APT signals come directly from weather satellites in polar orbit (NOAA 12 and 15 on 136.50 MHz, NOAA 14 on 137.62 MHz, NOAA 15 on 137.5 MHz, METEOR 3 on 137.85 MHz, etc) without intermediate processing by ground stations. These later upload software corrections, put the satellite on or off, etc but don't change the images content.

APT signals are transmitted on 137 MHz. This band is easily accessible on low cost receivers at the condition to use an IF FM Filter offering a 30 to 50 kHz bandwidth. Indeed, many all-band receivers and scanners come with either a too narrow (15 kHz) or too wide FM filter (230 kHz). 

Many multi-mode decoding software also support this mode of transmission. Most of these satellites pass twice over the same region in 24 hours, and almost always at the same time, once from North to South and once from South to North direction.

The APT signal  includes a synchronisation pulse in raster lines (called telemetry) that is scanned from Earth, this is the information displayed on the left and right of each picture.

The APT signal is easily recognizable to it modulated frequency (FM) and carrier tonality ranging between 1500-2500 Hz. New satellites have a 2400 Hz carrier like this NOAA 14 APT signal. 

In APT mode, satellites transmit at the rate of 120 lpm alternating two channels, one for the visible image, the other for the infrared image. This is through the decoding software that you select one or another channel.

The resolution is 4 km/pixel. Images are recorded in gray scale but software like WXSAT or SIAMIV can process them in false colors.

Russian weather satellites only transmit infrared images at 120 lpm. The sound of their transmission is thus slightly different from the other weather satellites like on this record of a RESURS 01-N4 signal with an IOC of 382. On their side, METEOR 3-05 uses a higher carrier closer to 2500 Hz with an IOC of 382 as well.

- WEFAX and HRI mode : WEFAX stands for Weather Facsimile. Like APT, images start and stop with tones as for faxes transmitted on shortwaves (SW FAX or HFFAX). WEFAX is today supported by most polar and  geostationary satellites. It uses the standard voice audio channel with an AM carrier at 2400 Hz modulated with a 1.6 kHz video signal. 

Normally charts on HFFAX are B/W, reason for which a decoding on 2-bit depth is enough. METEOSAT on the contrary transmits its images in gray scale, at the rate of 240 lpm alternating visible and infrared scans. Each tone corresponds to a specific shade of gray encoded while the satellite observes the Earth. Start tones for METEOSAT are modulated at 300 Hz (same as HFFAX) and also includes some chart info (like telemetry data). The stop tone is at 450 Hz, the same as HFFAX, and there are at last horizontal synchronisation bursts for each raster line easily recognizable to their "tick-tock".

These synchronization pulses begin with the visible scan with a burst of 1040 Hz (the "tick"), followed by the IR scan burst at 832 Hz (the "tock"). 

Due to this special periodical modulation, the composite audio signal shows a very recognizable FM tone onto 1691.0 MHz (channel A1) and 1694.5 MHz (channel A2) carriers.

A satellite like METEOSAT transmits its pictures every half-hour to ground tracking stations. These raw data are reformatted in real time, engineers add political boundaries and label each image, then transmit the corrected image back to the satellite where it is retransmitted again to the Earth at 1.691 GHz to the attention of all users.

At receive, WEFAX images are cut into 800x800 pixel sections and annotated. A line of 800 pixels is transmitted in 250 ms, hence a complete picture is constructed in 3m33s.

WEFAX signals are transmitted by METEOSAT, GOES, INSAT, GOMS geostationary satellites, each of them covering a dedicated sector of the world (for example, METEOSAT 8 covers Europe and Africa, METEOSAT 5 and INSAT cover Indian ocean, GOMS covers Russia, GMS covers Far East, and GOES covers America).

Like EUMETSAT birds, all these satellites transmit two types of images :  WEFAX (analog mode) and HRI or PDUS (High Resolution digital Images, encrypted). WEFAX images are the easiest to record. If you don't have a satellite dish tuned on 1.69 GHz (similar to a TV dish of at least 1m in diameter), you can use a SHF Yagi, a quadrifilar helix or even a GPS antenna called "micropatch flat antenna" 22.5 cm (9") long. In all cases your antenna must be connected to a 1690/137 MHz downconverter (see below) or directly to a WEFAX decoder as we explained previously.

Surrounding the MSG-1 satellite, alias METEOSAT 8, two WEFAX images recorded on 137 MHz and respectively transmitted in analog and ADTPEC mode by NOAA 14 satellite.

- MSG mode : the geostationary MSG-1 satellite, alias METEOSAT-8 is operational since the end 2002 and remplaces METEOSAT 7 at the longitude of 0°. It transmits images in low and high resolution (LRIT and HRIT) in C-band between 3.7 and 4.2 GHz.

- HRPT and CHRPT modes : these are two high resolution transmission modes used by satellites in polar orbit transmitting on the frequency of 1.69 GHz. Due to a multispectral analyze occupying between 5 and 10 channels, the resolution reaches 1.1 km/pixel but images use up to 120 MB of disk space. To reserve to experts. 

Some brands like Timestep provides the require digital hardware to decode LRIT, HRIT, HRPT and CHRPT modes.

Receiver

If you begin in this activity as a listener and are thus fan of ham radio, in order to receive APT signals from weather satellites you can start in buying a second-hand receiver covering VHF and UHF bands with the appropriate IF filter to get a 30-50 kHz bandwidth. You can find on the second-hand market receivers at a few hundreds euros. If you use a directional antenna, considering the required precision and its weight, the rotator will be often the most expensive part of your installation (300-1500€). But you can already begin with a fixed antenna like the Quadrifilar Helix that works very well, even when the satellite is only 20° above the horizon.

An amateur radio, thus licensed and authorized to transmit on ham bands, will see the problem under another angle, the word is appropriate, Hi ! If this is of your concern, you can buy an all-band transceiver covering all the spectrum from HF to UHF bands. But often radio amateurs prefer to use dedicated systems - especially in the event of breakdown - and buy several transceivers, one to work in HF bands and one or several to work in V/UHF bands.

To receive WEFAX on 1.69 GHz you can use your APT receiver and add in a downconverter 1690/137 MHz, but often received images display a poor quality and resolution. To improve your images you can buy a WEFAX receiver like the model sold by Timestep.

The ideal receiver or transceiver, all bands and modes, to place on your desktop does not exist (yet) as it should cover all frequencies between 150 kHz and 4 GHz and should be able to work in modes like CW (Morse), voice (LSB, USB, AM or FM), and must be able to decode digital USB modes with special filters (e.g. RTTY, FSK, PSK31 or SSTV).

A WinRADIO WR-3000i GUI running on a PC and tuned on VHF aviation bands. Its frequency range is 150 kHz to 1500 MHz in AM and SSB.

But I am happy to tell you that the market progresses. There are scanners (all bands and modes receivers) and portable transceivers to which you can add interfaces to reach this objective. Scanners are probably the cheapest solution at the condition to find a model offering a bandwidth suited to APT signal (about 40 kHz). Very few satisfy to this requirement. Name AOR 5000 (10 kHz - 3 GHz, 1900 €.

Better, with the introduction of Digital Radio Mondiale (DRM) you can buy today "DC to daylight" receiver cards for PC like the cheap blackbox ICOM PCR-1000 (100 kHz - 1.3 GHz, $629 at Universal Radio) or the much more expensive WinRADIO WR-3500i DSP (150 kHz to 2.6 GHz , $2495 to WinRADIO) or WR-3700i DSP (150 kHz to 4 GHz, $2995 to WinRADIO) offering a wide coverage in most modes (AM, FM, USB, LSB, CW, FSK). Coupled to an appropriate antenna and optionaly a rotator, you have all you need to pick up signals from satellites and all civilian or military services transmitting in shortwaves and microwaves in good conditions. Check the website of major manufacturers (some permit to subscribe to a maillist like WinRADIO to name one of them) as well as specialized magazines for more information.

Remember that if you can use freely a scanner, listening to some reserved frequencies (military among others) is prohibited if you cannot proved a professional interest in using these bands.

To receive Inmarsat, the most efficient solution is to use dedicated receivers like Miteq Inmarsat Pilot or a competitor. This prevent you to buy or to build a downconverter and the signal will be of excellent quality.

This receiver uses digital techniques (DSP) as well as a frequency synthesizer. It is compatible with most maritime and aeronautical applications.

Downconverter and demodulator

Only geostationary satellites like METEOSAT transmit on high frequencies and in digital modes. To avoid you acquiring a specific receiver for the 1.69 GHz band or a WinRADIO card quite expensive with its FAX module (at least $600), you can use your 136-138 MHz receiver, the one you use to receive transmissions from polar satellites (NOAA, METEOR, RESUR, SICH, OKEAN). Note however that most METEOR and all RESUR, SICH and OKEAN are not active in APT mode.

You only need to equip your VHF receiver with a 1690/137 MHz downconverter like the TV970 displayed below offering a 30 kHz bandwidth.. This converter must be placed directly below the dish (if the dish is wide enough it can be placed in the feedhorn too) because at so high frequencies signal losses are significant. Your installation must be completed with an Analog-to-Digital (A/D) converter to demodulate signals that you will connect to the serial port of your computer. Today this A/D adapter is replaced by the sound card that equips any computer (same principle as for SSTV) or you can build or buy an external device like MFJ-1213 adapter or the ones sold by Bonito Communication Technologies.

Then, in a classic configuration (all excepted WinRADIO) to read weather images you need to install a decoding software able to read APT, WEFAX and other WX modes. Among free programs let's highlight WXSAT and JVComm32/JVFax. A very performing multi-mode decoder, but commercial is Skysweeper Pro, that I reviewed shortly. The famous AMSAT website, HF-FAX as the one of Dave Ransom provide a long list of products too. 

At last, to be complete I remind you that is you are only interested in weather data, you can receive them for free using the APRS network, which is sometimes relayed by repeaters.

The future

METEOSAT will end the analog transmission end 2005, when MSG (METEOSAT Second Generation) satellites will be available. The same will happen with the GOES satellites. In all these satellites, digital receivers are used for demodulation of PSK, BPSK, and QPSK signals. Some receivers include an optional internal frame formatter able to support standard products like MSG HRIT, MSG LRIT, MTSAT LRIT, NOAA HRPT, GOES GVAR, M-22 AMB, GMS S-VISSR, DOD DMSP, Fengyun 1 CHRPT, and Fengyung 2 S-VISSR. 

LRIT/HRIT systems receive, archive, display and process digital LRIT and HRIT data from EUMETCast, MSG direct broadcast, GOES and MTSAT. Note that to receive EUMETCast and MSG direct broadcast, EUMETSAT recommends using of a DVB Receiver TechniSat SkyStar to DVB receiver. This device is available as an internal PCI card or an external USB unit. It is supplied with required software (drivers and T-Systems Business TV-IP).

Some LRIT receiver will accept either 137.5 MHz or 70 MHz input, but you could always use a down convertor 1690/137 MHz as currently.

LRIT and HRIT data are used in more and more fields : nowcasting, numerical weather prediction, climate monitoring, research... The latest MSG HRIT imagery is available every 15 minutes – twice faster than WEFAX and HRI systems. Coupled with the high quality and wide range of data available, this technology will allow major improvements in the forecasting of severe weather.

The polar orbiter from NOAA will be available in the next years. The first converged NPOESS satellite is expected to be available for launch in the latter half of the decade, approximately 2008, depending on when the remaining POES and DMSP (Military) program satellite assets are exhausted. NPOESS will provide significantly improved operational capabilities and benefits to satisfy the U.S.A.'s critical civil and national security requirements for space based, remotely sensed environmental data.

MetOp is the European first polar-orbiting satellites dedicated to operational meteorology. It represents the European contribution (ESA) to a new cooperative venture with NASA providing data that will be used to monitor our climate and improve weather forecasting. MetOp is a series of three satellites to be launched sequentially over 14 years, starting in 2005, and forms the space segment of EUMETSAT's Polar System (EPS). They will orbit close to 830 km of altitude. I have no idea whether MetOp will operate in analog mode but they seemed programmed to send digital transmissions as well.