This paper presents the design and implementation of an inexpensive and generic Spectrum Alerting System (SAS), based on Software Defined Radio (SDR), by using the RTL-SDR USB device and a low- power microprocessor Raspberry Pi. The main aim of the SAS is within a defence surveillance system for discovering rogue or unidentified signals in an attentive spectrum, sending alerts to the signals intelligence analyst. However, it can be employed in any kind of intelligence surveillance system. In this implementation, the SAS analyses a predetermined part of the electromagnetic spectrum to establish a baseline pattern, subsequently, monitoring the spectrum to discover rogue or unidentified signals within this spectrum. Detected rogue or unidentified signals are recorded and an email alert message is sent to the signals intelligence analyst for further investigation. Whilst currently, this message is an email, the alert message could be sent via other means such as Twitter, Facebook or other messaging agents. Moreover, the implementation and experimental results demonstrate the success and potential of the SAS as an inexpensive and generic choice for an intelligence surveillance system in both a benign and hostile environment.