GPS World, February 2017
AVIATION directionality In this way the aircraft can be maneuvered to point the antenna either towards or away from the RFI source Normally there is very little directivity making this a challenging search A direction finder is a significant improvement FIGURE 8 shows the L band antenna array used by a DF 4400 as installed on the bottom of the aircraft Newer generation spectrum analyzers with a good GNSS specific pre amplifier using digital sampling with a fast A D converter could provide useful capability However the subject is beyond the scope of this discussion and we focus here on comparing the CRPA approach with a standard direction finder The FAA Flight Inspection service conducted complementary flights during the Rockwell Collins live sky van testing The flights included orbits and a direct overflight of the RFI source This was complemented by additional laboratory calibration to ensure that results could be compared The sensitivity results of the CRPA approach are more meaningful in comparison with a generic directionfinder capability Since test data is only available for a top mounted CRPA the comparisons here are made for the preferred bottom mounted CRPA using engineering estimation The key finding was that while direction finding capability was quite comparable between the CRPA system and the DF 4400 for CW the CRPAsystem outperforms the DF 4400 by a significant margin when encountering broadband signals This is considered to be a significant improvement given the expected nature of RFI sources During the FAA overflight the aircraft did not manage to detect the broadband signal Consequently the values given here are reconstructed from laboratory analysis TABLE 1 compares the estimated achievable sensitivities In view of the limitations of the data analysis performed these values must be interpreted with caution In general we can conclude that the direction finding sensitivity of the CRPA system is relatively insensitive to the encountered modulation of the RFI FEBRUARY 2017 WWW GPSWORLD COM GPS WORLD 53 FIGURE 8 Cubic antenna array
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