) L3f 2006-05-25 to 2006-06-26 0.90 m (262) 1.18 m (91) 0.16 m (14054) L3g 2006-10-25 to 2006-11-27 0.73 m (236) 0.84 m (161) 0.16 m (20342) L3h 2007-03-12 to 2007-04-14 0.68 m (154) 0.82 m (72) 0.14 m (12732)

mean 0.62 m 0.79 m 0.20 m

Table 3.3 summarizes the elevation differences of the GLA12 measurements, which are varying with investigated area and laser period, but the predicted elevation accuracies of 0.15 m (Zwally and others, 2002) cannot be achieved. Only over the flat plateau region (mean slope of 0.16°), the accuracies are close to the predicted value. For the coastal region (mean slope 0.75°) and the mountainous region (mean slope 1°), the mean elevation differences are about three to four times higher. The standard devi-ations against the laser periods with regard to the three areas (Figure 3.12 A) and the slope (Figure 3.12 B) confirm this. The standard deviations of the crossover elevation differences is largest in the moun-tainous regions (except for L3e and L3h). The lowest standard deviation can always be found at the plateau, where the mean slope is also lowest. This can be confirmed by Figure 3.12 B, where standard deviations are lowest with slopes between 0 and 0.5°. With increasing slope, the standard deviation of crossover elevation differences also increases, but in a non-uniform way. Aside from this, it is very obvious that the laser periods are very different in accuracy and precision.

0 0.5 1 1.5 2 2.5 3

L1 L2a L2b L3a L3b L3c L3d L3e L3f L3g L3h

Laser period

Standard deviation [m]

Coast Mountain Plateau

0 1 2 3 4 5 6

L1 L2a L2b L3a L3b L3c L3d L3e L3f L3g L3h

Laser periods

Standard deviations [m]

0 - 0.5°

0.5 - 1°

1 - 1.5°

1.5 - 2°

2 - 2.5°

A

B

Figure 3.12.: Standard deviations against the elevation differences of GLA12 data at crossover point with regard to the three areas (A) and the different slopes (B).