Southwest Airlines at IAD

[rbuxton]

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.

 

[RFI-EMI-GUY]

Your SDR receiver is off frequency.

Sent from my SM-T350 using Tapatalk

 

[rbuxton]

Your SDR receiver is off frequency.

Sent from my SM-T350 using Tapatalk

How do you determine that it is?

 

[IAmSixNine]

Try using your SDR on a good strong known frequency. Then see if its on or off using that.

 

[rbuxton]

Try using your SDR on a good strong known frequency. Then see if its on or off using that.

Local NWS is 162.55. SDR is right on target.

 

[RFI-EMI-GUY]

If you want precision use a transmitter that is synchronized to GPS. an analog simulcast 800 MHz voice channel is good. 900 MHz paging channels will work. Network HDTV channels have a pilot carrier tone (look that up). Your SDR software probably has an offset setting to center the channel. However cheap SDR receivers will wander around as they heat up.

 

[RFI-EMI-GUY]

Local NWS is 162.55. SDR is right on target.

The error you are showing in the OP is 5 KHz that is over 10 ppm which is a lot. Could the transmitter be that far off? Possible, but the subscribers would have a tough time of it trying to connect. Unless the shop programmed everything off channel 5 KHz.

If in your test, NWS 162.55 is measured a bit off target, at say ((162.55/460.7625) * 5000) = 1726 Hz (162.551726), that could mean your SDR is off. Make it dead on at an 800 MHz frequency and check again.

Do you have a signal generator that is calibrated? Make a test of that carrier.

The only reason I am stressing this is I see a lot of posts from first time SDR radio owners reporting very odd channel reception reports in UHF and 800 MHz.

 

[rbuxton]

If you want precision use a transmitter that is synchronized to GPS. an analog simulcast 800 MHz voice channel is good. 900 MHz paging channels will work. Network HDTV channels have a pilot carrier tone (look that up). Your SDR software probably has an offset setting to center the channel. However cheap SDR receivers will wander around as they heat up.

Well. My SDR seems to me to be right on the mark. Checked HD TV pilot freqs.





So,,, is 460.76875 a legit freq?

 

[RFI-EMI-GUY]

Does your SDS100 receive better and with less errors on the odd channel?

I would check and double check, with other equipment and then as a courtesy, call the licensee (licensee contact info on FCC site). It is entirely possible that a typographical error made its way to the radio shop. I have seen FCC licenses with gross errors.

Whether the radio models will resolve a 5 KHz channel center is a question, but I would expect they all do these days.

.76875 is a legit channel, it was .7675 your OP was concerning.

This from the FCC I/B pool:

460.7625 ......do 30, 61, 62, 69
460.76875 ......do 33, 61, 62
460.775 ......do 61, 62

 

[rbuxton]

Does your SDS100 receive better and with less errors on the odd channel?

I would check and double check, with other equipment and then as a courtesy, call the licensee (licensee contact info on FCC site). It is entirely possible that a typographical error made its way to the radio shop. I have seen FCC licenses with gross errors.

Whether the radio models will resolve a 5 KHz channel center is a question, but I would expect they all do these days.

.76875 is a legit channel, it was .7675 your OP was concerning.

This from the FCC I/B pool:

460.7625 ......do 30, 61, 62, 69
460.76875 ......do 33, 61, 62
460.775 ......do 61, 62

Explain "it was .7675 your OP was concerning."

 

[RFI-EMI-GUY]

Explain "it was .7675 your OP was concerning."

Quoted from your Original Post:

"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."

But then you also say:

"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."

So maybe there is no receiver problem? The station is actually on 460.76875?

 

[rbuxton]

Quoted from your Original Post:

"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."

But then you also say:

"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."

So maybe there is no receiver problem? The station is actually on 460.76875?

Yes. I ran a Excel sheet starting at 460.600 then add .0625 to each freq to come with my 6.25khz separated freqs. This list does not contain .7675. The list does contain .76875.



The freqs are so close that the SDS100 and the SDR pick up decoded voice on both freqs. When I zoom in tight on the SDR I believe that 460.76875 is the correct freq. So when you advise me that .7675 is not a allocated freq but .76875 is (at least that's what I think you're saying) then I am of the belief that .76875 is the freq actually in use.



Weird that NO ONE is licensed for it.



Also, is there a list of narrowband freqs on the FCC website? I could find it.

 

[RFI-EMI-GUY]

Yes. I ran a Excel sheet starting at 460.600 then add .0625 to each freq to come with my 6.25khz separated freqs. This list does not contain .7675. The list does contain .76875.



The freqs are so close that the SDS100 and the SDR pick up decoded voice on both freqs. When I zoom in tight on the SDR I believe that 460.76875 is the correct freq. So when you advise me that .7675 is not a allocated freq but .76875 is (at least that's what I think you're saying) then I am of the belief that .76875 is the freq actually in use.



Weird that NO ONE is licensed for it.



Also, is there a list of narrowband freqs on the FCC website? I could find it.

The .00625 channel centers are normally only used for NXDM FDMA licensees. Are you sure it is DMR you are scanning?

Otherwise, it could again be a typo that was propagated into the system programming.

The allocation tables in the FCC CFR47 Part 90 include all the narrowband freqs.

 

[rbuxton]

The .00625 channel centers are normally only used for NXDM FDMA licensees. Are you sure it is DMR you are scanning?

Otherwise, it could again be a typo that was propagated into the system programming.

The allocation tables in the FCC CFR47 Part 90 include all the narrowband freqs.

Yes, I am sure it is DMR as evidenced by the display on the SDS100 and DSD+.


So 6.25khz channel center spacing in the 460 Mhz part of the band is not applicable to every frequency? How would that work in a practical sense? A NXDM FDMA user would be assigned a freq of, say, 460.76875, and a non-NXDN FDMA user would be assigned a freq of, say, 460.7500 (consistent with the 25khz spacing of freqs.)???

 

[rbuxton]

The allocation tables in the FCC CFR47 Part 90 include all the narrowband freqs.

Thanks. I have a better understanding of the usage of the freq in question after reading the CFR. At this point I am confident that Southwest Airlines is using 460.76875 at Dulles Airport.



47 CFR 90.35 indicates limitations for 460.76875 that it is 6khz band width (DMR is) and used by an airline company (Southwest) at an airport (Dulles.)


Next, I'll call the administrator of the license to see if I can figure out why they're using 460.76875. There should be evidence of a license I would think. Right?

 

[rbuxton]

And on more thing. What is "......do" under Class Of Station in 47 CFR 90.35???

 

[RFI-EMI-GUY]

Yes, I am sure it is DMR as evidenced by the display on the SDS100 and DSD+.


So 6.25khz channel center spacing in the 460 Mhz part of the band is not applicable to every frequency? How would that work in a practical sense? A NXDM FDMA user would be assigned a freq of, say, 460.76875, and a non-NXDN FDMA user would be assigned a freq of, say, 460.7500 (consistent with the 25khz spacing of freqs.)???

Channels on 25 and 12.5 KHz centers like 460.000, .0125, 0250, .0375 etc are narrowband 12.5 KHz channels. These channels used by technologies such as analog and DMR.

Channels on centers like 460.00625, 460.01875, 460.03125, 460.04375 are generally FDMA channels using NXDN.

You could put NXDN on a 25 or 12.5 KHz channel center but it would waste the remaining bandwidth.

 

[RFI-EMI-GUY]

And on more thing. What is "......do" under Class Of Station in 47 CFR 90.35???

"ditto" as in same as above.

 

[RFI-EMI-GUY]

Thanks. I have a better understanding of the usage of the freq in question after reading the CFR. At this point I am confident that Southwest Airlines is using 460.76875 at Dulles Airport.



47 CFR 90.35 indicates limitations for 460.76875 that it is 6khz band width (DMR is) and used by an airline company (Southwest) at an airport (Dulles.)


Next, I'll call the administrator of the license to see if I can figure out why they're using 460.76875. There should be evidence of a license I would think. Right?

No, NXDN is 6.25 KHz bandwidth. It is FDMA technology and uses physically discrete 6.25 bandwidth. DMR is not 6.25 KHz.

Here is the confusion:

DMR is "6.25e" or 6.25 KHz Equivalent bandwidth in that it physically occupies 12.5 KHz bandwidth but because it provides two TDMA slots it is equivalent to 6.25 KHz FDMA.

https://en.wikipedia.org/wiki/Digital_mobile_radio#DMR_Tier_I

If Southwest is using a 6.25 KHz bandwidth channel for DMR, someone made a mistake.

Do you have the callsign?

 

[rbuxton]

No, NXDN is 6.25 KHz bandwidth. It is FDMA technology and uses physically discrete 6.25 bandwidth. DMR is not 6.25 KHz.

Here is the confusion:

DMR is "6.25e" or 6.25 KHz Equivalent bandwidth in that it physically occupies 12.5 KHz bandwidth but because it provides two TDMA slots it is equivalent to 6.25 KHz FDMA.

https://en.wikipedia.org/wiki/Digital_mobile_radio#DMR_Tier_I

If Southwest is using a 6.25 KHz bandwidth channel for DMR, someone made a mistake.

Do you have the callsign?

Interesting. Not sure what to do now to figure this out. On both the SDS100 and DSD+ I get DMR info on 460.76875 and 460.7675. And my reasonable deduction that 460.7675 is not a allocated freq and 460.76875 IS causes me to believe the freq is 460.76875.



Now, what mode? You say a DMR freq cannot be used on that freq. How a mistake like can be made is beyond me.



I do not have a callsign for 460.76875. I cannot find where this freq is licensed by anyone, anywhere.