Image Features

 Below is a raw image as received - most of the images shown elsewhere have been edited in some way, generally cropped to remove unwanted areas, and may have some additional processing such as histogram equalisation or brightness/contrast adjustment to bring out interesting details, etc.

Raw Image

This particular one comes from NOAA 17, a southbound, morning pass where there is sufficient illumination to give both visible and infra-red images. I've deliberately chosen an image which isn't one of my best so as to show some of the defects which may arise. The numbered features are as follows:

1] Visible image area - this is the portion of the image on the right and represents more or less what we might see if we were able to view the Earth from the satellite ourselves.

2] Infra-red image area - corresponds to the visible image, but is formed from the emitted thermal infra-red radiation from the Earth's surface (including clouds, water surfaces, etc.)

3] Visible image sync tone - the sync tone frequency here is 832 Hz and generates a narrow stripe of parallel lines.

4] Infra-red image sync tone - this is similar to the visible image but the sync frequency is 1040 Hz giving a narrower line pattern.

5] Visible image grey scale - normally an 8 step grey scale appears here corresponding to the range of brightness which occurs in the image. In the present instance, as image pre-processing took place before capture, few levels of the scale can be seen; most of the image would have been dark due to low illumination so the range has been compressed using a logarithmic amplifier.

6] Infra-red image grey scay - the same as for the visible image, but in this case due to low temperatures, an unprocessed image would have been almost white; an exponential amplifier was used to expand the range of available contrast into a suitable brightness range..

7] Calibration information - data bars contain encoded information about the measurement conditions; these can be used to derive for example, local temperatures from the infre-red image.

8] Timing marks - these appear every minute.

9] Data drop-out - unfortunately, my receiving system isn't perfect and images frequently suffer from blemishes. In this case, not so much loss of signal as disturbance of reception by an external, spurious signal, source unknown.

10] Noise background - this is the residual noise remaining from the receiver after the satellite has passed out of view, before the receiving system squelch circuit has detected that the signal is no longer present.

It should be borne in mind that this is the image as acquired with a particular software package [JVFAX 7.0], and whilst others will give essentially the same details, the appearance, for one reason or another, may not be quite the same as this.

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© Stuart Hill [Updated February 2003]