# Satellite-Based Sea Ice Navigation for Prydz Bay, East Antarctica
## 5 Discussion
In open drift (SIC between 0.4 and 0.6), ice will move at a speed approximately three times higher than the motion of close pack ice (SIC between 0.9 and 1.0), yet it is not hazard for ship navigation. When pack ice moves freely in response to the wind, its speed approaches nearly two percent of the wind speed [25]. Faster motion is expected when the ice surface becomes rough. The steep topography and the presence of glaciers in the Antarctica's coastal areas create katabatic winds that intensify sea ice motion. The effect of the wind extends to within 40 km off shore. A data set of satellite tracked sea ice motion in the Antarctic for the period 1992-2010 is presented [26]. It links the statistically significant trends in Antarctic ice drifts (in most sectors) to local winds. Maps of sea ice motion with 48-h span and 62.5-km resolution are generated by the Ocean and Sea Ice program of the Satellite Application Facilities (OIS-SAF) of EUMETSAT (The European Organisation for the Exploitation of Meteorological Satellites) for limited areas in the west Antarctica winter. The 25-km EASE-Grid (the Equal-Area Scalable Earth Grid (EASE-Grid)) weekly sea ice motion around the Antarctica, available from the National Snow and Ice Data Centre (NSDIC), shows limited ice motion in the study area; typically around 3 km/day with anomalies that can reach 12 km/day [27]. Such large-scale gridded data are not useful for tactical ship navigation planning. Ice motion products from SAR data, with remarkable precision compared to products from passive microwave or scatterometers, are not useful also because of their poor temporal revisit coverage. In all cases, ice motion tracking cannot be estimated accurately during the ice melt season or at locations where variable atmospheric conditions cause decorrelation of the radiometric signature from one day to the next. The above discussions deliver a rationale for using the MSLP and near-surface wind data to reveal information about the severity of local ice motion and hence justify a decision to order fine-resolution SAR data. The requirement for timely remote sensing data depends on the ice conditions revealed initially by the coarse resolution data. This issue can be illustrated using the key dates for each expedition (Table 3), which include arrival of the vessel to the ice-covered area, arrival to final destination and the acquisition time of the satellite images/products. It should be noted that in CHINARE2012 the journey ended on 3 December (arrival to the Zhongshan Station) after almost 6 days from entering the pack ice in Prydz Bay. However, RV _Xuelong_ took only 2 days to cross the pack ice and the polynya (Figure 8) and nearly 4 days to cross land-fast ice ahead as mentioned before. The table also shows that the most up-to-date satellite data (which were used to develop/or adjust the proposed route) were acquired on the same day (in 2011 and 2013) or 3 days before the actual time of the vessel arrival in the imaged scene (in 2012). The decision was mainly based on the severity of ice dynamics (change within a few hours or a few days). In the case of CHINARE2012, when a large polynya covered most of the Prydz Bay, MODIS data acquired 3 days before the vessel entry to the bay was sufficient for the route planning. On the other hand, when the bay was covered by nearly 100% ice concentration in 2013, Radarsat-2 SAR image had to be ordered so that the acquisition date coincided with the time of the vessel's arrival at the sea ice edge. The route plan for the CHINARE2011 mission was based on Envisat images acquired 4 then 3 days before the vessel encountered the ice then adjusted based on a newly acquired image (2 days before the vessel encountered the ice). The frequent changes in ice concentration, motion and re-distribution caused by the high wind speed in the Antarctic region warrants a need for higher temporal resolution of SAR imagery when planning for vessel routes.