Mapping an LTR System

Overview

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 using Uniden scanners scanners, although some of it will apply to all trunking scanners. Uniden scanners scanners 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 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.

1. FCC Licensing databases (Older version, but has some queries not in the new one)
2. 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 two possible approaches to mapping a STD LTR system.

Method #1

• Identify most of the Active HH's first, and then use method #2 for the rest. This method is faster and produces more interesting information right away.
• Program all of the frequencies under the same call sign, or even different call signs for the same Licensee in the same general location. Assign the bank trunking system as LTR.
• Enter each frequency twice as in the following example. At this point, they do not have to be in positions 1-20. The example will have them starting in 21.

   Pos  Frequency  Trunk Setting
   21   462.5000    off (conventional)
   22   462.5000    on
   23   463.2500    off (conventional)
   24   463.2500    on
   etc, etc
   

Note: if your scanner does not allow conventional and trunking in the same bank, then just program all the frequencies once as conventional, and note the active ones. Skip directly to Method 2 and enter these active frequencies first.

• Start monitoring. Take note of which channels are active. As you hear traffic on any of the conventional frequencies, quickly go to manual, and switch to the next position. I.E., if transmission is heard on 23, quickly switch to 24. If that frequency is trunked, you will see the TGID displayed. Most systems have one or more frequencies assigned as conventional, so don't be surprised if there is NO TGID displayed.
• Take note of the HH's display on each frequency. The active ones probably have users assigned, and the most frequently displayed HH will be the actual HH.
• In a few days, you will often have located most of the active HH's, and you can put these in their correct positions (1-20) to start getting a feel for the system. You can also start making a list of TGID's. Your scanner may allow you to add them as received. For example, the BC796D does this with the transfer key.
• If there is no traffic on one or more of the active frequencies loaded in their "correct" positions, go to method #2 and program all 20 positions with the each of these questionable frequencies to verify their correct position. This is also a good method to double check each frequency as you identify it.

Method #2

The second is more scientific, but may take a little longer. Since there can be up to 20 channels in an LTR system, program the lowest frequency into ALL 20 channels for that system in the the 1st 20 positions in the selected bank. When a transmission is heard, hit the "hold" button and observe the TGID. On the BC796D, hit the Trunk Button, and it will tell you which of the 20 positions received the transmission. This becomes more important as you try to find the GOTO frequencies. Repeat this several times to confirm audio is being received on this frequency for several different TGID's. If the audio is on talk groups A-05-GGG then that frequency is likely to be channel 05. Then repeat the process for the next highest frequency for all 20 channels.

Often, systems are designed to use only the ODD channel numbers, and are assigned from lowest to highest frequency in the system. If you use Butel software, you can easily log these TGID and their associated frequencies. Other software may do the same. If not, then use the hold button during active searching as above. The problem with logging is that you can get false TGID's, or ones from other systems that will easily fool you into thinking you have LCN's for the GOTO.

How Do I program my scanner to find the GOTO frequencies?

Once you have the active HH's identified using method #1, you use method #2 to obtain the less active GOTO frequencies. In this phase, the purist would have you program ALL 20 positions with one of the next unknown frequencies. However, you can just lock out all the known HH's and only program the remaining blank positions. If you do that, you should observe the transmission switch back to one of these locked-out frequencies when it is completed. Either way, you only hear transmissions on NEW frequencies & positions, and you wont' be distracted with things you have already identified. This part of the search process is very boring, and quite frustrating, but there really is no other way.

Once you hear a transmission, you must immediately push hold and then trunk on the BC796D. The actual scanner position should be displayed, and you then know to which position out of the 20 that frequency belongs. Other BC scanners have similar sequences. After verifying several times, you can then add that frequency to it's correct position, lock it out, and replace all the other unknown positions with the next frequency.

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 if your scanner allows mixed mode in any bank.
• During the entire mapping process, be sure your scanner is set to ID Search, and NOT ID Scan!

Conclusion

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.