STAR LIGHT

Introduction

STAR-Light will enable hydrologists to extend plot models of land-atmosphere energy and moisture transport processes to the circumpolar Arctic. Arctic energy balance experiments at sites that are both representative and accessible are used to develop Land-Surface Process (LSP) models, but extrapolating from these sites to the varying and mixed terrains of the circumpolar Arctic will require model
Baseline Aircraft:
Aviat Husky A-1B
calibration and validation that can be achieved only with frequent observations over broad regions beginning with spring thawing and ending with fall freezing. Once calibrated, these regional models will then serve either as improved lower boundaries of atmospheric models or as more reliable elements of integrated regional hydrology models.

At the heart of the LSP model are estimates of moisture stored in soil, vegetation, and snow. The quality of these estimates and the skill of model predictions can be significantly improved by assimilating near-daily observations of the moisture in the upper few centimeters of soil. The remote sensing hydrology community has converged upon 1.4 GHz brightness as the most effective observation for this purpose. Recent breakthroughs in radiometer technology, in LSP/Radiobrightness models, and in efficient schemes for assimilating radiobrightness have placed us on a path toward reliable long-term monitoring of changes in the amount, state, and spatial distribution of moisture stored within tundra throughout the Arctic. Essential elements of this vision are data from satellite radiometers and calibrated LSP/R models for arctic terrains.

Both the European Space Agency and NASA are developing 1.4 GHz synthetic aperture radiometers for low Earth orbit. Hydrologists are preparing for the advent of data from these instruments with extensive field campaigns to develop and calibrate LSP/R models, and to validate schemes for assimilating satellite data. The focus of these efforts has been prairie terrains. There are no proven LSP/R models for arctic terrains even though one could reasonably argue that remote sensing technologies are more vital to Earth system science in the Arctic. Mature LSP/R models for arctic terrains require collaborative campaigns involving arctic soil and snow hydrologists and remote sensing hydrologists supported by near-daily regional data from an airborne 1.4 GHz imaging radiometer. Only three such instruments are planned for the next decade – two in Europe and an enhanced version of NASA’s Electronically Scanned Thinned Array Radiometer (ESTAR) which flies on the NASA P-3 – a large, 4-engine turboprop aircraft. The high operating costs of the P-3, the conflicting schedules of instruments on the P-3, and the demand for ESTAR data in NASA’s many large field campaigns will greatly limit its use in seasonal and inter-annual investigations in the Arctic.

The PI’s research group has developed the first example of a compact, 1.4 GHz, Direct Sampling Digital Radiometer (DSDR). The proposed STAR-Light instrument will use seven 1.4 GHz DSDR receivers configured as a 2-dimensional synthetic aperture radiometer. STAR-Light will be sufficiently compact and robust to operate in the Arctic on a light aircraft or on an Uninhabited Aerial Vehicle (UAV). A design goal is that it fit within the performance and configuration limitations of a Super Cub. STAR-Light will be designed, fabricated, and tested over a 3-year period by the Space Physics Research Laboratory (SPRL) at the University of Michigan.

Synthetic Thinned Aperture Radiometer (STAR) Concept

10 element antenna array:

 
 

Different antenna baselines sample different spatial Fourier components of the scene:

 

Vi + j Vq =  Tb(f) F1(f) F2(f) exp(j 2 p sin f d / l ) df
 

The correlation between pairs of brightness signals as a function of the pair spacing essentially samples the scene at a spatial frequency that depends upon the pair spacing. The Fourier transform of the correlation is then the brightness of the scene. The equivalent filled aperture antenna for a 10-element array looks as follows:

 
Each element looks as follows: 

 
The 10 elements are put together as follows:

 
The STAR-Light sensor module and control module will be attached to the Aviat Husky A-1B as follows:
A close-up of the sensor module:

STAR-Light Specifications

System Specifications:

 
Aircraft Specifications:
Altitude => 150 m to 3 km 

Speed => 60 to 90 kn 

Power => 232 W with 208 W margin 

Weight => 177 lb with 24 lb margin