GSE Blog 

Iridium Push To Talk (PTT) is a newly commercialized service that is extremely useful for the market, but is also not well understood so lets learn a bit about how it all works.

To start with, Iridium PTT is not a "Nextel" style of phone where you simply push the "key" button and broadcast a message to your group and then go back to normal telephone calls and SMS. The Iridium PTT phone must be booted into one of two modes when it is powered on, either regular mode, or PTT mode, and to switch between the two, you must reboot again. Just to clarify, since this is the number one question:

• When you boot into PTT mode, you CANNOT make voice calls, data calls, SMS, LBS, or tracking.
• When you boot into regular/voice, you CANNOT receive/transmit PTT broadcasts.
• You must choose a mode, and if your application requires both simultaneously, you will need 2 Iridium phones.

So what does PTT do for you? Simply you push the "Key" button on the side of the Iridium PTT mode and broadcast a voice message to your group as long as you hold the "Key". You can allocate Iridium PTT phones to your PTT group and define the region to cover the territory you operate within. It is a relatively simple use case to replace VHF walkie talkie's with Iridium PTT and have much better coverage without the inherent maintenance of VHF. PTT can also operate in a data mode for machine applications.

To get into the network side of PTT, lets detail a bit about how it all works. The Iridium network is quite complex as the satellite's orbit above and therefore the spot beams covering you and your neighbor could be on a single spot beam on one satellite, which would be quite easy to broadcast a PTT voice channel. Or you could be on different spot beams on the same satellite, which is still quite easy to repeat on multiple spot beams on the same satellite. Lastly, you could be on different spot beams on different satellites. This is the most complex structure as the satellite's are quite intelligent, but are not a full network and database of the entire network so this is where the ground station comes into play. You always have to start with the most complex situation and work back from there so PTT channels run from the satellite, back to the ground station, and back to neighboring satellite's for re-transmission. There are other applications of PTT for military and specialized cases that do not follow this rule, and we are by far not in the logic of all PTT routing, but in general, commercial PTT use does route the traffic through the ground station.

Iridium Push-to-Talk