GPS World, May 2009

SYSTEM DESIGN TEST GPS Modernization FIGURE 1 L5 Demonstration payload mounted on access panel here in a critical yet often overlooked part of any satellite system The Navstar GPS arguably one of the crowning achievements of the space age has spurred a second wave of GNSS such as GLONASS Galileo Compass QZSS and IRNSS These new systems like GPS must operate in a few internationally sanctioned bands of spectrum within what is known as the L band These bands were selected by GPS for their favorable spacebased radionavigation properties Indeed these segments of L band L1 1559 1610 MHz L2 1215 1240 MHz and L5 1164 1215 MHz are considered the beachfront property of radio frequency spectrum Like any buildable lot near the water two truths apply to L band spectrum it is extremely valuable and they aint makin it anymore Thus spectrum allocated to radionavigation satellite services RNSS is subject to regulation to ensure peaceful coexistence of its users The primary regulatory body for RNSS spectrum is the International Telecommunication Union Radiocommunication Sector ITU R based in Geneva Switzerland Several governments including the United States have their own domestic spectrum regulatory agency in addition to complying with ITU R regulations Many other countries adopt ITU R rules for their domestic regulations The GPS L1 and L5 frequencies lie in bands designated for aeronautical radionavigation services ARNS Aviation users depend on these bands for safety oflife SoL services Although ITU R regulations protect all RNSS bands L1 and L5 receive additional protection due to the SoL services they support The Federal Aviation Administration counts on GPS signals in these protected ARNS bands to provide dual frequency ionospheric corrections for future aviation users with significant availability integrity and accuracy advantages Crowded Bands New Rules In response to a signifi cant increase in the number of fi lings in RNSS bands due to new GNSS services and signals the ITU R made a signifi cant regulatory rule change at its 2003 World Radio Conference ITU R Radio Regulation 5328B went into effect on January 1 2005 decreeing that new systems that fi le to operate in L1 L2 or L5 after that date must coordinate with other system providers having active fi lings in these bands Systems with active fi lings or that are already transmitting will have higher status in these bands than systems with new fi lings All systems with active fi lings or operational signals when the new rules went into effect in January 2005 were grandfathered with primary status The basic purpose of the coordination process is for a system provider with lower status to convince those with higher status that it will not cause harmful interference If harmful interference is claimed the newer system may be obliged to alter its signal or to move to another band The new system must complete the coordination process with every provider having higher status in each band This represents a potentially large investment of time and money for a new system particularly if one or more systems with higher priority claim harmful interference In addition to these new rules the ITU R also established regulations to discourage so called paper satellites or filings made with no intention of actually transmitting signals Although a nominal filing fee discourages nuisance applications the primary method of eliminating unused filings is a requirement to transmit signals from the designated orbit within seven years of receipt of the filing by the ITU R This initial transmission serves to bring into use the signals in the filing and secures the band for future use GNSS operators take ITU R rules very seriously A primary mission objective of the Galileo In Orbit Validation Element GIOVE A satellite launched on December 28 2005 was to secure Galileos ITUR filings Similarly China secured multiple L band frequency filings with the launch of its Compass M1 satellite on April 14 2007 GPS is no exception and always has filed for and obtained primary status for its signals The most recent GPS ITU R filing submitted in August 2002 covered modernized GPS signals and included a new signal in the L5 band The seven year BIU deadline for this filing was August 26 2009 As delays to the first launch of an L5 capable satellite put meeting this BIU deadline in jeopardy the GPS Wing took action to maintain the primary status of GPS in the L5 band Risk Management Decision Engineers at the GPS Wing had a short list of options for alternative L5 signal hosts Low Earth orbit LEO and geosynchronous satellites though much more numerous than GPS MEO satellites could not be considered because signals must be transmitted from the orbit listed in the ITU R fi lings in this case a circular orbit inclined at 55 degrees with a 12 hour period Although dediÅ GPS World May 2009 www gpsworld com 28