GPS World, October 2009

News developments GPS Galileo GLONASS Compass SYSTEM THE Glitches and Vulnerabilities A range of unrelated events in September show that GPS the worlds preeminent GNSS remains a work in progress T he fi rst in a series of deviations from normal GPS signal broadcasts during September was noted by researches at the University of New Brunswick among others around the globe who found that normal signals from the L1 and L2 transmitters on the GPS satellite PRN01 SVN49 were unavailable for more than two hours on the morning of September 4 The satellite did not transmit useful signals on L1 and L2 from about 12 00 to 14 11 UTC as reported by International GNSS Service stations in Europe The L5 test signal continued to be tracked by some receivers but not others One possible explanation for the inability to track PRN01 is that the satellite rejected an upload and automatically went into non standard mode resulting in GPS receivers being unable to track the L1 and L2 signals In other words the L1 L2 transmitters were still on but transmitting a non standard signal It is not known for sure what actually happened with the satellite but perhaps it is related to the ongoing issues with the signal refl ections on the satellite and that the GPS Wing was conducting further tests said Richard Langley GPS Worlds Innovation editor and professor at the University of New Brunswick Luckily the problem was short lived As to why some receivers continued to track the L5 signal but others did not Langley speculates that some receivers may need to acquire and track the L1 signal before they can track the L5 test signal Å FIGURE 1 Created from nearly 200 Envisat scenes this Arctic mosaic reveals that the most direct route of the Northwest Passage the orange line across northern Canada is fully navigable The blue line traces the Northeast Passage along the Siberian coast which is only partially obstructed by ice see story page 16 Envisat advanced synthetic aperture radar mosaic produced by the Danish National Space Center HDOP Warning On September 10 the U S Coast Guard Navigation Center USCG NavCen issued a high dilution of precision DOP warning for certain locations in the U S Asia and Oceania reporting that GPS users might experience a temporary degradation in GPS reception in parts of the southwest and central United States from 13 02 UTC to 13 23 UTC on September 11 The warning is based on a best four satellite scenario what the DOPs would be if we only used the best four satellites the combination providing the lowest DOP value of all the satellites in view at a particular location said Langley However most civil receivers these days track eight or 10 or all satellites in view I contacted the Coast Guard about this and they did another analysis and confi rmed DOP spikes for all inview users too Prompted by that I did my own analyses and found that with PRN31 out of action for the delta V and PRN01 not yet declared healthy only fi ve satellites above 5 degrees elevation GPS World October 2009 www gpsworld com 14