sterretje:
Add a field with a sequence number to the data packets. The sequence number will be incremented by the transmitter after every packet. If the receiver does not see the expected number in the packet, it knows that one or more packets are lost.You possibly want to cater for 'reboots' of the transmitter and receiver; e.g. store the last transmitted / received number in eeprom. Be aware that you can only write a single eeprom cell 100,000 times; without precautions, that means roughly 100,000 / 3 = 33,000 hours = ~3 years.
Can the now.minute() value not be used as a sequence number? In case of transmission every 20 minutes for example 19, 39, 59?
I am however not sure how to write an algorithm for that?