HUT-2D – Airborne Interferometric Radiometer
The HUT-2D instrument was finished in its current configuration in 2005. It consists of 36 L-band receivers installed in a U-shape configuration. When airborne, it is mounted below the fuselage of Aalto University’s research aircraft Skyvan pointing to nadir.
The technical characteristics of HUT-2D are shown in Table 1. The radiometer provides a swath of 95 % of the flight altitude, 14 x 15 pixels within the alias-free field of view, and an angular resolution of 3 to 6 degrees depending on tapering.
Table 1. Technical characteristics of the HUT-2D radiometer.
|
Parameter |
Value |
|
Center frequency |
Nominal 1413 MHz, adjustable within 1400 to 1427 MHz |
|
Bandwidth |
7 MHz |
|
Polarization |
Switchable dual linear polarisations |
|
Alias-free field of view |
± 25° |
|
Swath |
0.95 x altitude |
|
Boresight pointing |
Nadir |
|
Radiometric sensitivity |
0.8 K (integration time 4000 ms) 2.8 K (integration time 250 ms) |
|
Angular resolution at boresight |
3° (no tapering) 4° (triangular tapering) 6° (Blackman tapering) |
|
No. of pixels within the alias-free field of view |
14 x 15 |
|
Minimum incidence angle |
0° (vertical) |
|
Maximum incidence angle (center of pixel fully in alias-free field of view with maximum incidence angle) |
34° for pixels in image corner 23.5° for pixels in image linear axes |
Figure 1 shows the layout of the instrument. Each arm consists of 12 receiver units with antenna, RF-receiver and receiver control boards integrated on top of each other. Every arm has one distributed calibration unit and local oscillator & sample clock signal division unit. Additionally, one common centralized calibration unit and one local oscillator unit is required. The supply power to the instrument is distributed via the correlator and the supply power division node. The correlator is attached inside the aircraft cabin.
Figure 1. HUT-2D layout.
Figure 2 depicts the HUT-2D instrument when accommodated below the fuselage of our Skyvan aircraft. The size of the instrument is approximately 2 m by 2 m and its weight is 100 kg.
Figure 2. HUT-2D instrument below the aircraft fuselage.
The HUT-2D system has been a key element for the preparation and execution of the SMOS mission including the following highlights:
- the SMOS on-board calibration system was first tested on the HUT-2D radiometer
- the Corbella equation, which provides the theoretical basis for remote sensing by aperture synthesis, was verified using HUT-2D
- the SMOS external calibration strategy was verified by the first-ever image of galaxy by aperture synthesis
- the HUT-2D instrument onboard Skyvan provided the first end-to-end demonstration of the SMOS mission concept
- the first-ever SMOS-like sea surface salinity retrievals were made by HUT-2D
- HUT-2D onboard Skyvan was used to acquire airborne data for selected test sites in the ESA SMOS 2008 Rehearsal Campaign and the 2010 Cal/Val Campaign.
The HUT-2D system has been in operational use since 2006. The performance of the HUT-2D radiometer has been analyzed in detail based on measurements of well-known natural targets, such as cosmic background radiation and low-salinity water scenes.
Recent publications:
- Rautiainen, K., J. Kainulainen, T. Auer, J. Pihlflyckt, J. Kettunen, and M. Hallikainen, “Helsinki University of Technology L-band airborne synthetic aperture radiometer,” IEEE Transactions on Geoscience and Remote Sensing, vol. 46, no. 3, pp. 717-726, 2008.
- Kainulainen, J., K. Rautiainen, and M. Hallikainen, “First 2-D interferometric radiometer imaging of the Earth from an aircraft,” IEEE Geoscience and Remote Sensing Letters, vol. 4, no. 2, pp. 241-245, 2007.
- Kainulainen, J., K. Rautiainen, J. Lemmetyinen, J. Seppänen, P. Sievinen, M. Takala, and M. Hallikainen, Experimental study on radiometric performance of synthetic aperture radiometer HUT-2D – Measurements of natural targets, IEEE Transactions on Geoscience and Remote Sensing, vol. 49, no. 2, pp. 814-826, 2011.