# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### ES Function Requirements  
The ES function operates passively in a receive-only mode to monitor the RF environment around the ship. This function comprises automatic detection, analysis, identification, classification and angle-of-arrival measurement of received RF signals, especially radar signals. The analysis task from the above list specifically involves measurement of signal waveform parameters such as time of arrival, frequency, pulse repetition interval, pulse width and waveform modulation, and determination of threat radar parameters such as beamwidth and antenna scan revisit time. The ES function can often detect an approaching platform through its RF emissions before any other onboard sensor. Consequently, it serves to provide early threat alerts for emitters classified as hostile or unknown, and cues fire control systems if necessary with threat bearing information. Also, the EA function depends upon receiving information from the ES function to maximize its effectiveness, particularly threat bearing to enable it to point its beams accurately in the threat direction, and threat waveform parameters to allow optimization of the jamming technique. This support to the EA function is especially important when multiple threats are present simultaneously. The frequencies of operation listed in Table 1 for the ES function refer to the frequency range over which the ES function is required to detect RF emissions. Emitters of interest include communications systems that typically operate below 5 GHz, surveillance and fire control radars which usually transmit in the 1-12 GHz range, and active missile seekers which mostly radiate at frequencies above 8 GHz. As seen from Table 1, the typical instantaneous bandwidth required for the ES function is much higher than those listed for the radar subfunctions. This is largely a consequence of the potential need for an ES system to detect and properly characterize high bandwidth waveforms from emitters such as airborne synthetic aperture radars and low probability-of- intercept radars. It is worthwhile to note that unlike radar functions, there is no _a priori_ knowledge of the received waveforms to allow matched filtering, and consequently, the increased system noise power due to large instantaneous receiver bandwidths results in lower sensitivity (i.e., lower SNR) that can adversely impact signal detection and analysis. The ES function can mitigate this impact for weak narrowband signals by digitally reprocessing the data captured within the wide instantaneous receiver bandwidth through a bank of narrowband filters before the detection process occurs. The required receiver dynamic range for the ES function is based on the ratio of the strongest expected threat emitter power at the receiver input to an input level corresponding in strength to the receiver noise floor power. There is no EIRP requirement for the ES function, as it is operates in a receive-only mode. The beamwidth requirement indicated in Table 1 applies to the beam that is formed by the AESA upon receive. The value for this requirement reflects the accuracy to which angle-of-arrival (AoA) of incoming emissions detected within a receive beam must be measured to allow useful determination of threat bearing. It should be noted however that a beam does not necessarily have to be formed by the AESA in order to determine AoA. If a strong RF signal arrives with a sufficiently high SNR, then the phases of the signal detected on a relatively small subset of array elements can be directly compared and processed with an interferometric algorithm to determine AoA with sufficient accuracy to meet the requirement. For such signals, this technique is relatively simple to implement, and essentially allows instantaneous detection and AoA measurement over the entire beamwidth of an array element, which typically represents a broad range of angles.