This paper describes techniques that have been investigated in order to minimise RF interference effects between a 35.5GHz radar and a multi-frequency radiometer with one of its bands operating around 36.5 GHz. One of the main potential interference paths identified is direct coupling between the instruments' antennas: both instruments have high-gain, earth pointing reflector antennas. The instruments are to be accommodated on a small satellite and, unlike large multi- instrument platforms, it is not possible to use the satellite structure to isolate them. The radiometer antenna is scanned mechanically over a large sector and there are no feasible options for introducing baffles or shields in order to reduce the antenna coupling factor. Instead, techniques that have been looked at include frequency-domain filtering and time-domain blanking. Blanking is achieved using a PIN switch to isolate the input to the radiometer channel during the period when the radar is transmitting. Whilst the approach can reduce the effects of interference to acceptable levels there is the potential for the PIN switch to degrade the radiometer's performance in other ways. Although such switches have been used in Dicke switch radiometers there is little reported information on their use in total power radiometers as is proposed here. Therefore a programme of tests was conducted to investigate the stability and repeatability of a PIN switch placed in the front-end of a representative radiometer. These tests, the results obtained and the conclusions drawn are reported on.
The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer due to be flown in conjunction with AMSU-A on the TIROS-N series of satellites. The instrument provides two 'window' channels centered on 89 GHz and 150 GHz and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. AMSU-B is specified to have an antenna pattern half power beamwidth (HPBW) of 1.1$DEG +/- 10%, beam efficiency of >=95% over the main beam (2.5 times HPBW) and to control mispointing of the antenna beam to within +/- 0.1$DEG with a knowledge of +/- 0.05$DEG. The paper describes the techniques used to measure the performance of the AMSU-B antenna system and the results obtained. Using the techniques described, the antenna patterns of AMSU-B were measured over a dynamic range greater than 76 dB. Beam pointing was measured to within +/- 0.02$DEG.
The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer. The instrument provides two 'window' channels, one at 89 GHz and one at 150 GHz. Three channels centered around 183.31 GHz are used to determine the atmosphere's humidity profile. Double sideband operation improves the instrument's temperature sensitivity by reducing its effective noise temperature. However, because this implies that the atmosphere is sounded over two separate frequency bands it is necessary to determine how the sensitivity of the instrument varies across each passband, and also the difference between the sensitivities of the lower and upper sidebands of each channel. This is especially important for the 183.31 GHz channels where the sounding frequency relates to altitude in the atmosphere. This paper describes a test technique which was developed to characterize the variation in sensitivity of each channel of AMSU-B.
The Advanced Microwave Sounding Unit B (AMSU-B) is a five channel microwave radiometer due to be flown in conjunction with AMSU-A on the TIROS-N series of satellites. The instrument provides two 'window' channels centered on 89 GHz and 150 GHz and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. AMSU-B is a total power radiometer which is required to achieve a temperature sensitivity of 1 K to 1.2 K according to channel. It has a scan period of 8/3 seconds; during each scan it acquires 90 Earth view pixels, 4 cold calibration pixels and 4 hot calibration pixels. The calibration data are, in effect, used to determine the gain of the radiometer during that scan period. The sources used for calibration are an on-board, ambient temperature black- body target for the hot calibration, and deep space for the cold. The radiometric performance of the instrument is dependent on the accuracy to which it is calibrated during each scan period. The required calibration accuracy is +/- 1 K with a random component due to short term fluctuations of not more than +/- 0.2 K. This paper addresses the assessment of in-orbit error sources which effect calibration over the mission duration. The emphasis has been placed on errors in the bias values over life and errors associated with orbital variations of the hot and cold calibration target. The conclusions of the assessment are presented.
The Advanced Microwave Sounding Unit - B (AMSU-B) is a five channel radiometer. It provides two 'window' channels centered on 89 GHz and 150 GHz, and three channels centered on 183.31 GHz which are used to determine the atmosphere's humidity profile. The paper addresses the principal requirements for the instrument and presents the results obtained for the Engineering Model instrument.