ERS – ENVISAT CROSS-INTERFEROMETRY RESULTS OVER THE MACKENZIE RIVER DELTA, CANADA

Urs Wegmüller, Maurizio Santoro and Tazio Strozzi

Gamma Remote Sensing AG, Worbstrasse 225, CH-3073 Gümligen, Switzerland, http://www.gamma-rs.ch, wegmuller@gamma-rs.ch

ABSTRACT

An ERS-2 – ENVISAT Tandem (EET) pair acquired over the Mackenzie River in Northern Canada was analyzed. The interferogram showed high coherence over many parts of the scene, but we failed in generating a DEM over this area because of high spatial phase gradients which prevented us from finding a reliable unwrapping solution. In spite of this the interferometric phase and coherence contain interesting information over this area.

1. INTRODUCTION

The Mackenzie River Delta at 68° northern latitude is characterized by mixed taiga and tundra landscapes with many lakes and river arms. In the context of global warming permafrost and the ecological systems and their changes over time are studied. Near the delta the Mackenzie River and its many arms cover a significant area. In this area hardly any larger vegetation is present. Away from the river there are many small lakes. Between the lakes the land is covered by shrubs and scattered trees.

In 2002 ESA launched the ENVISAT satellite with the Advanced SAR (ASAR). ENVISAT is operated in the same orbits as the ERS-2, preceding ERS-2 by approximately 28 minutes. One of the ASAR modes, namely IS2 at VV-polarization corresponds closely to the ERS SAR mode, except for the slightly different sensor frequency used. A unique opportunity offered by these two similar SAR instruments operated in the same orbital configuration is EET cross-interferometry (CInSAR). At perpendicular baselines of approximately 2 kilometers the look-angle effect on the reflectivity spectrum compensates for the carrier frequency difference effect. In order to specifically collect data suited for EET CInSAR ESA conducted between 27 September 2007 and 12 February 2008 and again in winter 2008/2009 dedicated ERS2 – ENVISAT Tandem Campaigns.

In our study we processed an EET CInSAR pair acquired on 10-Mar-2009 with a perpendicular baseline of 2247m over the Mackenzie River Delta. The objectives were to assess the potential of EET CInSAR to generate a DEM for this area and to see if other information of interest for permafrost research may be found in this special long-baseline short-interval interferogram.

2. DATA AND PROCESSING

For our study we used an EET pair acquired on 10-March 2009 with a perpendicular baseline of 2247m. The relevant parameters of the interferometric pairs considered are summarized in Table 1. To cover a larger area a longer data strip (> 300km along track) was processed.

Table 1 INSAR parameters. dt stands for the time difference, B⊥ for the perpendicular baseline and dDC for the Doppler Centroid difference.

Dates dt B⊥ dDC 20090310_20090310 28 min. 2247 m 344 Hz

For this area north of 60deg. Northern latitude the SRTM 3” DEM is not available. As height reference for the interferometric processing we used either a constant height or the Canadian Digital Elevation Data (CDED, available at http://www.geobase.ca). For further discussion of the EET CInSAR methodology it is referred to [1,2]. The differential cross interferogram of the EET pair acquired on 10-Mar-2009, relative to the CDED heights, is shown in Figure 1 together with the corresponding EET “coherence product”, a composite of the coherence (red), the average backscattering (green) and the backscatter change (blue). Good coherence in many parts confirms the adequacy of the EET pair for CInSAR and the processing applied.

_____________________________________________________ Proc. ‘Fringe 2009 Workshop’, Frascati, Italy, 30 November – 4 December 2009 (ESA SP-677, March 2010)

Figure 1 EET CInSAR (10-Mar-2009, dt= 28minutes, B⊥= 2247m, dDC= 344Hz) differential phase relative to CDED (left, a color cycle corresponds to a phase cycle) and RGB composite of the coherence (red) backscattering (green) and backscatter change (blue, right) of Mackenzie River Delta. Yellow boxes indicate Sections 1 to 4 which are discussed in more detail below. The size of this area is approximately 100km x 300km

3. RESULTS

In March soils and lakes in the Mackenzie Delta region are frozen and snow covered. In the area of the river arms with hardly any larger vegetation high coherence is observed. The small lakes show also high coherence and typically a constant differential interferometric phase for the entire small lake. For the land between the lakes with higher shrubs and northern taiga forest much lower coherence is observed. For this area it was therefore not easily possible to derive a reliable EET CInSAR DEM.

In the rather flat part to the north the phase changes slowly. So far we failed with our attempts to unwrap the differential interferometric phase. The reason is that the spatial phase gradients in the differential interferogram are still very large.

In the following the EET CInSAR signatures and its expected potential for DEM generation and other applications is discussed for several smaller sections representing different land cover regimes of the Mackenzie River area.

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4

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2

3.1. Kugmallit Bay

In the bay area the sea is frozen at this time of the year. The EET CInSAR coherence is high over the coastal sea ice (Figure 2). For this area a constant reference height was used for the DINSAR processing. The CInSAR phase variations observed relate therefore either to height variations of several meter or to cm scale movements occurring during the 28 minutes between the ERS2 and ENVISAT acquisition. Such movements may be related to tidal changes. Fractures in the ice are visible in the backscattering and coherence.

Figure 2 EET CInSAR differential phase (left, a color cycle corresponds to a phase cycle) and RGB composite of the coherence (red) backscattering (green) and backscatter change (blue, right) over Kugmallit Bay (section 1 indicated in Figure 1). Area is 16km x 22km.

Figure 3 EET CInSAR differential phase (left, a color cycle corresponds to a phase cycle) and RGB composite (right) showing the meandering Mackenzie River (section 2 indicated in Figure 1). Area is 16km x 22km.

3.2. Mackenzie River

Near its delta the meandering Mackenzie River occupies a very broad river bed. This area is most likely regularly flooded, as indicated by the missing shrubs and trees. Overall this area is very flat. Nevertheless, significant phase variations are found in the differential interferogram (Figure 3). This is related to the very high sensitivity of the EET CInSAR phase to elevation with a height ambiguity of only 4.2m per phase cycle. Furthermore, the area includes many water bodies. Pure ice is quite transparent to the microwaves and therefore the main scattering originates from the lower side of the ice, which is the solid ground in the case of fully frozen lakes or the ice – water interface in the case of partly frozen lakes and river arms. This interpretation is supported by the high backscattering observed which is typical for partly frozen water bodies [3]. Unwrapping in this area is complicated by the very abrupt phase changes caused by the water bodies. It is not fully clear on what ambiguity the correct unwrapped phase of the water body shall be. The horizontal linear distortion visible in the upper part of Figure 3 is related to a jump in the CDED heights used for the DINSAR processing. The coherence is overall high except for the areas with high phase gradients.

Figure 4 EET CInSAR (10-Mar-2009, dt= 28minutes, B⊥= 2247m, dDC= 344Hz) differential phase (left, a color cycle corresponds to a phase cycle) and RGB composite of the coherence (red) backscattering (green) and backscatter change (blue, right) showing northern lakes in the Mackenzie river delta area (section 3 indicated in Figure 1). The size of this area is 16km x 22km.

3.3. Tundra lakes

Section 3 includes an area with tundra lakes. Over the lakes the coherence is high and the phase relatively constant, but it differs between lakes (Figure 4). Outside the lakes the phase varies strongly. As a result of this strong phase variation and of shrubs and sparse forest

present in this area the coherence is partly low in this area. Phase unwrapping appears to be extremely difficult in this area. As a consequence it is not likely that we succeed in generating a CInSAR DEM for this area. What might be of interest is to further investigate backscattering, coherence and phase over the lakes. Determining the depth of the lakes and the depth to which the lakes are frozen would be of interest [3].

3.4. Richardson Mountains

Further south, in the Richardson mountains the topography starts to vary much stronger. Here the differential CInSAR phase shows very high phase gradients (Figure 5). The DEM used as reference is clearly not accurate enough to be used in support of the unwrapping of the interferometric phase. Furthermore, the slopes and maybe also vegetation result in significantly reduced coherence levels. In this hilly terrain the potential of EET CInSAR is limited.

Figure 5 EET CInSAR (10-Mar-2009, dt= 28minutes, B⊥= 2247m, dDC= 344Hz) differential phase (left, a color cycle corresponds to a phase cycle) and RGB composite of the coherence (red) backscattering (green) and backscatter change (blue, right) showing a section of the hills south of the Mackenzie river delta (section 4 indicated in Figure 1). The size of this area is 16km x 22km.

4. CONCLUSIONS

In this contribution we reported on EET CInSAR results achieved over the Mackezie River in Northern Canada. An EET pair with a perpendicular baseline of 2247m was used to generate a differential cross interferogram and a CInSAR coherence product over a 100km x 300 km area. Much of the area is relatively flat and the CInSAR coherence is high for much of the area. Nevertheless, generating a CInSAR DEM was not achieved. Local high phase gradients and phase jumps

related to the thickness of the ice of partly frozen tundra lakes prevented us from getting a reliable solution.

While failing so far in deriving a DEM over this area the CInSAR phase and coherence product show a good potential to discriminate different land covers in this remote area. Considering that global warming is expected to influence the permafrost in this area, having tools to assess changes in this remote area is of interest.

5. ACKNOWLEDGMENTS

This work was supported by ESA under contract 22526/09/I-LG on ERS-ENVISAT Tandem Cross-Interferometry Campaigns: Case Studies. ERS and ASAR data copyright ESA (CAT 6744). Canadian Digital Elevation Data (CDED) is from http://www.geobase.ca.

6. REFERENCES

[1] Wegmuller U., M. Santoro, C. Werner, T. Strozzi, A. Wiesmann and W. Lengert, “ERS-2 Zero-Gyro-Mode Data Application Showcases“, Proc. FRINGE 2007 Workshop, Frascati, Italy, 26. – 30. Nov., 2007 (http://earth.esa.int/workshops/fringe2007).

[2] Wegmüller U., M. Santoro, C. Werner, T. Strozzi, A. Wiesmann, and W. Lengert, “DEM generation using ERS–ENVISAT interferometry“, Journal of Applied Geophysics Vol. 69, pp 51–58, 2009, doi:10.1016/j.jappgeo.2009.04.002.

[3] Hirose T., M. Kapfer, J. Bennett, P. Cott, G. Manson, abd S. Solomon, Bottomfast ice mapping and the measurement of ice thinckness on tundra lakes using C-band synthetica aperture radar remote sensing, Journal of the American Water Resources, Vol. 44, No. 2, Apr. 2008.