HUTRAD – Airborne Multifrequency Radiometer

The Helsinki University of Technology RADiometer (HUTRAD) for remote sensing was constructed in the late 1990’s and consists of a non-imaging subsystem and an imaging subsystem. The non-imaging subsystem operates at six frequencies between 6.8 and 94 GHz, with vertically and horizontally polarized channels at each frequency. The imaging subsystem operates at 93 GHz and is dual-polarized. The main technical characteristics of HUTRAD are close to those of the AMSR Earth observation instrument. HUTRAD is accommodated onboard our research aircraft Short Skyvan.

Figure 1. The HUTRAD system accommodated in the rear cargo bay onboard Aalto University’s research aircraft Skyvan.

Design Criteria

The HUTRAD system was designed to meet the following requirements (motivation given in brackets):

• The main technical parameters, including frequency, polarization, and incidence angle, are identical to those of space-borne instruments (geophysical algorithm development)
• The main measurement parameters can be programmatically changed (flexible use of radiometer)
• The receivers are modular (easy service)
• Temperature stabilization of the radiometer is good (eliminates output drift, improves sensitivity)
• The receivers tolerate external interference (airborne measurements in populated areas)
• A reliable calibration method is used (correct brightness temperatures)
• A data localization system is included (airborne and surface data can be reliably combined)
• For imaging channels, the platform’s angular motion is compensated for (nondistorted image).

Summary of Technical Characteristics

The HUTRAD consists of the following subsystems:

• Non-imaging dual-polarized subsystem for frequencies 6.8, 10.65, 18.7, 23.8, 36.5, and 94 GHz
• Imaging dual-polarized subsystem for 93 GHz
• Ancillary instrumentation for visual target recording, aircraft navigation, and localization of scientific data.

The receivers for the 6.8 to 36.5 GHz range are Dicke-type radiometers, whereas the 93 and 94 GHz receivers are total power radiometers. The temperature control is based on Peltier elements that provide rapid heating/cooling, thus effectively reducing receiver output drift. The temperature stability of the critical receiver components is better than ± 0.1 K during a typical data collection flight. The incidence angle is 50º off nadir. The antenna 3 dB beamwidth varies with frequency from 5º to 3º for the non-imaging channels and is 1.6º for the imaging channels. The integration time is programmable and the sensitivity is better than 0.6 K (integration time 0.5 s) for all channels. Calibration of the receivers is based on using hot (ambient temperature) and cold (boiling nitrogen temperature) absorbers on the ground before and after each data collection flight. Additionally, the 93 and 94 GHz total power receivers employ an in-flight calibration system (ambient temperature only) in order to correct for the output drift. The imaging subsystem provides vertically and horizontally polarized images of the target. From a nominal altitude of 1 km the swath width is 1.3 km, allowing examination of spatial variations in the brightness temperature.

Description of Subsystems

The non-imaging subsystem consists of the low-frequency unit (6.8, 10.65, and 18.7 GHz receivers) and the high-frequency unit (23.8, 36.5, and 94 GHz receivers). A Dicke type construction was chosen for all receivers operating in the 6.8 to 36.5 GHz in order to minimize the influence of the internal noise and gain fluctuation. For the 94 GHz receiver a total power configuration was chosen due to the high loss and cost of Dicke switches at high frequencies. In order to maximize the performance to cost ratio the lower frequency radiometers (6.8 and 10.65 GHz) are direct receivers, whereas the others are superheterodyne receivers. To improve the sensitivity and reduce costs the 94 GHz system has a double-sideband receiver. The rest of the superheterodyne channels employ single-sideband receivers. All the receivers have antennas with separate outputs for vertical and horizontal polarizations. The 36.5 GHz radiometer is a polarimetric radiometer capable of simultaneous measurement of all four Stokes parameters.

The 93GHz receiver is a dual-channel receiver (horizontal and vertical) operating in the total power mode. In order to produce an image of the test site the whole receiver is moved using two servo motors. The scan motor is used to perform across-track movement of the receiver footprint and the pitch motor is used to compensate out the pitch angle variation of the aircraft.

The tasks of the main computer are to run the user interface, to save radiometer data, and to regulate the inside case temperatures of the receiver. The motion controlling computer controls the movements of the two motors so that a desired scan pattern is accomplished. The motor control routine running is activated periodically (2ms) by interrupt pulses generated by the video texter/timer card.

Physically these blocks are placed in two separate units, the electronics unit and the instrumentation unit. Inside the electronics unit are both the computers and the interface electronics. The motors and the receiver system (including the 93GHz receiver, DC-amplifiers, heaters and fans) are assembled in the instrumentation unit.

Data Storage and Handling

Data from each receiver of the non-imaging subsystem is transmitted over a local-area network to a file server, where the data is stored on a hard disk. The data can be monitored in real-time in order to provide a rough estimate of the quality of the measured data. The file server is running the Linux operating system, which provides multi-protocol networking and true multi-user workstation capabilities, while at the same time providing various network services needed to control time synchronization and data storage on the network. The file server is equipped with a GPS receiver, which acts as a primary network timing source. The geopositioning data produced by the GPS receiver is also stored to provide information about the actual flight path taken during the measurement. The time synchronization precision has been measured to be a few milliseconds. Scientific and ancillary data from the imaging subsystem is stored on a hard disk.

Temperature Stabilization

The inside temperature of the receivers is stabilized in order to minimize output drift. The temperature of the radiometers is measured with PT100 resistive sensors from at least three different locations inside each receiver, and any of these sensors can be used as the input for the temperature regulation system.

Regulation of the temperatures is implemented as separate threads in the measurement software. The software approach enables easy modification of the regulator parameters, and makes it possible to develop sophisticated interference detection and fault diagnosis methods. Each temperature regulator controls a Peltier/heating element, thus enabling reliable measurements during unstable climatic conditions.

HUTRAD accommodation on Skyvan aircraft

The Short Skyvan twin-engine turboprop aircraft is equipped with a relatively large (1.5 m by 1.9 m) rear cargo door and a large cabin (cross-section 2 m by 2 m), which make it ideal for remote sensing instrument installation.

The HUTRAD radiometer system is accommodated in the rear cargo bay of the aircraft, separated by a wall from the passenger cabin where the operating panels are located. HUTRAD looks backwards along the flight track; the rear cargo door is removed during data collection. Each receiver has its own measurement and control computer. The control computers are connected to the local area network, which carries data and control information between the radiometers and the file server and monitoring work station. The nominal aircraft speed during data collection is 55 m/s.

Calibration

The receivers are calibrated on the ground immediately before and after a measurement flight by using two calibration targets that consist of absorbing material. The calibrators are placed in front of the antennas resulting in recorded brightness temperatures equal to the physical temperatures of the absorbers. One of the targets is at ambient temperature (temperature measured before and after calibration) and the other is cooled to 77 K using liquid nitrogen. In addition, the 93 and 94 GHz channels employ an in-flight calibration system that provides the ambient temperature reference whenever desired.

For the non-imaging subsystem, the physical temperature of the hot target is measured by traditional thermometers. The calibration for each receiver is repeated a minimum of three times. The quality of the calibrations is evaluated. Additionally, the non-imaging 94 GHz total power radiometer employs an in-flight calibration system in order to compensate for the drift of the measured brightness temperature vs. time. The drift is due to minor changes in the inside temperature and the receiver gain. A hot (ambient temperature) target is placed by a motor in front of the antenna immediately before and after data collection over a test site. During data collection the absorber is outside the field of view of the antenna. The physical temperature of the in-flight absorber is measured using a PT100 sensor and the results are stored on the radiometer server. Assuming that the receivers are linear a first-degree calibration curve is determined from the hot/cold calibration procedure. For 94 GHz data additional drift corrections are done.

The calibration procedure of the 93 GHz imaging subsystem consists of on-ground calibrations before and after every data collection flight and, additionally, of in-flight calibrations before and after every measurement site. On-ground calibration is performed using both ambient temperature and nitrogen-cooled absorbers. In-flight calibration employs only ambient temperature absorber. During calibration of the imaging subsystem the computer samples the outputs of the receiver several times. The quality of calibration is checked by counting the standard deviation of these samples. If the STD exceeds the user-set limit value the calibration is discarded. The temperature of the hot calibration absorber, which varies with ambient temperature, is measured repeatedly using a PT100 temperature sensor.

Video System

Motion video on the target is recorded and displayed at the monitoring panel. The video subsystem is equipped with a text generator, which inserts date and time information together with user-editable commentary text into the video image.

Navigation System

Remote sensing flights have strict navigation requirements. Both positioning accuracy and accuracy of the actual flight track of the aircraft have to be better than 10 meters. To meet these requirements, a custom made navigation system was designed and installed in the Skyvan research aircraft. The system is based on real time differential GPS with an accuracy of about 2 meters. The navigation system provides the pilots with a graphical display with symbolic map presentation, off-track indication and numerical navigation data. A custom system has allowed flexible mission-specific software development. Optimal presentation of data has significantly improved flight accuracy and reduced mission time and costs. The navigation computer is also a part of the local area network, allowing communication with the scientific workstations. In-flight target selection and flight planning is possible based on a moving map display in the cabin.

Data Localization System

Redundant position data is logged both on server and navigation computer. Scientific data localization is performed using the GPS time signal. All non-imaging radiometer data is time tagged using the airborne local area network clock. The network has its own GPS receiver dedicated for server clock synchronisation. The receiver timing pulse-per-second (PPS) signal is used for the purpose. The achieved time synchronisation precision was measured to be on the order of milliseconds. Also direct time synchronisation is possible for other instruments that are able to utilize the PPS signal.

Fully Polarimetric 36.5 GHz Radiometer

To enable polarimetric measurements, the 36.5 GHz receiver of the HUTRAD system was modified into a polarimetric radiometer, Fully Polarimetric Radiometer (FPoR). Additionally, a Fully Polarimetric Calibration Standard (FPCS) was developed for the FPoR. The current versions of the FPoR and FPCS were completed in 1999 and 2001, respectively. An extensive set of laboratory and airborne measurements has been performed to verify the function and define the characteristics of the system.

Figure 2. Polarimetric 36.5 GHz radiometer.

The FPoR measures all four Stokes parameters, which allows the determination of the polarization state of the brightness temperature signature. Detection of the target surface or inner structure anisotropy is possible. An example of such a structure is sea surface; presently, the detection of wind vectors by determining the height and orientation of sea waves is the most promising application of polarimetric radiometry. Compared to sequential measurement of Stokes parameters used by the polarimetric radiometers of the first generation, the simultaneous measurement applied by the FPoR improves the obtained sensitivity. To calculate the third and fourth Stokes parameters, the FPoR applies analog correlators based on commercially available analog multipliers. The third and fourth Stokes parameters describe, respectively, the linearly and circularly polarized components of the brightness temperature.

The calibration system FPCS is used to illuminate the antenna of the polarimetric radiometer with accurately calculable Stokes vectors. This enables the complete end-to-end calibration of the receiver. The FPCS is based upon microwave blackbody targets in ambient and boiling nitrogen temperatures, a freestanding metal wire grid polarizer, and a phase retardation plate. Associated mathematics to calculate the calibration coefficients have been developed, as well as calibration software.