NAV-ISS-20 Rockblock Ring Alerts - Poll for SBDRING

[briellelaw]

Right now we successfully check the ring alert status with 'AT-CRISX',
but this just tells us the most recent Type of alert that was received (telephony vs SBD)
not if there is a pending ring alert.

We actually need to poll for the SBDRING announcement sent from the rockblock.
However, the announcement is only sent when the rockblock is in 'command mode'
I think this requires us to be sending an AT command at the time.
I think we have to send an SBDRB command prior to polling for SBDRING,
because the announcement is sent to the last known location of the rockblock
https://docs.rockblock.rock7.com/docs/faqs

Potential solution is to create another function that sends a simple AT command
and just waits until the SBDRING announcement is received.

Another solution is to make use of the 'timestamp' -which is a hex value representing
the Iridium system time (# 90ms frames from the last epoch). But based on experimentation,
this value doesn't seem to change - at least within a 5 second polling interval.
https://www.rock7.com/downloads/ATC_Iridium_ISU_AT_Command_Reference_MAN0009_v5.pdf

[briellelaw]

For Oct launch (and likely any upcoming launches in 2021) we will not be utilizing the SBDRING feature.
This would require a major refactoring of our bbb_satellite_listener code. In the interest of time, this may or may not be revisited in the future.