**1. Introduction**

Antarctica is the most hostile and remote continent on Earth, possessing a large area of over 14 million km2. Overall, 98% of the continent is covered by flowing ice sheets, causing many areas to be inaccessible from the ground [1]. The Antarctic continent and overlying ice sheet both play crucial roles in the recent sea level rise in response to global warming [1]. Crucial questions include how is Antarctica changing on regional and continental scales, what drives and controls Antarctica's changes and what is Antarctica's impact on climate and sea level changes. To answer the questions proposed by the Scientific Commission on Antarctic Research (SCAR), Antarctic and Southern Ocean Science Horizon Scan and the Antarctic Roadmap Challenges (ARC) Project [2,3], observations in Antarctica with high accuracy, resolution and reliability are urgently needed in order to improve our understanding of the continent and ice sheets and predict their changes and influences through numerical modeling.

Airborne surveys have been used to study Antarctica since the 1960s [4,5]. Multidisciplinary instruments onboard airborne platforms provide an e fficient and flexible means of data collection over Antarctica, satisfying various requirements for polar exploration [6]. As a bridge between ground-based and space-borne observation, airborne surveys can acquire data with high e fficiency in areas that are hard to access from the ground and achieve more accurate and higher resolution measurements relative to satellite remote sensing. Crucially, we cannot ye<sup>t</sup> collect ice-penetrating radar (IPR) data from orbital platforms due primarily to interference from the ionosphere. With these advantages, airborne surveys have been continuously and extensively used in Antarctica for the past several decades. A number of large international airborne campaigns have been launched to survey Antarctica including the early SPRI-NSF-TUD (Scott Polar Research Institute, the National Science Foundation and the Technical University of Denmark) survey [7], AGAP (Antarctica's Gamburtsev Province [8]), IMAFI (Institute–Moller Antarctic Funding Initiative [9]), PolarGap [10], ICECAP (International Collaborative Exploration of the Cryosphere by Airborne Profiling [11–13]) and OIB (Operation IceBridge, https://www.nasa.gov/mission\_pages/icebridge/index.html) surveys, as well as airborne surveys by German Polar 5 and Polar 6 in Dronning Maud Land and Dome F, respectively e.g., [14]. These projects have mainly focused on measuring the geometry and properties of ice sheets, subglacial topography and the geological structure of the continent. Based on airborne survey data, three important datasets, Bedmap (Antarctic Bedrock Mapping [15]), ADMAP (Antarctic Digital Magnetic Anomaly Project [16]) and AntGG (Gravity and Geoid in Antarctica [17]), have been compiled and gradually updated through international e fforts. These datasets provide primary knowledge of ice thickness, bedrock topography and the geological settings of Antarctica.

China has recently become involved in Antarctic aviation. In 2015, a BT-67 airplane called Snow Eagle 601 was deployed to operate in Antarctica for Chinese National Antarctic Research Expeditions (CHINAREs [13]). An airborne IPR, a gravimeter, a magnetometer, a laser altimeter, a camera and a GNSS were configured and integrated on the airplane, o ffering powerful capabilities for aerogeophysical investigations. Meanwhile, as a branch of ICECAP, an international campaign of ICECAP/PEL was initiated by China to survey Princess Elizabeth Land (PEL), the largest data gap in Antarctica, using the Snow Eagle 601 airborne platform along with collaboration with the USA and the UK. In the past five austral seasons, the airborne platform has been continuously applied to survey PEL and other critical areas in East Antarctica including the Amery Ice Shelf, Ridge B, the West Ice Shelf, the Shackleton Ice Shelf and the George V Coast.

This paper reviews the scientific designs and operations of Snow Eagle 601 in Antarctica from the past five austral seasons from 2015 to 2020. First, we introduce the Snow Eagle 601 platform and ground-based scientific equipment. Next, we describe scientific operations supporting the aerogeophysical surveys including aviation support, survey design and data collection and processing. Finally, we summarize the progression of the airborne survey and discuss potential developments for airborne instruments and possible regions for additional survey in Antarctica for future years.

#### **2. Snow Eagle 601 Airborne Surveying System**

#### *2.1. Airborne Platform*

The Snow Eagle 601 airplane is a modified DC-3 aircraft, now known as a DC-3T or, more formally, a BT-67. Standard improvements include new Pratt & Whitney (East Hartford, CT, USA) turbine engines, a modern avionics system, fuel system and structural reinforcements among others. Modifications of the aircraft for polar operations include combined ski/wheel landing gear, an oxygen system, an air conditioner and a large cargo door. Ice-penetrating radar (IPR) antennas are mounted beneath the wings, a tail boom is used for a magnetometer and rolling doors beneath the fuselage are opened in flight to support a laser altimeter and a visual camera. GNSS antennas are mounted on each wing over the center of gravity (CG) and forward of the CG. Structural enhancements, electrical

conduit and junction boxes to support power and data connections to science instruments are also installed on Snow Eagle 601.

Currently, the scientific instruments on the aircraft emphasize aerogeophysical investigations including an IPR made to be functionally similar to the High Capability Airborne Radar System (HiCARS) developed by the University of Texas Institute for Geophysics (UTIG) [13,18] for deep ice-penetrating capability, a GT-2A gravimeter [12], a CS-3 magnetometer, a Riegl LD90-3800-HiP laser altimeter, an Elphel NC353L downward-looking camera and a JAVAD dual frequency, four channel, carrier-phase GNSS receiver (Figure 1 and Table 1). Redundant interfaces were also reserved for the installation of more instruments in the future. More detailed descriptions of Snow Eagle 601 and the airborne scientific systems can be found in Cui et al. [13].

**Figure 1.** The Snow Eagle 601 platform and instruments.


**Table 1.** The Snow Eagle 601 airborne instruments and their typical performance and observing targets.

Two to four days were needed to install the instrument suite onto the aircraft including two days for the gravimeter warm-up and auto-calibration before flights began. Ground testing of the instrument suite was required to confirm functionality before the first flight test. Normally, a short (~3−4 h) flight test would be conducted to test the operation of the suite, emphasize the high power radar amplifier and to calibrate the magnetometer. At the end of a season, airborne instruments can be de-configured from the airplane in one day.

#### *2.2. Base Stations*

As shown in Figure 2, two GPS base stations and a magnetic base station were installed beside the landing area to enable the differential processing of airborne GNSS data and to record variations in the magnetic field due to solar activity. GNSS and magnetic data from observatories in Zhongshan Station were also recorded during each flight as a backup for the instruments running at the skiway. When flights were conducted at other Antarctic stations, available GNSS and magnetic data from the nearest base station was requested for potential use. Gravity ties between the aircraft parking area and absolute gravity stations were measured at least twice during each season for airborne gravity data processing.

#### **3. Aviation Support**

## *3.1. Aviation Groups*

Snow Eagle 601 is owned and managed by the Polar Research Institute of China (PRIC, Shanghai, China) but the airplane is registered in Canada; aviation operation and maintenance services are provided by Calgary, Alberta-based Kenn Borek Air Ltd. (KBA, Calgary, AB, Canada). KBA is a commercial company with a long history of complex polar aviation operations; only a three-person KBA crew is required for all aviation operations of the aircraft.

The PRIC determines annual missions and establishes the schedule for both the scientific and logistical operations of Snow Eagle 601. A field team from the PRIC and collaborative institutes is organized to support flight missions and carry out airborne surveys with separate scientific and logistical groups, each typically composed of six members. The scientific group is split into two sub-groups with three people managing flight operations (FOP) and the other three managing base operations (BOP). The main duty of FOP is to install and maintain airborne instruments, design flight plans and acquire data on each flight. The main duty of BOP is to conduct data processing and quality control (QC), operate and maintain base stations and prepare media and flight documents for FOP.

In a typical mission cycle, BOP initiates magnetic and GPS base station recording while FOP starts a pre-flight reference measurement on the aircraft gravimeter and starts recording GPS on the aircraft receivers at least half an hour before the aircraft moves. During the airborne survey (about seven hours), FOP will manage and monitor data acquisition on the airplane while BOP monitors the base instruments. Generally, only two of the three FOP members will fly on a given flight as each additional operator reduces survey flying time by about 15 to 30 min (80–160 km range) because the fuel load is limited by the aircraft weight. After landing, FOP and BOP require about an hour to complete a post-flight gravity reference measurement and recover all media including GPS data that are left to record for 30 min after landing. BOP collects media from the magnetometer and GPS base stations. After the post-flight data collection, both airborne data and base station data are moved to the BOP office. BOP requires about five hours to carry out data download, processing, quality control (QC), document printing and scanning and archiving. BOP and FOP discuss data QC results together to confirm that all instruments performed nominally and the collected data are of the desired quality before confirming the next flight.
