Editing Automatic picture transmission (section)

== Receiving images ==
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An APT signal is continuously broadcast, with reception beginning at the start of the next line when the receiver is within radio range. Images can be received in real-time by relatively unsophisticated, inexpensive receivers during the time the satellite is within radio range, which typically lasts 8 to 15 minutes.

{{As of|2004}} there were almost 5,000 APT receiving stations registered with the [[World Meteorological Organization]] (WMO). It is unclear what percent of the total user-base this represents, since registration is not a requirement, and was only available after 1996.

=== Software-defined radio ===

A popular device used to receive the APT signals is a [[Software-defined radio]] (SDR). Many of these are cheap, and allow reception of frequencies from 500 kHz up to- in some cases- 7 GHz. [[Open-source]] software is available to set up the receiver, and since many have [[Plug and play]] it makes reception of APT accessible to the non-specialist. A cheap and reliable example is the RTL-SDR v3/v4, as it can reach up to 2.56 MHz in bandwidth which more than adequate for the reception of the APT signals (34 kHz). The SDR can decode on all the modulations, in this case it would be the [[Frequency modulation|NFM]].

=== Radio receiver ===
The bandwidth required to receive APT transmissions is approximately 34&nbsp;kHz. Most older scanners (police and fire type receivers) are the standard 15&nbsp;kHz bandwidth which were designed to support voice transmissions. Newer VHF general coverage receivers are equipped with multiple IF bandpasses; some are, but not limited to: 6&nbsp;kHz, 15&nbsp;kHz 50&nbsp;kHz & 230&nbsp;kHz(broadcast FM). Use of a receiver with too narrow a bandwidth will produce pictures that are saturated in the blacks and whites, as well as possible inversion. Too wide, and the noise floor of the receiver will be too high to acquire a good picture. For the amateur enthusiast, a computer controller receiver is the best option to allow the software to automatically tune and set the required modes for proper reception. There are also dedicated APT receivers made specifically for computer control and APT reception. Specifically, [[Icom Incorporated|ICOM]] PCR1000, PCR1500 & PCR2500 will produce excellent results. Searching on the web for "NOAA APT (RECEPTION or RECEIVER)" will produce a wealth of information on receivers, software, and antennas.

=== Antenna ===
APT images from weather satellites can be received with a right-hand [[circular polarization|circular polarized]], 137&nbsp;MHz antenna. Normally, there is no need to have the antenna follow the satellite and a fixed position antenna will provide good results.

The three most frequently recommended antennas are the [[turnstile antenna|crossed dipole]], [[Dipole antenna|V dipole antenna]] or the [[helical antenna|quadrifilar helix antenna]] (QHA or QFH).

=== Displaying the images ===
Years ago,{{when|date=January 2015}} to receive APT images, a specialized decoder was required in addition to the receiver to display or print images, much like HF [[WEFAX]] (serving the maritime community). Often both receiver and decoder were combined into one unit.

Nowadays, with the advent of personal computers, all that is required is dedicated software such as WXtoIMG (many of which offer "free" versions [http://www.geo-web.org.uk/soft.php]) and a sound card. The sound card acquires and digitizes the [[Slow-scan television|slow scan]] video (in the audible range) coming from the speaker, phones, or line-out of the receiver, and then the software will process the various visible and infrared channels of the AVHRR sensor. 

==== Enhanced images ====
Since each channel of the AVHRR sensor is sensitive to only one [[wavelength]] of light, each of the two images is [[luminance]] only, also known as [[grayscale]]. However, different materials tend to emit or reflect with a consistent relative intensity. This has enabled the development of software that can apply a color palette to the images which simulates visible light coloring. If the decoding software knows exactly where the satellite was, it can also overlay outlines and boundaries to help in utilizing the resulting images.