If you are taking snapshots it will not interrupt any availability of your server. When you are taking AMIs however, you are given the option if you want to reboot the instance.
If you choose not to reboot the instance when taking an AMI, it will not have the latest data. If you choose to perform the reboot, your instance will be temporarily unavailable while the AMI is being created.
So, If you choose to take Snapshots with Cloudranger you shouldn't experience any unavailability or downtime. You can easily schedule when you would like your daily backups to be taken, and also customise the retention periods for these backups based on your requirements.