Made by IBM, AN/ALR-502 is the Canadian designator for AN/ALR-47.The AN/ALR-47 countermeasures receiving set was originally developed for the US Navy's anti-submarine warfare S-3A aircraft. The equipment is automatic and features an eight port (four per wingtip) antenna array, digital processing and selectable frequency band limits, tuning speeds and signals of interest. Twin superheterodyne narrowband receivers are used. These incorporate an intermediate frequency discriminator, out-of-band rejection logic and voltage-controlled oscillators that are tuned directly by computer control. The receiver channels are designed for high sensitivity and there is logarithmic compression of video outputs. The characteristics of the video outputs from the four monopulse receiver channels are derived in a signal comparator that converts the data to digital format. All subsequent signal processing is performed digitally under computer control. The two antenna groups each comprise two pairs of cavity-backed planar spiral broadband antennas, arranged in a near orthogonal disposition for monopulse direction-finding.
Control of ALR-47 is by means of computer programs that provide for receiver tuning, signal processing, and initiating warnings of significant contacts or known threats identified by stored signal characteristics. Provision is made for human intervention and manipulation of the software in the course of a mission to meet specific operational needs. Examples of this are facilities for filtering out unwanted signals of various types so that the overall processing and display load is reduced. Similarly, parameters such as scan speed and frequency scanning limits, pulse repetition frequency, pulse-length, or bearing limits can all be varied at the discretion of the operator.
The "Information Fusion Concepts" white paper provides additional details on the system. "At present, the ESM system uses wing-tip antenna arrays to automatically detect, locate and identify targets passively at long range and in a multidensity signal environment. ESM processed signal information is presented to the Non-Acoustic Sensor Operator (NASO) stations and to the Tactical Navigation (TACNAV) operator. Incoming signals are compared with those in the computer library and any unknown or threat ID is confirmed or verified by the operator, who then would have to provide this to the fusion function. The MSDF prototype developed for the CPF made this ID validation automatic by assigning beliefs to the various platform IDs and combining that with the information garnered from the other sensors in a remarkably efficient manner. It is planned to have the Aurora's ESM range increased during the upgrade.
Since the ESM is a passive sensor, there are no real mission-related restrictions on its use to provide inputs to the fusion function. The scenarios studied in the CPF MSDF demonstration prototype have shown that the first few ESM contacts are the most important contacts leading to a definitive platform ID. This should not be surprising, since it is the sensor that has the most processing capability on board the Aurora, capable of giving the smallest list of possible platform IDs. The only concern that has to be addressed is the subsequent use of its declarations about emitter type and/or platform ID, since one would like to keep open the possibility that the emitter is actually uncatalogued and that the best fit is not really very good given the existing database. This entails keeping a fair level of ignorance (in the Dempster-Shafer sense of the word), and this can only be achieved by screening out excessively repetitive ESM declarations. This would not be a problem on the Aurora if the operator is advised to enter ESM information very selectively.
The ESM bearing accuracy and signal characterization leading to emitter (and/or platform) ID are classified but the bearing accuracy is substantially worse than for the radar. One can therefore anticipate that an ESM report coming from a bearing where a fleet is in close formation can be challenging, depending on the geometry of the fleet with respect to the Aurora ".
Reference:1) http://longsword.weebly.com/avionics.html
2) http://www.janes.com/articles/Janes-Radar-and-Electronic-Warfare-Systems/AN-ALR-47-warning-system-United-States.html
3) Information Fusion Concepts For Airborne Maritime Surveillance and C2 Operations by P. Valin ,É. Bossé and A. Jouan dated May 2006. Document #P525407