Editing Automatic picture transmission (section)

== Transmission ==
[[Image:NOAA APT Frame Format.gif|right|thumb|300px|The APT transmission format]]
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=== Structure ===
The broadcast transmission is composed of two image channels, [[telemetry]] information, and synchronization data, with the image channels typically referred to as Video A and Video B. All this data is transmitted as a horizontal scan line. A complete line is 2080 [[pixels]] long, with each image using 909 pixels and the remainder going to the telemetry and synchronization. Lines are transmitted at 2 per second, which equates to a 4160 words per second, or 4160 [[baud]].

=== Images ===
[[Image:NOAA_19_APT_Image.jpg|left|thumb|An APT image of western North America in visible and infrared bands]]On NOAA [[Polar Operational Environmental Satellites|POES]] system satellites, the two images are 4&nbsp;km / pixel smoothed 8-bit images derived from two channels of the [[advanced very-high-resolution radiometer]] (AVHRR) sensor. The images are corrected for nearly constant geometric resolution prior to being broadcast; as such, the images are free of distortion caused by the curvature of the Earth.

Of the two images, one is typically long-wave [[infrared]] (10.8 [[micrometers]]) with the second switching between near-visible (0.86 micrometers) and mid-wave infrared (3.75 micrometers) depending on whether the ground is illuminated by sunlight. However, NOAA can configure the satellite to transmit any two of the AVHRR's image channels.

=== Synchronization and telemetry ===
Included in the transmission are a series of [[Synchronization in telecommunications|synchronization]] pulses, minute markers, and telemetry information.

The synchronization information, transmitted at the start of each video channel, allows the receiving software to align its sampling with the baud rate of the signal, which can vary slightly over time. The minute markers are four lines of alternating black then white lines which repeat every 60 seconds (120 lines).

The telemetry section is composed of sixteen blocks, each 8 lines long, which are used as reference values to decode the image channels. The first eight blocks, called "wedges," begin at 1/8 max intensity and successively increase by 1/8 to full intensity in the eighth wedge, with the ninth being zero intensity. Blocks ten through fifteen each encode a calibration value for the sensor. The sixteenth block identifies which sensor channel was used for the preceding image channel by matching the intensity of one of the wedges one through six. Video channel A typically matches either wedge two or three, channel B matches wedge four.

The first fourteen blocks should be identical for both channels. The sixteen telemetry blocks repeat every 128 lines, and these 128 lines are referred to as a frame.

=== Broadcast signal ===
The signal itself is a 256-level [[amplitude modulated]] 2400[[Hertz|Hz]] [[subcarrier]], which is then [[frequency modulated]] onto the 137&nbsp;MHz-band [[Carrier wave|RF carrier]]. Maximum subcarrier modulation is 87% (±5%), and overall [[RF]] [[Bandwidth (signal processing)|bandwidth]] is 34&nbsp;kHz. On NOAA POES vehicles, the signal is broadcast at approximately 37[[dBm]] (5 watts)<ref>{{cite web |url=http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c4/sec4-2.htm |url-status=dead |archive-url=https://web.archive.org/web/20080706174801/http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c4/sec4-2.htm |archive-date=2008-07-06 |title=NOAA KLM USER'S GUIDE Section 4.2}}</ref> [[effective radiated power]].