Unmanned Systems Technology 003 | UAV Solutions Talon 120 | Cable harnesses | Austro Engine AE50R and AE300 | Autonomous mining | AUVSI 2015 show report | Transponders | Space systems

70 Focus | Transponders an aircraft. They must also be able to tolerate the extreme operating conditions as a result of the low temperatures at high altitudes, as well as the g -forces acting on them during aircraft manoeuvres. Safety requirements also call for UAV transponders to have a high level of reliability, which has to translate into an extremely low failure rate. Another important safety requirement is resistance to EMI. These requirements for UAV transponders can often make such subsystems comparatively more expensive to develop and procure than those used by ground and sea vehicles, although the adoption of COTS technology does hold increasing promise in this regard. For air transponders, they must be sensitive to weak interrogation signals because the signals diminish in strength the further they travel from the transmitter. A UAV transponder must therefore be able to discriminate between the interrogation signal and the background electronic noise. At the same time, a UAV transponder has to be powerful enough to return a signal that can be detected and recognised by the interrogator. In the ground vehicle domain, transponders are less constrained by the SWaP requirements for UAV versions, increasing the feasibility of adopting COTS technology for them. Such solutions can include the use of commercially available military or civilian radios, which can then be integrated into the vehicle and housed in a ruggedised custom chassis to protect the transponder from the effects of vibration and shock as the vehicle moves across rough terrain. The chassis should Summer 2015 | Unmanned Systems Technology Transponders enable UAVs to be tracked using radar and satellite (Courtesy of Micro Systems) UAV transponders can often be more expensive to develop than those used by ground and sea vehicles, although COTS technology has promise here and therefore increase the range. Moreover, VHF and UHF radio links can be relatively ‘data heavy’, allowing a transponder to handle a lot of information. For example, UHF transponders can carry of the order of 230 kbit/s worth of data. Satellite communications (Satcom) is also proving attractive for transponder technology. It is already used in the UAV domain, and may be adopted for ground- and sea-based vehicles. Satcom systems use a number of specific frequencies – typically from 5.925 GHz up to 40 GHz – and, like VHF/UHF transponders, Satcom transponders can carry a lot of data depending on the frequency used. However, specific unmanned vehicle frequency bands are not fixed, so for example the section of the radio spectrum used in the US for UHF transponders may not be the same as that in another nation. This can cause challenges when military unmanned vehicles are deployed abroad, so one solution is to give the transponders the ability to use a number of different radio channels within a specific segment of radio spectrum. Once the vehicle is deployed, these can be configured by the user to ensure they meet with local requirements. Similarly, the spectrum itself can become congested by other users of comms systems, so a lot of transponder designs include a frequency-hopping function, which allows them to switch between parts of the spectrum within the segment in order to use parts of the spectrum that are not already occupied. The use of digital radio techniques, which rely on software, greatly eases this process, with the user being able to program the transponder to this end. Design requirements In terms of design considerations, requirements vary according to whether a transponder is for an airborne, ground or sea vehicle. As with all airborne subsystems, UAV transponders must meet stringent standards to ensure they can be safely operated on board