NOAA KLM User's Guide
Table J.3-1 contains a summary of scan initiation and FOV information for the microwave instruments.
|Instrument||Scan Period (seconds)||Integration Start Delay after 8 sec sync pulse (ms)||IFOV + dead time (ms)
(see Note 2)
|Earth Pixel integration period (ms)||IFOV (degrees)||Separation between adjacent Earth IFOVs (degrees)|
|AMSU-A1||8||+3.55||202.516||165||3 3/10||3 1/3|
|AMSU-A2||8||+3.26||202.50||158||3 3/10||3 1/3|
(see Note 1)
|19||18||1 1/10||1 1/10|
(see Note 3)
|19||19||1 1/10||1 1/9|
|Notes: AMSU-A1 and AMSU-A2 are a step and stare instrument. AMSU-B is a continuous slew instrument.|
AMSU-A is a cross-track, line-scanned instrument designed to measure scene radiances in 15 discrete frequency channels. At each channel frequency, the antenna beamwidth is a constant 3.3 degrees (at the half power point). Thirty contiguous scene resolution cells are sampled in a stepped-scan fashion (i.e., the instrument's FOV rotates to a data collection position, stops, collects data, then moves to the next collection position, stops, collects data, etc.) every eight seconds, each scan covering 50 degrees on each side of the subsatellite path. The AMSU-A instrument starts at earth position 1, then goes sequentially to earth position 30, then to the cold calibration view position and then to the warm load view position (See Figure J.3-1). These scan patterns and geometric resolution translate to a 50 km diameter cell at nadir and a 2,343 km swath width from the 833 km nominal orbital altitude.
In the step scan, the AMSU-A antenna steps and stops at each beam position for a period equal to the sample period, plus a settle time, sufficient to insure a maximum jitter (percentage overshoot/undershoot of the antenna step). For AMSU-A1, a jitter up to +10% is allowed for any 10 ms period in the first .33 of the step period. Otherwise, the jitter will be less than or equal to ±5%. For AMSU-A2, a jitter up to +10% is allowed for any 20 ms period in the first .33 of the step period. Otherwise, the jitter will be less than or equal to ±5%. The step time for the 30 earth view beam positions will be equal.
Each channel of the AMSU-A instrument is considered to form a beam. All main beam axes of the AMSU-A will be coincidental, i.e., they will be pointing in the same direction at the same time for any given beam position. In the following paragraphs, if only one beam is discussed, it is inferred to represent any and all beams.
The AMSU-A beams will have cross-track scanning. All beams will scan in a plane perpendicular to the spacecraft orbital velocity vector. Note: The spacecraft velocity vector is pointing in the direction towards the reader (out of the page). The sense of the scan will be counterclockwise as one looks along the spacecraft orbital velocity direction (i.e., the antenna scans from the sun direction through nadir to the cold space direction and repeats as shown in Figure J.3-1).
The AMSU-A beams will scan the earth viewing sector a total of 96.66 degrees (±48.33 degrees from nadir) on beam centers. There will be a total of 30 beam positions (30 resolution cells on the earth surface), to be called cell numbers 1 through 30, from Sun to anti-Sun. There will be 15 cells on either side of nadir. The beam center position of each cell is separated from the adjacent cell along the scan direction by 3.33 degrees (there will be a non-cumulative step tolerance of ±0.04 degrees).
There will be four beam positions selectable by command, to provide a cold (space look) calibration position. The primary cold calibration beam position will nominally be at 6.66 degrees from the sun X velocity plane in the nadir direction. The three alternate cold calibration positions will nominally be at 8.33, 10.00 and 13.33 degree declinations.
The location of the beam positions (earth viewing) in time with respect to the frame synchronization pulse for AMSU-A1 and AMSU-A2 are illustrated in Figures J.3-2 and J.3-3, respectively.
Figure J.3-4 shows the size of the AMSU-B footprint over the entire Earth scan.
AMSU-B is a cross-track, line scanned instrument designed to measure scene radiances in 5 channels. At each channel frequency, the antenna beamwidth is a constant 1.1 degrees (at the half power point). Ninety contiguous scene resolution cells are sampled in a continuous fashion, each scan covering 50 degrees on each side of the subsatellite path. These scan patterns and geometric resolution translate to a 16.3 km diameter cell at nadir at a nominal altitude of 850 km.
All antenna beams for AMSU-B scan in a plane perpendicular to the instrument's baseplate, containing nadir in the x-z plane. The maximum deviation from the scan plane will be less than or equal to 0.1 degrees. The direction of scan motion is from sun (+z) to nadir (+x) to antisun side (-z). During earth scan, the cross track motion will be continuous with the angular velocity constant to within ±2%. All antenna beams will scan ±48.95 degrees about nadir with reference to the beam axis. The scan period will be 8/3 seconds, in order to maintain a relationship with the AMSU-A scan pattern.
The scan plane and motion for the AMSU-B instrument are shown in Figure J.3-5.
The Microwave Humidity Sounder (MHS) is a self-calibrating microwave radiometer, observing the Earth with a field of view of ±50 degrees across nadir, in five frequency channels of the millimeter-wave band (89-190 GHz). MHS, together with the complementary AMSU-A instruments, provides the operational microwave sounding capability for the NOAA-N, -P meteorological satellites.
MHS is a cross-track, line-scanned instrument. Ninety contiguous scene resolution cells are sampled in a continuous scan, covering 49.44444... degrees on each side of the sub-satellite path, with an antenna beam width of 1.11111... degrees at half power point. These scan patterns and geometric resolution translate to a 17-km diameter cell at nadir from the 870 km nominal orbital altitude.
In scan mode, the MHS reflector performs the scan profile. This is a predefined position versus time profile which incorporates the Earth view and the two calibration targets, as shown in Figures 126.96.36.199-1 and J.3-6. Three predefined profiles are provided: profiles 0, 1 and 2; and these are the same except for small changes in the position of the Space view.
Amended November 2, 2005
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