Yes, I am also quite impressed at the amount of thought that has been put into this concept. I would be quite interested in helping in designing these systems, as I have been following some of the necessary technologies for years.
Jim<p>I foresee a stiff plastic or metal pipe, tapered to a point at one end, which can be driven by force into soil or into a lawn, so that it ends up to be approximately flush with the plane of the soil. Once placed, a smaller cylindrical container, as well as active elements, if needed, can be slid into the metal pipe, from above.<p>From the linked article: <a href="https://opaque.link/post/dropgang/" rel="nofollow">https://opaque.link/post/dropgang/</a><p>"This challenge is met by Dropgangs in various ways. The primary one is that the documentation of each dead drop is conducted in minute detail, covering GPS coordinates, photos of the surrounding and the location, as well as photos of the concealment device in which the product is hidden (such as an empty coke can). The documentation however increases the risk for the Dropgang since whoever creates it would be more easy to identify by surveillance. In addition, even great documentation still requires the customer to understand it and follow it precisely, which can lead to suspicious behavior around the dead drop location (staring at photos, visually comparing them to the surrounding, etc)." [end of partial quote]<p>Ordinarily, smartphones that use GPS, don't use accurizing features, such as WAAS. (Wide Area Augmentation System).<p>See <a href="https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System" rel="nofollow">https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System</a><p>WAAS correction data is probably already available on the Internet.
"Accuracy[edit]
The WAAS specification requires it to provide a position accuracy of 7.6 metres (25 ft) or less (for both lateral and vertical measurements), at least 95% of the time.[2] Actual performance measurements of the system at specific locations have shown it typically provides better than 1.0 metre (3 ft 3 in) laterally and 1.5 metres (4 ft 11 in) vertically throughout most of the contiguous United States and large parts of Canada and Alaska.[3] With these results, WAAS is capable of achieving the required Category I precision approach accuracy of 16 metres (52 ft) laterally and 4.0 metres (13.1 ft) vertically. [end of partial quote]<p>WAAS might be described as a form of differential GPS. If the location as computed by the smartphone was improved by WAAS, the statement above indicates an accuracy within about 1 meter.<p>IR-specific retroreflectors to greatly simplify things.<p>The article describes complicated systems using Bluetooth or WiFi to help locate these dead-drops. While they are certainly innovative, they add cost and complexity to the hardware involved. I have thought of a much-cheaper system that I feel is sufficiently secure and simple for common use.<p>Light-retroreflectors are commonly made from Scotchlite <a href="https://en.wikipedia.org/wiki/Retroreflective_sheeting" rel="nofollow">https://en.wikipedia.org/wiki/Retroreflective_sheeting</a> or plastic molded corner-cubes. If a rather small (say, 1/4 in diameter) sphere covered with retroreflector material was held up from the insert, possibly by a short, thin stiff wire, the sphere could be visible, but not excessively obvious even during the daylight. It would be easy to find this device with a flashlight in the dark. For added security, an infrared-transmitting plastic (such as is often used to cover IR-activating remote controls, such as <a href="https://www.eplastics.com/plexiglass/acrylic-sheets/ir-transmitting" rel="nofollow">https://www.eplastics.com/plexiglass/acrylic-sheets/ir-trans...</a> ) could be used to ensure that only IR is retroreflected back to a searcher.<p>Ordinary smart-phone camera arrays are not only sensitive to human-visible light (generally described as 400-700 nanometer wavelength), but are also sensitive to near-IR wavelengths. If a smartphone camera was combined with a directional IR LED, substituting for the white light LED lamp used for photography, and aiming in the same direction, a user would be able to see (through the camera display) the IR-specific reflections from an IR-limited retroreflector, and this would probably be doable both during the day and at night. A person operating such a camera would "look like" he was doing photography, or perhaps playing a game. Somebody watching, even at night, could not see the IR. The IR 'searchlight' could be a narrow-beam device, perhaps with a full-angle of 16 degrees or so (typical for a narrow-beam IR LED), so it wouldn't be particularly obvious even if watched through an IR viewer. (If the IR LED itself was shielded from direct view.)<p>One advantage of this technique is that the searcher could identify the target from a very long distance away, perhaps many tens of meters, and thus approach it in a more "innocent" fashion. No obvious "searching" would have to be performed in the open. And, the person who placed the dead drop could ascertain its status without later needing to approach it closely.<p>This technique could be combined with Bluetooth or WiFi techniques, too. The retroreflector could normally be retracted, and only raised if the proper Bluetooth or WiFi signal was heard. Or, perhaps, the target would contain an exposed IR LED, which would activate from an battery only if the proper signals were heard. The resulting dead-drop would be virtually impossible to find.