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發表於 29-3-2016 18:09:52
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StructureThe broadcast transmission is composed of two imagechannels, telemetryinformation, and synchronization data, with the image channels typicallyreferred to as Video A and Video B. All this data is transmitted as ahorizontal scan line. A complete line is 2080 pixels long, witheach image using 909 pixels and the remainder going to the telemetry andsynchronization. Lines are transmitted at 2 per second, which equates to a 4160words per second, or 4160 baud.
ImagesOn NOAA POES system satellites,the two images are 4 km / pixel smoothed 8-bit images derived from twochannels of the advanced very-high-resolutionradiometer (AVHRR) sensor. The images are corrected for nearly constantgeometric resolution prior to being broadcast; as such, the images are free ofdistortion caused by the curvature of the Earth.
Of the two images, one is typically long-wave infrared (10.8micrometers)with the second switching between near-visible (0.86 micrometers) and mid-waveinfrared (3.75 micrometers) depending on whether the ground is illuminated bysunlight. However, NOAA can configure the satellite to transmit any two of theAVHRR's image channels.
Synchronization and telemetryIncluded in the transmission are a series of synchronization pulses,minute markers, and telemetry information.
The synchronization information, transmitted at the startof each video channel, allows the receiving software to align its sampling withthe baud rate of the signal, which can vary slightly over time. The minutemarkers are four lines of alternating black then white lines which repeat every60 seconds (120 lines).
The telemetry section is composed of sixteen blocks, each 8lines long, which are used as reference values to decode the image channels.The first eight blocks, called "wedges," begin at 1/8 max intensityand successively increase by 1/8 to full intensity in the eighth wedge, withthe ninth being zero intensity. Blocks ten through fifteen each encode acalibration value for the sensor. The sixteenth block identifies which sensorchannel was used for the preceding image channel by matching the intensity ofone of the wedges one through six. Video channel A typically matches eitherwedge two or three, channel B matches wedge four.
The first fourteen blocks should be identical for bothchannels. The sixteen telemetry blocks repeat every 128 lines, and these 128lines are referred to as a frame.
Broadcast signalThe signal itself is a 256-level amplitude modulated 2400Hz subcarrier,which is then frequency modulated onto the 137 MHz-band RFcarrier. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz. On NOAAPOES vehicles, the signal is broadcast at approximately 37dBm (5 watts)[1]effective radiated power.
Receiving imagesAn APT signal is continuously broadcast, with receptionbeginning at the start of the next line when the receiver is within radiorange. 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 stationsregistered with the World Meteorological Organization(WMO). It is unclear what percent of the total user-base this represents, sinceregistration is not a requirement, and was only available after 1996.
Radio receiverThe bandwidth required to receive APT transmissions is approximately34 kHz. Most older scanners (police and fire type receivers) are thestandard 15 kHz bandwidth which were designed to support voicetransmissions. Newer VHF general coverage receivers are equipped with multipleIF bandpasses; some are, but not limited to: 6 kHz, 15 kHz50 kHz & 230 kHz(broadcast FM). Use of a receiver with too narrowa 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 receiverwill be too high to acquire a good picture. For the amateur enthusiast, acomputer controller receiver is the best option to allow the software toautomatically tune and set the required modes for proper reception. There arealso dedicated APT receivers made specifically for computer control and APTreception. Specifically, ICOMPCR1000, PCR1500 & PCR2500 will produce excellent results. Searching on theweb for "NOAA APT (RECEPTION or RECEIVER)" will produce a wealth ofinformation on receivers, software, and antennas.
AntennaAPT images from weather satellites can be received with aright-hand circular polarized, 137 MHz antenna.Normally, there is no need to have the antenna follow the satellite and a fixedposition antenna will provide good results.
The two most frequently recommended antennas are the crosseddipole and the quadrifilar helix antenna (QHA).
Displaying the imagesYears ago,[when?] toreceive APT images, a specialized decoder was required in addition to thereceiver to display or print images, much like HF WEFAX (serving themaritime community). Often both receiver and decoder were combined into oneunit.
Today,[when?] withthe advent of personal computers, all that is required is dedicated software[which?] (many of which offer"free" versions) and a sound card. The sound card acquires anddigitizes the slow scan video (in the audible range) comingfrom the speaker, phones, or line-out of the receiver, and then the softwarewill process the various visible and infrared channels of the AVHRR sensor.Most software will automatically save every image and publish processed imageonto the website of choice, putting up a new image on every pass of an APTsatellite.
Enhanced imagesSince each channel of the AVHRR sensor is sensitive to onlyone wavelengthof light, each of the two images is luminanceonly, also known as grayscale. However, different materials tend to emit or reflectwith a consistent relative intensity. This has enabled the development ofsoftware that can apply a color palette to the images which simulates visiblelight coloring. If the decoding software knows exactly where the satellite was,it can also overlay outlines and boundaries to help in utilizing the resultingimages.
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樓主: VR2GY
發表於 26-2-2016 19:00:18
