The Advanced Microwave Sounding Unit are flown onboard NOAA-KLM satellites. The instrument has been operational since 1998. The AMSU contains two modules: A and B. The A module (AMSU-A) has 15 channels (see Table 1) and is mainly designed to provide information on atmospheric temperature profiles while the B module (AMSU-B) allows for profiling the moisture structure. The AMSU-A has an instantaneous field-of-view of 3.3^o and scans ± 48.7^o from nadir with 15 different viewing angles at each side. The AMSU-A measures thermal radiation at microwave frequencies ranging from 23.8 to 89.0 GHz. Atmospheric temperature profiles are primarily based on the measurements obtained at channels near 60 GHz, which is an oxygen absorption line. In particular, the AMSU-A sounding channels (3-14) respond to the thermal radiation at various altitudes because of their weighting function distributions, whereas channels 1 and 2 are primarily designed for obtaining the information on surface properties. Since the satellite provides a nominal spatial resolution of 48 km at its nadir, the temperature perturbations from synoptic to mesoscale can be reasonably depicted. In addition, several AMSU imaging channels at frequencies of 31.4, 89 and 150 GHz are utilized to determine cloud liquid and ice water contents because they directly respond to the emission from liquid droplets and the scattering from ice particles.

The AMSU-B instrument contains five channels from 16-20 (see Table 1) and provides the atmospheric profile of moisture. However, the radiances from the first AMSU-B onboard NOAA-15 are severely contaminated by the S-band data transmitters and the Search and rescue Repeater (SARR). It appears that AMSU channel 17 and 19 are mostly affected by RFI (Atkinson 2000).  The levels of RFI affecting the AMSU-B are routinely monitored by NOAA and United Kingdom Meteorological Office (UKMO), with the UKMO proving updated correction tables every 2 months along with recommendations to NOAA on whether the current corrections table need to be updated. The RFI correction is only needed for the AMSU-B onboard NOAA-15.

Table 1. AMSU instrument characteristics

The ATMS instrument is a passive micro- and milli-meter wave radiometer. It employs a direct RF-gain, total power, RF-detection receiver at the lowest frequencies, and a direct RF-gain, total power, heterodyne receiver at the high end of the frequency coverage. The requirements for the ATMS are detailed in references (Northrop-Grumman, 2001c), and is summarized here. The ATMS collects microwave radiometric data in 22 channels from 23.8 to 183.3 GHz; the number and frequency coverage of the channels are listed in Table 1.

ATMS Instrument: Assembled and Exploded Views

Table 2: ATMS Channels Characteristics

During the cross-track scan period of 8/3 seconds, ATMS scans 105.45 degrees (about 2,200 km useful ground swath) with 96 Earth-viewing beam positions as shownschematically in Fig 2; four beam positions will cover the cold load and four more will cover the warm load. All the channels are expected to be closely aligned within the limits listed in Table 2. The ATMS is a cross-track scanner, with scanning direction from the sunward to the anti-sun side of the instrument. The ATMS employs a continuous-motion drive, so that the field-of-view of each channel is smeared across the scene during the integration time. Scan synchronization with CrIS (which employs an 8 second scan) is useful for data correlation.

FIGURE 2. SCHEMATIC VIEW OF THE SCAN GEOMETRY FOR ATMS

The footprint on the ground at nadir and at the edge of the scan is given in Table 1. The spacing between adjacent measurements is 17.6 km along track; across track it ranges from 16.4 km at nadir to 67 km at the edge of the scan (for an orbital altitude of 828 km; Northrop-Grumman, 2001b).

Cold space and hot load calibration are accomplished through the antenna during each scan. The purpose of the ATMS is to collect specialized data to permit the calculation of the moisture, temperature and pressure vertical profiles of the Earth’s atmosphere. This purpose is accomplished by measuring the atmospheric properties both at window and at sounding frequencies. For the ATMS the sounding channels are those at 23.8 GHz, and the families at 53-58 and 183 GHz; the window channels are at 31.4, 50-52, 88.2 and 165.5 GHz. The data are processed and delivered in the form of Raw Data Records (RDRs), Sensor Data Records (SDRs), and, after merging with the data from the CrIS, Environmental Data Records (EDRs).The radiometric channels of the ATMS measure polarization in the quasi-horizontal and quasi-vertical state. These states are defined as follows:

Q_V=T_Vcos²(q)+T_Hsin²(q)     Quasi Vertical      (1)

Q_H=T_Vsin²(q)+T_Hcos²(q)     Quasi Horizontal   (2)

where, following the conventional definitions, T_V and T_H are respectively the Brightness Temperatures of the Vertically polarized (i.e., with electric field vector perpendicular to the orbital velocity direction) and Horizontally polarized (i.e., with electric field parallel to the orbital velocity direction) components of the incoming radiation, and q is the scan-angle away from nadir. The polarization and field-of-view characteristics for each of the ATMS channels are given in Table 3 The uncertainty of the beamwidths must be within 10% of the stated value, with a variation of 10% or less among channels which are required to have the same beamwidth.

Table 3: ATMS Sensor Characteristics

Notice that from the above equations, the measured Q_V and Q_H reduce to T_V and T_H , respectively, at nadir where q is zero, and to T_V and T_H when q is 90^o . For non-zero q away from nadir, the ATMS receives a linear combination of the pure vertical and horizontal components. As the mirror scans the antenna beam across the Earth, each of the polarized channels receives a varying combination of the horizontally and vertically polarized signals. The total electric field vector of the received wave lies in a plane perpendicular to the propagation direction and has a polarization angle f_p with respect to a reference line, defined as the intersection of the tangent (at beam center) plane and the plane of incidence. The total electric field vector rotates (i.e. the polarization angle f_p changes) as a function of the scan-angle q. For the quasi-vertical component, f_p=q , while for the quasi-horizontal component, f_p=90-q.

The warm calibration load consists of a metallic substrate (aluminum or magnesium). In its front surface are machined two orthogonal series of parallel triangular grooves, so as to form a two-dimensional array of uniform pyramids.  The pyramids are coated with a layer of absorbing Eccosorb epoxy (a microwave absorbing material).

The temperature of the load is not controlled; it is isolated from the structure of the ATMS  and left free to fluctuate around the nominal ambient temperature of 300 K. There are seven Platinum Resistance Thermistors (PRTs) embedded in the body of the warm load to measure its physical temperature.