KE7IZL
Member
I was reading this specification here https://www.ncei.noaa.gov/pub/data/satellite/publications/podguides/N-15 thru N-19/pdf/2.1 Section 4.0 Real Time Data Systems for Local Users .pdf where it lists the 16 fields of the telemetry strip of the image.
but it neglects to say how to actually INTERPRET the values there. Of particular interest to me is the Channel ID field.
I know that just as any other part of the image, each field contains a value between 0 and 255 because it is composed of pixels. However I also know that due to the analog nature of the image transmission, the range of values that exists in this field would need to be quantized into larger steps (like maybe all values between 0 and 4 should be interpeted as 0, values between 5 and 9 as 1, values between 10 and 14 as 2, etc). This way each value in the field could be represented as a range of pixel values so it would be more resistant to error (like just being one pixel value above or below the intended value would not result in a complete change in the value of the field's data).
However, I don't know how to interpret these fields. For one, I don't know what quantization steps are used to encode values in any of the fields, other than the brightness steps (which have 9 brightness values, starting at the darkest gray above black, going up to white, and then back down to black). The calculations of these are easy. They are step_val/8*255. The first 8 steps have step_val going from 1 to 8, and the ninth step has step_val at 0.
I don't know how the other telemetry fields are calculated though. This is not mentioned at all in the PDF file I linked to above. And that appears to be the only official APT specification that NOAA has made public. Has anybody managed to reverse engineer how to interpret the various telemetry fields in a NOAA APT transmission? In particular I'm interested in the Channel ID field.
but it neglects to say how to actually INTERPRET the values there. Of particular interest to me is the Channel ID field.
I know that just as any other part of the image, each field contains a value between 0 and 255 because it is composed of pixels. However I also know that due to the analog nature of the image transmission, the range of values that exists in this field would need to be quantized into larger steps (like maybe all values between 0 and 4 should be interpeted as 0, values between 5 and 9 as 1, values between 10 and 14 as 2, etc). This way each value in the field could be represented as a range of pixel values so it would be more resistant to error (like just being one pixel value above or below the intended value would not result in a complete change in the value of the field's data).
However, I don't know how to interpret these fields. For one, I don't know what quantization steps are used to encode values in any of the fields, other than the brightness steps (which have 9 brightness values, starting at the darkest gray above black, going up to white, and then back down to black). The calculations of these are easy. They are step_val/8*255. The first 8 steps have step_val going from 1 to 8, and the ninth step has step_val at 0.
I don't know how the other telemetry fields are calculated though. This is not mentioned at all in the PDF file I linked to above. And that appears to be the only official APT specification that NOAA has made public. Has anybody managed to reverse engineer how to interpret the various telemetry fields in a NOAA APT transmission? In particular I'm interested in the Channel ID field.
