Group for Earth Observation - wxtoimg.infowxtoimg.info/sampler.pdf · National Space Centre next May Read about the disappearing perennial Arctic sea ice Peter Wakelin writes about

Inside this issue . . .

Register for GEO’s firstSymposium at theNational Space Centrenext May

Read about thedisappearing perennialArctic sea ice

Peter Wakelin writesabout India’s SpaceProgramme

David Taylor discussesMeteosat-8 (MSG-1)reception and answersyour questions on thebest PC configuration touse

Learn about Europe’sEnvisat satellite and itsamazing radar Imagery

Follow our guide tosetting up your ownweather satellite groundstation, and receive yourown satellite images

The Snows ofKilimanjaro - FrancisBell discusses satelliteimaging of snow in thetropics

Arthur Andrews writeson the Australian scene

Build a portable,collapsible QFH antenna

and much more ...

The Independent Amateur Quarterly

Publication for Earth Observation

and Weather Satellite EnthusiastsLAUNCH ISSUE

Group for Earth Observation

ContentsIntroducing GEO Francis Bell 2GEO Symposium - Leicester - May 1, 2004 3The Disappearing Perennial Arctic Sea Ice NASA 4Official GEO Internet Discussion Group 5Snows of Kilimanjaro (How Band Aid Got it Wrong) Francis Bell 6Meteosat Second Generation John Tellick 7Free Amateur and Educational Access to MSG EUMETSAT 9The Polar Low Phenomenon Les Hamilton 10Watching the Weather Cedric Roberts 13Unusual Solar Activity Peter Wakelin 14May the FORCE be with You Francis Bell 15MSG Update - John Tellick’s Questions Answered David Taylor 16The Indian Space Programme - part 1 Peter Wakelin 18European Publications - Free to GEO Readers Francis Bell 20Copy Deadline for the Next Issue 21Satellite Frequencies and 2-line Elements 21Advanced Synthetic Aperture Radar from Envisat 22Setting the Correct Time on Your Computer David Taylor 25Cover Images 28Comet Training Modules Peter Bartlett 30View from the Other Side Arthur Andrews 31Improved W/V Data from MSG-1 David Taylor 33Massive B-15A Iceberg Snaps in Two NASA 34Earth Imaging News Peter Wakelin 36In Tray - Readers’ Letters and Questions 37Satellite Predictions 38Joining the Group for Earth Observation 39EUMETSAT Operations New Bulletins 40Software Showcase Douglas Deans 41First Steps in Setting up an APT Ground Station Les Hamilton 42Portable/Collapsible Quadrifilar Helix Antenna Chris van Lint 46

GEO is led by

Francis Bell Nigel EvansClive Finnis Ray GoddenLes Hamilton John TellickPeter Wakelin

CONTACT INFORMATION

Public Relations and EducationFrancis Bell, Coturnix House, Rake Lane, Milford, Godalming, Surrey GU8 5ABEnglandTel: (01483) 416 897e-mail: [email protected]

General InformationJohn Tellicke-mail: [email protected]

Articles and Images for PublicationLes Hamilton, 8 Deeside PlaceAberdeen AB15 7PWScotland, UKe-mail: [email protected]

Membership and SubscriptionsNigel Evans,‘Eventail’, 7 Gloster Ropewalk, Aycliffe, Dover CT17 9ESEngland, UKe-mail: [email protected]

Meteorological ConsultantPeter Wakeline-mail: [email protected]

Publisher

Group for Earth Observation Ltd.

The Group for Earth Observation Limited is acompany in England and Wales limited byguarantee and not having share capital.Company number 4975595.

The registered office is Coturnix House, RakeLane, Milford, Godalming, Surrey GU8 5AB

PrintingToptown Printers, Vicarage Lawn,Barnstaple, Devon EX32 7BN.

Responsibility for ConstructionalProjects and SoftwareEvery effort is made to ensure that the technicaland constructional articles published in thisQuarterly are correct. However, the ultimateresponsibility is with the reader to ensure thesafety of constructions and for any interfacingwith other equipment. GEO cannot acceptliability for shortcomings in any published designor any constructions carried out by members orother third parties.

Copyright GEO © 2004The contents of this Quarterly remain theintellectual property of the Group for EarthObservation Ltd. and its contributors. Copying oftext or images, either from the printed Quarterlyor from any electronically stored source, isforbidden without permission from the editor.

Welcome to your first issue of the GEO Quarterly, the officialmagazine of GEO, the Group for Earth Observation. GEO isa new organisation aimed towards everyone with a keeninterest in weather satellites and Earth imaging.

GEO has been conceived as a wholly amateur group, which willremain totally independent of any commercial interests in theweather satellite and Earth imaging fields. GEO is totallydedicated to serving both educational users and the amateurcommunity.

Now turn to page 2 and read Francis Bell’s ‘Mission Statement’on The Group for Earth Observation. Learn of our hopes andaspirations, and the bright future we all see ahead for theRemote Imaging Community.

Inside this launch issue you will find numerous articles aimedat newcomers to the hobby. Arthur Andrews reminisces abouthow he started out in Australia. Cedric Roberts explains whatfired his interest in the weather. John Tellick traces theMSG-1 story from its launch in 2002 up till the present.Francis Bell suggests an Earth-imaging project to test yourskills.

GEO Quarterly No 1GEO Quarterly No 1 Group for Earth ObservationGroup for Earth Observation Launch IssueLaunch Issue

Page Page 22 Check www.geo-web.org.uk for all the latest news

Welcome to this first publication of‘GEO Quarterly’. In this introduction Iwill be as brief as possible because thecontent of the following pages will bemuch more interesting and will relateto the practical aspects of our interestsin live weather satellite reception.

The background to this new groupstems from a wish to establish anamateur organisation interested in thereception of live weather satelliteimages directly from space. The historyof amateur weather satellite receptiongoes back over 20 years, to the timewhen Muirhead printers and dedicatedframestores were used for recording asection of an image. Today however,with readily available fast computersand sophisticated software, togetherwith a second generation of weathersatellites beyond APT and WEFAX dataformats, a second look at our interestgroup / hobby seemed timely. Hence,the new group: ‘Group for EarthObservation’, associated with theappropriate acronym ‘GEO’ and itsquarterly publication.

In the very long tradition of amateurinterests—the best example must beamateur radio—our intention is toexperiment, free from any commercialassociation, and to help other membersinterested in direct reception of weathersatellite signals. You may have seenpublicity material submitted to ShortWave Magazine, the RSGB’s RadComand the National Space Centre’s BNSCDirectory. These published texts areexactly in line with the abovestatements of our plans and rationale.

Without wishing to excessively flatterthose people who have agreed to leadthis new group, there is a wealth ofintellectual background within thegroup, embracing disciplines such asRF, astronomy, computing, meteor-ology, oceanography, publishing and awide experience of weather satellitereception.

We have decided to make this firstGEO Quarterly publication freelyavailable, but of course, to maintain ourgroup and cover the costs of thequarterly publications (quarterlyimplies four times a year) we do need asubscription membership. If you havenot yet joined then read this publicationand then judge whether £18 a year ismoney well spent. Visit our web site atwww.geo-web.org.uk which is readilyaccessible and will be under constantreview as this GEO Group develops.

Alternatively, write to:

FJB (GEO),Coturnix House,Rake Lane, Milford,Godalming GU8 5ABEngland, Great Britain.

for information about joining the GEOgroup.

Forgive me if I repeat this but GEO hasa passion for being an amateur basedgroup. And here comes the big benefit:

EUMETSAT have just written tome stating that amateur andeducational users of theirsatellite services do not have topay any license fee for thereception of their encryptedsatellite data streams. Here Imust express particular thanksto John Tellick who deservesmost of the credit for making thispossible. For over a decade now,John has campaigned tirelesslywith EUMETSAT, the MetOffice and other Agencies onbehalf of weather satelliteenthusiasts.

I cannot understate the benefit of thiswhich I and others have worked forwith EUMETSAT. Herewith my publicthanks to the EUMETSAT Council andto our UK Meteorological Office for their

support of the amateur and educationalcommunities who explore the scienceand technology of what is possible forthe reception and display of the newgeneration of weather satellite images.

A cautionary word here. I believe thatGEO is in a privileged position in theamateur community and I ask that thisstatus is not compromised. I ask thatnobody distributes or publishesmaterial, text or images, where thecopyright resides elsewhere. Most of ourmaterial is directly available in thepublic domain so there is no problem,but it is courteous and perhaps a legalrequirement in some cases to have thepermission of the author or copyrightholder before submitting material forprinted publication or display on theInternet.

GEO will not support any exploitationof the intellectual property ownedoutside the public domain or withoutthat owner’s permission.

Judge our first publication by reading itand then decide whether you wish tojoin us. To join, visit our web site orwrite to FJB as shown above.

If you think our new GEO group with itsquarterly publication plus its supportfor amateur and educational users isworthwhile, then please recommendGEO to your friends and colleagues. Wedo need an extensive membership tomake GEO viable and worthwhile.

I cannot finish without expressing myadmiration for the commitment andskills of those people (friends),responsible for the formation of GEO.The same goes to contributors to thispublication who have made it soworthwhile. I look forward to thesuccess of the group and the excitingtimes ahead.

Enjoy reading and exploring the rest ofyour first GEO Quarterly.

Francis Bell

Page Page 33

Launch IssueLaunch Issue Group for Earth ObservationGroup for Earth Observation GEO Quarterly No 1GEO Quarterly No 1

Check www.geo-web.org.uk for all the latest news

The dawn of a new era wassignalled with the success-ful launch of Europe’snewest geostationary satel-lite, MSG-1 on August 28,2002. MSG-1 is the first ina new series of all-digitalEuropean geostationarymeteorological satelliteswhich will replace the orig-inal Meteosat series whichhas been operational since1977.

MSG is a cylindrical,spin-stabilised satellite,rotating at 100 rpm arounda principal axis which isaligned nearly north-south.The spin is also a necessaryfeature for the operation of MSG’s Earth scanningradiometer, the Spinning Enhanced Visible andInfrared Imager (SEVIRI).

This instrument scans the Earth every 15 minutes toproduce full-globe images in eleven spectral channelsranging from visible light (VIS) through Infrared (IR).These images have a resolution of 3 km at the sub-satellite point (SSP) while a selected area, includingEurope, is provided as a high resolution visible image(HRV) with 1 km resolution at the SSP.

MSG satellites transmit two end-user imagingchannels. High Rate Information Transmission (HRIT)on 1695.15 MHz* provides all twelve SEVIRI channelsat full resolution every 15 minutes. Low RateInformation Transmission (LRIT) at 1691 MHz*contains a subset of five of the SEVIRI spectralchannels at reduced resolution every 30 minutes plus,in addition, relays of foreign satellite data (GOES-Eand -W, Meteosat-5 IODC and GMS/MTSAT imagery)every 3 hours.

MSG-1 also carries a Geostationary Earth RadiationBudget (GERB) instrument, the first time this hasbeen flown on a geostationary satellite. Its data, bothalone and combined with that from SEVIRI, willprovide vital new information on diurnal variationsinto the radiative balance of the planet. It will alsohave important applications regarding climate studiesand monitoring.

John Tellick

Launch of MSG-1© 2002 ESA/CNES -

Arianespace/Photo ServicesOptique CS

Failure of the MSG-1 Power AmplifierFollowing its successful launch and placement intogeostationary orbit, MSG-1 commissioning tests beganin late September 2002. But in mid-October, thedownlink under test ceased when the Solid StatePower Amplifier (SSPA) being used unexpectedlyswitched off and could not be reactivated. Subsequentimaging tests proved the SEVIRI instrument and theraw data downlink chain to be operating correctly, butit was eventually decided not to disseminate userservices directly owing to doubt over the remainingSSPAs.

The EUMETSAT EUMETCast ServiceEUMETSAT had already been testing a new dataretransmission service for the Advanced TIROSOperational Vertical Sounder (ATOVS) data fromsensors aboard NOAA polar orbiting satellites.Following the SSPA failure, EUMETSAT were obligedto pursue an alternative means of disseminating theMSG-1 data and trials via EUMETSAT's MulticastDistribution System (EUMETCast)] began at the endof April 2003. These were transmitted byEUTELSAT's Hot Bird-6 satellite stationed at 13°Eusing the digital video broadcast (DVB) format used bydigital satellite TV. This service was due to becomefully operational as the primary 0° longitude service inlate January 2004 with MSG-1 renamed Meteosat-8and located at 3°W.

System Hardware RequirementsThe main component of the hardware system is theTechniSat SkyStar-2 PCI DVB receiver card. This is afree-to-air digital satellite-TV receiver that is installedinside your computer (there is also a USB version).The SkyStar unit can also receive data transmissions,including satellite Internet services. The receivingantenna recommended by EUMETSAT for most ofEurope is an 85 cm offset satellite-TV dish, provided agood quality standard Universal digital satellite TVLNB is employed. Good quality satellite TV co-ax cablesuch as CT100 is required between the dish and thereceiver. Care should be taken when stapling androuting this cable as digital signals can suffer lossesdue to sharp bends or cable constriction.

Aligning the dish to Hot Bird-6 is not too difficult if youhave a sensitive satellite-TV level meter at the dish.One of the SkyStar installation set-up screens includesa signal-strength meter which allows you to ‘fine tune’your dish set-up.

Software RequirementsTo receive the EUMETCast service from Hot Bird-6you also require software called tq®-TELLICAST tohandle the DVB data stream. This software isavailable either from T-Systems in Germany for €60plus handling charges, or from EUMETSAT, who sendyou purchasing details once you have applied for theservice. To be able to purchase this software you mustfirst successfully register with EUMETSAT for MSG



dissemination using the form EUM/OPS-MSG/REG/01which is available on the EUMETSAT website at:

http://www.eumetsat.de/en/dps/helpdesk/registration.html

Once you have been successfully registered, you will beissued with the username and password required forinstallation of the tq®-TELLICAST software andnotified that you may now apply for the software.

To assemble, save and display the files generated bythe T-Systems software you will also require DavidTaylor’s MSG Data Manager software. This can beobtained from David Taylor for £44 at:

http://www.satsignal.net

It is also strongly recommended that you download andread EUMETSAT’s technical document EUM TD 15,Issue No 3. A link is available from our website at:

http://www.geo-web.org.uk/eumet_docs.html

as it contains much useful information concerning theEUMETCast service.

The Fully Operational ServiceThis began in January 2004 and contains the complete12-channel SEVIRI HRIT data every 15 minutes plusfull LRIT data every 30 minutes, as well as foreignsatellite relays. With the exception of the WMOrequirement of ‘free’ 6-hourly HRIT and LRIT datasets, this data will be encrypted from March 2004. Toreceive the full encrypted data sets an Aladdin eTokenPRO decryption unit (a USB dongle) will be required,together with associated software to control access tothe data in accordance with the EUMETSAT DataAccess Policy and your user category as applied for atregistration. The current price is around £40 for thedongle and software. All users who have applied for theEUMETCast service will be contacted regardingapplication for the decryption unit and software beforethe service is encrypted in March 2004.

C-band MSG-1 EUMETCast ServiceAlthough Europe is now catered for with the Ku-bandEUMETCast service via HotBird-6, EUMETSAT also havean obligation to supply AfricanStates with MSG meteorologicaldata under the EU-fundedPUMA Agreement, which hadequipped African States withMSG receiving stations.

For technical reasons, C-band

transmission was preferable and, following tests,EUTELSAT’s Atlantic Bird-3 satellite stationed at5°W was chosen. These tests proved successful, and atrial service began late in 2003. The C-band beamcovers the whole of Europe and Africa, all of which,with the exception of parts of Morocco, southernAlgeria, Mali and Niger, lie within the 39 dBWfootprint.

The format of the C-band dissemination will be DVBEUMETCast, the same as for the Hot Bird-6 Ku-Banddissemination. The data will be uplinked to AtlanticBird-3 via the Fucino ground station in Italy and willbe a re-broadcast of the Hot Bird EUMETCast service.

Access to the C-band EUMETCast trial disseminationboth within and outside EUMETSAT Member States issubject to applications using the same MSG ImageData Service registration form described above.

The same T-Systems tq®-TELLICAST software will berequired, as will MSG Data Manager to deal with filemanagement and image viewing.The Atlantic Bird-3 C-band footprint over Europe andAfrica is only 39 dBW compared with Hot Bird-6'sKu-band spot beam of 53 dBW over SE England andcentral Europe, This means that a significantly largerantenna will be required to receive this service, andEUMETSAT recommend a 2.4 metre dish.

The Atlantic Bird-3 C-band EUMETCast footprint alsoextends into the eastern part of the USA at reducedlevels and EUMETSAT recommend that a dish size of3.7 m will be required in most US locations.

* No longer relevant to MSG1 due to SSPA failure.

C-band MSG-1 EUMETCast ServiceAlthough Europe is now catered for with the Ku-bandEUMETCast service via Hot Bird-6, EUMETSAT alsohave an obligation to supply African States with MSGmeteorological data under the EU-funded PUMAAgreement, which had equipped African States withMSG receiving stations.

The Skystar-2 card (left) and USB unit (right)

A typical 85 cm offsetsatellite-TV dish

The Ku-band signal power level foot-print forHot Bird-6. Most of central Europe and S and

SE England lie within the 53 dBW contour.

Page Page 55



For technical reasons, C-band transmission was preferable and, following tests, EUTELSAT’s Atlantic Bird-3satellite stationed at 5°W was chosen. These tests proved successful, and a trial service began late in 2003. The C-band beam covers the whole of Europe and Africa, all of which, with the exception of parts of Morocco, southernAlgeria, Mali and Niger, lie within the 39 dBW footprint.

The format of the C-band dissemination will be DVB EUMETCast, the same as for the Hot Bird-6 Ku-Banddissemination. The data will be uplinked to Atlantic Bird-3 via the Fucino ground station in Italy and will be a re-broadcast of the Hot Bird EUMETCast service.

Access to the C-band EUMETCast trial dissemination both within and outside EUMETSAT Member States issubject to applications using the same MSG Image Data Service registration form described above.

The same T-Systems tq®-TELLICAST software will be required, as will MSG Data Manager to deal with filemanagement and image viewing.The Atlantic Bird-3 C-band footprint over Europe and Africa is only 39 dBW compared with Hot Bird-6's Ku-bandspot beam of 53 dBW over SE England and central Europe, This means that a significantly larger antenna will berequired to receive this service, and EUMETSAT recommend a 2.4 metre dish.

The Atlantic Bird-3 C-band EUMETCast footprint also extends into the eastern part of the USA at reduced levelsand EUMETSAT recommend that a dish size of 3.7 m will be required in most US locations.

* No longer relevant to MSG1 due to SSPA failure.

Footprint diagram for Atlantic Bird-3 C-band transmission

Next Page

Arne van Belle from s’Gravendeel in the Netherlands received this MSG-1 image via the Eumetcast service at 08:44 UT on May 8,2003. The image was decoded using David Taylor’s MSG Data Manager. After remapping, channels VIS006 (blue), VIS008 (green)and IR016 (red) were combined into this colour composite image using GeoSatsignal.

This selection of channels gives, generally, the most realistic colour scheme: it can be used to differentiate ice and snow from water.The high cold clouds are tinged cyan, as is snow in the Alps.

A striking feature of the image is Cyclone ‘Manou’ centred over Madagascar.

MSG-1 Trial Data, © EUMETSAT, 2003

Page Page 77



When you examine your NOAAchannel-4 infrared images duringthe winter months, you may fromtime to time spot what looks like aminiature cyclone in the BarentsSea or the North Atlantic Ocean.What you are observing is almostcertainly a polar low.

Just such a phenomenon wasobserved by Ferdinand Valk inDecember 2003, and his image isreproduced opposite. The polar lowis the whorl of cloud which fills theBarents Sea and obscures thesouthern part of the island ofNovaya Zemlya.

What is a Polar Low?During long Arctic winter nights,cold air builds up to create what isknown as the polar front, theboundary between cold air to thenorth and warmer air to the south.A polar low is a small cyclone withwindspeeds near to or above galeforce, which forms in the cold airfar north of the polar front itself.

The European Polar Low WorkingGroup [1] defines any cyclone to thenorth of the polar front, and whichhas a spread of less than 2000 kmacross, as a ‘meso-scale cyclone’ or‘mesocyclone’. The classical polarlow is included as a subtype,restricted to very intense maritimesystems stretching up to 1000 kmand where the near-surface windsexceed 15 metres per second (30knots).

A polar low forms only over thesea, and exclusively during thewinter months: it arises when apacket of cold polar air movesacross relatively warmer water(such as that provided by theNorth Atlantic Drift as it sweepspast Iceland and into the BarentsSea). Typically, these polar lowsshow considerable similarity totropical hurricanes but they aregenerally only of the order of 400to 800 km across: some may span

Figure 1 - NOAA 17 - 09:14 UT on December 21, 2003 (© Ferdinand Valk)

as little as 100 km. The polar lowillustrated in figure 1 spannedaround 900 km. Figure 2 shows thesynoptic sea-level pressure chartfor the region. The long, trailingcomma-shaped tail seen huggingthe northern coastline of Russia’sKola peninsula is a characteristicfeature of the polar low. As thestorm matures, spiral cloud bands,often with a clear eye at the thecentre of the cloud vortex, lookremarkably similar to a tropicalcyclone and provide the polar lowwith its alternative appellation,that of Arctic hurricane.

For many centuries, seafarers inthe north Atlantic and Arcticoceans have recounted tales offierce storms that seemed to appearfrom nowhere, often accompaniedby strong winds and snowfall [3]. Itis highly probable that ships whichdisappeared without trace over theyears may have been the victims ofthese unpredictable, short-livedcyclones. Polar lows show a very

characteristic pattern and life cycleduring which they produce severeweather consisting of heavyprecipitation (usually falling assnow) coupled with strong surfacewinds.

Eventually, most polar lowsencounter the coasts of Norway,Scotland and Denmark where theiractivity rapidly diminishes. In fact,Northern Scotland bore the bruntof just such a polar low in February1969 during which a gust of windmeasured during a violent snow

Figure 2 - Sea-level isobarsat 12:00 UT on December 21, 2003

Les Hamilton



Figure 3 - NOAA 17 - 07:33 UT on December 20, 2003



shower at Kirkwall airport in the Orkney Islands wasrecorded with a speed of 118 knots (218 km/h) .

It was not until satellite imagery arrived on the sceneduring the 1960’s that a better understanding of thesestorms was realised. Because of their northern positionand small scale, and the fact that they appear in thedark winter season, AVHRR-NOAA IR images provideone of the best ways of detecting polar lows.

Today we know that polar lows are a frequent featureof the relatively warm ice-free waters around thecoasts of the Nordic countries, in the Labrador Sea, theGulf of Alaska and the Sea of Japan. And of course,they are also a feature of Antarctic polar waters.

Lifespan of a Polar LowA polar low can brew up and disperse completely againin a surprisingly short period of time: typically thephenomenon lasts only between 12 and 36 hours.

The illustrations on the right trace the appearance anddispersal of the December 21 polar low. Exactly oneday before Ferdinand Valk spotted this system, aNOAA-17 channel-4 HRPT segment revealed no signwhatever of any activity of this nature (figure 3)although there was clear evidence that streamers ofpolar air were heading southwards over the BarentsSea. Early the following day, NOAA-12 imaged acompact polar low encircling the area (figure 4), butjust six hours later (figure 1) it was already clearlybeginning to show signs of decay. The final image inthe sequence came from a NOAA-16 transmission inthe early hours of the following day (figure 5), by whichtime the storm had all but exhausted itself, leavingbehind only a few isolated fragments of bright cloud toindicate that it had ever existed.

Further Examples of Polar LowsInterestingly, a particularly well formed polar lowappeared in precisely the same region almost exactlyone year ago, on December 20, 2002 (figure 6).

Although polar lows are generally associated with theArctic, they can form in the waters of the northAtlantic, in the region between Greenland and Norwayand sometimes, rarely, even as far south as the FaeroeIslands. Furthermore, a polar low embedded in asoutherly moving airstream may on occasions trackinto the North Sea. We need look back only as far asJanuary 2003 to remind ourselves of one of theserelatively rare visitations when much of Britain andthe Low Countries experienced paralysing blizzards asa polar low made landfall. The culprit coursed downthe North Sea, and is illustrated in figure 7, aNOAA-17 channel-4 APT image acquired shortlybefore noon on that day.

Like their much larger tropical cousins, polar lowstend to decay rapidly when they reach land, due to the

loss of the energy supply provided by the relativelywarm sea they are traversing. But before they do, theycan produce severe weather conditions over the landand this was one such occasion. The cyclone advancedsouthwards over the North Sea and caused severe

Page Page 99




blizzards over much of eastern Britain as well as overlarge parts of Holland and Belgium (figure 7) [2]. Atsome locations on the east coast of England, windspeeds reached force 10!

From that afternoon chaos reigned throughout GreatBritain. Stanstead Airport was closed while thousandsof travellers were delayed at Heathrow Airport asflights were cancelled; the London underground cameto a halt; schools were closed; many workers trying tomake their way home after work found themselves intraffic gridlock for many hours; thousands sufferedpower cuts; snowploughs struggled to keep roads open,with many short journeys taking hours and there werenumerous road accidents as up to 15 cm of snow fell inmany places. Yet in the corresponding NOAA-17 imagethe following day, only a tenuous streamer, possibly aremnant of the cyclone’s ‘comma’ tail, remained to tellthe tale (figure 8).

References and Further Reading1. The European Polar Low Working Group

http://www.meteo.uni-bonn.de/mitarbeiter/GHeinemann/eplwg/eplwgop.htm

2. 'Polar low' boven zuidelijke Noordzeehttp://www.knmi.nl/~floor/artikelen/zenit/polarlow/

3. Weather Onlinehttp://www.weatheronline.co.uk/feature/wf261103.htm

4. Cloud Structure (of the Polar Low)http://www.zamg.ac.at/

docu/satmanu4.0/satmanu/manual/PL/pl1.htm

ImagesFigure 2Synoptic chart created with Digital AtmosphereFigures 3, 4, 5, 6Reproduced by courtesy of the SMIS NOAA HRPT telemetry archive at:http://d902.iki.rssi.ru/dataserv/engl/oper_e.shtml

Figure 7 - A polar low brings blizzards to Britain and the Low Countries (NOAA 17 - 11:08 UT - January 30, 2003

Figure 8 - Next day, the polar low has completely disappeared(NOAA 17 - 10:43 UT - January 31, 2003)

The images were processed using David Taylor’s HRPT Reader, thenconverted to orthographic projection using his Groundmap program,which also added country outlines.Figures 7 and 8NOAA-17 APT images received by Les Hamilton and processed usingCraig Anderson’s Wxtoimg software



The fact that these satellites approach the observer,pass high overhead then recede towards the oppositehorizon superimposes a significant and constantlyvarying Doppler effect on to their transmittedfrequencies (the same as you hear when a police-car

Perhaps you have visited one of the rallies this summerand seen weather satellite images on display. Maybe afriend with a PC has let you into the secret that you toocan receive such images, in real time, from the skies.Or you may have discovered the hobby when browsingthe Web. Either way, armed with a personal computer,suitable antenna and dedicated receiver for the137 MHz band, you could be running your own groundstation in next to no time and for relatively little cost.

The SatellitesThere are currently three American satellitesdedicated to the return of so-called Automatic PictureTransmission (APT) weather images of our planet.Operated by the National Oceanic and AtmosphericAdministration (NOAA), the NOAA-12, NOAA-15 andNOAA-17 satellites occupy low Earth orbits840 kilometres in altitude and pass completely roundthe planet once every 104 minutes. As the satelliteorbits are fixed in space with respect to the Sun, therotation of the Earth carries every part of the globewithin the view of any of these satellites at least twiceper day.

ReceiversIn order to gain access to the signals from thesesatellites, you require a receiver tuned to the 137 MHzband. There are several options for Europeanenthusiasts; the dedicated Proscan II receiver, the RX2(which is only available in kit form) or the CzechRX137-141 or a scanner. Details can be found at:

http://www.geo-web.org.uk/hard.html

Fig.1 - A cased RX2

Fig.2 - The Proscan II

Les Hamilton

siren approaches then recedes from you). Receiversmust be able to accommodate this and a bandwidth of40 kHz is required. Both Proscan and RX2 receiverswere specifically designed to combat this but themajority of scanners were not.

Consequently, many scanners are totally unsuitablefor weather satellite imagery, and even those that are(e.g. the Icom PCR 1000) produce inferior resultscompared with the Proscan and RX2.

The AntennaSignals from the NOAA weather satellites aretransmitted with right-hand circular polarisation, andyou therefore require an antenna specificallyconstructed to receive this type of signal. Use of amakeshift antenna will certainly allow you to hear theaudio signal through your receiver, but it will not bepossible to decode it into a satisfactory image.

Most APT enthusiasts utilise one of two basic designsof dedicated antenna, the crossed dipole (turnstile) orquadrifilar helix (QFH). Turnstile antennas can bebought for around £30. Commercial QFH antennas arenotoriously expensive, but many enthusiasts buildtheir own from PVC conduit tubing, junction boxes and

co-axial cable. Chris van Lint’s popular QFH project,originally published some years ago is reproduced onpage 46, and takes just an afternoon to construct.

The antenna should be mounted with clear horizons allaround. The best situation as at the top of your TVmast (above the TV antenna), but loft-mountedantennas can give excellent results as can an antennamounted on a stake in the garden, just a metre or so offthe ground. So long as there are no obstructionsbetween the antenna and the satellite, nor largenearby metallic objects to cause signal reflections, goodresults can usually be obtained.

Fig.3 - A turnstile antenna Fig.4 - A QFH antenna

Page Page 1111



‘Recording’ radio-button, make sure that the ‘line-in’option is ticked in the ‘Show the following volumecontrols:' browser, then click OK to reveal theRecording Control panel (figure 6).

The usual input channel for recording from a wxsatreceiver is line-in, so this must be activated by firstlychecking the 'Select' box in the 'Line balance:' panel,and then elevating the volume control slider to mid-scale (this will require more careful adjustment later).However, depending on the input levels of yoursoundcard, you may prefer to make use of themicrophone input channel instead.

SoftwareThe simplest package, but the most popular amongbeginners, is Wxsat authored by Christian Bock ‘wayback in 1995. The program’s appeal lies in its ability todecode the satellite transmissions into images in realtime, while simultaneously recording them on to yourhard-drive in the form of WAV files. Although thegeneration of images requires a degree ofexperimentation and effort by the user, recording WAVfiles is relatively straightforward.

Open Wxsat’s <Recording> menu and select the thirditem from the foot, labelled ‘Test’. This reveals awindow flanked by a grey line at both top and bottom.These two lines represent the maximum permittedamplitude for an incoming satellite signal. Exceedthese levels and the signal will be ‘clipped’, maskingout much of the bright cloud detail as full white. Thislevel must be adjusted during reception of a live

The Display UnitThese days, weather satellite images are almostalways displayed on the screen of a personal computer.But before you can view the weather pictures, youmust first convert the received radio signals intoimages, and for this an interface is required. There aretwo options: either a purpose-built decoder in the formof a PC card or external unit, or the PC’s existingsoundcard. The simpler option (and also the cheaper) isthe latter, and this is the route described in this article.

Your receiver will possess an audio output and signalsfrom this must be fed into the PC soundcard, generallyvia the line-in socket, though the microphone input isanother possibility—which one you use depends on thesignal level output by the receiver itself. Check whichworks best for your set-up.

Connecting It All TogetherOnce your antenna is safely mounted where you intendto deploy it, use good quality screened co-axialdownlead (RG 59 is ideal) to connect it with your

Fig.6 - The soundcard recording control with theline-in input muted (left) and active (right).

Fig.5 - How the units connect

Fig.7 - Wxsat’s recording-level test screen

receiver. The receiver in turn must be connected toyour PC soundcard by a length of screened audio lead.Suitable short audio leads with a jack-plug at each endare readily available at rallies and computer fairs.

Preparing the PC Soundcard for RecordingFirst-timers frequently stumble at the very firsthurdle. Although everything appears to have been setup correctly, no audio signal is heard from the PCspeakers and frustratingly, no images are produced.This outcome is almost always the result of anincorrectly configured soundcard.

By default, computer sound-cards are configured forplaying back audio through the PC speakers. Often thecard’s recording mode will not have been initialised. Dothis by means of the program Sndvol32.exe which youcan find in your Windows folder. If in doubt as to whereto find this, click on your PC’s START button andselect ‘Find’. Type sndvol32 in the 'Named:' field, 'Lookin:' should be drive C:, then click the 'Find Now' button.When the filename Sndvol32.exe appears in thebrowser window, double-click on it to open it. Sndvol32first appears with the words 'Volume Control' in itstitle-bar. Click the <Options> menu, followed by<Properties> to reveal the properties screen. Click the



Fig.8 - NOAA 17 - 11:12 UT on July 13, 2003

Fig.9 - NOAA 17 - 11:12 UT on July 13, 2003

This is the same image as shown above. This time the inputlevel was far too high and clipping has occurred, destroyingtonal detail in the clouds, and washing out the IR frame.

satellite signal, something which may be accomplishedby adjusting the Sndvol32.exe volume slider position,or by placing a variable potentiometer (volume control)into the audio line feeding the soundcard. Themaximum amplitudes in the signal from the receivermust not be allowed to break either grey line.

Figure 7 shows part of a NOAA wave sample with apeak-to-peak value of ± 0.7, which is more thanadequate for the production of good images. Figure 8was produced from the same NOAA-17 signal as thetest screen above. Notice how the image displays a fullrange of tones from brightest white through full blackand that both land and cloud show excellent tonalvariation. Figure 9 shows the same transmission, butmost of the cloud structure in the visible (left) framehas been lost because of ‘clipping’. For the same reason,the infrared image (right) is almost devoid of detail.Although this is an extreme example, for the purposeof illustration, readers do frequently submit imageswith very washed-out clouds like those in figure 9.

The main reason for this would seem to be an attemptto enhance land detail, which is often quite dark, byincreasing the input level. This practice is notrecommended, as it destroys most the cloud detail.Instead, the balance between light and dark tonesshould be adjusted after the image has been created,using the ‘gamma’ feature of an image editing programsuch as Paintshop Pro.

The Usefulness of WAV FilesTo actually record a WAV file while a screen imagebuilds up you must select the appropriate decodingmode. Click the <Parameters> menu to reveal thedefault list of decoding options. Start by selecting thetopmost item from the list; this will produce twinNOAA images as shown in figure 8, with a visible lightimage on the left and an infrared image to its right.Other options on the list permit decoding of individualvisible or infrared images, as well as allowing the userto combine them as a false-colour image.

To initiate a recording session, select<Start at Subcarrier> from the <Recording> menu, followedby <Save Bitmap and Wave> to prime Wxsat to record aWAV file as it displays the image each time it detectsa NOAA weather satellite APT signal.

The great advantage of saving WAV files as well as theactual bitmap images is that the WAV file may berepeatedly decoded through Wxsat (or another programsuch as Satsignal or Wxtoimg). This allows parametersto be adjusted until you judge that the final image isthe best that can be achieved.

Help With WxsatThe Wxsat program comes complete with an extensive‘Help’ file extending to more than 350 kb in size. Thiscontains dozens of pages of information, many withsupporting diagrams and screenshots to help you

customise the program to meet your requirements.Wxsat may be downloaded from the HF-FAX siteat:

http://www.hffax.de/WX_Satellite/WXSat/inst259e.exe

The splendid NOAA-17 APT image on the facingpage was acquired by Hartmut Schaksmeier ofRemscheid, Germany at 11:33 UT on November 13,2003 and depicts the first serious winter storm to hitGreat Britain. Northern Ireland and western Scotlandfaced the brunt of its power with winds of 120 kph.The image was processed with the software packageWXtoImg, which will be described in a future issue.

Fig 10 - A WXSAT help screen dealing with histograms

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