Spatial Resolution and Sampling

Figure 3 illustrates the instantaneous field of views (IFOVs) of the GMI channels and their translation from one scan to the next. The figure illustrates the relative size of the channel footprints, the common off-nadir angle for all channels, and the inter-scan distance resulting from the spin rate and spacecraft ground speed. (The figure does not reflect the instantaneous projections of the beams per the feedhorn GMI channel footprints in the Figure 3 layout.) For comparison between the GMI and DPR instruments, the IFOV for the DPR radars (identical for both radars) is illustrated. For channels 1 through 7, the IFOVs enable spatial contiguity in the cross-scan dimension. For channels 8 and 9, the IFOVs satisfy a minimum coverage in the cross-scan dimension of 50%.

Figure 3. GMI channel footprints.

The choice of sampling times for the GMI is governed by the desire to achieve “Nyquist” spatial sampling in the alongscan dimension of the swath. In addition, samples from individual channels must be co-registered on the Earth surface. Sample times are slightly larger than integration times due to latencies inherent to the digital sampling electronics. To satisfy the Nyquist criterion, all channels will be sampled at a minimum of two times as the GMI scans through a single IFOV. To guarantee co-registration on the Earth, sample times for each channel will be integral multiples of each other. Electronic time delays between channel sampling will account for the fixed angular differences in along-scan beam pointing due to the multiple feedhorn design.

Rain Mapping

The GMI radiometer will serve as a transfer standard in two contexts: as a radiometric transfer standard for the other radiometers of the GPM constellation, and as a precipitation transfer standard for the retrievals of the GPM constellation. Both transfer standards represent areas of scientific research.

In the first context of the radiometric transfer standard, the GMI radiometric calibration will serve as a reference for other radiometers. In this method, the brightness temperature calibration of constellation member radiometers will be adjusted to achieve a common basis with that of the GMI. This technique will reduce precipitation retrieval differences between sensors due to biases from inter-sensor calibration. Referencing calibration to the GMI requires that the GMI maintain excellent calibration and calibration stability through its life. In operation, the ability to create a common calibration from the GMI will depend upon statistical data from spacecraft intersection events, i.e. the viewing of common earth scenes. Such intersection comparisons are complicated by factors such as intervening clouds and precipitation, different look angles, different footprint sizes, and time delays and sample spatial separations specific to each intersection.