Formatting, improvement ideas

README.md

@@ -1,6 +1,6 @@
 Long range, low power messaging system based on Meshtastic relays suitable for underground communications.
 
-<figure>
+<figure style="text-align: center"
     <img src="24-02-10 13-07-46 3107.jpg" alt="Vangelis node placed by a cave stream." width="200px">
 </figure>
 
@@ -11,7 +11,7 @@ Natural caves are challenging as it comes to communications due to irregular cav
 * [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.
 
-<figure>
+<figure style="text-align: center"
     <img src="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.">
     <figcaption><em><center>Vangelis architecture overview.</center></em></figcaption>
 </figure>
@@ -25,7 +25,7 @@ The **Vangelis** project expands on all of the above by using autonomous low-pow
 * Range of surface relays limited only by LoRa radio reach, practically up to a few kilometers in mountainous areas
 * Consumer smartphones connecting over Bluetooth to relay nodes using [Meshtastic app](https://meshtastic.org/downloads/) to send and receive messages
 
-<figure>
+<figure style="text-align: center"
     <img src="24-02-10 13-12-22 3110.jpg" alt="A caver reading message on a smartphone standing next to a relay node.">
     <figcaption><em><center>Typical usage scenario.</center></em></figcaption>
 </figure>
@@ -34,17 +34,20 @@ The **Vangelis** project expands on all of the above by using autonomous low-pow
 
 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;
+* **surface node** 
 * **cave node** powered from a LiPo battery and intended to operate without recharging for the whole period of a cave operation
 
-<figure>
+<figure style="text-align: center"
     <img src="24-02-12 14-24-37 3136.jpg" alt="Three small cave nodes and two larges surface nodes placed next to each other.">
     <figcaption><em><center>Comparison of cave nodes (left) and surface nodes (right).</center></em></figcaption>
 </figure>
 
 ### Surface node
 
+Relay node intended to be used outdoors and thus not subject to size and weight limitations. The prototype is 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. The node can be equipped with a GPS receiver.
+
 Bill of materials:
+
 * [Clear Hinged Lid IP67 Waterproof ABS Electrical Enclosure Case junction box](https://www.ebay.co.uk/itm/304321459053)
 * [WisBlock Meshtastic Starter Kit](https://store.rakwireless.com/products/wisblock-meshtastic-starter-kit)
 * OLED Display [Solomon SSD1306 - RAK1921](https://store.rakwireless.com/products/rak1921-oled-display-panel)
@@ -63,6 +66,7 @@ Notes:
 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.
 
 Bill of materials:
+
 * [RAK WisBlock Mini Base Board RAK19003](https://store.rakwireless.com/products/wisblock-base-board-rak19003)
 * [RAK LoRa Core Module RAK4631](https://store.rakwireless.com/products/nordic-nrf52840-ble-core-module-for-lorawan-with-lora-sx1262-rak4631-rak4631-c)
 * [RAK barometric pressure sensor RAK1902](https://store.rakwireless.com/products/rak1902-kps22hb-barometric-pressure-sensor)
@@ -78,13 +82,29 @@ Bill of materials:
 * [USB-C male to female coupler](https://www.amazon.co.uk/dp/B0B18SLFD4)
 * [Rubber USB-C plugs](https://www.ebay.co.uk/itm/144021649084)
 
-<figure>
+<figure style="text-align: center">
     <img src="24-01-29 11-35-50 3052.jpg" alt="Yellow, small 3D printed plastic case with antenna.">
     <figcaption><em><center>Assembled TacMesh case.</center></em></figcaption>
 </figure>
 
-### Barometric pressure
-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.
+### Future improvements
+
+#### Barometric pressure
+
+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. The surface node would then operate as the barometric reference for the whole network. With this improvement, the Meshtastic application would see the nodes identified by their depth rather than merely names, e.g.:
+
+1. 0 m relative (surface node)
+2. -50 m
+3. -100 m
+4. -200 m
+etc
+
+Messages sent to the channels could be also identified by the depth of the respective relay node, thus indicating that team X has reached -100 m, then -200 m etc.
+
+#### Cable communications
+
+Long, tight crawls are especially challenging for connecting using relay nodes due to relatively short (~10 m max in our tests) radio range and high risk of the nodes being displaced or damaged by cavers moving with bags in confined space. In theory, the nodes could be also connected using a serial cable connected to the USB-C port, which could replace the radio link through the problematic tunnel. The Meshtastic firmware currently supports [serial communications](https://meshtastic.org/docs/configuration/module/serial/) but not exactly for node-to-node links. The length of such a hypothetical link and how it would need to be powered is also unknown.
+
 ## 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