What is Rattlegram in Amateur Radio?

When I looked at the voice repeater schedule of IO-86 satellite for this weekend, I noted that there are several special voice repeater schedules for Rattlegram, being the tenth anniversary of the satellite. What is Rattlegram? It is a free mobile application available for both Android and iOS platforms, which can send text messages over radio waves using handheld transceivers. In that way, the combination of radio and mobile phone will function like a digital-walkie talkie even if there is no digital mode available in the radio. You may be aware that usual digital modes in amateur radio utilise special cables and sound card based devices to encode the audio needed for digital modes. Some radios have that facility partially built-in so that they can be connected to a computer with software like WSJT-X installed with USB cable.

In case of Rattlegram, the app encodes text into an audio signal which sent through the radio. At the receiving end, the audio from the radio is picked up by another phone with the Rattlegram app and decodes the message. The technology is called COFDM meaning coded orthogonal frequency-division multiplexing. It allows text messaging without the need for a cable or internet connection. Hence it is suitable for emergency communications when internet service is not available, as in case of natural disasters. Rattlegram is built on open-source software, with the modem implementation and code available on GitHub. Rattlegram can send UTF-8 (Unicode Transformation Format – 8-bit) text messages with up to 170 bytes over audio in about a second.