Snowscat on an Aerial Tramway
Scatterometers emit electromagnetic waves to measure reflection. SnowScat is a fully polarimetric scatterometer which emits microwaves in the range of 9–18 GHz (1.5 cm to 3 cm). It is used to study the interaction of these waves with different snow types. SnowScat analyses snow mass, snow structure and snow-pack morphology such as stratification, grain size and type. This is done by taking measurements of the physical properties of the snow pack in addition to collecting data from meteorological instrumentation, using on-site observations and remote sensing equipment.
In 2014 the European Space Agency (ESA) awarded a contract to GAMMA and the WSL Institute for Snow and Avalanche Research (SLF) to develop tomographic capabilities for SnowScat. The instrument takes multiple images of the snowpack at differing angles and heights using an aerial tramway. From these, a three-dimensional image of the snowpack can be calculated. Our goal is to see snow layers without digging a pit and validate models to measure the snow parameters, which can then be applied on current and future satellite missions.
SnowScat was developed and built in 2008 by GAMMA Remote Sensing AG (http://www.gamma-rs.ch) within the ESA ESTEC project KuScat, Contract No. 42000 20716/07/NL/EL. It supported the Phase A of the ESA CoReH2O satellite mission through campaign data acquired in Northern Finland within the ESA project NoSREx (Nordic Snow Radar Experiment) and ESTEC contract 22671/09/NL/JA/ef.
2014 - 2020
In 2009, SnowScat was initially tested on Weissflujoch in Davos, Switzerland. It was then moved to Sodankylä in Finland for 4 years starting from the autumn 2009 in preparation for the satellite mission COREH2O. It was at the Finnish Meteorological Institute in Lapland where the snow pack on the site had low density and water content. Microwave observations and direct measurements were also performed in Finland, and during the 2012-2013 winter period an experiment of vertical time-domain snow profiling was initiated to resolve the scattering contributions from the snow layers of different physical properties.
For information on SnowScat in Finland see: http://blogs.esa.int/campaignearth/2013/03/19/ready-set-snow/
After Finland, SnowScat was installed on a tower above a maize canopy in Flevoland in the Netherlands from July to September 2013. Here the backscatter response of vegetation was characterised in response to variations in naturally occurring moisture.
Since November 2014 the equipment has been set up at Davos Frauenkirch in preparation for winter 2015.
SnowScat System Specifications
Power: 230V, max ~ 60W
Weight: ~ 40 kg
Temperature Range: -40° C to 40° C
Antennas: Dual pol, < 10° (3dB)
Antenna cross-pol: < -25 dB
Frequency: Stepped CW from 9.15 to 17.9 GHz
Incidence angle: -40° to 110°
Azimuth angle: -180° to 180°
Polarization: HH, HV, VV, VH
Dynamic range: Receiver dynamic range > 80 dB with the 16 bit ADC
Signal bias: < 0.5 dB
Gain characterization: Internal calibration, Calibration 8'' sphere
Control: Remote Control through Ethernet; standalone
Data storage: Internal, external through Ethernet
RFI: Frequency blacklist
Updates made to SnowScat
Previously in Finland the instrument worked with a viewing geometry similar to a satellite. However, the ability to move the instrument around presented the opportunity to have a closer look at the layering of snow. An Aerial tramway was constructed as seen in Figure 3. This allows SnowScat to scan along a linear track roughly perpendicular to the line of sight.
The requirements of this new setup was:
- Vertical Profiling: The new system has the ability to point straight down with no disturbances from the scaffolding.
- New Tilt Sensor: Provides independent measurements of the current sensors attitude.
- The rail: This makes interferometric and tomographic images possible by aperture synthesis which combines signals from a collection of measurements to produce an image with a high resolution.
The new setup means a three-dimensional tomograph can be constructed with a combined resolution of each individual measurement. The tomograph can be used to analyse the snowpack and provide valuable results for future research.