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| Figure 3.5 ETM+ Sample Timing - Format 1 |
The ETM+ data stream is composed of a continuous succession of major frames. A major frame contains the data for an entire period of one complete scan of the ETM+ scan mirror. A major frame is partitioned into minor frames - a specific pattern for organizing groups of ETM+ data words. This pattern is based on the architecture of the Landsat 7 auxiliary Electronics module (AEM) that samples, digitizes, and groups analog video signals from the ETM+ scanner to form scene data. The minor frame data structure is 85 words (8 bits) in length consisting of 16 separate groups of 5 words, 4 data words from band 6 and a one spare.
The line sync code is generated by the AEM at the beginning of each new scan line and inserted into minor frame zero which is also called scan line start (SLS). The time code pattern preempts all video data except band 6 although the band 6 data for the first minor frame is invalid. The six minor frames immediately following the SLS minor frame describe the spacecraft time code as illustrated in Figure 3.5. The time code data preempts all minor frame video except band 6 data. The band 6 data alternates between the odd and even detectors for each successive minor frame and are synchronized to odd pixel data for the first minor frame. The first valid band 6 data are from the even detectors and occur in minor frame two.
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| Figure 3.5 ETM+ Sample Timing - Format 1 |
Scene data transmission for the other bands starts at the minor frame boundary immediately following the time code and continues until start of the next end of line pattern code which is mechanically/optically triggered. For reference, 6313 minor frames of scene data are nominally transmitted during any given scan cycle. The digitized scene data can be organized into either of two minor frame scene data formats as depicted in Figures 3.5 and 3.6. Bands 1-5 are allocated to format 1 while bands 7 and 8 are allocated to format 2. The two band 6 data streams allocated to formats 1 and 2 are low gain and high gain respectively.
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| Figure 3.6 ETM+ Sample Timing - Format 2 |
