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Global Positioning update

Thu, 08/31/2000 - 8:00pm
Jeffrey Krauss, Precision Locator and President of Telecommunications and Technology Policy
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By Jeffrey Krauss, Precision Locator and President of Telecommunications and Technology Policy

Some recent events will affect the Global Positioning Satellite System (GPS) and the frequencies it uses. First, the government turned off a signal degradation that caused intentional errors. Then the World Radiocommunication Conference (WRC) allocated additional spectrum for GPS and for the European system Galileo. And now, the focus has shifted to interference control, and that affects new technology that could be used for video and broadband distribution within the home.

The GPS system was designed and built for the military, but civilian uses have begun to dominate. In the past, the military had a concern that, in an armed conflict, not only allies but also the enemy could use the GPS satellite signals to determine positions. So the system was designed with two kinds of radio signals. One kind, intended for use by the U.S. military, is encrypted. The other kind, not encrypted, has been transmitted with intentional errors.

Two kinds of errors were introduced. The satellite clock frequencies were made to fluctuate, and the transmitted satellite locations, or ephemeris parameters, were slightly wrong. This was called selective availability (SA). The result was that civilian GPS receivers could be expected to have horizontal location errors of up to 100 meters.

On May 2, 2000, the government turned off SA. This was largely a recognition of the widespread deployment of terrestrial "augmentation" systems. These terrestrial transmitters send out calibration signals that allow the more sophisticated GPS receivers to correct the SA errors.

Additional spectrum

The WRC conference, a forum sponsored by the International Telecommunications Union (ITU) every few years, is where governments get together and agree on the uses of the radio spectrum. The latest WRC gave early Christmas presents both to GPS and to a European competitor, called Galileo, that is still on the drawing boards. GPS satellites transmit down in the 1559–1610 MHz band, located in the radio spectrum about halfway between the 800 MHz cellular frequencies and the 1800 MHz PCS frequencies. But there isn't enough room there to accommodate Galileo as well. And in addition, GPS supporters wanted to add a frequency for improved performance and reliability. The result was an agreement to allocate 24 MHz of spectrum centered at 1202 MHz for Galileo, and another 24 MHz centered at 1176 MHz for GPS. These frequencies are now used by narrowband aeronautical radionavigation signals called Distance Measuring Equipment (DME); the spread spectrum GPS and Galileo signals will be able to coexist with some DME signals, but others will have to be relocated.

With this new spectrum allocation, Galileo can proceed with its plans to deploy a satellite system that achieves location accuracy of one meter.

Interference

The FCC has been considering the potential for interference to GPS receivers from two different kinds of transmitters. All satellite receivers are vulnerable to interference from terrestrial sources, because the satellite signals are very weak. Earth stations at cable headends and even DBS receivers use parabolic dish antennas to strengthen the received signals and reject interfering signals coming from other directions. But handheld GPS receivers with tiny antennas don't have this capability. So eliminating the sources of interference becomes very important.

First, the FCC has adopted strict out-of-band emission limits for radios to be used for wireless Internet access in the new 700 MHz allocation. Thirty megahertz of spectrum in the bands now occupied by TV channels 60 to 69 will be auctioned later this year, and the second harmonics of TV channels 65, 66 and 67 fall into the band used by GPS. So the FCC limited the emissions from these radios into the 1559–1610 MHz band to be far lower than for other bands.

Second, the FCC has a proceeding underway on UltraWideband (UWB) devices. UWB transmitters use pulsed technology that occupies extremely wide bandwidths, but at very low power levels—so low that any interference is unlikely. A typical UWB signal might, for example, occupy the 1500 MHz of bandwidth from 500 MHz to 2000 MHz. This covers TV broadcast frequencies, cellular and PCS phones, radars and other communications services. But the UWB signals are very, very weak. Typical uses might include short distance, high data rate wireless networks within a building, and ultraprecise radars to locate victims in collapsed buildings.

While FCC approval of UWB devices above 2 GHz seems certain to occur, GPS users have mounted a major lobbying effort to prohibit them below 2 GHz, in order to protect against interference to GPS receivers. Interference tests are being planned, but this is more likely to play out as a political, rather than technical decision at the FCC. Because UWB technology offers the possibility of broadband wireless in-home networks to distribute video and data, we'll stay tuned on this one.

jkrauss@cpcug.org

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