Difference between revisions of "Mapping an LTR System"

Revision as of 15:24, 1 January 2011

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, 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.
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 Licensing databases (Older version, but has some queries not in the new one)
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

Although Object-oriented programming article has not been written yet, it should be similar to Uniden DMA

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!
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.

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.

@@ Line 48: / Line 48: @@
     <LI>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.
 <li>During the entire mapping process, be sure your scanner is set to '''ID Search''', and NOT '''ID Scan!'''
+<li>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.
 </UL>
-Some additional comments based on a slightly different approach:
-Observations from using a Pro-97 in Colorado. Tedious, but doable.
-. 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.
-. Pro-97 TGID and home LCN reading is useful but is susceptible to noise. Readings taken from a long, strong transmission are valid. Other readings might not be. Noise can cause false TGID display. I've seen brief inaudible blits of LCN 26 or LCN 32 and other nonsense. Logging these TGIDs over many different transmissions will give the picture of what the valid LCN really is. (By logging I mean I'm using a spreadsheet and typing everything I hear so as to be able to correlate things later.)
-. The systems I am finding seem to have the idle strobe enabled. Suspecting this I did a very slow Tune-step search thru the band of interest logging all strobing signals. Band plans have standard channel spacing so stepping to the next valid frequency isn't too hard.
-Tune (Funct-Tune - up/down arrow) will find the channel. Using the programmed Search function won't (it seems to be deaf and we're trying to confirm presence of a periodic short burst). Sit on the channel for 5 minutes or so to catch any slow-rate strobes.
-Log the frequencies where strobes are found. If possible, listen awhile and log TGIDs and most revealing of all - the CWID (that's the morse code station identifier) if it is sent. The systems I've seen have the CWID on its own TGID which for the Pro-97 reveals the LCN of the channel by its home channel number as well as the LCN reading in the Pro-97's field.
-. With CWID you can look up the FCC license info for the system. (Or not. I found one channel identifying itself with an invalid, not on record, callsign. Confirmed the ID several times. No record. ) Using the FRN license info, you can then get additional frequencies that might be part of the system. Listen to them to detect activity and LCNs you might have missed the first time thru.
-.5 Without CWID you can use the FCC lookup by frequency to get some candidate companies and their alleged transmitter locations.
-. With the spreadsheet you have a frequency, maybe some TGIDs and words, and a possible owner name for the system and an LCN for that channel.
-. After collecting the freq and probable LCN and owner you can sort the spreadsheet by owner and then by LCN to create a trial map of the system.
-. Then test the trial map by listening on each channel (manual) for awhile logging TGIDs. You should expect to see TGIDs that are homed on that channel. What you are really looking for are the other TGIDs that pop up there due to their home channel being busy. These TGIDs will tell you that there are other LCNs in the system. For example, I have a trial map showing LCN 01, 17 and 18 for a system. If TGID 0-04-004 pops up on channel 01 this tells me that I am missing a frequency that belongs to channel 04.
-. In some cases, there may be frequencies that fit into the LCN map but aren't actually part of the system. It is possible to have a single-channel system that IDs itself as having a certain LCN. It seems to me that the only way to sort this out is to listen to that channel and watch for other non-homed TGIDs showing that this channel is indeed part of a system. (It could take awhile to reach a conclusion)
-. Not all channels of a system will have the same callsign. But they will probably have the same owner.
-. Not all freqencies listed on a license are on the same system. Not all frequencies listed are in use. Sometimes frequencies list to different owners and the active/expired info isn't sufficient to resolve the ambiguity.
-. TGID number is not sufficient to prove that it is the same user on the same system. Chances are pretty good, for instance, that every system has a 0-01-001 TGID. You have to listen to the users and perhaps learn voices, names and background noise or signal characteristics (ping-on-transmit, digital data racket, hum, etc) to be sure it's the same users showing up on the different channels.
-. It's likely that certain "styles" help you sort a system. For instance, CWID being on 0x-255 on each channel vs 0x-253. Or that the even LCNs are used but the odd ones aren't.
-. Figuring out who has what TGID is straightforward enough to a certain level. For instance recognizing a car towing service is simple. Identifying WHICH company it is could be impossible just by listening. It might take field work so as to be at a location that's aired in order to put eyes on the radio user to get the company name.
-. Foreign language skills might be required. Many truck drivers hereabouts only speak Spanish. I can track a little of this. One TGID on a particular system has been entirely in Russian. I can recognise their voices and pickup a name here and there but I can't tell what business they are in. (taxi most likely?)
 ==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.
 [[Category:Trunktracking Topics]]