Commercially-available radiolocation devices?

[mountainpenguin]

I have been wondering if the lora mesh might be worth trying. cheap module
modules are around £10 each plus some kind of power source and can be left running for a long time. scattering gps versions over the surface and compare the signal strength to the (potentially multiple) underground nodes to target multiple leads at multiple locations. just remember to remove them all after.
the TX power is small and its either 400 (with license) or 868 so very poor rock penetration but it would be interesting to experiment

 

[Stuart France]

I'm not quite clear how LoRa can be useful in a radiolocation scenario. It is all on 868MHz licence free in the UK. LoRa won't go through rock at all, or reflect around loads of corners going down a bendy passage, and it's not particularly directional either without specialised aerials.

The power output of LoRa modules is significant. A typical transmitter will pull 50-100mA when transmitting a packet. The "low power" average power claim refers to the transmitter spending most of its life asleep between "events" and drawing only a microamp in that state. The killer is that receiver has to be powered the whole time listening just in case a transmitter transmit something - and it might consume 20mA or more continuously. That might not seem very much, but it is in the context of AA alkaline batteries inside a small enclosure.

Meshtastic technology has been tried with some success for digital comms (sending text messages not speech) in a rescue scenario over modest distances, a few hundred metres, but it uses up a lot of repeater stations in a realistic caving scenario. As to using Meshtastic to implement comms in a long-distance rescue in (say) Aggy or Daren or Draenen, that is for the birds.

I've used Seeed modules to connect microprocessor stuff to local Wi-Fi and they work fine - until they don't. At that point someone had to attend in person to power the thing off and back on. So we overcame the travel nuisance issue by adding another microprocessor simply to watch the Seeed module and reset it when evidence suggested it had ceased operating. Let's hope they've done a better job with the module that you've suggested.

It is advisable not to believe in what you read in datasheets and other manufacturer blurb. One LoRa module manufacturer told me that a buyer had transmitted from Calais to Dover with LoRa, and someone else had transmitted across several counties from a hang-glider. Well maybe, but caves don't have 20 miles of water with tall cliffs at either end, and they don't fly either.

 

[Relict]

What sort of depth of rock is thought to be between your cave and the surface directly above? Obviously there are 'serious issues' with signal strength loss the further the transmitter-receiver are apart since magnetic moment reduces by the distance cubed, not distance squared as with light illlumination. Also an error in levelling the underground transmitter loop (or the regularity of its windings) will introduce a displacement of the apparant surface position directly above the transmitter (the so-called Ground Zero) as determined by the surface person using the receiver set.

You need to ask "has it any hope of working well enough - if at all" before building something.

I've lent my gear to the Forest of Dean people, possibly permanently - we'll see, as they're in the Second Entrance business. The distances they're interested in measuring are of necessity quite short and thus any Ground Zero positioning error there would be insignificant.

It's about 9 m but we don't know if it's all rock as there may be intervening void.