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The plan is to create cheap ultrasonic anemometer with good stability in all weather conditions and reasonable accuracy around 1 m/s or less.

Unfortunately my knowledge in this area is lacking but Im slowly learning and i hope that one day we will have working product. The principle is quite simple. Transmitter sends ultrasonic signal to receiver which measure time it took for this process. I first tried to use HC-SRO4 ultrasonic arduino module

which is cheap an aveliable. I did some measurements and got accuracy of 1%. I thought this will be enough until i did some calculations. As it turns out i would need repeatability of less than 1 us but in reality my results were +- 5 us. Then I came up with neet idea to make 100 measurements, order it by time and use only 40-50 on that list to calculate average. That did improve results but not near enough for needed accuracy.

I found out that HC-SRO4 transmitter uses 40KHz and has amplitude of 25us and that this is just not enough to accurately measure time of flight - "tof". I search online and find couple of similar projects. I read them many times since its difficult topic for me. My knowledge did improve but im still not certain that i understand it well enough. The most important part of the design is analog component that amplifys signal detect envelope and phase changes. Counting phase changes should give much more repeatable results.

There are three projects aveliable online

1. one from Carl https://code.google.com/archive/p/mysudoku/downloads
2. one from Hardy Lau https://www.dl1glh.de/ultrasonic-anemometer.html
3. one from Lukas https://soldernerd.com/arduino-ultrasonic-anemometer/

I also find some simple design but im not sure if it worked well http://n1qq.com/blog/2015/ultrasonic-anemometer-part-1/
there is also one on easyeda (site that im familiar and know how to use) https://easyeda.com/AndySmithersHome/ultrasonic-anenometer but once again i dont know how well it worked. I send msg to the author but unfortunately did not get response.
Here is also interesting link with calculations http://trekker.customer.netspace.net.au/wind.htm im interested in 3 transducer type.

The on from Lucas probably has the best documentation and is also one of the latest, but it uses pic32 microcontroler that im not familiar with at all. I would rather use arduino compatible MCU. There is user @Antiath that commented on Lukas blog quite a lot and from what i understand build the project using teensy 3.6 which is arduino compatible. Unfortunately i couldn't reach that user aswell.

My plan is to first make analog component with one sensor as receiver and using the HC-SRO4 as transmitter. Im planing to replicate analog part from Lukas and from Andy and then compare them. For now I would use simple ATmega328 just to try out but migh change it for more capable arduino compatible MCU or even PIC32.

The design of the anemometer is yet to be determinated but im thinking of using the one with bouncing beam from flat surface since most comercial anemometer system use them

I also have one broken ultrasonic anemometer from WeatherFlow. Im pasting PCB if it may be useful to somebody. Let me know if you want any details about it.

Once or twice a day data is send twice in matter of 30-60 seconds of one another, while second upload contains only one measurement. Im not sure exactly where the problem occurs or if its a problem with my code or SIM800 library.

The second upload contains only one measurement so wind speed and gust are identical. This should not have happen. I tried to add locking mechanism in loop function but that didn't solve the problem. Now I added second if condition in senddata() hoping it will solve it...

It could be caused by random arduino board reset. Im looking into it. Opinions welcomed.