Automatic Picture Transmission
Encyclopedia
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.
.
(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.
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.
subcarrier
, which is then frequency modulated onto the 137 MHz-band RF carrier
. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz. On NOAA POES vehicles, the signal is broadcast at approximately 40dBm
(10 watts) effective radiated power
.
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.
, 137 MHz 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).
Today, with the advent of personal computers, all that is required is dedicated software (many of which offer "free" versions) and a sound card. The sound card acquires and digitizes the slow scan video (in the audible range) http://www.markroland.com/engineering/APT/APT_sample.mp3 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. Most software will automatically save every image and publish your processed image onto the website of your choice, putting up a new image on every pass of an APT satellite.
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.
Soviet / Russian satellites transmitting APT
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'
.
Transmission
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 baudBaud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
.
Images
On NOAA POES system satellites, the two images are 4 km / pixel smoothed 8-bit images derived from two channels of the Advanced Very High Resolution RadiometerAdvanced Very High Resolution Radiometer
The Advanced Very High Resolution Radiometer is a space-borne sensor embarked on the National Oceanic and Atmospheric Administration family of polar orbiting platforms . AVHRR instruments measure the reflectance of the Earth in 5 relatively wide spectral bands...
(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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
(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 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 2400HzHertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
subcarrier
Subcarrier
A subcarrier is a separate analog or digital signal carried on a main radio transmission, which carries extra information such as voice or data. More technically, it is an already-modulated signal, which is then modulated into another signal of higher frequency and bandwidth...
, which is then frequency modulated onto the 137 MHz-band RF carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz. On NOAA POES vehicles, the signal is broadcast at approximately 40dBm
DBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
(10 watts) effective radiated power
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
.
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 during the time the satellite is within radio range, which typically lasts 8 to 15 minutes.there were almost 5,000 APT receiving stations registered with the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
(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.
Radio receiver
The bandwidth required to receive APT transmissions is approximately 34 kHz. Most older scanners (police and fire type receivers)are the standard 15 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 kHz, 15 kHz 50 kHz & 230 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 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 polarizedCircular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization in which the electric field of the passing wave does not change strength but only changes direction in a rotary type manner....
, 137 MHz 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
Turnstile antenna
A turnstile antenna is a set of two dipole antennas aligned at right angles to each other and fed 90 degrees out-of-phase. The name reflects that the antenna looks like a turnstile when mounted horizontally. When mounted horizontally the antenna is nearly omnidirectional on the horizontal plane...
and the quadrifilar helix antenna
Helical antenna
A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane...
(QHA).
Displaying the images
Years ago, 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.Today, with the advent of personal computers, all that is required is dedicated software (many of which offer "free" versions) and a sound card. The sound card acquires and digitizes the slow scan video (in the audible range) http://www.markroland.com/engineering/APT/APT_sample.mp3 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. Most software will automatically save every image and publish your processed image onto the website of your choice, putting up a new image on every pass of an APT satellite.
Enhanced images
Since each channel of the AVHRR sensor is sensitive to only one wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light, each of the two images is luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
only, also known as grayscale
Grayscale
In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...
. 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-8TIROSTIROS, or Television Infrared Observation Satellite, is a series of early weather satellites launched by NASA, beginning with TIROS-1 in 1960. TIROS was the first satellite that was capable of remote sensing of the Earth. This initial remote-sensing effort was significant because it enabled Earth...
, launched December 21, 1963 - Nimbus 1, launched August 28, 1964, was the first application satellite
- First NOAA polar-orbiting vehicle to use it was TIROS-NTIROSTIROS, or Television Infrared Observation Satellite, is a series of early weather satellites launched by NASA, beginning with TIROS-1 in 1960. TIROS was the first satellite that was capable of remote sensing of the Earth. This initial remote-sensing effort was significant because it enabled Earth...
, launched on October 13, 1978, and it has flown on all NOAA polar-orbiting vehicles since then. - Also flown on the Soviet METEORMeteor (satellite)The Meteor craft are weather observation satellites launched by the USSR. The Meteor satellites were designed to monitor atmospheric and sea-surface temperatures, humidity, radiation, sea ice conditions, snow-cover, and clouds.-Meteor 2-21:...
weather satellites.
Current status
NOAA satellites transmitting APT- NOAA-18
- NOAA-17
- NOAA-15
- NOAA-19
Soviet / Russian satellites transmitting APT
- Meteor 2-21
- Meteor 3-5
Future
With the improvement in electronics, analog transmission systems are giving way to digital transmissions systems. The MetOpMetOp
MetOp is a series of polar orbiting meteorological satellites operated by the European Organisation for the Exploitation of Meteorological Satellites. The satellites are all part af the EUMETSAT Polar System. It is intended to replace the soon to be retired TIROS network...
program, a collaboration between NOAA and EUMETSAT, has switched to Low Rate Picture Transmission
Low Rate Picture Transmission
The Low Rate Picture Transmission is a digital transmission system, intended to deliver images and data from an orbital weather satellite directly to end users via a VHF radio signal...
(LRPT) for its new polar-orbit satellites and NOAA has stated they will not fly APT transmitters on POES vehicles after NOAA-N'
NOAA-N'
NOAA-19, designated NOAA-N prior to launch, is the last of the United States National Oceanic and Atmospheric Administration's POES series of weather satellites...
.
See also
- EUMETSAT
- High Resolution Picture TransmissionHigh resolution picture transmissionWeather satellite pictures are often broadcast as High Resolution Picture Transmissions , Color High Resolution Picture Transmissions for Chinese Weather Satellite transmissions or Advanced High Resolution Picture Transmissions for EUMETSAT Weather Satellite transmissions...
(HRPT) - RadiofaxRadiofaxRadiofax, also known as weatherfax and HF fax , is an analogue mode for transmitting monochrome images. It was the predecessor to slow-scan television...
- Weather satelliteWeather satelliteThe weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...
s
External links
- NOAA POES spacecraft status
- http://personales.ya.com/metbee/apt_format.html
- http://www.sat.dundee.ac.uk/
- http://www3.sympatico.ca/konecny/weather.htm
- http://www.digitalham.co.uk/weather/satellite_pictures.php