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8.3.1.9.1 Data Characteristics

Although the Microwave Humidity Sounder (MHS) is a new instrument on NOAA-N and -P, it shares some commonality with the AMSU-B that it replaces. Therefore, similarities with the AMSU-B Level 1b format were maintained as much as possible. For example, a number of MHS Level 1b fields that are similar to AMSU-B Level 1b fields (e.g., calibration coefficients, earth location data, earth FOV counts) have the same byte offsets as their corresponding AMSU-B fields. Also, the MHS Level 1b record length is 3,072 bytes, which is the same as the AMSU-B Level 1b record length. However, in addition to some obvious telemetry differences, nomenclature used by the instrument manufacturer in MHS documentation is used in this document for consistency. For example, the channels are referred to as H1, H2, H3, H4 and H5, as opposed to 16, 17, 18, 19 and 20, respectively, as is the case for AMSU-B.

The MHS instrument, and its associated interface unit (the MIU) on the NOAA satellites, can operate in a variety of different modes and output several different packets, or formats, of data. The MHS Level 1b format given in this document is applicable for any mode of the MHS instrument and for the "nominal" modes of the MIU, i.e., the modes in which the MIU passes through its received MHS data without replacement with its own telemetry data. The MHS Level 1b data will not contain any MIU telemetry. (Note: the MIU is a NOAA-specific piece of hardware. Therefore, references to the MIU and how it affects the data stream, are only applicable to the MHS data from the NOAA satellites.)

Table 8.3.1.9.1-1 shows the nine MHS modes in which packet data is output and the three types of packets that are output in these modes.

An empty science data packet has the same format as the science data packet. However, except for the initial 39 bytes of housekeeping data, the packet is empty - i.e., zero filled. In fixed view mode, the instrument is not scanning, but is set, or fixed, at one view position. Therefore, all of its normal views of earth, space, and the on-board calibration target (OBCT) are of this fixed view position instead. When an empty science data packet is received or when the instrument is in fixed view mode, calibration is unable to be performed. Therefore, scans generated in either of these two situations are marked as unusable.

Technically, memory dump mode is not an actual mode of the MHS. Instead, the MHS can be commanded to perform a memory dump during most of its modes. The extended memory data packet generated during a memory dump supersedes the packet normally output during that particular mode. For the sake of simplicity, this document treats memory dump mode as a unique MHS mode.

According to Table 8.3.1.9.1-1, when MHS is in "self-test" mode or "memory dump" mode, it will output an extended test data packet or extended memory dump packet, respectively. In all other modes, including "scan" mode, it will output a (possibly empty) science data packet. The instrument will normally be in "scan" mode. Each different type of packet output by MHS results in a different type of data record output to the MHS Level 1b data set. This document provides Level 1b format specifications for all three types of data records. However, no matter the type of data record, they all share the same basic three-part organization: a header section, a packet data section, and a trailer section. The format of the header and trailer sections are identical across the three types of data records. The header section is composed of the first 29 bytes of the data record, which comprise the fields "Scan Line Number" through "Scan Line Quality Flags [Time Problem Code]", inclusively. The trailer section is composed of the last 238 bytes of the data record, which begins with the field "Main Bus Select Status" and continues through the "<Zero Fill>" padding at end of record. The content and format of the packet data section varies depending on the type of data record. In all cases though, the actual data from the particular MHS packet within this section begins at the same byte offset (1481). Additionally, in the case of data records containing either extended test data packets or extended memory data packets, the packet data is inserted exactly as received from the spacecraft without modification. (Some of the data in a science packet is slightly re-ordered, to align with common fields of the AMSU-B Level 1b data record, with a few additional fields inserted.)

As mentioned above, an empty science data packet and a science data packet have the same format. The difference is that most of the content of an empty science data packet is, as its name implies, empty. In terms of a Level 1b data record containing an empty science data packet, the following fields are zero filled (empty):

"Scene (Earth View) Data" (bytes 1481-2560) through "OBCT View Position Validity Flags" (bytes 2686-2686)

"Status Word" (byte 2727) through "Science Packet Spare Words" (bytes 2787-2831)

However, an empty science data packet does contain valid housekeeping data. So, in a data record containing an empty science data packet, the "Mode and Sub-commutation Code" field (byte 2687) through the "Raw Current Consumption Data" field (bytes 2720-2725) contain valid

Amended March 29, 2004

 

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