W4UVV
Member
I've been sleuthing the FCC licenses for Southwest Airlines at Dulles Airport and have come up with that not a one of the licensed freqs are used.
However, closely watching the waterfall near 460.7625 (a freq licensed to Southwest) I found 460.7675 (a freq NOT licensed to Southwest, nor is it licensed to anyone anywhere) with activity and concluded that this was a DMR single freq system used by Southwest Airlines. But then I wondered about the narrowbanding in the UHF band which leads me to this question.
Is 460.7675 a correct frequency? My understanding is that there is 6.25 khz between freqs so 460.7675 would not be an allocated freq.
If I run out a list of frequencies starting with 460.000 at 6.25 khz intervals there is no 460.7675 in that list. The closest freq is 460.76875. Then I zoom in tight on the SDR waterfall and lo and behold it looks like the center freq is 460.76875.
What fooled me initially was that I was receiving voice and data (Cap+, Color Code 12) on the SDR (DSD+) and the SDS100 on freq 460.7675. And I receive the same on 460.76875, which I am lead to believe is the correct frequency.
The odd thing of all of this is the Southwest Airlines is not licensed for 460.7675 or 460.76875 at Dulles Airport. Nonetheless, Southwest Airlines is using 460.76875. How is it not shown in the FCC licenses? Beats me.
If someone else can chime in on this subject I'm all ears.
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FYI 460.76875 mhz. is a legitimate assignable frequency in compliance with the FCC 450-470 mhz. frequency band plan i.e., 12.5 khz. spacing and 6.25 khz spacing. Your comments regarding Southwest Airlines frequency license are correct.
If Southwest Airlines does not have a license for operating on this frequency, it is in violation of FCC regulations. It would be a simple matter for Southwest Airlines to submit a license modification application to change operatation on 460.76875 mhz. My frequency database shows no licensee in VA currently assigned to this frequency.
The radio installer/maintainer also has some accountability. But as long as Southwest Airline comms work ok it's happy.
As you might suspect exceptions may occur and have occurred and may and do result in certain FCC licensed frequencies not returned doing a FCC General Menu search. Here are two examples: In early 2000 when installation of STARS radio system began VITA and the state paid contractor consultant then after the fact both suddenly realized that the currently licensed vhf frequencies available were not enough to support a statewide radio system...another PPPPPPP situation. That resulted in an emergency application to the FCC from VITA for additional vhf frequencies for STARS installations. Fortunately for VA, at that time there were a few vhf frequencies available in the 151 mhz. and 152 mhz. range which quickly were licensed to VA. VITA received a FCC letter authorization for immediate use but subsequently these licensed frequencies did not appear in a FCC General Menu search. The second example is about 15+ years ago FCC selected frequencies in the 450-470 mhz. range, of which certain khz. spectrum ranges were auctioned to the general public for operation in VA. These frequencies are available in the ULS system but not easy to find and again not returned doing a General Menu search.
Good job on your analysis!
John
W4UVV