Mapping an LTR System
Logic Trunked Radio, or LTR, is typically used by private business. It can be more difficult to unravel, since it tends to grow as users are added, while Public Safety Trunked Radio Systems tends to have most of the users defined and allocated to a specific Logical Channel Number , or LCN. Specifically, a certain subset of frequencies are defined as Home Channels (HH), and others are used for "overflow" conversations, sometimes called "GOTO" and "FREE" frequencies. As users are added, the number of assigned home frequencies increase, and presumably the FREE decrease. Sometimes, new frequencies are added with a different call sign.
The following explanation and tips are for LTR Standard systems primarily using Uniden scanners, although some of it will apply to all trunking scanners.
- Uniden scanners with Banks ( BC796D, BC780XLT, and BC898T ) must have the frequencies loaded into a bank in the exact order and channel number that is used by the LTR System. For example, if there are 3 HH channels 01, 03, 05, they must be programmed into positions 101, 103, 105 for a bank that starts with 101. Any bank can be used, but the order must be the same. This order requirement also applies to Object Oriented Scanners
- For Uniden DMA scanners with Dynamic Memory (ex. BC246T, BC346XT BCD396XT, BR330T, BCT15, BCD996XT), you just need to enter the LCN number next to the LCN frequency. The order is not important as long as the LCN number is entered next to the frequency.
This is NOT true for Radio Shack scanners (except the Pro-97 and Pro-2055). Some Radio Shack trunking scanners can read the idle bursts from the repeater to get the repeater number. However, less than half of the systems use idle bursts, and some systems will broadcast idle data bursts only on channels used as home channels and not on those used for "overflow" conversations
How Do I find LTR systems?
First, check the Radio Reference Database for your general location to see if there are any already identified. If there are none listed, you can do a search of the FCC database for your area:
FCC ULS License Search
Most LTR Systems are in the 451-465 range. Restrict your search to Radio Service Code YG-Industrial/Business Pool, Trunked, although some non-trunked systems have been converted to trunking without registering the change with the FCC. Also, look for systems that have five or more frequencies, and check the licensee information for the FCC Station Class Codes. If it is "FB2" that usually means it is a standalone dedicated or community repeater operation, not LTR. However, if it is "FB4, FB6, or FB8", that is a good clue that the operation is LTR. If you use the Radio Reference Database, it usually identifies trunked systems immediately.
How Do I find the correct Home frequencies in a known LTR System?
The problem with a small system that grows is that it may only have 3 out of 10 home channels assigned to users in the beginning. Finding those are easy because they are used frequently and consistent HH appear. When home channel is in use, the user will be sent to either another inactive HH, a GOTO, or FREE channel. When that happens, the Talk Group ID (TGID) will show up on the GOTO channel as the HH channel, so the actual channel number is not displayed. For example, if HH 01 is busy, TGID 0-01-001 is sent to HH 09. TGID 0-01-001 will display on HH 09, NOT 0-09-001. To make matters worse, if there are no users assigned to a GOTO channel, you will never find the correct channel number by looking at the TGID's displayed. TGID's should be in the form: A-HH-GGG, where HH is the LCN/Home Repeater, and GGG is the User Group Nbr.
There are several possible approaches to mapping a STD LTR system, and they vary with each type of scanner
LTRs may also be mapped and/or tracked using software programs
Although an Object Oriented Scanners (OOP) article has not been written yet, it should be similar to Uniden DMA
Monitoring the System
After you have identified most of the frequency HH positions, you can put them in their correct positions, and start to observe the trunking process. Another suggestion would be to replace all the blank positions with some frequency that is easy to recognize, like 460.000, that is NOT part of the call sign frequencies. If you notice a transmission is cut off, and/or the scanner is quickly jumping back and forth to 460.000, you know that you haven't found all the frequencies yet.
If this happens, you may have to try Locating additional LTR frequencies In once case, the frequency was registered 30 miles away, but was actually in use at the site being mapped. It was NOT registered for that location. Some have indicated that licensees can get together and share frequencies, none of which is registered with the FCC.
Some observations and comments:
- The “A” in the TGID is either 0 of 1. Typically, a particular system will not mix them, so if you get mostly 0-HH-GGG, and only a few 1-HH-GGG, it is likely that the “1” TGID's are errors, or from another system. Also, some TGID's will show HH's greater than 20. This is generally due to noise on the signal.
- If you press “hold” on a TGID, and it stops transmitting, it is probably from another system, or another HH that was sent there. If an active conversation seems to show different frequencies flashing but there is no sound, the scanner is trying to switch to the correct position on your scanner, but the correct frequency is not there.
- Many systems have one or more conventional non-trunked frequency in the system. This can be determined if no TGID shows when you are locked on a frequency and a transmission is heard in the ID search mode. This frequency must be placed in any channel higher than 20 for bank scanners, or in a conventional group for DMA scanners.
- During the entire mapping process, be sure your scanner is set to ID Search, and NOT ID Scan for Uniden, or Open for Radio Shack and GRE scanners.
- LTR is business radio. Hence the traffic is predominantly during business hours. Trying to map a system during nights and weekends will be less effective.
Mapping LTR systems can be frustrating and time consuming but this FAQ will make it a little easier. Finding the most active channels is not that hard, so take a break and enjoy your efforts before going on to the more difficult part.