## 2 Materials and Methods
### Design
#### 2.1.1 Hardware
In v1.0 of the Cryologger, an MPL3115A2 (NXP Semiconductors, Eindhoven, The Netherlands) temperature (\(\pm 1\)\({}^{\circ}\)C) and pressure (\(\pm 0.1\) kPa) sensor was included to measure conditions within the enclosure. In v2.0, to reduce the overall component count, the internal temperature sensor of the RTC (\(\pm 3\)\({}^{\circ}\)C) was used instead to monitor conditions inside the enclosure. Data telemetry is achieved using the RockBLOCK 9603 transceiver (Ground Control, Las Vegas, NV, USA), which is based on the Iridium 9603N module (Iridium Communications, McLean, VA, USA). An external helical antenna (M1621 HCT-P-SMA; Maxtena, Germantown, MD, USA) was used in v1.0 of the beacon to achieve a better signal-to-noise ratio under adverse transmission conditions. In v2.0, it was determined that the RockBLOCK integrated patch antenna was suitable for this use case, which helped to reduce overall costs. Power is provided to the Cryologger from a 7.4 V 38 Ah (273.6 Wh) lithium-thionyl chloride (LiSOCl2) battery pack (TLP-93121-B-AL1; Tadiran Batteries, Lake Success, NY, USA). The LiSOCl2 chemistry was selected because it is specifically engineered for operation in extremely cold conditions as low as \(-40\)\({}^{\circ}\)C. This battery pack also contains hybrid-layer capacitors that absorb power spikes from the Iridium transceiver during transmissions, which prevents damage to the cells and prolongs the operational life of the pack. However, the disadvantages of this chemistry are its cost, and that it is classified as a dangerous good and must be transported according to stringent regulations [37]. Battery voltage is monitored using a simple resistive voltage divider connected to one of the microcontroller's analog inputs. A step-down voltage regulator (Pololu Corporation, Las Vegas, NV, USA) is used to convert the voltage to 3.3/5 V to be compatible with Arduino electronics. The power consumption of the Cryologger is approximately 40 mW while idle and 130 mW during GNSS signal acquisition, and it can peak as high as 2.25 W while transmitting data via the RockBLOCK transceiver. When in deep sleep, power consumption decreases to approximately 1 mW. Once deployed, the Cryologger ITB has a maximum lifespan of up to 4 years at a 1-h sampling and 3-h transmission schedule (i.e., 35,000 measurements and 12,000 transmission cycles; see Section 3). However, the effective lifespan is dependent on several factors, including whether the RockBLOCK transceiver has an obstructed view of the sky, which can result in much longer transmission times and greater power consumption. When combined with extremely cold temperatures, further reductions in power efficiency can occur and lead to premature battery depletion. To protect the electronics from the elements, the ITBs are housed in rugged Nanuk cases (Plasticase, Terrebonne, QC, Canada) (Figure 1b). These cases are impact resistant, have an IP67 rating against water ingress, and are rated to temperatures as low as \(-30\)\({}^{\circ}\)C. In v1.0, the electronics were also placed in a 3D-printed case for extra protection, which was not possible in v2.0 due to time constraints before deployment. To improve the grip of the case on the ice surface and prevent the beacons from being blown off the iceberg by the wind, a simple solution of wooden boards with screws drilled into them was affixed to the bottom of the case with rope. The wood also provides added buoyancy if the beacon falls into the water and acts as an insulating layer between the case and the ice surface.