## 2 Materials and Methods
### TMI 1B11 V8 (Gpm05)
#### 2.1.1 Algorithm Development
Based upon an analysis of the on-orbit receiver gain, it was determined that the gain was stable over a period of 20 s. Thus, an algorithm was implemented that uses a running average of 9 scans (current scan plus 4 scans backward and forward) for the hot load measured temperature (Th), the hot load counts (_Ch_), and the cold counts (_Cc_) to compute the gain and offset [7]. In Figure 2, the calculated gain is shown with and without the scan averaged algorithm. The fourth modification in the final version was based on identifying and removing RFI in the cold counts. It was reported in [10; 11] that there was an RFI intrusion coming from a geostationary  
Figure 1: Scan bias correction for channel 10.65 V for Ta = 170 K (**blue solid line**) and Ta = 280 K (**red dashed line**). Figure 2: Smoothed (**bottom panel**) and non-smoothed (**upper panel**) calculated gain for channel 10.65 V and orbit # 92412.  
satellite through the CSR. This led to an increase in the cold count observations, which corrupted the calibration process and associated errors in the scene brightness temperatures. Thus, in V8, an algorithm was implemented that compares along-scan and cross-scan gradients to the normal maximums and determines the calibration state of the scan by combining the two results. Therefore, if a scan is flagged as RFI, the algorithm looks forward and backward to find nearest valid scans to apply a linear interpolation. This algorithm uses previous and post orbits to perform the calibration near orbit boundaries. Figure 3 demonstrates the effectiveness of the TRMM V8 processing, by comparing the scan mean cold load counts with or without corrections [7]. The fifth change in V8 was to identify and correct the solar intrusion in the hot load counts. During the reprocessing of the TMI data, it was discovered that under certain solar geometry conditions, the radiometer hot counts (\(C_{h}\)) did not follow the measured hot load physical temperature (\(T_{h}\)) as expected, which indicated that there was a radiometric calibration anomaly associated with the hot load. To illustrate this effect, consider Figure 4 that shows a time series of TMI \(C_{h}\), \(T_{h}\), \(C_{c}\) (cold counts), calculated forward- transfer function gain and calculated forward-transfer function intercept in panel (a) to (e) respectively, for one orbital solar cycle (expressed as phase from orbit midnight). Therefore, a hot load correction procedure was developed and applied in 1B11 V8 that resulted in several lookup tables for each channel of the hot load physical temperature corrections (\(\Delta Th\)) that were based on solar coordinates for different solar array orientations and two different spacecraft yaw values (yaw 0 and 180). For example, Figure 5 shows the hot load correction look-up table for channel 10.65, V-pol during the post-boost time [12]. In addition, Figure 6 shows the calculated gain with or without the hot load correction. Figure 3: Cold counts with (**bottom panel**) and without (**upper panel**) Radio Frequency Interference (RFI) correction for channel 19.35 H and orbit # 87738. Figure 4: TMI parameters for 10.65 V channel during TRMM orbit # 87736: (**a**) hot counts, (**b**) measured hot load physical temperature, (**c**) cold counts, (**d**) receiver gain and (**e**) receiver offset. Figure 5: The hot load correction table for channel 10.65 V at yaw 0\({}^{\circ}\), during the post-boost time. Figure 6: The calculated gain for channel 10.65 V with (**bottom panel**) and without (**upper panel**) hot load correction (orbit #87736). The last major change was the MR emissive antenna correction. From the beginning of the mission, it was discovered that TMI Tb observations were warmer than expected for ocean scenes because of a slightly emissive MR. In 2001, [PERSON] et al. [2] developed a constant emissive MR correction, which was implemented in 1B11 V6, to remove the orbit-mean Tb bias, which was based on intersatellite comparisons with the Special Sensor Microwave Imager (SSMI).