70 · MT 3/2022 C4ISR Forum This signal will also produce radiation at frequencies of 11.2GHz (twice the frequency of the signal), 16.8GHz (three times), 22.4GHz (four times) and so on. The strength of these signals will diminish as they increase in frequency. Thus, the strength of the sixth harmonic will be a fraction of the original signal. Harmonics can be problematic if they crowd out another part of the electromagnetic spectrum. For example, the fourth harmonic of a 2.4GHz garage door opener will be 9.6GHz. This latter signal will be right in themiddle of X-band (8.5-10.68GHz), a frequency reserved by the International Telecommunications Union (ITU) for radar systems and military SATCOM, not garage door openers. The ITU is tasked with globally regulating use of the electromagnetic spectrum. Having the fourth harmonic of our garage door opener in X-band is not a problem if the signal power level of this signal is so low that it does not cause any interference. Given the power level of the average garage door opener, this is unlikely to be a problem. Dan noticed that some L-band (1.215-1.4GHz) harmonics seem to cause interference to Sentinel radar pictures. L-band is also a popular radar frequency. An L-band radar transmitting on a frequency of 1.350GHz would have a fourth harmonic of 5.4GHz, which sits squarely in the C-band frequency detected by the Sentinel satellites. This would depend on the fourth harmonic signal being sufficiently strong to cause interference to the satellites’ SARs. However, if the radar’s transmissions are sufficiently powerful, this might be a possibility. Dan says he has seen instances in which L-band interference has been spotted on the Sentinel SAR pictures. Potential Impact The appeal of open-source intelligence has gained momentum in recent years. The advent of satellite imagery made commercially and freely available to the public and commerce kick-started this trend in the late against targets in Saudi Arabia. The radar’s activation was almost certainly conducted to enhance protection of Dhahran against these attacks. It is important to note that the satellites are not detecting the radar’s main beam, known as the main lobe. Instead, they are detecting the radar’s sidelobes. The main beam is the RF energy transmitted from the radar, which hits the target and returns to the radar as an echo. It is the main lobe which the radar’s processor analyses. This lets the radar compute a target’s speed, height, bearing and even identity. All main lobes produce sidelobes, which are beams of RF energy radiating out at an angle above, below and each side of the main lobe. The power of these lobes diminishes the further away they are angled from the main lobe. Assuming the main lobe is transmitting at an angle of 0°, a sidelobe at a 45° angle will be much less powerful. A radar is pointing its main lobe into the sky to find targets. The main lobe may be pointed in a completely different direction from the gaze of the satellites’ SARs. However, some of the sidelobes will be in the direction of the satellites’ radars. It is these sidelobes the Sentinels’ SARs detect. By detecting the sidelobes, the satellites locate the radar. For example, by seeing the green and red strips of interference on Sentinel radar pictures of Syria, and working out where they meet, one can determine the presence of a C-band radar. This is how the locations of the AN/MPG-65 radars in the Middle East were determined. Dan emphasises that his discovery “can only tell you if there’s an active battery or other (C-band) electromagnetic source at the site.” Some satellite communications systems use C-band. It may be possible to use Dan’s techniques to determine their location and when these systems are transmitting. Furthermore, he has observed that some radars in other frequencies may cause similar interference on the Sentinel radar pictures. This is the result of frequency harmonics, which cause interference. A device transmitting radio waves will produce harmonic radiation. Let us suppose we own a C-band radar transmitting on a frequency of 5.6GHz. Bellingcat’s Radar Interference Tracker clearly shows interference on Sentinel SAR radar pictures caused by C-band radars deployed in Ukraine. The interference is indicated by the discoloured strips on the image. (Image Belligcat)
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