Space & DAB: spring-clean cases

Wednesday 22nd February 2012
Space debris & DAB noise. Courtesy:wigpen.blogspot.com &http://www.tbi.univie.ac.at/~stefanie/research.html

As Europe and the US start to take some action to clean up the low earth orbit (LEO) of man-made hardware junk, in the UK digital radio broadcasting desperately needs to clean up to its performance. Australia suggeststhat software may be the answer.

Last year's European Space Surveillance conference in Madrid seems to be producing some action over the pressing issue of the risk to earth from the amount of man-made jettisoned and orbiting rocketry and satellite components threatening to interfere with working satellites and the International Space Station.

NASA monitors at least 16,000 of these objects if larger than 10cm in diameter. If operational spacecraft or satellite collides with one of these, serious, costly damage results. The satellite is often destroyed. But worse the collision itself generates thousands more junk fragments.

That epitomises the now retired NASA scientist (left) Donald J. Kessler's syndrome of 1978 scenario, where thedensity of  objects in low Earth orbit is high enough that collisions between objects cause a cascade – each collision generating debris, increasing the likelihood of further collisions, so debris distribution in orbit renders both satellite use and space exploration unfeasible for many generations.


“It has become essential to be aware of the existence of this debris and the risks that are run by itsproliferation,” says Claude Nicollier, (right) astronaut and EPFL professor. The Swiss Space Center at EPFL is launching CleanSpace One, a project to build the first prototype in a family of “de-orbiting” satellites that will clean up space debris.

The target for the initial CleanSpace One launch will be either Switzerland’s first orbiting object, the Swisscube picosatellite, put in orbit in 2009 or TIsat, launched in July 2010. The cleanup satellite has three major challenges to overcome, each needs development of new technology that in turn, could be used down the road in other applications.

After its launch, the cleanup satellite will have to adjust its trajectory in order to match its target’s orbital plane. To do this, it could use a new kind of ultra-compact motor designed for space applications that is being developed in EPFL laboratories.

In range of its target, travelling at 28,000 km/h at an altitude of 630-750 km, CleanSpace One (left)  will grab and stabilise it – a tricky mission at these speeds, particularly if the satellite is rotating. Scientists plan to develop a gripping mechanism inspired by plant or animal example. Coupled, CleanSpace One will “de-orbit” the unwanted satellite, heading back to  Earth’s atmosphere, where the two satellites will burn up on re-entry.

"We want to offer and sell a whole family of ready-made systems, designed as sustainably as possible, that are able to de-orbit several different kinds of satellites,” explains  (right) Swiss Space Centre director Volker Gass. “Space agencies are increasingly finding it necessary to take into consideration and prepare for the elimination of the stuff they’re sending into space. We want to be the pioneers in this area.”

Design, construction and maiden space voyage CleanSpace One, will cost about SFr10m. Depending on the funding and industrial partners, this first orbital rendez-vous could take place in 3-5 years.

Satellite operators can avoid big debris and armour satellites to withstand smaller piece impacts. But if not cleaned up, low-Earth orbit will become too perilous for people and satellites reports the New York Times. John L. Junkins (left) a professor of aerospace engineering at Texas A&M University notes “No one will insure a space launch.” The United States has about 500 pieces of large space junk, Russia about twice the number. “Things the size of a Greyhound bus,” he said.

Taking down five or six of the large intact objects each year would be enough to halt the cascade effect, he said. Eliminating 10 a year would quickly reverse the trend.

NASA is financing research to come up with some solutions. Raytheon is studying if a high-altitude balloonmight carry a machine that would shoot puffs of air into the path of orbiting debris. That slight increase in atmospheric drag could force junk to fall back to Earth.  

Star Technology and Research, with $1.9m NASA funding is to develop and test technologies for ElectroDynamic Debris Eliminator spacecraft — (right) Edde,  With by a 6-mile-long wire or tether, that generates energy as it is pulled through the Earth’s magnetic field, Edde will approach junk, catch it in a disposable net and move it to a lower orbit, where air friction would coax the item to re-enter the atmosphere. Edde, in orbit, would then move on to its next target.

Technology however is not the only problem.  Space junk still belongs to the country or origin. US trying to clean up a Russian rocket, would have Russian protests. A ground-based laser capable of pushing satellites around in space, would have strong weapon connotations.

All new US satellites are now accompanied with plans to bring them safely out of orbit.  A code of conduct for nations has been suggested. But as Europe has been putting together a set of ground rules that the US calls them too restrictive, such a code may be a long time in appearing or being given global assent.

It is however probably as important to earth's survival as the current concern over global warming.


UK  DAB NEEDS A SOFT CLEAN-UP?

In the UK the spring clean that is being  urged is a cleaner digital radio before the threatened analogue switch off.  It is suggested the UK take a leaf out of the Australian digital radio experience book, where software has held the key to improving noise clean up.

Authors Richard Morris (left) from Signal Broadcast, Wayne Dickson from  BTC Pty Ltd (below) , and Paul Harfoot from Rohde & Schwarz note it is "now over 15 years since DAB digital radio switched on in the UK. Cover and listenership has increased steadily since and industry is starting to plan for the DAB Switchover.

"When this happens, major stations will switch off FM services and rely solely on their DAB transmitters.

The UK government has stated it will announce the date switchover will occur when certain conditions have been met: that is when 50% of listening is to digital services, and when DAB coverage matches current FM coverage.



Much work is being planned to improve coverage. Industry is planning to increase the power of many existing transmitter sites and to build hundreds of new ones. But this work is expensive. It may also be possible to increase the effective coverage of existing transmitters without altering the radiated power, suggest the authors.

Their study shows the important of MER performance of digital radio transmitters, as a system with a poor MER figure will have an impact on the reception area of radio signals. With modern transmitter designs MER figures greater than 33dB can now be achieved, generating a more robust signal and providing better in building coverage

.

Digital radio coverage is limited by noise. It is the carrier to noise ratio at the receiver which determines if the audio content can be decoded. If the received signal is above a certain threshold the receiver can perfectly recreate the transmitted signal. If the signal is below that threshold it is rapidly degraded.  It is desirable to reduce the amount of noise in the system.

The authors record that in 2009 Commercial Radio Australia commissioned high power digital radio transmitters in five major cities. Using the conventional “shoulder measurement” the on-air transmitter performance looked satisfactory with a shoulder height exceeding 37dB. However, when the output was monitored a different picture emerged.

The results obtained using the Rohde Schwarz (right)  an ETL  script shows that the transmitted signal contains a significant amount of phase noise, even though the shoulder height measurement would not suggest any problem. The MER varies across the bandwidth of the signal between 23.5dB and 17.5dB with an RMS figure of 19.8dB.

Any phase noise present in the transmitted signal will have an effect on receiver performance – and this can be shown graphically in the loss of margin available to cope with incoming interference when phase noise is present in the transmitted signal.

The effect of MER on reception varies with the carrier to noise requirements of the receiver which in turn depends on the quality of the transmission path.  In a Gaussian channel, the DAB receiver only requires a signal to noise ratio of 8dB.  In a complex reception environment with significant multipath effects and contributions from many DAB transmitters the carrier to noise requirement increases significantly – with a figure of 15dB required in some cases.

In the Australian case the poor MER figure resulted in signals  less resistant to manmade noise such as generated by electronic items in a home or office environment. In other countries with denser digital radio networks,  as the UK, the poor MER figure would result in poorer performance in areas of strong co-channel interference.

With an MER with an RMS value of 19.8dB and with MER figures reducing to 18dB on some carriers, the effect is to erode the margin available to resist co-channel interference by up to 1.7dB overall, and by over 2dB on some carriers.  This is enough to have a significant effect on the reception of signals near the edge of coverage.

Fortunately, the issue discovered in Australia was relatively easy to resolve. A software update was developed which considerably improved the performance of the transmitters in service, and the MER was improved to a more acceptable figure of 25.8dB. This result also improves considerably the ability of systems to survive co-channel interference. Perhaps the UK should look to software solutions too.

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