Offline Tracker Device concept.

RF Options		Frequency	Data	Distance
Lora	****	915/868 Mhz	4	4
nRF24L01	***	2.4 Ghz	4	4
RF	***	433 Mhz	1	5
BLE	**	2.4 Ghz	4	1 - 2
WiFi	*	2.4 Ghz	4	1 - 2

Distance Measurement	Challenge
Signal Power	Low reliability
Ping Time	Based on the time it took to send and receive a data back.
Triangulation	Multiple trackers must.
GPS	Expensive, high power consumption

Direction Determination	Challenge
Multi Frequencies	Most likely expensive, interference might be an issue.
Antenna Array	Calculation required. High clock speed might be required = Power
Triangulation	Might give rough estimation.
Ultrasonic Sound	Ultrasonic audio, requires multiple microphones, works in close distance (which is fine)

	Other Comments
1	UI	Screen isn't a must simple LED is fine, can be turned on while tracking.
2	Mesh	Mesh might be a good idea, neighboring devices would increase reliability, although means more power.
3	Mesh	Mesh can be done in 2 ways, just relaying anything or like a router might keep neighboring device list with ids.
4	DIY	Parts should be easy to get for common Joe
5	Existing Tech	There is a project called meshtastic, exiting network might be utilized in favor.
6	Power	Tracking devices should be able to run at least a week.
7	Portability	Tracker device, shouldn't be heavy to carry around. Desired form factor is something in between Walkie Talkie / Cellphone to smart watch.

	Device Identification
1	Trackers should have an ID, how to assign these id's are challenge.
2	Main device can be paired and can be assigned through.
3	Main device might have a bluetooth or wifi, for configuring through a phone.
4	Impersonation might be an issue. Something like a mac adress could be useful.

	Expansion
	I have another idea for emergency communication protocol, might be merged with that.
	Emco