Passive Radar Utilizing Existing Infrastructure (e.g., 5G)
A central theme is the exploration and viability of using existing, ubiquitous infrastructure, particularly 5G cellular networks, as a form of passive radar. This approach leverages the widespread deployment of cellular towers and their inherent radio frequency (RF) emissions to detect and track objects without dedicated radar hardware.
- "Depending on node density of a 5G network (think street lamp cells), it is not outside of the realm of possibility that you're going to be able to obtain radar derived point clouds from cellular networks doing double duty as phased array radar networks." - toomuchtodo
- "There's a whole host of radar research using OFDM/ Wifi (I wrote a paper on the topic a while back where i implemented it with some software defined radios)." - polalavik
- "There are proposals for the 6G standard to support Integrated Sensing and Communication(ISAC)... So the hardware might natively be able to support gait recognition. The use cases given are UAV detection and localization." - gene-h
- "Micro-5G base stations on street lamps: Key to proliferating mmWave technology" - toomuchtodo (linking to an article)
- "It underscores how important cybersecurity is in mobile, IoT and Wi-Fi systems. A few critical exploits chained together is all it takes for physical surveillance or bio-sensing" - knetl
Limitations and Practicality Compared to Dedicated Radar
While the concept of leveraging existing infrastructure is intriguing, a significant portion of the discussion focuses on the limitations and practical challenges when comparing it to traditional, dedicated radar systems. Concerns are raised regarding resolution, range, signal processing complexity, and cost-effectiveness for specific applications.
- "i don't think this is intended to track the type of folks who leave their AIS broadcasting." - zomiaen (implying it's not for sophisticated tracking of elusive targets)
- "AIS, like ADSB, is secondary surveillance - not radar. It's a mechanism for cooperative targets with functioning electronics to identify themselves and provide operational information. However, it does not detect uncooperative entities or those not equipped with the electric transponders." - jchulce (drawing a distinction with AIS)
- "You're telling me Ukraine can't get dedicated radar? A non weapons package that any Western nation would sell to them without reservation?" - timewizard (questioning the necessity of improvised solutions)
- "The transmitters are often not very powerful compared to dedicated radar systems and worse, they transmit energy in the horizontal plane rather than upwards where the planes are. The frequencies involved are much lower, which inherently leads to less radial accuracy unless you use VERY large antennas." - WJW (explaining technical limitations of using broadcast signals)
- "Many of those problems are mostly terrible when trying to detect aircraft at long range though, and largely go away for short range surface use like in port. I'm still not entirely sure why for a port, which is stationary and requires tons of infrastructure investment anyway, this system would be preferable to a normal civilian type radar system." - WJW (questioning the cost-benefit for port surveillance)
- "Drones and missiles would be detected even later, since the antennas of cell towers are designed not to radiate any energy upwards (there's usually no cell phones high in the air, so that energy would just be pure waste)." - WJW (highlighting directional limitations of cell antennas)
- "Gait recognition is extremely difficult to do with any accuracy even with high resolution video in semi-controlled environments. Doing with with opportunistic 5G signals sounds far fetched." - brk (skeptical about high-resolution use cases)
- "Sure, but not as well as a dedicated radar system and at much higher cost." - WJW (regarding missile/drone interception)
AIS as a Complementary or Alternative System
The Automatic Identification System (AIS) is discussed as a point of comparison and as a potentially simpler or easier-to-use alternative for certain maritime detection tasks. The discussion touches upon its limitations as well, particularly its reliance on cooperative broadcasting.
- "No? It's significantly smarter and easier to use AIS." - timewizard (advocating for AIS)
- "AIS is not mandatory for all vessels, and in any case it can fail both on the vessels themselves and in the control center." - WJW (pointing out AIS limitations)
- "For example, AIS won't show you an enemy's invading fleet, and ADSB won't show incoming missiles." - jchulce (highlighting what AIS and similar systems cannot do)
Defense-in-Depth and Improvised Capabilities
The idea of using multiple, layered sensing systems for enhanced defense and the potential of utilizing readily available commercial off-the-shelf (COTS) hardware for military applications are explored, particularly in the context of the Ukraine war.
- "It can also be used for defense in depth. Each additional sensing system which must be disabled before an attack is an additional barrier." - timschmidt
- "Ukraine war shows improvised capability from cots hardware can have a meaningful impact. Probably easier to get 5g cell tower infrastructure than dedicated military radars." - denkmoon
- "Of course they can, though not as easily as you seem to think I suspect. Radar technology is very secret. Regardless, it’s a matter of numbers. I’m sure the Ukranians would love to field unlimited predator drones if they could. In reality, they field DJI and other commercial drones en masse because of availability. Only relatively recently have they got their own cheap mass produced drones online. These drones are also easier to destroy than a predator, so by your logic, why would they have invested in this? Wunderwaffe never wins wars, logistics do." - denkmoon (emphasizing mass availability of COTS over advanced tech)
Stealth Technology and Radar Countermeasures
A significant portion of the conversation revolves around stealth aircraft, radar absorbing materials (RAM), and how different radar systems, including passive and multistatic approaches, interact with stealth technology. The effectiveness of stealth against various detection methods is debated.
- "The longer range versions of multistatic radar are used to detect stealth aircraft. All that careful stealth geometry to minimize direct reflections doesn't help much when the emitters and receivers are in different locations." - Animats
- "No but the highly classified radar absorbing compounds that stealth aircraft are wrapped in definitely help :)" - 4gotunameagain
- "The radar absorbing compounds of stealth aircraft are highly optimized for specific wavelengths (usually X-band) and fall off heavily outside that frequency band. Similarly, the radar cross section of stealthy aircraft is highly optimized for specific purposes (usually evading GBAD in the forward direction) and rapidly falls off in other scenarios." - WJW (detailing stealth limitations)
- "Most 'stealth' aircraft are actually fairly visible from other directions." - WJW
- "Well, you don't even need a radar. Tamara sensor could detect B-2, when it had it's onboard radar on." - Grayskull (referencing passive detection of active systems)
- "stealth does in general go out the window when you turn on your radar. It's much like dressing in black and then running around with a flashlight at night." - rcxdude
- "Doubt: the APQ-181 radar on the B-2 is a Ku band radar, about 15 GHz. Tamara is about 1 GHz. This is entirely incompatible frequency ranges." - chipsa (challenging a claim about stealth detection)
- "Also, the APQ-181 is a LPI radar, which means it’s specifically designed to avoid correlation of signals such that you can track by the signals emitted." - chipsa (explaining Low Probability of Intercept radar)
Regulatory and Security Concerns (ITAR, Export Controls)
The discussion touches upon the potential regulatory hurdles and national security implications associated with advanced RF sensing technologies, including their classification and export controls.
- "I seem to recall reading (on HN, no less) that advanced passive radar technology is classified as munitions, by the US Government and is under export controls?" - supportengineer
- "Yes, they are on the BIS Commerce Control List. It doesn't need to be particularly advanced to be export controlled." - syedkarim (confirming export controls)
- "I was reading about that and was really interested in trying it - got quite close to buying some kit (KrakenSDR) - then it seemed that particular capability got removed suddenly a couple of years ago due to ITAR regulations, or at least legal types getting worried about ITAR..." - logifail (anecdote about ITAR affecting amateur capabilities)
Emerging RF Sensing Applications Beyond Navigation
Beyond just tracking ships or aircraft, the conversation explores a broader range of potential applications for RF sensing, including human detection, gait recognition, and disaster monitoring.
- "They said it is high-enough resolution to perform gait recognition." - ImPostingOnHN (about a startup using 5G as radar)
- "Also flood forecasting" - nelox (linking to a news article)
- "5G signals can be used to track pedestrians on the street, not just ships in the port." - janpmz
- "Me? I just want a car to be able to detect me so they don't run me over." - blendo
The Busy Electromagnetic Spectrum and Future Standards
A general observation is made about the increasing density of RF signals in the environment, with upcoming standards like Wi-Fi 802.11bf explicitly designed for sensing capabilities, indicating a broader trend in RF utilization.
- "It underscores how important cybersecurity is in mobile, IoT and Wi-Fi systems." - knetl
- "The gap between the people demanding these systems and those who design it it is so large, it’s vulnerable to corruption in infinite ways, let’s be honest." - soundpuppy
- "I wouldn't go so far as to call this RF 'pollution', but it is a reminder that the EM spectrum is getting a lot busier." - blendo
- "See 802.11bf" - throw0101b (referencing Wi-Fi sensing standard)