# Satellite-Based Sea Ice Navigation for Prydz Bay, East Antarctica
## 5 Discussion
The case of CHINARE2011 illustrates the importance of availability of SAR data on daily basis even if the ice cover does not appear to change much. The ice scene in the three Envisat images acquired on 22, 25 and 27 November 2011 (the three images at the right side in Figure 7) does not change but the appearance and propagation of the thin bright line in the three images proved to be of particular importance as described before. In general, one SAR image per day would be sufficient. This revisit frequency has been available with the launch of Sentinel-1A in April 2014 and Sentinel-1B in in April 2016, comprising a constellation of the two satellites. The two-satelliteconstellation could offer a 6-day exact repeat cycle. The SAR revisit frequency will be further enhanced with the launch of Radarsat Constellation Mission (RCM) scheduled in 2018 [28]. The increased revisit frequency will be a useful and welcomed improvement for route planning. _Question 3: What is the required level of accuracy of ice information?_  
Accuracy of existing SIC maps ranges from 5 to 20%, depending on the concentration-based categories of the ice cover, i.e., open drift, closed pack/drift, compact and consolidated, etc. [29]. The accuracy increases as ice age or thickness increases. This level becomes acceptable as long as the ice field is not subjected to severe changes at short time scale. Less accuracy is usually attached to scenes of dominant thin ice-types such as open and closed pack drift; usually encountered in polynys and marginal ice zones. Nevertheless, the accuracy is not a crucial issue in this case since navigation through thin ice is not a challenge for ice breakers. As for the ice type information obtained through subjective analysis of remote sensing data (SAR or optical sensors), the accuracy cannot be established. The navigational route plan can then rely on such unverified analysis, supported by SIC maps and ship observations. When changes in the ice cover take place at a fast rate under severe weather conditions the question of the accuracy becomes irrelevant and any planned route will be subject to adjustments; which can sometimes be substantial. _Question 4: Is there a need for data integration from different sensors?_  
A single-channel SAR may not be very useful in discriminating between open water and thin ice cover as in the case of polynya (Figure 9). The radar backscatter from thin ice and open water overlap heavily because each surface has a wide range of backscatter signature. Signature of thin ice is affected by: (1) surface salinity, which varies significantly with thickness during the early ice growth stage, as well as the presence of frost flowers; and (2) surface roughness, which is manifested in a few forms such as rafting. On the other hand, open water exhibits a range of backscatter signature starting from very low values when the water surface is calm to high values when the surface becomes turbulent in response to wind action. This ambiguity does not exist in optical remote sensing data (e.g., in MODIS images) since the surface roughness does not affect the radiance from sea water. Integration of optical and radar imagery is important in this case. Another supporting observation is the identification of the iceberg in the SAR image (Figure 9 near the middle of the image) compared to its vague appearance in the MODIS image. Large grain size of the iceberg's snow cover trigger high backscatter in SAR data [30]. Acquisition of multi-polarization SAR data is recommended to solve the ambiguity of SAR signature of open water. Recent studies concluded that backscatter in cross-polarization SAR is always low regardless of the wind-driven surface roughness [31]. These data will be more available for commercial use in the future.