Editing Automatic picture transmission

The '''Automatic Picture Transmission''' ('''APT''') system is an analog image transmission system developed for use on [[weather satellites]]. It was introduced in the 1960s and over four decades has provided image data to relatively low-cost user stations at locations in most countries of the world. A user station anywhere in the world can receive local data at least twice a day from each satellite as it passes nearly overhead.

== 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]].

== 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.

== History ==
* Developed by the National Earth Satellite Service
* Tested on [[TIROS|TIROS-8]], launched December 21, 1963
* [[Nimbus program|Nimbus 1]], launched August 28, 1964, was the first application satellite
* First NOAA polar-orbiting vehicle to use it was [[TIROS|TIROS-N]], launched on October 13, 1978, and it has flown on all NOAA polar-orbiting vehicles since then.
* Also flown on the Soviet [[Meteor (satellite)|METEOR]], Sich, Resurs and Okean weather satellites.

== Current status ==
Due to APT being phased out, there are very few satellites that still transmit APT. Previous satellites such as [[Sich-1]] and the [[Meteor (satellite)|METEOR]] series of satellites operated on APT, but have since reached the end of their operational lifespans. 
;NOAA satellites transmitting APT
* [[NOAA-18]]
* [[NOAA-15]]
* [[NOAA-19]]

== Future ==
With improvements in electronics, analog transmission systems have given way to digital transmissions systems. [[NOAA-19]], called [[NOAA-N']] prior to its launch on 6 February 2009, is the last satellite to carry an APT system.<ref>{{Cite web |url=http://noaasis.noaa.gov/NOAASIS/ml/future.html |title=Future NOAA Polar Orbiting and Geostationary Satellite Systems - NOAA Satellite Information System (NOAASIS); Office of Satellite and Product Operations |access-date=2015-04-27 |archive-url=https://web.archive.org/web/20150502135907/http://noaasis.noaa.gov/NOAASIS/ml/future.html |archive-date=2015-05-02 |url-status=dead }}</ref> The [[MetOp]] program, a collaboration between [[NOAA]] and [[EUMETSAT]], has switched to [[Low Rate Picture Transmission]] (LRPT) for its new polar-orbit satellites.

Since the [[NOAA-20]] satellite, most of the NOAA satellites have their lowest transmission frequency at 7812 MHz in the [[C band (IEEE)|C Band]], making reception by the general public impractical since unlike older schemes (APT, LRPT, [[LRPT|HRPT]]) they require a [[dish antenna]] with a specialised [[antenna feed]] etc.

== See also ==
* [[EUMETSAT]]
* [[Radiofax]]
* [[Weather satellite]]s

== References ==
{{Reflist}}

{{refbegin}}
* NOAA's KLM User's Guide: [https://web.archive.org/web/20070316190349/http://www2.ncdc.noaa.gov/docs/klm/html/c4/sec4-2.htm Section 4.2 – APT System]
* [https://web.archive.org/web/20070802041925/http://grin.hq.nasa.gov/ABSTRACTS/GPN-2003-00026.html GPN-2003-00026 – Automatic Picture Transmission]
* [https://history.nasa.gov/SP-168/p14.htm Exploring Space With A Camera]
* [https://web.archive.org/web/20070228193410/http://noaasis.noaa.gov/NOAASIS/ml/meteor.html NOAASIS – NOAA Satellite Information System for NOAA Meteorological / Weather Satellites]
{{refend}}

== External links ==
* [https://web.archive.org/web/20070208205042/http://www.oso.noaa.gov/poesstatus/index.asp POES spacecraft status] NOAA
* [http://www.sat.dundee.ac.uk/ NEODAAS Dundee Satellite Receiving Station]
* [http://www3.sympatico.ca/konecny/weather.htm APT images received at Thirteen Island Lake Ontario Canada]
* [http://www.geo-web.org.uk/soft.php Decoding Software for APT Satellite Reception]

[[Category:Broadcast engineering]]
[[Category:Spacecraft instruments]]
[[Category:Satellite meteorology]]
[[Category:Satellite broadcasting]]