Communication between unmanned systems and their ground control station (GCS) is one of the most important considerations one must make for successful unmanned systems operations. Most unmanned systems today use communications in the radio frequency portion of the electromagnetic spectrum, between 3 kHz and 300 GHz. In some wireless communication cases, the frequency used by the communication system is limited by the frequency allocations made by national governments (Mazar, 2008). However, the exact communication frequency of each unmanned system can be determined by the medium in which the frequency will be traveling in. When choosing the frequency of communications, looking at the medium that the waves must travel through is important in order to make sure that the communications you send make it to their intended target with as little disruption as possible. This includes accounting for things like reflection, diffraction, scattering, or other disturbance.Transmission Medium – Microfilament Transmission LineOne of the most reliable transmission mediums for communications of unmanned systems is through transmission lines. The use of a transmission line in communications eliminates the loss of amplitude that radio waves would otherwise suffer from in free space (Coleman, 2004). However, transmission lines are ineffective if the receiver is too far away from the source or when the line is made out of a material that is not a good conductor of radio waves. An operator of a system can compensate for the weaknesses of transmission lines if they know the exact properties of their line so they can adjust their frequency accordingly if need be.CyPhy’s PARCOne particular unmanned aerial vehicle (UAV) that makes use of a transmission line for communications in the Persistent Aerial Reconnaissance Communications platform (PARC) developed by CyPhy shown in Figure 1. The PARC is a commercial quadrotor UAV that makes use of a tether for communications and power supply that is similar to a tether system on an unmanned underwater vehicle (UUV) (Rees, 2012). The PARC’s transmission line is made from a patented microfilament technology that “carries power and Ethernet communication between the GCS and PARC vehicle” (Ascent Vision, n.d.). The direct connection of the UAV to the GCS ensures that the communication line is secure and limits the abilities of external interference of communications by jamming or spoofing (Rees, 2012). The use of the microfilament tether for communications is also reliable, ensuring that communications do not experience a delay between the GCS and the PARC UAV. Transmission Medium – Air/VacuumLikely, the most common of transmission mediums is Earth’s atmosphere. Although, transmitting RF through Earth’s atmosphere can present several unique challenges for unmanned systems communications. One of the problems encountered when transmitting through the atmosphere are the effects of atmospheric distortion. The two layers of Earth’s atmosphere that tend to cause the most disturbance to communications are the ionosphere for frequencies below 3 GHz and the troposphere for frequencies above 3 GHz (Ippolito, 2008). For instance, on a rainy day, a radio signal above 3 GHz might see more signal loss in the atmosphere than on a clear day due to the interaction between the water droplets and the waves.JunoAn example of an unmanned system that communicates through free space is the Juno spacecraft. Launched by the National Aeronautics and Space Administration (NASA) on August 5, 2011, Juno is an interplanetary mission to Jupiter to study its atmosphere and magnetosphere (Greicius, 2017). In order to communicate with Earth, Juno utilizes NASA’s Deep Space Network (DSN), a set of radio antennas located in three strategic places on Earth. Communications between the DSN and Juno are done in the X-band between 7 and 11.2 GHz. These X-band transmission travel at the speed of light, however, due to the massive distance between Earth and Jupiter it takes the signal approximately 48 minutes to travel one way (Lakdawalla, 2016). Because there is a delay in communications, any maneuvers or changes to the satellite that must be communicated are planned out in advance unless it becomes necessary to make immediate decisions for the health of the spacecraft. All transmissions from Juno are made using one of its five antennas: “the high-gain antenna (HGA), medium-gain antenna (MGA), toroidal low-gain antenna (TLGA), and two low-gain antennas (LGAs)” (Mukai, Hansen, Mittskus, Taylor, Danos, & Kwok, 2017) which are shown in Figure 2. Transmission Medium – WaterBy far one of the most difficult mediums to communicate in is water. There are several problems with underwater communications including reflection, refraction, and energy dispersion (Lloret, Sendra, Ardid, &Rodrigues, 2012). In addition to these problems, communications underwater often occur at lower frequencies, which means that there is a lower data rate than the typical unmanned system in an alternate medium such as air (Lloret et al., 2012).YellowfinThe Yellowfin is an unmanned underwater vehicle (UUV) developed by the Georgia Tech Research Institute (GTRI) for autonomous collaborative observation and monitoring of the ocean (West et al., 2010). The Yellowfin UUV communicates using several different methods. For long-range underwater communications, the Yellowfin uses primarily acoustic communications which are made possible by an acoustic modem made by The Woods Hole Oceanographic Institution (WHOI) called the WHOI micro-modem (Bogle, Melim, & West, 2011). The WHOI micro-modem “allows the Yellowfin to communicate underwater over fairly long distances at very low bandwidth” (West et al., 2010). Additionally, the Yellowfin can effectively communicate underwater using RF at medium distances (West et al., 2010). Due to the nature of acoustic communications, signals are subject to multi-path scattering which can result in signals reflected off of the ocean surface or seafloor reaching the receiver before the direct signal (Bogle, Melim, & West, 2011). However, Bogle, Melim, & West (2011) note the problem of multi-path scattering in acoustic communications lessens as the vehicle’s depth and range from the receiver increase.