I am new to this world, I started working with astroberry a few weeks ago, and I realized that to have a correct functioning of the system , I would need an RTC.
What could be the best solution?
RTC module?
gps module?
Smartphone / tablet?

If you're online use NTP, otherwise I'd go for the GPS, plenty of USB GPS dongles that are supported by the PI.

Hi, i bought and installed the small DS3231 RTC module as described here: www.andreavinci.it/blog/en/2018/02/22/ra...231-real-time-clock/ It costs just around $2 (maybe less on ebay/ali) and you just plug it to the RaspberryPi connector to the appropriate pins. It works well.

Of course, GPS is even better (but it won't work when testing indoors). I bought the BN-880 GPS module for $12 on Aliexpress and it works well with GPSD. You just need to connect it to the right pins on RPi again, which may require soldering in this case. But you can also try some USB receiver, which is easier. I also tried the BN-880 connected externally via a USB-to-serial adapter and it worked "Plug-and-Play" with a good adapter, unfortnately, GPSD hangs after a short time when used with a cheap Prolific PL2303 USB-to-serial adapter.

I would like to use a gps as a rtc module. In this days I'm testing my smarthpone with an app, netGPS.

I just received this www.amazon.co.uk/gp/aw/d/B07LBWF1P7?psc=...pop_mob_b_asin_title

Works well with pi4 GPSD