Actually the GPS signal can be manipulated, who would have guessed otherwise? The system being a American Military Defense innovation initially, this is one of the strategies implemented to prevent enemies using the system against US targets. The other system implemented was the selective availability (SA) restriction imposed on the signal for civil, e.g. non military, use. Today a large variety of digital gadgets are equipped with a GPS receiver, ranging from in car navigation systems to mobile phones and cameras. This was kicked of by the former president Bill Clinton’s decision to lift the imposed selective availability (SA) restriction in 2000 (Prasad 2005, p.7). Following the SA removal, civil and commercial GPS accuracy increased from around 100m to somewhere between 3m and 15m (Pendleton 2002 as cited in Spencer Spencer 2003, p.56).
However to come back to the temporal and local jamming of the GPS signal holds still a very important status in the strategy of US military action. This is that the European system Galileo is still under construction and its partial launch will not be until 1012 or beyond. The other functioning system is the Russian Glonas. However this is not covering the entire planet with constant signal as it only operates from 18 satellites (2008) covering Russia. In this sense the US holds a monopoly on this location based information system.
Image taken from Wikipedia
The jamming of the signal is normally not know to the public and only speculated over. However it is very likely that it is used in current war zones, like Iraq and Afghanistan. There are reports over this jamming to be found on the internet.
Computerworld has an article on the subject quoting some GPS experts on the matter. “Sam Wormley, a researcher at Iowa State University in Ames and manager of an authoritative GPS resources and accuracy Web site, said that the Pentagon "definitely" has the capability to jam civilian GPS signals in a given area without interfering with more precise military signals. Wormley said that's because the military signals occupy a different and smaller slice of the GPS frequency band than that used by the civilian signals.” The jamming most likely is achieved through a slight desincronisation of the clocks. For military purpose this can easily be decoded.
There are very funny discussions going on out there on the web around the possibility of jamming satellite signal. A good one is on yahoo.answers.com, where some guy accuses his neighbor ‘Joe’ to jam his satellite dish, because when ever Joe is home the guy thinks his TV signal is disrupted.
Thinking this further, how do we know that the actual position is correct? As we have seen in the introduction of this post, as well as in last weeks new Argos catalogue, consumer GPS products have become immensely popular and everyone needs to know where they are. Whether this is true or not in this case is probably not that important. So to say, we don’t know if the represented location on Google Earth is actually the true position as in lat long, yes we can see that this image shows the street we’re in, but the structural framework of the Lat Long coordinate is not necessarily the ‘right’ one. But I guess this is the question of the artificially impose grid that we can only virtually refer to and belief in as a convention.
So next time you end up in New York, rather than the planned Newark because of a spelling mistake while typing it into the gadget, you can blame the US for temporarily jamming your specific satellite. But if you are after your neighbor here are some web stores where you can purchase your own satellite jammer to annoy your ‘Joe’.
However I wanted to link a creepy James Bond extract, where the space craft swallows the satellite, but you guess it is not out there yet. So if anyone has this sequence laying around please upload and link it here.
However I therefore link to a very boring but scientific clip that actually visualises the GPS signal availability in Kabul during the course of one day. The scientist, Richard Langley, a professor of geodesy and precision navigation at the University of New Brunswick has observed the predicted position of the satellite versus the actual signal strength in the are and there seems to be clearly a jam. However, that was recorded back in 2001, but most certainly this has taken place before and after, as well as in other places than Kabul too.
Clip by Richard Langley - Kabul.GPS.Visibility.mov