Guardrail/Common Sensor (GRCS)
System 3 / GRCS (Minus)
The AN/USD-9(B) Special Purpose Signal Detection System, commonly called the Guardrail Common Sensor System (GR/CS or GRCS) is a combined communications intelligence (COMINT) and electronics intelligence (ELINT) system, or more simply a signals intelligence system (SIGINT). GRCS was a product improvement to the Improved Guardrail V system. Initial development was split into the fabrication of systems intended for deployment to Europe (System 4) and to Korea (System 3). Operational requirements meant that in the end System 3 was deployed before System 4, but without some of the planned upgrades, leading to System 3 sometimes being referred to as GRCS (Minus) or GRCS (-).
System 4 major enhancements over the Improved Guardrail V include the following:
- Integration of CHAALS for COMINT high accuracy TDOA/DD COMINT emitter location which provides support to COMINT targeting operations
- AQL for ELINT collection/emitter location/target processing and ELINT TDOA
- Receiver pooling for flexible allocation of intercept assets
- Digital temporary storage recording (DTSR) and a digital audio bus for audio recording and audio distribution support to the operator positions
- SIGINT related software upgrades for combined for processing of combined COMINT/ELINT missions and for TDOA operator tasking
- Integration of GPS for highly accurate navigation updates
- Re-engine, modified RC-12K aircraft with PT-67A engines, increased payload capacity/take-off climb margin required for SIGINT mission capacity, including new SIGINT wing tip pods
- Integration of third IDL for three platform ELINT and TDOA COMINT missions
A number of upgrades related to these fundamental enhancements were also a necessary part of the GRCS program. For example GRCS employs 4 main frame computers versus one computer in the Improved Guardrail V system, with integrated software interfaces that are required between COMINT, ELINT and CHAALS. These computers and several new special processors support the new SIGINT capabilities and precision time difference of arrival (TDOA) emitter location capabilities and provide the required additional processing power and storage to manage the approximate additional 500K lines of code entailed in GRCS.
Two different airframes were used for systems 3 and 4. The RC-12H used on system 3 was a derivative of the RC-12D that was used for Improved Guardrail V. The reason for this variation hinged on the fact that the RC-12K could not be made available in the delivery time frame. The RC-12H had the appearance of the RC-12K including the new pods, but had the smaller PT-6A-41 engines and a 15,000 pound gross maximum weight limit. The RC-12K aircraft used for GRCS System 4 had larger engines and beefed up wings and additional empennage modifications for stabilization and a 16,000 pound gross maximum capacity that was capable of carrying the added CHAALS and AQL sensors.
The technology for many of the system upgrades such as Temporary Digital Storage Recording (DTSR), GPS hardware, AQL and CHAALS came from other programs. For example, the DTSR came from an Air Force program. This upgrade allows the operators to efficiently record and play back audio signals. The recorded signal data is stored on magnetic disks for instant recall. Signals can be designated for automatic recording and the operator can do play back while recording is still in progress.
With this capability the operator can do a computer search for recorded signals based on time, frequency, or operator position etc. Recording automatically takes place when a directed search intercept occurs, if that is desired. An audio bus for Intercommunication and Signal Distribution and a receiver pooling capability were added as part of the signal handling upgrade to better support the SCARS signal acquisition automation. One of the many system software upgrades incorporates connectivity for allowing in-coming messages into the system computerized message data base.
Guardrail Common Sensor systems 3 and 4 were part of the same product improvement program, but differed in the amount of hardware deployed and the fielded capabilities. Although System 3 was tested with a full complement of hardware, it was delivered without AQL and CHAALS and their assigned mainframe computers. The RC-12H airframes were the available aircraft at the time of integration, but did not have the capacity for the full mission nor were production AQL and CHAALS units available at the time, therefore GRCS System 3 was deployed with only 6 platforms and with no CHAALS targeting capability or ELINT capability installed.
The subsystem differences between System 3 and System 4 are:
- System 3 IPF does not have the CHAALS or AQL subsystems or their maintenance facilities
- System 3 IPF does not have the third data link that is required to support CHAALS three aircraft missions
- The System 3 IPF has two main frames and CPO for two additional computers and other AQL/CHAALS specific equipment
- The System 3 payloads have full provisions, but only Improved Guardrail V equipment installed
- The System 3 AGE has provisions but lacks the extra AQL/CHAALS ground support equipment
- System 3 has 6 RC-12H platforms versus 9 each RC-12K's used on system 4
In addition to the above defined basic System 4 specified upgrades, a group of quick reaction capability enhancements were added to system 4 late in the program. These include microwave intercept and downward frequency intercept extensions, special signal receiver capability, expanded multi channel, proforma enhancements and Smart File Cabinet/FasTrack smart map capability.
GRCS Systems 3 and 4 were part of the Army's worldwide electronic warfare forces and provided airborne, stand-off collection and location of tactical SIGINT signals. Redeployment of GRCS Systems 3 and 4 needed to have advance preparations or at least an advanced party to consider the best location and to insure that line of site to the planned mission flight tracks has been properly considered. Although the system had the organic support that was needed for flight operations, it required a landing strip within range of the planned orbits. It also required an Integrated Processing Facility (IPF) location that was within line of sight of the planned mission flight tracks.
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