IEEE Geoscience and Remote Sensing Magazine - March 2014 - 10

figure 1. G1X UAS on approach to land at SLW site in Antarctica.

Radars have been used to sound ice, map both deep and
near-surface internal layers, and image the ice-bed interface with fine resolution.
Most of the radars used for sounding ice during the
1960s and 1970s were incoherent. A few attempts were
made to develop coherent radars for sounding ice in 1970s
with very limited success because of the lack of suitable
inexpensive technologies [16]. The first solid-state coherent radar sounder was developed at the University of Kansas in the late 1980s [17]. It was redesigned, upgraded,
and widely used for measurements over the Greenland
ice sheet as a part of NASA's Program of Arctic Assessment (PARCA) initiative [18-21]. Following the successful demonstration of a fully coherent low-power radar for
sounding ice sheets, coherent radars were developed by

table 1. G1X technical sPecifications.
Parameter

Value

units

Dimensions/Geometry
Length

2.84

Height

1.11

2.06

WinG
Area
Span

5.29

Aspect ratio

11.75

n.d.

PoWer Plant
Engine

Desert Aircraft 100 cc

Max power

7060

WeiGhts
Fuel

2.72

Payload

9.07

Empty

26.76

Max takeoff

38.55

Performance

Cruise speed

m/s

Range

100

Endurance

min

Takeoff/landing distance

other groups and are currently used for measurements
over the Greenland and Antarctic ice sheets [22-23].
At the University of Kansas, we developed radars with
4-15 element cross-track arrays and multiple receivers for
airborne and surface-based measurements [8, 21]. These
radars include synthetic aperture radar (SAR) and array
processing capabilities [8, 23] and have been successfully
used to demonstrate 3-D imaging of ice sheets [24-26].
These systems have also been used to sound several challenging areas of the ice sheets, including outlet glaciers
and ice sheet margins [27-28], as well as for high-altitude
measurements from long-range aircraft [29]. However, as
mentioned previously, the performance of radars operating at frequencies of 50 MHz or higher severely degrades
over fast-flowing glaciers due to rough surface scatter
and volume scatter. A few attempts have been made to
develop coherent airborne HF radars operating at frequencies as low as 1 MHz and as high as 30 MHz [30-35]
for sounding glaciers with temperate ice; these have been
shown to be effective in sounding temperate ice under
favorable conditions. Normally, low-frequency radars
are operated with long, resistively-loaded wire antennas
trailing behind the aircraft. Although SAR processing can
be used to reduce beamwidth in the along-track direction, the large beamwidth of trailing long-wire antenna
in the cross-track direction results in significant reflections from the walls of the glaciers, which degrades radar
performance. A UAS equipped with low-frequency radars
that can be flown over closely-spaced lines-as close as
5 m at 14 MHz-in the cross-track direction for synthesizing a 2-D aperture is needed to sound fast-flowing glaciers with fine resolution.
3. Platform overview
The G1X unmanned aerial system (UAS) is a mid-wing,
semi-autonomous, high-aspect ratio aircraft that has
been modified by the University of Kansas specifically
for scientific missions throughout the cryosphere. The
G1X UAS platform has a 5.3 m wingspan, 2.84 m fuselage length, and weighs approximately 38.5 kg when fully
instrumented and fueled. When operating at a cruise
speed of 28 m/s, the platform has a range of about 100 km
for approximately one gallon of fuel. Additional range
and endurance can be enabled with a supplemental fuel
tank. The aircraft can be configured with either wheels or
skis; recent field trials in Antarctica were all on skis. Takeoff and landing performance is verified at 90 m or less.
Figure 1 shows a photograph of the G1X on approach to
land at the SLW snow runway in West Antarctica. Table 1
shows the G1X's technical and performance information.
The integration of HF/VHF antennas for operating the
radar on a small UAS drove aircraft requirements. The
antenna's physical length requirement demanded a relatively high-aspect ratio and wing span (Table 1). Winglets
were used to improve the stability characteristics of the
aircraft. The physics-based model, pilot rating, and system
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