On my return trip through Chicago, I had more time to break out multiple scanners and try to figure out some of what I heard on my last trip.
First off, the 163.4750 MHz frequency does NOT appear to be the TSA on further listening. It is, in fact, the Immigrations and Customs Enforcement operations at the International Terminal at O'Hare Airport. However, it gets strange in that all the traffic on this repeater is also heard on another repeater on 163.7500 MHz, located somewhere nearby. These frequencies are quite busy with announcements as to incoming flights and baggage movement (thus my thinking it was TSA at first). I also heard one unit calling CHARLIE 100 about taking a subject under arrest to a holding cell. CHARLIE 100 is the Customs & Border Patrol National Law Enforcement Communications Center in the Orlando, FL area.
Next, I discovered the 165.8500 MHz frequency is actually the input to the ICE/CBP repeater on 163.6250 MHz, which does appear to be at ORD. I heard more than a few units switch between the 163.6250 and 163.4750 frequencies.
The 169.1625 MHz repeater does appear to be TSA, but have not confirmed the input. 172.1500 MHz is definitely in use by the TSA at ORD.
And finally, while searching, I got a couple of brief hits on 167.0375 MHz. I don't know who that might have been. All the frequencies listed in this blog entry were using APCO P-25 digital mode and un-encrypted.
