What technologies to choose for a telemetry collection project?

Question

Electronics

Good afternoon, colleagues!
There is a problem that I really want to solve. Now I am at the stage of choosing technologies and I want to consult with experienced specialists.
There is a network of sensors that is located in the countryside (fields, hills, forest and no network). The sensors transmit their coordinates and a couple of dozen bytes of telemetry to the hub. These sensors are worn by people and they move at speeds up to 6 km/h (walking). Since there is no network, the exchange of information takes place over the air.
Sensors should be no larger than a pack of cigarettes and have low power consumption.
There is a hub, which should be made in the form of a tablet with a diagonal of 7-8 inches, with touch (preferably) or button control. The tablet receives information from the sensors and displays the coordinates of the sensors with telemetry on the map. Information is exchanged every 1,2,5,10,... seconds, depending on the setting. Sensors receive information with all settings from the tablet.
In the beginning, I plan to make a prototype for testing.
Kaka I see the solution to this problem:
1. development of sensors. This point is the simplest and most obvious for me. GPS module + radio module (it will be written in detail about it) + STM32 as a control MK (I chose it, because I am familiar with them).
2. Tablet development. It’s more difficult here, because you immediately want cheapness, compactness, the ability to quickly create a prototype and not have delays when working with a radio network.
This is the first time I have encountered such a task and the choice fell on Raspberry Pi + Display / touch + radio module + power + Linux on board.
I'm not sure if this is the best option, I will be glad if you advise something more optimal.
Criteria:
-low cost
-work with the radio network without delay
-installation of the program to display the coordinates on the map/grid.
- connection of the display and radio module.
3. Development of a radio network. The most difficult stage. Of all the available bands, 433 MHz is the best (27 MHz - you cannot create industrial networks, which is a pity). The wavelength is only 70 cm, the output power is not more than 10 mW (more is prohibited), which is not enough for rough terrain, since we would like a range of 2-4 km. I would close my eyes to the limitation and put a 5W transmitter on the tablet, and 1W on the sensors. Data transfer will occur with time division, the tablet will synchronize all sensors relative to its timer.
This is where my most important question arises: how to increase the data transfer range for this case? I understand that the laws of physics cannot be deceived, and at a frequency of 433 MHz I have a certain wavelength, data rate, etc. , but you can use a more sensitive receiver, noise-immune encryption and other means to increase the range. I would appreciate any information on this matter.
Thank you for your attention!

Answer 1

Throw out a self-made tablet from this joy and take a serial one, plug a USB TV tuner into it and the SDR receiver is ready. If this is a collection of telemetry, it makes no sense to send something to the collectors.
Synchronization from the tablet timer - in the bath, you have GPS.
For data transmission, use noise-like signals with a wider spectrum and coherent accumulation. The speed will be low, but the signal can be pulled out from under the noise.

Answer 2

The reception range can be increased due to the type of receiving antenna collinear, ideally to build an RF network model in CST Microwave Studio, where all the initial data will be antennas with RF transceiver equipment. As for speeds, telemetry, if it is not broadcasting video / audio streams, then it does not need a broadband path, again, how many bytes are planned to be transmitted from sensors and in what form, from here, consider the speed.

Answer 3

Regarding the radius, it would be an excellent option for each "sensor" to play the role of a bridge between another sensor that is further away and a tablet, the network can be expanded to an incredible scale, under the control of stm32 a very real option.