# On the Energy Performance of Iridium Satellite IoT Technology
## 4 Current Consumption Model for Iridium Satellite IoT  
This section presents a current consumption model of a device that periodically transmits a message (e.g., a sensor reading) with payload size \(L\), once every time interval, \(T\), by means of the Iridium SBD service. We assume that the device is by default in sleep mode in order to save energy, except when it executes the operations required for message transmission. Based on the current consumption model, the section also shows how the device battery lifetime and energy efficiency of data delivery can be calculated. In order to provide a realistic model, we performed current consumption measurements on a real device hardware platform, called RockBLOCK Mk2 (Rock Seven Mobile Services, Whiteley, UK). This platform uses the Iridium 9602 module, and supports the Iridium SBD service, with a maximum transmitted message size of 340 bytes. The transmit power used by the device is 32 dBm. Note that the device includes a supercapacitor, which acts as an energy reservoir and allows to buffer the highly pulsed nature of the device internal circuits. Figure 2 illustrates the experimental setup used for the measurements. The RockBLOCK device (left) was controlled via AT commands from a computer, while an Agilent  
Technologies N6705A power analyzer (right) was used to measure the current consumption of the device. Measurements were performed in an outdoor scenario, in line-of-sight conditions (i.e., without obstacles between the RockBLOCK device and the Iridium satellites). Figure 3 shows the location of the RockBLOCK device, and the rest of the experimental setup, in the mentioned outdoor scenario. We next provide the current consumption pattern and main characteristics for the considered Iridium IoT device for a successful message transmission, for \(L=1\) byte (see Figure 4). The device is initially in a sleep state, then it becomes active to perform a successful message transmission, and finally, it returns to the sleep state. Remarkably, while the device is active, it spends an average of 6.0 s involved in tasks that do not correspond to payload transmission such as device processing, protocol overhead, etc. This time is significant, even when compared with the nominal data transmission time of up to 2.72 s for the maximum payload size allowed in SBD (i.e., 340 bytes). Recall that, as mentioned in Section 3, despite the theoretical user capacity of 1.78 kbit/s, Iridium SBD offers an actual user capacity of 1 kbit/s. Payload transmission time is determined as the payload size in bits divided by the user capacity of 1 kbit/s. On the other hand, the device consumes a greater amount of current after transmission has finished, during which the device returns to the sleep state and its current consumption decreases slowly. Such a slow decrease is due to the supercapacitor included in the device. The average current consumption of the considered successful message transmission (computed since  
Figure 3: Outdoor scenario used for the current consumption measurements of the RockBLOCK Mk2 Iridium IoT device. Figure 2: Experimental setup for measuring the current consumption of the RockBLOCK Mk2 Iridium IoT device. the device receives an AT command to start message transmission, until 3 min later), denoted as \(I_{OK}\), is 7.66 mA. The current consumption of the device when it returns to sleep mode tends asymptotically to be 77.0 \(\mu\)A, after more than 24 h in the absence of communication activity. We next focused on the current consumption behavior of the Iridium IoT device when it performs a message transmission in apparently good visibility conditions, but the message cannot be successfully delivered, for \(L=1\) byte (Figure 5). In line-of-sight conditions, such an event may mainly occur due to an intersatellite handover or due to contention for communication resources.