*3.2. Logistical Support*

The scientific operations of Snow Eagle 601 mainly rely on logistical support from the Chinese Zhongshan Station and the Russian Progress Station Skiway located in the Larsemann Hills along Ingrid Christensen Coast, East Antarctica. The distance between the skiway and Zhongshan Station is about 10 km. The skiway is maintained by Progress Station during the aviation operations of Snow Eagle 601 based on collaboration between Russia and China. A temporary camp is built beside the skiway during aviation operations of each year (Figure 2). Custom-designed cabins and containers with different functions are configured for accommodation and living, power supply, field data processing and the storage of aviation accessories and scientific instruments (Figure 2). Ground support vehicles including Arctic Trucks, PistenBully snow vehicles and snowmobiles are used to transport personnel and equipment between Zhongshan and the Progress Skiway, maintain the camp and support field work.

Daily weather forecasts are provided by Zhongshan Station, Progress Station and Davis Station; the Antarctic Mesoscale Prediction System (AMPS, https://www2.mmr.ucar.edu/rt/amps) is additionally monitored regularly by the flight crew. The crew will choose from multiple flight plans based on all available weather reports.

**Figure 2.** Field camp for aviation and scientific operations of Snow Eagle 601 located along the Russian Progress Station Skiway.

#### **4. Airborne Survey and Data Processing**

#### *4.1. Flight Plan Design*

Flight plans are designed based on pre-season experiment designs that consider the scientific significance of each line in the context of previously completed surveying and flight limitations (e.g., maximum range and weather conditions) in target areas. Before a flight mission, two or three potential flight plans are prepared for the pilots to choose according to weather conditions. Key waypoints along a flight route are provided in the flight plans as well as surface elevation, flight range and flight altitude at each waypoint. As flight plans are created using projected coordinates but the aircraft is flown using geographic coordinates, the distance between each waypoint is kept to less than 50 km to minimize the resulting track error.

A target area can rarely be comprehensively surveyed on one flight. Systematic flight planning and survey organization allows an airborne survey to return to the target area and fill in coverage systematically and is robust against weather and equipment failures. This approach also allows elegance in analysis. We use a system of data organization developed by the University of Texas Institute for Geophysics that maps our survey plans and survey metadata into the organization of the collected data. The organizing elements are termed PSTs (project/set/transect). The project is generally defined according to the name of surveyed area or scientific campaign (e.g., PEL, representing Princess Elizabeth Land). The set indicates the survey platform used and the version of the instrument suite. For Snow Eagle 601, the first three letters in the set are taken from the final three letters of the aircraft tail number (C-FGCX); the last two numbers identify the version of the instrument suite used (e.g., GCX0g, representing the seventh version of the first acquisition system on C-FGCX). The transect represents particular survey lines within a survey area defined by the project, with orientations planned based on the target areas. Transects are typically either planned as radial lines for maximum range or as grids of perpendicular lines over an area of interest. For the latter, X lines are typically planned to run along the glacier flow and perpendicular Y lines are planned to run across the ice flow. X and Y lines are numbered sequentially from one to a number high enough to cover the survey area with a chosen line spacing; the final digit in each transect is used to denote whether the flown line is new (with an 'a' designation) and the final digit will be incremented from a to z for each repeated line or extension to the line flown on subsequent flights. PSTs divide a continuous flight into di fferent transects by turning points and are unique transects of data collected at a discrete location and at a particular time. PSTs are imported into a data acquisition system by FOP during the flight and they are very useful for future data processing, organization and interpretation. Therefore, PSTs are clearly noted in a flight plan. The original definition of PST nomenclature can be found in UTIG's technical report database [18].

Chosen experiment designs depend on the given scientific objectives. While a radial survey plan is the most e fficient way to explore large unknown areas, it also has few crossovers useful for evaluating data consistency and results in very long baselines that can result in degraded GNSS solutions. Radial lines were chosen for the first season flown by the Snow Eagle 601 airborne platform to quickly cover all of Princess Elizabeth Land. Gridded survey plans are generally chosen for investigating critical targets with a high spatial resolution requirement or where many crossovers are needed for data validation. Disadvantages to grids include longer transit lines to the survey areas, many turns and heavy logistical support required. For small outlet glaciers and ice streams, a gridded survey is the best choice as it can focus on and cover the target region in detail and also follow glaciological features.

The flight parameters cannot usually be the best for all of the instruments onboard so compromises must be made in the flight plans, often depending on the prior goals of the airborne survey. The airborne IPR should be flown at a constant height of over 500 meters above the surface to avoid saturating the low gain radar channel but preferably lower than 1500 meters to guarantee a su fficient penetrating capability. The maximum range of the laser altimeter is 1500 m, depending on surface and ambient conditions. Straight and level flight are preferred for gravity and magnetics and it is best if crossovers occur at similar elevations. During the past five austral seasons, with the highest priority of mapping the geometry and subice properties of ice sheets by airborne IPR in PEL, the Snow Eagle 601 flew at a height of ~600 m over surface most of the time, usually requiring consistent flight elevation changes to match changes in surface elevation. However, the flight height decreased to ~60 m when the signal attenuation in the ice was known to be high and penetrating capability was especially needed over areas with complex ice properties or deep subglacial valleys where the subglacial bedrock was di fficult to detect because the IPR signals could be significantly attenuated or reflected away.
