Currently, Pi checks the hours, minutes, and seconds before sending it to GCP to make sure received radio packet is within 5 seconds of the current time. This doesn't entirely prevent the replay attack because a hacker could capture the packets from the day before and re-transmit the packets the next day at the very same time it was captured the day before. We must check the date as well. However, this requires that the Uno send a different NMEA sentence than GPGLL. We probably want GPRMC instead of GPGLL.
Currently, Pi checks the hours, minutes, and seconds before sending it to GCP to make sure received radio packet is within 5 seconds of the current time. This doesn't entirely prevent the replay attack because a hacker could capture the packets from the day before and re-transmit the packets the next day at the very same time it was captured the day before. We must check the date as well. However, this requires that the Uno send a different NMEA sentence than GPGLL. We probably want GPRMC instead of GPGLL.