Frequency-modulation retransmission lessons-learned in Korea
by CPT Michael Sohn and 1LT Thomas Martin
“Strike 6, this is Warrior 6, over … (static) … Strike 6, this is Warrior 6, over … (static). …”
For a Signal officer in an infantry division, this is one of the worst things you could hear – the commanding general unable to communicate with his brigade commanders. Now imagine the difficulty commanders have controlling the battle when they can’t talk over their frequency-modulation radios farther than 15 kilometers away without using FM retransmission.
This scenario may seem unlikely, but in Korea or other places with mountainous terrain, it’s an obstacle that must be constantly overcome.
The 2d Infantry Division conducts exercises that require FM communications stretching 40 km by 60 km in an operations area with six major intersecting terrains. These missions require multiple FM retrans nets that link more than 17 command posts for the division’s command and control. With the extensive use of FM retrans operations in Korea, Company C, 122d Signal Battalion has learned some important lessons concerning FM retrans operations that can maximize FM communications for warfighters.
In mountainous regions like Korea, the key terrain for Signal is on hilltops that provide good communication coverage for both FM and mobile-subscriber equipment. With limited hilltops available, 122d Signal Battalion is sometimes forced to co-locate FM retrans with MSE systems. However, when the FM retrans team is located close to a radio-access unit, the FM retrans team experiences increased interference and the division’s FM nets suffer as a result. After investigation, and with the assistance of Communications-Electronics Command at Fort Monmouth, N.J., the reason for this condition was discovered.
During Ulchi Focus Lens 2002 (the annual peninsula-wide exercise) and this year’s warfighter exercise, one of the division’s FM retrans teams was co-located with a remote RAU team. The missions required FM communications all the way from Seoul to Camp Casey and beyond – a distance of 60 km. The FM retrans team experienced severe interference on the command net during these two exercises. The retrans team checked all equipment but didn’t discover any equipment errors. During a similar exercise, the battalion placed the same network on the ground but without the RAU co-located on the hilltop with the FM retrans team. During this exercise, the division command net had almost zero interference and FM communication was flawless.
Our investigation showed that FM radios (RT-1523E) and the RAU radios (RT-1539) operate on the same frequency band, so the transmitting signals from the RAU can cancel out the transmitting signals from the FM (Single-Channel Ground and Airborne Radio System) radio. Furthermore, if the incoming SINCGARS signal is low, then the more powerful RAU’s transmitting signal will overwrite the smaller received SINCGARS signal and the FM retrans team will not receive the intended signal (see figure below).
If the incoming SINCGARS signal is low, the more powerful RAU's transmitting signal will overwrite the smaller received SINCGARS signal.
When this occurs during frequency-hopping operation, the FM retrans team will hear static and the distant stations won’t be able to communicate through the retrans team. This condition verified why the retrans teams experienced interference during two division-level exercises.
Another issue associated with mountainous terrain is the need to change net identification while maneuvering around the operations area. Because a retrans team set up in the standard F1-F2 configuration receives one net ID (F1) and transmits another net ID (F2), warfighters must change net ID based on location so they can talk with everyone. This presents a significant problem to C2, particularly when units are conducting maneuvers and convoys, because FM communication isn’t truly seamless and users must know when and where to change over to the other net ID. Using an F1-F1 retrans configuration and consequently eliminating the need to change net ID can resolve this problem.
An F1-F1 retrans configuration, in contrast to an F1-F2 configuration, essentially acts as a signal repeater rather than a signal retransmission. Using a new data function available on the RT-1523E model radio, a retrans team can designate one radio as a “receive only” radio and the other radio as a “transmit only” radio. This means that when someone talks on the net, the “receive only” radio picks up the signal and repeats it through the “transmit only” radio. Using an F1-F1 retrans configuration, 2d Infantry Division now has seamless communications and has essentially eliminated the need to change net ID while maneuvering through the operations area, allowing unit commanders to concentrate on the battle.
FM communications is the warfighter’s most important communications asset. Without FM, warfighters can’t effectively maneuver and they can’t fight. Retrans operations are an essential part of the FM mission requirements for 2d Infantry Division, or any other unit in a mountainous region. Using an F1-F1 retrans configuration and avoiding co-locating FM retrans assets with MSE will maximize the overall FM communications support for the warfighters.
http://www.gordon.army.mil/AC//Fall02/FMKorea.htm