GMI Concept and Design

The Ball Aerospace concept for the GMI is illustrated in Figure 2. The GMI employs an offset parabolic antenna with an aperture size of 1.2 m. The antenna subsystem includes four feedhorns serving the nine channels. Each frequency is allocated an independent feedhorn with the exception of a shared feedhorn for the 18.7-GHz and 23.8-GHz channels. The antenna subsystem and receiver electronics rotate at 32 rotations per minute. A stationary thermal shroud, with an opening to cold space, surrounds the rotating instrument subsystems. The instrument will be responsible for its own momentum compensation. The control circuitry and logic governing instrument spinning and momentum compensation is contained within the instrument controller assembly. The instrument controller assembly and momentum wheel, providing momentum compensation, are mounted beneath the shelf supporting the GMI sensor.

Figure 2. Ball Aerospace GMI concept.

The four-point technique provides a polynomial fit to the instrument response that typically, in other spaceborne radiometers, is estimated by a linear fit. The conventional hot and cold targets are measured as per the typical total power calibration scenario. However, in addition, a noise diode is switched on momentarily during the hot load and cold sky target views, providing two additional temperature points available for calibration. The benefit of the technique is a more accurate instrument calibration.

The annular ring on the instrument deck is constructed of a metallic-coated material of low heat capacity and characterized by low emissivity at infrared and microwave wavelengths. The ring serves to radiatively isolate the hot load target from the instrument deck while the instrument deck rotates beneath it. In addition, the hot load and annular ring have enveloping shrouds to prevent solar heating either directly or through reflection. This design will help minimize temperature gradients within the hot load. All channels of the GMI will be calibrated to an accuracy of 1.35 K or better where this calibration value applies to the GMI main lobe temperature.