Natural caves are challenging as it comes to communications due to irregular cavity shapes, presence of water and general difficulty of installing any kind of equipment in an environment where everything has to be carried on human's back over hundreds of meters vertically. There are numerous existing solutions, each with its own challenges:
* Wired telephone is a reliable solution, however poses a significant investment due to the above challenges, plus it is easily damaged in confined or gravel passages.
* [Through-the-earth radio communications](https://en.wikipedia.org/wiki/Through-the-earth_mine_communications) using very low radio frequencies (HeyPhone, Nikola, CaveLink) are the most popular _ad-hoc_ communications solution during cave rescue or expeditions . Obtaining a reliable link at depths below hundreds of meters however is a lottery as many factors from geology to weather impact attenuation even at low frequencies.
* Underground communication links based on radio repeaters was discussed at least since 2014[^1]. _Sybet_ came up with industrial solution [SPELLCOM](https://sybet.eu/batnode/) using radio repeaters to relay voice communications over underground cavities.
<imgsrc="drawing.svg"alt="Schematic drawing of a cave with three relay nodes placed on tunnel bends and cavers communicating to end nodes using their mobile phones.">
The **Vangelis** project expands on all of the above by using autonomous low-power repeaters for relaying text messages rather than voice over low-power radio transmission:
* Radio transmission using [LoRA](https://en.wikipedia.org/wiki/LoRa) modulation using [Meshtastic protocol](https://meshtastic.org/docs/introduction) for routing
* Low-power radio transmission with maximum 200 m line-of-sight range for underground nodes
* Long-term autonomous operations of each relay node
* Low weight and portability of the nodes
* Range of surface relays limited only by LoRa radio reach, practically up to a few kilometers in mountainous areas
The system is composed of two node types: surface and underground. Each node operates in router-client mode, which allows both client device (smartphone) connection over Bluetooth and message relaying to other nodes over LoRa.
* **surface node** powered from a 18650 cell with [PV charging](https://docs.google.com/document/d/12GIY24vLKLABg2RUTPP6yMzokr44GMzJOE4p7ngaCbI/edit#heading=h.9lmvuqjahqxl), equipped with a 3-5 dBi antenna; typical usage scenario is to be installed at cave entrance and relay messages further to another such node at a base camp;
* **cave node** powered from a LiPo battery and intended to operate without recharging for the whole period of a cave operation
* [Outdoor LoRA antenna](https://store.rakwireless.com/collections/outdoor-antennas) any antenna can be used, two prototype nodes were built with 3 dBi and 5 dBi antennas
* 3D-printed antenna holder, STL for RAK [3dBi Fiberglass Antenna](https://store.rakwireless.com/products/3dbi-fiber-glass-antenna) or [5dBi Fiberglass Antenna](https://store.rakwireless.com/products/5dbi-fiber-glass-antenna-supports-863-870mhz) are available
* [5V 1.25W 250 mAphotovoltaic panel](https://www.ebay.co.uk/itm/113383833070), 110mm x 69mm, 2.4mm
* OLED display is optional. It's mostly useful when pairing smartphone with Meshtastic over Bluetooth using a random PIN code, but since these are impractical underground, the nodes are configured to use no PIN at all. Since it also displays last packet information etc it's a nice to have.
* Choice of the antenna is of paramount importance for outdoor LoRA range. Poor antenna will limit the range to tens or hundreds of meters at best, even in line of sight. With RAK 3-5 dBi antennas we easily get kilometers long range. It does not matter for underground nodes as their range is limited by rock anyway.
Durable, waterproof case made of 3D-printed plastic, indented to be operated in hostile environment and simplified operations. Prototype weight ~130 grams. The only user interface is the power button on the top, that is big enough to be operated in gloves. The WisBlock board has a green, blinking LED which is visible through the plastic case and serves as an indicator that the device is live and transmitting. The case has an USB-C port on the bottom that in normal conditions is closed with a rubber seal.
* [IPEX to RP-SMA connector](https://store.rakwireless.com/products/ipex-to-sma-connector)
* [BLE-PCB Bluetooth antenna](https://store.rakwireless.com/products/ble-pcb-antenna-5-5dbi)
* [TacMesh Waterproof Enclosure](https://www.thingiverse.com/thing:5923930/) 3D printed from PLA, [assembly instructions](https://youtu.be/gpnivx2jVRk)
* [Tactile 4x4x1.5 mm push button](https://www.amazon.co.uk/dp/B08F7V2Y66) not installed in the cave prototypes, mostly needed for node reset for firmware upgrade
As cave nodes are located through the cave, they have no means of determining their own location as GPS signal is unavailable. Barometric pressure sensor allows to [correlate the pressure](https://en.wikipedia.org/wiki/Barometric_formula) seen by a node with altitude above mean sea level ([AMSL](https://en.wikipedia.org/wiki/Height_above_mean_sea_level)) as long as at least one surface node is equipped with GPS receiver _and_ barometric pressure sensor.
## Footnotes
[^1]: M. D. Bedford; G. A. Kennedy [Modeling Microwave Propagation in Natural Caves Passages](https://ieeexplore.ieee.org/abstract/document/6933914/), IEEE Transactions on Antennas and Propagation, 2014
