Is there an easy way to determine the altitude of a low-flying aircraft?
After looking over my options, I’m going to try an antique optical rangefinder.
I bought it on Ebay. I’m currently waiting for it to arrive.
Background
I was awakened last night by yet another low-flying helicopter, here in the DC ‘burbs.
The noise from these ranges from merely obtrusive, to loud enough to rattle the windows. Below is a recording of one of the several that passed overhead today, taken from my back porch. It doesn’t quite stop conversation, but you do have to raise your voice a bit.
This is normal for the DC area. There are a lot of military and other government high officials stationed in this area. These folks tend to get shuffled from place to place via helicopter. Unfortunately, one of the well-used north-south routes passes directly over the Town of Vienna.
Is it really that loud, or is flying low?
In theory, nothing should be flying below 1000′, in my area.
But in the past, that has been an issue. I recall that, many years ago, some Vienna Town Council members complained to various authorities about noise from low-flying aircraft, and got the “minimum 1000′ for the TOV” as part of the answer.
This got me thinking about measuring a passing helicopter’s height.
(Luckily, I am hardly the first person to have had an interest in this. Luckily, I say in hindsight, because that way, my Google inquiries would not attract undue attention from the authorities.)
Turns out, there is no good way for an amateur on the ground to measure the height of an over-flying helicopter. At least, none that I’ve come across.
But seriously, how hard can this be.
If nothing else, think of it as a way to rule out bad pilot behavior (low flight altitude) as an explanation for a loud helicopter fly-over. (With the obvious alternative explanation being “that was a loud helicopter”. Which, given that these may be military aircraft, is always a possibility.)
So, are those overflights loud because they are loud aircraft, or are they loud because they’re flying well below 1000 feet?
Optical rangefinders that won’t work
First, there are “laser rangefinders”, not intrinsically different from a laser tape measure, just more oomph and maybe some specialized optics. But first, I ain’t pointin’ no laser at no aircraft, period. Let alone a low-flying (likely military) helicopter. Plus, the ones available for civilian use (e.g., laser tape measure, laser golf or boating rangefinder, rangefinders for hunting big game) probably won’t work for this use anyway, owing to the small visible target. I get the impression these laser rangefinders (e.g., for golfers) can find the range to a hillside or location on an open lawn, but they aren’t designed to find something as optically small as a helicopter flying at 1000′.
I’m also brushing aside all the military “passive-optical” (coincidence and stereoscopic) rangefinders. These are WWII-era and earlier tech with mirrors, prisms, and such. If nothing else, aside from having to own one (they tend to be big, to get you the best separation of the two lenses), you’d have to have the forethought to have it handy, and set up, just as the helicopter was flying by. Plus, those are all expensive military collectibles now.
A vintage civilian non-laser coincidence rangefinder, via Ebay
Source: Ebay.
I can vaguely recall hand-held purely optical rangefinders, from the pre-laser era. These are the vastly smaller, and likely less accurate, analogs of military coincidence rangefinders. But they worked the same way, using two widely-separated lenses, then measuring how much you need to move the image from one eyepiece, until it coincides with the image from the other.
I bought one on Ebay. Above, you see a RangeMatic 1000.
This allows you to measure distances to 1000 yards, with some modest degree of accuracy. It looks like it should be more than adequate to allow me to identify helicopters flying at 500 feet, rather than at 1000 feet. It looks like the difference between 150 yards and 300 yards is about an eighth of a turn of the dial.
This, if it works, will give me the line-of-sight distance to the helicopter. That only tells me the height of the helicopter if it flies directly overhead. I’m going to need to add some sort of mounting and an inclinometer. The line-of-sight distance, plus the angle of elevation above the horizon, should allow me to infer the height of the helicopter over ground. (In fact, that’s easy enough that I don’t even have to look it up. Height above ground is the sine of the angle of elevation, times the straight-line distance to the object.
Thus ends this task, until my Ebay’ed optical rangefinder shows up in the mail a few days from now.
Estimating overflight height by apparent size.
The very crudest golfing range finders work by using the height of the pin (the stick-with-flag that marks the hole). These pins are a standard size, and the simplest golf rangefinders simply place the apparent size of the pin on a scale — the smaller it is, the further you are away from it.
Other purely optical methods seem chancy. In theory, if I could identify the model of helicopter, I could infer distance by measuring how how big the over-flying helicopter appears.
This is more work than I care do do.
Can I determine the height of a passing helicopter, purely from its sound?
Source: Reference BBC. Photo by Joe Pettet-Smith
First, an interesting historical side-note. Listening for approaching aircraft is not a new idea. As I understand it (likely from seeing it on YouTube), in parts of Great Britain, big, cast concrete parabolic sound reflectors still stand along the coastline. These concentrate (and effectively, amplify) incoming sound waves. These were used to detect the sound of incoming aircraft while they were still miles offshore, prior to the implementation of radar during WWII. Reference BBC
This is one of those weird things that is clearly possible, from first principles. Maybe not even terribly difficult, as a one-off proof of concept. But for which you can buy no ready-made unit.
Sound travels about one foot per millisecond. Two microphones, 100′ apart, would therefor experience about a 100-millisecond (or one-tenth-second) difference in when they “heard” a sound at ground level.
For this approach, I’d use some microphones, some recording gear, and the speed of sound, to triangulate where a near-surface sound is coming from, based on when (precisely) that sound shows up, at microphones placed at known locations perhaps 100′ apart.
The theory is easy: https://en.wikipedia.org/wiki/Acoustic_location
Start with the concept of a gunfire locator or gunshot locator. These (typically) use a widely-distributed set of microphones to detect and locate gunshots. Once a gunshot is detected, these use “standard triangulation methods” to estimate the direct and distance to the gunshot.
(There are crowdsourced versions of these: https://github.com/apispoint/soter, but that seems limited to categorizing a noise as a gunshot, not pinning down the location.)
Substitute helicopter noise for gunshot, and do the math in 3-D instead of assuming location on the ground, and that’s what I’m after. Something that will give me a fairly precise location of a helicopter flying overhead. From the noise of it alone. So that I may then calculate the height above ground, from that location.
In two dimensions, you only need two microphones — think, two ears — to identify the direction that a sound is coming from. Per Wikipedia, that’s all about the lag between the time the sound hits one ear, versus the other. To quote:
Where:
- is the time difference in seconds,
- is the distance between the two sensors (ears) in meters,
- is the angle between the baseline of the sensors (ears) and the incident sound, in degrees
- c is the speed of sound
But that only works (pins down a unique direction) if you’re working in two dimensions. And one pair of microphones provides no clue as to distance. Just direction.
If you work through what you do need, to pin it down in three dimensions, a minimum rig would need four microphones, arranged like the corner of a cube. This provides a pair of microphones in each of three dimensions. The further apart the better, as these are going to be used to estimate a helicopter height of maybe 1000′.
The rest should be math.
But this solution involves a lot of hardware, no matter how I figure it. Four microphones or recording devices, wires to connect them to a central station, and a four-track sound recorder.
This would be a difficult and expensive solution, so I’m not going to pursue it further unless the RangeMaster 1000 fails to do the job.
Conclusion
I’ll have to wait for my antique optical rangefinder to arrive before I can bring this to a conclusion.
My belief is that a simple hand-held “antique” optical rangefinder, plus something to measure the angle of elevation, should provide all the accuracy I need to distinguish helicopters flying at or about the 1000′ ceiling, from putative “low flying” helicopters at (say) 500 feet.
My guess is that these helicopters are merely loud, not low. But I should be able to validate that with this simple bit of equipment.
