An SDR would need to receive the entire 902-928 band all at once. The DTRs and DLRs spend no more than 90ms on any given frequency in the hopset of 50 frequencies. That's about 11 hops/second. The hopset is spread out over the entire 902-928MHz band.
It is theoretically possible to capture a DTR or DLR transmission with a near field receiver but you would still need to decode the VSELP digital. While the DTRs and DLRs are technically not encrypted, they are hard enough to monitor that it's not going to be done with a consumer grade receiver. The only inexpensive and practical method to monitor DTRs and DLRs is to have one yourself, AND it has to be programmed to the same hopset, talkgroup ID, and 4-digit Profile ID Number (PIN) (if using Profile ID mode channels).
I purposely keep the default public group channels programmed into my DTRs in addition to my secure channels. I periodically hear people using the default public group channels in my travels. The DTRs and DLRs will talk to each other right out of the box at the factory default settings and people seem to use them at the factory defaults like FRS bubble packs. I am unable to monitor any DTRs and DLRs that have been custom programmed to make them secure.
The bottom line is they are safe from scanners.
