Can we trust our GPS devices?
In recent years, we have become increasingly dependent on applications using the Global Positioning System (GPS), such as railway control, highway traffic management, emergency response or commercial aviation. But in a very short news release, the American Geophysical Union (AGU) warns us that we can't always trust our GPS gadgets because 'electrical activity in the upper atmospheric zone called the ionosphere can tamper with signals from GPS satellites.' However, new research studies are under way and 'may lead to regional predictions of reduced GPS reliability and accuracy.' But read more...
Before going further, you can see above an artist rendition of the next-generation Global Positioning System satellites known as GPS III. (Credit: Lockheed Martin). Here is a link to a larger version of this illustration. And here are some more details from this Lockheed Martin's Global Positioning System page. "On May 15, 2008,the U.S. Air Force Space and Missile Systems Center, Los Angeles Air Force Base, Calif. awarded a team led by Lockheed Martin a $1.46 billion contract to build the next-generation Global Positioning System Space System program, known as GPS III. This program will improve position, navigation, and timing services for the warfighter and civil users worldwide and provide advanced anti-jam capabilities yielding superior system security, accuracy and reliability."
This brings us back to our subject: GPS reliability. This is the focus of several scientific reports included in the latest issue of Space Weather, an AGU publication. Here is a link to a special section about 'Space Weather Effects on GPS.' If you're an AGU member, you'll be able to read 7 scientific reports. Otherwise, you'll only have access to the abstracts.
One of these reports is available for free, "Space Weather and the Global Positioning System" (Anthea Coster and Attila Komjathy, Space Weather, June 6, 2008). The authors give a short description of the other articles featured in the special section mentioned above. "Papers in this section describe the use of GPS as a monitor of space weather events and discuss how GPS is used to observe ionospheric irregularities and total electron content gradients. Other papers address the implications that these space weather features may have on GPS and on Global Navigation Satellite System (GNSS) operations in general. Space weather impacts on GPS include the introduction of range errors and the loss of signal reception, both of which can have severe effects on marine and aviation navigation, surveying, and other critical real-time applications."
The whole article is worth reading, but if you don't have enough time to read it in its entirety, I suggest you focus on the paragraph named "Space-Based Ionospheric Measurements." Here is the beginning. "Ground-based GPS receivers allow for good data coverage over land but not over the oceans. [...] Putting GPS receivers in space is one way of addressing this lack of coverage. The idea of using GPS receivers in space to sense properties of the atmosphere grew out of earlier work in the remote sensing of planetary atmospheres -- in the 1970s, the atmospheres of Mars, Venus, and Jupiter were probed using the technique of radio occultation. Planetary occultation is when a smaller astronomical body passes behind a larger astronomical body, wholly obscuring its view. Similarly, radio occultation can be thought of as when the line of sight to a satellite is obscured by a planet or, in the case of GPS, the Earth."
But how GPS signals can be accurately tracked? "For GPS, a satellite in low-Earth orbit (LEO) tracks the signal from a GPS satellite. The LEO satellite typically orbits between 500 and 800 kilometers above Earth's surface, and GPS satellites are in nearly semisynchronous orbits (they travel around the Earth approximately twice a day) at an altitude of approximately 20,200 kilometers. From the perspective of the LEO satellite, GPS satellites rise and set several times a day. As the occultation occurs, the signal that is measured from the GPS satellite is refracted, or "bent," by differing amounts as it propagates through different layers of the atmosphere. By measuring the amount of refraction as the GPS satellite is rising or setting, scientists are able to reconstruct properties of the different layers of the atmosphere (e.g., relative humidity and temperature profiles in the troposphere, and electron density in the ionosphere)."
A final note: if you own -- or use -- a GPS device, bookmark this excellent article.
Sources: American Geophysical Union news release, June 9, 2008; and various websites
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