Automatic Picture Transmission
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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.
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[edit] Transmission
[edit] 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.
[edit] Images
On NOAA POES system satellites, the two images are 4km / 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 curve 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.
[edit] Synchronization and telemetry
Included in the transmission are a series of 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 1/8th max intensity and successively increase by 1/8th 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.
[edit] Broadcast signal
The signal itself is a 256-level amplitude modulated 2400Hz subcarrier, which is then frequency modulated onto the 137MHz-band RF carrier. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34KHz. On NOAA POES vehicles, the signal is broadcast at approximately 40dBm (10 watts) effective radiated power.
[edit] Receiving images
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 while the satellite is within radio range, which is typically 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 percentage of the total user-base this represents as registration is not a requirement and was only started in 1996.
[edit] Radio receiver
Add copy, explain need to receive FM signal of 34KHz bandwidth throughout the 137MHz range. Potentially include copy about how FM scanners can be used but will cause noise in the image due to their insufficient bandwidth.
[edit] Antenna
APT images from weather satellites can be received with a right-hand circular polarized, 137MHz antenna. Normally, there is no need to have the antenna follow the satellite and a fixed position antenna will provide good results.
The two most frequently recommended antennas are the crossed dipole and the quadrifilar helix antenna (QHA).
[edit] Displaying the images
Add copy, explain how originally the images were printed much like radiofax, but that today a personal computer soundcard is used to digitize the signal and display the image on the computer's monitor.
[edit] 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.
[edit] History
- Developed by the National Earth Satellite Service
- Tested on TIROS-8, launched 21 December 1963
- Nimbus 1, launched 28 August 1964, was the first application satellite
- First NOAA polar-orbiting vehicle to use it was TIROS-N, launched in 13 October 1978, and it has flown on all NOAA polar-orbiting vehicles since then.
- Also flown on the Soviet METEOR weather satellites.
[edit] Current status
- NOAA satellites transmitting APT
- NOAA-18
- NOAA-17
- NOAA-15
- NOAA-12
- Soviet / Russian satellites transmitting APT
- Meteor 2-21
- Meteor 3-5
[edit] Future
With the improvement in electronics, analog transmission systems are giving way to digital transmissions systems. The MetOp program, a collaboration between NOAA and EUMETSAT, has switched to Low Rate Picture Transmission (LRPT) for its new polar-orbit satellites and NOAA has stated they will not fly APT transmitters on POES vehicles after NOAA-N'.
[edit] See also
- Weather satellites
- Radiofax
- High Resolution Picture Transmission (HRPT)
- NOAA
- EUMETSAT
