Data transmission

Scientists who observe animals using transmitters have high telephone bills. The reason: to date, the transmitters have been transmitting their measurements as SMS messages via the GSM mobile radio standard. Mobile tariffs, which also include roaming in Africa, are expensive but cannot be avoided if researchers want to track animals such as fruit bats, storks and cuckoos in Africa.

The transmitters send their data to the receiver station in outer space and receive command signals from it (A). The receiver communicates with a control centre in Moscow (B).

With Icarus, data transmission costs will at least be less expensive in the future. This is because the new Icarus transmitters will no longer be sending their measurements via the mobile network but to a separate receiver station on board the International Space Station ISS. A data packet is approximately 220 bytes in size and will be transmitted in three and a half seconds. The data will be coded using the so-called CDMA method. With this technology, several data streams can be transmitted simultaneously in a common frequency range. The mini-transmitters not only forward their sensor data, they can also store several gigabytes of data. To do this, however, the transmitter must first be collected again.

Travelling in a vertical direction, the data packets reach the receiver station located hundreds of kilometres away in space. In a horizontal direction, in contrast, the data packets are not only weakened by the atmosphere but are also absorbed by the vegetation and the landscape. This means that the signals from the transmitters can only be captured on Earth over short distances and also only within the transmission window that lasts a few seconds. The data is therefore transmitted in unencrypted form although a simple encryption is technically possible.

The transmitters have a built-in time switch. This recognises the approximate intervals at which the receiver appears in the sky above the transmitter and signals when it approaches. The radio receiver on the transmitter then awakens and checks at regular intervals whether it is receiving the transmission signal that the ISS is constantly sending in the direction of earth. The transmitter can use this signal and its own GPS data to calculate the path of the ISS and thus determine the moment at which it will be possible to exchange data. Until then, the transmitter falls back into a deep sleep.

The transmitters are in standby mode for most of the time (1). It switches on at certain times and checks whether the signal from the ISS is being received (2, 3). It then calculates when it can send its data (4). While it is in the ISS reception window it transmits its data (5). The ISS confirms when it has received the transmitter’s signals (6). The transmitter then goes back into its energy-saving mode (7).

At the determined time, it awakens again and transmits its data to the receiver station out in space. If necessary, the ISS can adapt the configuration of the transmitter via radio signals. Each transmitter generally transmits its data once a day to the ISS. The ISS stores the data and transmits it at the next radio contact with the ground station to the Russian control centre in Moscow. The control centre forwards the data to the Icarus user data centre where the data is processed and made available to scientists in the Movebank database. As a result, more than 24 hours can pass between measurement and publication in Movebank.

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