about-himawari-8.md

+# About Himawari-8
+
+JMA provide great detail about the Himawari-8 mission in the [Himawari User's Guide] (http://www.jma-net.go.jp/msc/en/support/index.html)
+
+# Overview
+
+Himawari-8 is the successor to the Japan Meteorological Agency (JMA) Multi-functional Transport Satellites (MTSAT-1R and MTSAT-2) meteorological satellites. The primary meteorological sensor on board Himawari-8 is the Advanced Himawari Imager (AHI). AHI has improved spatial, spectral and temporal capabilities compared to the imagers carried by the MTSAT and Geostationary Meteorological Satellite (GMS) series.
+
+Himawari-8 is stationed above the equator at longitude 140.7 °E. 
+
+## Observation Schedule
+
+AHI has two main scan modes - Full Disk (full field of view) and target area. These modes can be interleaved (ie Full disk is suspended while it scans other target areas). Every 10 minutes, **with the exception of 0240 and 1440**, there will be a full disk scan; while this scan is occurring, there will be multiple observations of Japan (4 x 2 sectors), multiple targeted observations (severe weather), and ~40 observations of landmarks to assist with navigation/registration. 
+The following graphic shows the AHI scan pattern that is repeated every 10 minutes
+
+![](http://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/fig/ahi_obs.png)
+
+## Channel Information
+
+AHI has 16 channels in the range 0.4 - 13.3 μm with three spatial resolutions: one channel (3) has 500 m resolution, three channels (1,2,4) have 1000 m resolution, and 12 (5-16) channels have 2000 m resolution. 
+
+### Detector Information
+
+Each AHI channel is composed of a number of detectors. AHI has approximately 7800 detectors active at one time. When the various backup combinations are counted, AHI has over 38000(!) detectors. Each detector has its own spectral response function (SRF). If a detector SRF is poorly characterised it will appear as out of family and result in banding: discontinuities between lines running approximately east-west. As the number of detectors increases, the likelihood of an out-of-family response by one of the detectors increases. JMA have undertaken extensive analysis to reduce banding, though some banding is still observed in a number of bands. This banding is important to note when considering spatial statistics (e.g. calculation of the standard deviation of a neighbourhood).
+
+### Signal to noise, bit depth and precision
+
+The amount of energy emitted/reflected by the Earth depends upon the scene being observed (e.g. cloud, ice, ocean, green pasture, sandy desert and, notably, fire) and changes with wavelength. Similarly the efficiency of materials used to manufacture detectors varies with wavelength. Detector efficiency varies non-linearly (so a detector may be very sensitive for warm scenes, but be less sensitive for cold scenes).
+
+The range of a detector is selected to observe particular features (in the IR, coldest expected to warmest expected temperature; in the VIS, darkest expected to brightest expected). A detector's bit depth determines how many levels or steps the detector range is divided into. A bit depth of 8 would have 2^8 levels (256). The AHI has channels with bit depths of 11, 12, and 14 (2048, 4096 and 16384 levels, respectively). 
+
+### Spectral Response Functions
+
+JMA have made the [SRF data available here] (http://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/srf_201309/AHI-08_SpectralResponsivity.zip). JMA have also made a report on [AHI8 performance available here] (http://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/doc/AHI8_performance_test_en.pdf AHI8_performance_test_en.pdf). Note that the "Dynamic Range" column should be read as the saturation value (above which the detector is insensitive). 
+
+In the case of the 3.9 μm channel, the saturation temperature is 401 K. According to the GOES-R ATBD: 
+
+> the minimum detectable size of a fire burning at an average temperature of 800 K 
+> is approximately 0.004 km2 at the sub-satellite point in clear sky conditions. 
+> The elevated saturation temperature of 400 K in the 3.9 μm band (Channel 7) limits 
+> the number of saturated fire pixels to less than 5% of all observed fires.
+
+The Spectral Response Functions (SRF) are plotted (by JMA) below. 
+![](http://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/fig/AHI_SRF_VNIR_201309.png)
+![](http://www.data.jma.go.jp/mscweb/en/himawari89/space_segment/fig/AHI_SRF_IR_201309.png)
+
+## Navigation
+
+AHI data are delivered to the Bureau after being reprojected into the geostationary projection. The reprojection process used by JMA uses information from the landmark observations. Each image has a set of header information that includes navigation and projection information. The location described by the projection is the centre of the pixel (not the upper left corner as per some GIS applications). This is particularly important to note when considering multiple channels with different resolutions. 
+
+ JMA project the AHI data onto a fixed projection. The parameters of the projection are held within the AHI data stream, but are expected to remain unchanged. The proj4 string that corresponds to the parameters within the AHI data stream is:
+
+`"+proj=geos +lon_0=140.7 +h=35785863 +x_0=0 +y_0=0 +a=6378137 +b=6356752.3 +units=m +no_defs".`
+
+The corresponding OGC Well Known Text (WKT) is:
+
+```
+PROJCS["unnamed",
+    GEOGCS["unnamed ellipse",
+        DATUM["unknown",
+            SPHEROID["unnamed",6378137,298.2570248822722]],
+        PRIMEM["Greenwich",0],
+        UNIT["degree",0.0174532925199433]],
+    PROJECTION["Geostationary_Satellite"],
+    PARAMETER["central_meridian",140.7],
+    PARAMETER["satellite_height",35785863],
+    PARAMETER["false_easting",0],
+    PARAMETER["false_northing",0]]
+```
+
+Information about the stability of the AHI navigation is available from the JMA navigation monitoring page (http://ds.data.jma.go.jp/mscweb/data/monitoring/navigation.html).