The mobile sound ranging array:
Solid ground for target acquisition
Soldier Modernisation talks to Alex Koers, co-founder and director, Microflown AVISA
CASTLE acoustic subarray on Fennek recce vehicle
Microflown AVISA is a deep tech company providing an acoustic layer to the battlefield.Exploiting the benefits of its own proprietary (and universal) sensing platform, the CASTLE acoustic subarray, the company has implemented the concept of Passive Acoustic Distributed Sensing.
For networking, IP based MANET radios are used. Hence, sheer firmware defined capabilities can be provided that include:
- localization of indirect fires
- gunshot localization
- short range air defense
- tank hunting
Over the last decade, the company spent more than 400,000 person hours on developing a capability that experiences a surge in interest. Soldier Modernisation spoke to Alex Koers about their innovative solution.
Q: Within Europe a lot of companies and countries have one eye on what is happening in Ukraine. Has the on-going conflict affected how Microflown AVISA is working?
A: Yes it has, as the Ukraine war doesn't only show the brutality of the Russian artillery, but also the way the Russians operate.
With both aerial surveillance and Electronic Warfare being omnipresent, active target acquisition sensors like weapon location radars have become vulnerable.
But moreover, with the Russians also applying shoot & scoot when firing, the chances of hitting the enemy on the position it fired from are decreasing rapidly.
NATO is turning to the concept of the pre-emptive strike. Our Mobile Sound Ranging Array helps in gradually building the “kill web”. We prioritize on that.
Reconstruction of ballistic curve
Q: Who are your competitors?
A: Technically speaking weapon locating radars. We have surpassed the current generation of sound ranging systems that were developed around the turn of the century.
Q: What are the main differences between the new technology and the previous generation of sound ranging?
A: First of all, previous generation sound ranging sensor nodes are large. Their typical shape is an equilateral triangle, the leg length around 10 meters. By necessity, the system is ground based, which is problematic in times where soldiers need to remain on the move all the time.
Our Acoustic Multi Mission Sensor, a “molehill” shaped sensor with 23-30 cm footprint, equals and exceeds the capability of such a triangular shaped sensor node in terms of acoustic performance.
But given its footprint, our capability can be installed on mobile platforms, like ground based vehicles and, in the future, multi-copters.
Secondly, traditional sound ranging makes use of muzzle blast noise only, having a blind eye on rockets that don't generate muzzle blast noise.
For sure, rockets are an important threat that need to be addressed if possible.
Our target acquisition strategy is based upon capturing the 3D shockwaves that are generated by both howitzer shells and rockets, as long as they fly at supersonic speed.
Q: How does that target acquisition strategy work?
A: From the tip of the shell or rocket, a 3D shockwave front starts to propagate following the direction of the round being fired.
Keep in mind that the sound source in this case is approaching the receiving sensor posts with the speed of the round, not the speed of the sound.
This mitigates one of the main disadvantages of acoustic sensing, as sound waves don't travel with the speed of light.
We know, based upon forward models we made and validated, when, where and under what 3D elevation angle such a shockwave front will hit a CASTLE sensor post.
By combining all these arrows, our engineers have developed the algorithms that can reconstruct the corresponding ballistic curve in space and time.
Q: So what allows you to compete with weapon location radars?
A: Well, in essence, a weapon location radar tries to estimate the point of origin out of the ballistic curve it is trying to compute as well.
But nowadays drones provide the target coordinates, with pin-point precision, in real time with eyes on target. They have taken the position a radar used to have.
The only mission left is to tell the drone pilot when to take off and where to fly.
We can do so, just like a weapon locating radar does. One could argue both sensor categories are artillery shell tracking sensors, with different pros and cons and, not unimportant, significant differences in total cost of ownership.
Q: So sensor fusion, is that the way forward?
A: Yes, of course our MSRA can alert and cue a weapon locating radar, keeping it “down” as long as possible and allowing it to use a beam with a small top angle, minimizing the threat of hostile direction finding sensors localizing the radar.
But maybe also EO/IR systems should be considered in future sensor grids. Like our approach, the optical approach is passive and distributed, so in essence a similar concept in another domain. “Numbers matter” nowadays.
Q: Can you elaborate on the costs?
A: A weapon locating radar may cost 15 million Euros to buy. The annual operating costs include depreciation, cleanroom maintenance and labour costs. It adds up to 3-4 million Euros per year. Radars need to be defended as well.
The military plans for 25 years at least. It is a long-term financial commitment.
Our capability is in the price range of these annual costs, and, not unimportant, can be considered “unattended”.
Q: Don't you need a dedicated formation of soldiers for the MSRA?
A: It depends. Some nations indeed prefer this approach, with dedicated vehicles and well-trained soldiers, adding to the operational costs. But frankly, we believe it is recommendable to make use of “in fleet vehicles” that are driving around anyway. The organizational challenge is how to bridge the “stovepipes” in the Army.
Our CASTLE subarray can also be loaded with firmware to detect incoming direct fires and airborne threats.
This provides close proximity awareness for each of these vehicles and, moreover, the possibility to fuse data with on board optical systems. Edge processing as it could be.
And when needed, CASTLEs on some vehicles along the front line should be assigned to become a sensor post in the MSRA.
Some sensor posts may be jammed, some sensor posts would like to remain in a silent mode. The “luxury” of having access to enough sensor posts makes the idea a more robust contingency planning. But also from a signal processing point of view, having the ability to select among sensor nodes makes the MSRA better performing.
Keep in mind that 3D shockwave front creates a “spatula” type of “boom carpet” as we call it, not the “circle” with a gradually decreasing noise level. With dynamic targeting, the enemy right in front of you may decide not to fire straight ahead, but fire far to the left or right. It affects the direction the “boom carpet” is progressing.
Q: Any other controversial ideas?
A: Well, I would not say controversial, but maybe a bit counter intuitive. In our concept, we assume a ballistic curve that has of course both a beginning, the point of origin, and an end, the point of impact.
We can improve the localization accuracy of the point of origin by localizing the impact, and fitting the ballistic curve on this end. “You need to be lucky enough to be shelled upon”. Our concept is a bit fuzzy, optimizing between accuracy and timeliness.
Q: What about integration with other systems?
A: We do offer our own AVISA C2. But hooking up with overarching systems is a rather straightforward task.
Q: What is on your roadmap?
A: More than we can chew! We need to prioritize. But the categorization of the indirect fires has our focus. We already provide information on the trajectory, connecting the dots between point of origin and point of impact. The big question is if we can differentiate between howitzers and rockets. And yes, “we can”.
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