GPS a rischio per il rumore solare?

Capita spesso che certe notizie di carattere molto tecnico si facciano strada sulle home page dei quotidiani online, magari per un risvolto sensazionalistico un po' forzato. Questo pomeriggio Repubblica.it parlava di uno scienziato italiano o di origine italiana che avrebbe previsto per il 2011 possibili blocchi dei satelliti GPS a causa degli effetti solari. Leggendo la storia si capisce che lo scienziato stesso, Alessandro Cerruti, esorta a prendere provvidementi potenziando i sistemi.
Incuriosito ho fatto delle ricerche e ho trovato la fonte originale, un lancio del New Scientist di due giorni prima. Repubblica.it la riporta in modo sostanzialmente corretto, ma con qualche coloritura di allarmismo che la rivista inglese, con il consueto understatement, omette. Cerruti ha basato le sue valutazioni, annunciati in questi giorni in un convegno sulla radionavigazione, su una accurata analisi dei segnali dai satelliti GPS correlati a quelli dei flussi solari alle frequenze dei "burst" radio associati ai brillamenti, i temibili flares. Ovviamente si sapeva che il tempo geomagnetico, le tempeste nella ionosfera, possono avere un impatto sui satelliti e le loro comunicazioni. Cerruti ha stabilito un collegamento diretto, lineare, col sole, attraverso le particelle emesse durante i brillamenti. Il rumore generato da queste particelle ha un picco proprio tra le frequenze 1,2-1,6 GHz, in cui opera il sistema GPS. In occasione del prossimo massimo solare, intorno al 2012, ci si può aspettare che queste esplosioni interferiscano a lungo con la ricezione e quindi con il calcolo della posizione. Il problema è che per allora è facile che molti aerei facciano affidamento solo sui satelliti di radionavigazione per "fare il punto". New Scientist sottolinea che questi effetti non sono ancora stati registrati perché gli apparati GPS sono diventati un bene di consumo in una fase di minimo solare. Su Repubblica Cerruti stesso avverte che le reti satellitari stanno evolvendo e non è detto che tra qualche anno il sistema funzioni bene anche rapporti di segnale/rumore più efficienti di oggi.

Solar flares will disrupt GPS in 2011
14:29 29 September 2006
NewScientist.com news service
Jeff Hecht

Navigation, power and communications systems that rely on GPS satellite navigation will be disrupted by violent solar activity in 2011, research shows. A study reveals Global Positioning System receivers to be unexpectedly vulnerable to bursts of radio noise produced by solar flares, created by explosions in the Sun's atmosphere.
When solar activity peaks in 2011 and 2012, it could cause widespread disruption to aircraft navigation and emergency location systems that rely heavily on satellite navigation data. Particularly intense solar activity occurs roughly every 11 years due to cyclic changes to the Sun's magnetic field – a peak period known as the solar maximum. Solar flares send charged particles crashing into the outer fringes of the Earth's atmosphere at high velocity, generating auroras and geomagnetic storms.

Radio noise
Charged particles from solar flares also produce intense bursts of radio noise, which peak in the 1.2 and 1.6 gigahertz bands used by GPS. Normally, radio noise in these bands is very low, so receivers can easily pick up weak signals from orbiting satellites. In 2005, however, Cornell University graduate student Alessandro Cerruti discovered a puzzling failure in GPS reception while operating a receiver at Arecibo Observatory in Puerto Rico.
Along with Paul Kintner, from the university's electrical engineering department, Cerruti traced the problem to a radio burst induced by a solar flare. They found that GPS receivers operated by the US Federal Aviation Administration (FAA) and the Brazilian Air Force experienced similar disruption during this burst of solar activity. The researchers say the problem has escaped detection before because GPS systems have spread in popularity during a time of relatively low solar activity.

Drowned out
Discovering the disruption was surprising. "[Other] people will be surprised at the next solar maximum," Kintner says. Both the number and intensity of radio flares will increase and could drown out GPS signals during this period, he says.
This may be a problem for aircraft navigation. The FAA uses GPS receivers for air traffic control, which Kintner says "will certainly fail" during these intense solar flare radio bursts, which could cause signals to drop by up to 90%, for hours at a time. Although planes can fly without GPS, outages force the FAA to increase the distance between aircraft and slow take-offs and landings, delaying flights.
GPS is also used for emergency rescues and also to synchronise power grids and cellphone networks. One solution, says Kintner, would be to increase the strength of GPS signals. But this would mean redesigning GPS satellite hardware and software. Cerruti presented details of the problem at a meeting of the Institute of Navigation on 28 September. Details will be published in a forthcoming issue of the journal Space Weather.

Related Articles
Satellites could navigate by X-ray stars
04 September 2006
Pentagon's satellite-saving plan could backfire
15 August 2006
Code cracked on Galileo satellite system
15 July 2006
Weblinks
Paul Kintner, Cornell University
Institute of Navigation
Space Weather

Questo invece è il riferimento alla presentazione di Cerruti sui risultati delle sue analisi. Se interessa, qui c'è un altro suo articolo sulla ricezione della nuova costellazione Galileo.

Observed GPS and WAAS Signal-to-Noise Degradation Due to Solar Radio Bursts
A. Cerruti, Cornell University

GPS signals, systems, and navigation accuracy are vulnerable to a variety of space weather effects that are caused mostly by the ionosphere. However, the sun, which is sometimes a strong radio source, is the cause of GPS signal interference presented here. The first direct observations of GPS L1 (1.57542 GHz) signal-to-noise ratio degradation on two different models of GPS receivers due to the solar radio burst associated with the 7 September 2005 solar flare are presented.
Signal-to-noise ratio data from three identical, collocated receivers at Arecibo Observatory and also from four identical receivers of a different model located in Brazil, were available at the time of the solar radio burst. These receivers were all in the sun-lit hemisphere and all were affected similarly. The maximum solar radio burst power associated with the 7 September 2005 flare had a peak intensity of about 8,700 solar flux units (1 SFU = 10-22 W/m2-Hz) RHCP at 1,600 MHz, which caused a corresponding decrease in the signal-to-noise ratio of about 2.3 dB across all visible satellites. Only the right-hand, circularly polarized (RHCP) emissions affected the GPS signals.
To confirm the effect, the solar radio burst associated with the 28 October 2003 flare was investigated. Although polarization data were not available for this event, the maximum degradation at GPS L1 was about 3.0 dB, and a degradation of 10 dB was observed on the semi-codeless L2 signal for a solar radio burst of 13,600 SFU.
The event analyzed herein can be used to scale historical solar radio bursts of 80,000 SFU. Decreases of 12 dB (21 dB) in the L1 (L2, semi-codeless) signal-to-noise ratio are implied along with loss of tracking for inadequately designed GPS receivers. Since solar radio bursts affect all satellites in view of a receiver, all receivers in the sun-lit hemisphere, the new Galileo navigation system, and all space-based augmentation systems such as WAAS and EGNOS, they are a potential threat to life-critical systems.

Tags: radioascolto, radiofonia, radio, dxing.