Checking Database for Full Accuracy

[WikiChess]

Hi all,

I'm new to this and I imagine this question comes up a lot, so apologies in advance. I'm monitoring the PG County MD FiRST Simulcast P25 trunked system, and there seem to be some calls that drop off out of nowhere. I only have one RTL-SDR in my setup, so I suppose this could be related to that, but (assuming my understanding of trunked systems is correct) I would think the dongle should be able to follow a call all the way through before going back to the control channel to listen for another (I'm aware this setup will miss some calls, but I would think it wouldn't drop them in the middle of transmission). Could it be that the database is simply missing frequencies? Even when there's not an ongoing call, I can see what look like calls occurring at several frequencies within the system band. If this is the case, how would one go about checking whether those frequencies are part of the system but just not in the database?

 

[maus92]

Hi all,

I'm new to this and I imagine this question comes up a lot, so apologies in advance. I'm monitoring the PG County MD FiRST Simulcast P25 trunked system, and there seem to be some calls that drop off out of nowhere. I only have one RTL-SDR in my setup, so I suppose this could be related to that, but (assuming my understanding of trunked systems is correct) I would think the dongle should be able to follow a call all the way through before going back to the control channel to listen for another (I'm aware this setup will miss some calls, but I would think it wouldn't drop them in the middle of transmission). Could it be that the database is simply missing frequencies? Even when there's not an ongoing call, I can see what look like calls occurring at several frequencies within the system band. If this is the case, how would one go about checking whether those frequencies are part of the system but just not in the database?

No. The traffic channel is defined in the data transmitted over the control channel, so an SDR based solution should have no issues following a voice call. The software only needs to know the primary control channel. Additionally, all frequencies used by the FIRST - PG Simulcast site are listed in RRDB and are correct. The issue is probably in your configuration, and / or the user has lost connectivity for some reason. The other frequencies that you see in use are probalbly from other sites or systems nearby that also utilize the 700 public safety band, like adjacent FIRST sites, the PG County system, the DC system, etc.

 

[Jeff19]

Hi and welcome,

What software are you using to monitor the system? As you mentioned, the RTL-SDR has a frequency bandwidth of 2.4 MHz, so if parked on the control channel on the high end (773.49375) it looks like the bottom two frequencies could not be voice monitored on a single dongle setup. But this doesn't explain drops in the middle. If using SDRTrunk try adjusting the gain settings, or changing to auto if not already. Also, if using SDRTrunk for clarification the software does not jump back in forth; it can simultaneously monitor the control channel and any voice channel within a 2.4 MHz spread. As maus mentioned the frequencies in database are confirmed/correct so something else is going on.