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48 finding a way to avoid the problems with capacitative noise between the VCSELs that limit the power in an array. Their technique allows four arrays to be used together, each with a different frequency, in a single beam with an overall power consumption of just a few watts. The link budget for 20 km is -40 dBm in clear weather, which provides a very effective mechanism for the data transfer. However, the optical link suffers from much higher attenuation, with rain or fog rendering the link impractical in some geographical regions or potentially at certain times of the year, such as during the monsoon season in south-east Asia. The beam spread at 20 km is around 6 m, which for the downlink makes for a very compact receiver. The challenge is that the uplink also has to potentially accept a 6 m wide beam, which could be a problem on the Solara. The FSO transceiver would also have to accurately track the position of the ground station to ensure a connection, so the control of the transceiver mounting has to be closely coupled with the autopilot software and the sensors on the craft. The transceiver mounting control would also have to be tightly coupled to the variations in the signal strength, presenting a networking challenge within the aircraft as the signal strength would have to be part of the control loop of the mount positioning and potentially of the aircraft control system. Titan even sees the Solara 50 being used to provide cellphone coverage for an area of more than 6500 sq miles (16,800 sq km), replacing more than 100 base stations. How this relates to the number of simultaneous users and the available power from the base station has yet to be determined, and this will have an impact on the area coverage, the height, the algorithms for controlling the flight path and the backhaul capacity. At the moment, however, the project at TriLumia is on hold, as the company is looking to use its VCSEL technology for Lidar remote sensing applications in vehicles, and is working with developers of driverless cars to use it as a key sensor (see Platform one). Nonetheless, with funding, the company is open to continuing the development of the technology. Power management within the Solara 50 is key, and complex management algorithms are needed to ensure that the power from the solar cells can be stored in the batteries and delivered to the propulsion units, communications links and to the base station. Intelligent integrated energy harvesting power management devices have been designed to acquire and manage the microwatts and milliwatts generated from dc sources such as solar cells. This starts with a highly efficient dc-dc boost converter/charger that requires only microwatts of power to begin operation, and so can capture the maximum amount of light available. Once started up, it extracts the power from low-voltage output devices such as single- or dual- cell solar. The boost converter can be switched on with an input voltage as low as 330 mV, and once running it can continue to harvest energy down to an input voltage of 80 mV. The heart of this process is the programmable maximum power point tracking sampling network, which optimises the transfer of power into the power management chip. For solar cells that operate at a maximum power point transfer (MPPT) of 80% of their open- circuit voltage, a resistor divider can be set to 80% of the dc input voltage and the network will control the input voltage to operate near the ideal reference voltage. That’s the simplest version; an alternative is to have the external Rain or fog can render the link impractical in some regions or times of the year, such as the monsoon season in Asia The free space optical link used in TriLumina’s laser relies on a collection of technologies November 2014 | Unmanned Systems Technology