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Satellites reception

METEOSAT satellite dish.

Reception of weather images (II)

Let's take the ideal case where you have a directional antenna fixed on a small rotator. Once the hardware part installed you must install a satellite tracking software like WXTrack that supports the UNI-TRAC or the old Kansas City Tracker interface.

Among software products name Nova for Windows. All them require the last Keplerian data file or TLE describing satellites orbital parameters.

When the various modules are connected, powered, and their status checked, you can chase the satellite of your choice. Even if your antenna is not directional you need of a tracking software to locate the satellite in real-time. In this case of course your computer is not linked to your antenna, and it simply simulate a real-time tracking.

Signals that you can collect are numerous : from telemetric data (satellite temperature, orientation, current, etc) transmitted in AX.25 mode (packet), to digital encrypted images or analog signals transmitted in clear in SSB, SSTV, AM or FM mode or in dedicated modes like APT and WEFAX in the case of weather satellites.

A low orbit satellite crosses the sky in about 10 to 15 minutes, quite rapidly. So it is prudent to prepare your manipulations in advance because if you miss a passage you 'd have to wait at least 1 hour to make a new attempt. Hopefuly, today tracking software and decoders make the job easier, and it is often sufficient to depress a button to start an acquisition process, the storage of images being automatically handle by some applications.

At left, reception of weather images transmitted by the geostationary satellite METEOSAT 6 on 1691 MHz on April 15, 1997 with a 90 cm parabole equipped with a 137 MHz converter placed just below it. At center, an image transmitted on 18 May 1998 on 137.620 MHz by the polar orbit satellite NOAA 14. The image was decoded with WXSAT. At right, the reception of WEFAX signals transmitted by NOAA 14 on 137.620 MHz via the WinRADIO card WR-1550i equipped with the FAX module. All images were received in B/W then postprocessed in false colors. Documents F1DTU, NOAA and WinRADIO.

Weather satellites reception

Weather satellites transmit their data continuously on several frequencies, in various transmission modes and resolutions. There are four main transmission modes :

- APT used by polar orbit satellites, WEFAX and HRI (PDUS) used by geostationary satellites

- MSG used by METEOSAT of the second generation.

- We must add modes more difficult to decode like HRPT or CHRPT that require more sophisticated equipements.

The two main modes are APT and WEFAX. Their differences are the modulation mode, respectively FM and AM, the rate of transmission of 120 lines/minute (lpm) vs. 240 lpm and the Index of Co-operation (IOC) of 576 vs. 267.

- APT mode : APT stands for Automatic Picture Transmission. This is the oldest and the simplest FAX transmission mode by satellite. It enables a fully automated and unattended reception of weather pictures based on special start and stop tones recognized by the decoder.

Picture of Europe transmitted in APT mode by a NOAA satellite. The B/W raw image has been postprocessed in false colors.

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.

Europe and Maghreb pictured by METEOSAT 7 on June 3, 2004 at 18h TU.

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.

The receiver

As listener (SWL), to receive APT signals from weather satellites between 136-138 MHz - the easier way to begin - you can buy a radio receiver covering VHF bands equipped 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.

Easier to handle and sometimes cheaper, for some decades there are receivers covering all the spectrum. There are first scanners that are all bands and all modes receivers. The ideal is to find a model offering a bandwidth suited to APT signal (about 30 kHz). Very few satisfy to this requirement. Among the base models, there are AOR 5000 (10 kHz-3 GHz, 1900 €) and AOR 5001 (40 kHz-3.15 GHz, about 4500 €).

In the digital age, a very appreciated solution by amateurs because very complete and compact is the Software-Defined Radio, aka the SDR system. Among these SDR solutions name RTL-SDR, SDRPlay, DX Patrol, HackRF, SDR Sharp (which software is free) as well as Orbitron if you already own a receiver. Some interfaces are not larger than a USB stick like the Funcube dongle.

To join : APT Group on Facebook

To read: Receiving NOAA Weather Satellite Images, RTL-SDR

At left, a WinRADIO G303i interface card tuned on VHF band in VK5ZAI's ham shack. At center, the SDR# plugin for RTL-SDR interface tuned on a NOAA satellite emitting at 137.910800 MHz in WFM mode. At right, the Funcube Pro+ dongle is one of the smallest SDR receiver working from 150 kHz to 1.9 GHz to plug in the USB port of computer. The opposite port is connected to a coaxial cable to a discone, a quadrifilar, a helicoidal or a dish antenna.

You can also purchase an external receiver for computer like the old box ICOM PCR-1000 (100 kHz-1.3 GHz, $629) and its successors (PCR1000-02, PCR1500-30 and PCR2500) but which production is discontinued, the external receiver WinRADIO WR-G31DCC "Excalibur" (9 kHz-50 MHz, $850) or the much more expensive WinRADIO card WR-3500i DSP (150 kHz-2.6 GHz , $2495) or WR-3700i DSP (150 kHz-4 GHz, $2995 ) offering a wide coverage.

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

For the amateur radio, thus licensed and authorized to transmit on ham bands, there is no all modes transceiver covering all frequencies between 150 kHz and 4 GHz. However, he can use a desktop or portable transceiver covering HF bands to UHF (e.g. ICOM IC-7000, IC-9100, Kenwood TS-2000, Yaesu FT-100, FT-857D, FT-991, etc), although most amateurs dismiss this solution as a possible defect will immobilize the rig and prevent any traffic on all bands during several weeks. The alternative is to use a classic HF transceiver and add it the appropriate VHF or UHF converter.

That said, most amateurs prefer to purchase several transceivers, each being dedicated to some bands, in order to cover all the spectrum from LW or HF bands to UHF and higher if needed (see this ham shack). The amateur can begin with a portable transceiver like Yaesu FT-2800M or with the more recent model FT-1900R or FT-2900R covering the 2-m band in emission and from 137 to 174 MHz in receive.

Of course, whatever the solution, these systems need to be connected to an antenna tuned on the specific working frequency and the interface must be completed with a decoding program to vizualize weather images like WXtoImg or HRPT Reader.

If you use a SDR system, you also need a mean to transfer the audio to the decoding software. There is the virtual audio cable, a small tool allowing to exchange very easily audio flows between applications without having to use the speaker, the microphone or a intermediate converter. Another program is VB-Audio Software.

To watch : How to take NOAA satellite pictures using RTL-SDR for under 50$

Receiving NOAA weather satellite using SDR# and WXtoImg

WR-G31DDC 'EXCALIBUR' Demonstration Videos, WinRADIO

9 kHz to 50 MHz Digital Receiver

Above, the graphic user interface of the WinRADIO WR-G31DDC "Excalibur" running on a PC. This interface is combined with an external receiver covering in continuous from 9 kHz to 50 MHz. Below, the GUI of the WinRADIO WR-3000i card tuned on VHF aviation bands. Its frequency range is 150 kHz to 1500 MHz in AM and SSB.

About the antenna, you can use a fixed antenna like a discone or quadrifilar helix that works fine even when the satellite is only 20° above the horizon. If you use a directional antenna, the ideal is to use a rotator but considering the required precision and its weight, this motor will be often the most expensive part of your installation (300-1500€). Simpler and cheaper, you can also set up a small home-made crossed-dipole cut for the 136-138 MHz band.

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 or a SDR interface including UHF bands.

At last, for your information, to receive Inmarsat at 1.5 GHz, besides SDR or WinRadio cars and scanners (cf. this video),the most robust and efficient solution is to use a dedicated receiver 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. Instead of 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 a 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 built-in in 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.

Above, a basic satellite reception system sold by Comelec : this solution listed at 450 € includes an analog weather receiver, a 1690-137 MHz downconverter and a grid dish offering a 24 dBi gain. Of course there are configurations ten times more expensive depending on performances of your receiver and antenna systems. Below, if you are only interested in receiving weather messages, here is a dump extracted from "ProMeteo" sold by Bonito Communication Technologies with his interface "BoardTerminal '98". This tool allows you to select radio frequencies of weather centers transmitting messages like SYNOP, telex and other faxes and to display datas in a readable format on your PC.

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 ended the analog transmission end 2005, when MSG (METEOSAT Second Generation) satellites were 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.

MetOp in low orbit over Earth. Doc T.Lombry

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 was available in 2008, the others depending on when the remaining POES and DMSP (military) program satellite assets will be exhausted. NPOESS provides 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 that will be launched sequentially from 2005 over a period of 14 years. They will constitute the space segment of EUMETSAT's Polar System (EPS). They will orbit close to 830 km of altitude. MetOp seem programmed to send digital transmissions as well.

The price of the installation

The minimum investment for a satellite receive installation is hard to evaluate because many factors affect the price : your taste for digital and computers, for manual works, your skills in electronics, your current equipment (you probably own a computer, maybe a SDR receiver or a VHF receiver too), and the type of émission that you wish to decode.

The receiver or the dedicated decoder, and the suitable antenna represent certainly the two most important expenditure of your installation. If a second-hand VHF receiver can easily be found in advertisements for a price between a few tens and a few hundred euros, in the best case the helix or the dish antenna will be sold at the same price as the receiver, all the more if you add it a rotator and install it on a mast or a heavy-duty steerable mount, hence the interest to built it oneself.

Globally, that you use a VHF transceiver or a SDR receiver, let's say that you must count on an investment of about 450 € minimum, the computer in addition (see above the material sold by Comelec). Of course there is no maximum price. Timestep provides all the required hardware for prices ranging between 1000-3000 €.

Using such installations you can't miss any more the next crossing of Nimbus satellite and consorts.

Good hunting and... good receive !

For more information

Amateur radio equipment (links, on this site)

Space Communications with Mars (on this site)

Dish antennas (on this site)

Ham software review (on this site)

APT Group on Facebook

Dave Ransom Software

David Taylor Satellite Tools

Remote Imaging Group (RIG)

HF-Fax (info, products and live cams)

Rigpix (ham hardware)

Data Meteo (Italian)

World at Phone

EUMETSAT

NOAA

425org Weather Satellite Images

Amsat News

WinRADIO

ICOM

F1AGW website (in French)

F1DTU website (in French)

ON8IM website (in French)

ISS Fan Club

Build Your Own Satellite Ground Station, Makezine

An Introduction to Amateur Satellites, W0ECC (PDF on AMSAT website)

Monitoring NASA communications (frequency list)

Klingenfuss' Super Frequency List  (Broadcasting and utility frequency list on CD-ROM)

Interferences receiving APT/WEFAX images (EUMETSAT).

RTL-SDR Tutorial: Receiving and decoding data from the Outernet, RTL-SDR

Antenna : MegaLoop ML200, MegaLoop FX, Quadrifilar Helix Antennas (Gogan.org), Seavey Antenna, Diamond MR-77 avec connecteur PL, Tram 1185 bi-bande, Thiecom, Timestep

Coaxial cable :  RF coaxial cable (M/F SMA to SO-239 to link a doggle to a whip antenna)

Microwaves : UHF-Satcom, Amateur DSN Group (Yahoo!)

Weather balloon monitoring : SondeMonitor

DSP software: FFTDSP, Skysweeper Pro, Spectrum Laboratory, Spectran, Spectrogram, Raven, Adobe Audition, MatLab

Multi-mode decoders: Skysweeper Pro, Spectrum Laboratory, WXSAT, JVComm32/JVFax, WinSat Pro, Wavecom

Satellite tracking software (licence): Satellite related Software (AMSAT), Nova for Windows

SDR interfaces and software : Funcube, RTL-SDR, SDRPlay, DX Patrol, SDR Sharp, HackRF

SDR receivers on Amazon : RTL2832U+R820T, 48-863 MHz, RTL R820T+8232, 100 kHz-1.7 GHz

Software (satellite tracking) : AMSAT shareware, Stoff, KF2BD, N1VTN, Pocketsatplus, ZL3AD, SatScape, WXSAT, WXTrack

Software (predictions) : Real Time Position of Amateur Satellites over Europe, AMSAT Pass Prediction Calculator, AMSAT Pass Prediction Calculator, AMSAT Pass Prediction Calculator, Heavens-Above, Spaceweather

Orbitron, GoogleSat Trak.

Tutorial : RTL-SDR, Planetary.org

Virtual Audio Cabling: VB-Audio Software, Virtual Audio Cable

Weather hardware: MScan, Qurom, Comelec, Bonito Communication Technologies, Seavey Antenna, Timestep

Weather image decoder: WXSAT, SIAMIV, DARTCOM, WXtoImg, HRPT Reader

TLE and predictions : Real Time Satellite Tracking and Prediction, 3-line TLE (CelesTrak), ISS Latest TLE, Satellite Tracking

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