radio architecture allows easy reconfiguration of the radio for other frequencies such as S- and Ka-band with interchangeable radio frequency (RF) front-end slices. The uplink signal is first digitized by the analog-todigital converter (ADC) after down-conversion and amplitude leveling by the voltage variable attenuator (VVA) in the front-end electronics. The tracking loops implemented on Iris are fully digital second-order phased-lock loops (PLL) with configurable loop bandwidth (LBW) settings. The coherent turn-around approach takes the derived frequency from the uplink carrier signal and feeds this to a direct digital synthesizer (DDS) to tune the downlink phased-lock oscillator (PLO). This approach provides a simple method with fewer components that allows miniaturization while maintaining the necessary frequency agility from the transponder. The main oscillator is a 50-MHz temperature compensated crystal oscillator (TCXO). This reference signal is also used to generate the local oscillator (LO) frequency for block down-conversion in the receive chain. A simplified block diagram of this architecture is shown in Figure 3, and Table 2 lists some key specifications of the Iris V2.1 transponder. RADIO DESIGN This section provides a brief design description of the hardware, software, and firmware elements that comprise the SDR. The hardware elements are broken into the digital processing, the RF front-end electronics, and the power supply. Table 1. SLS EM-1 CubeSat Missions Using the Iris Transponder Figure 1. Iris version 1 (INSPIRE) and version 2 (MarCO) transponder stacks. Mission name Figure 2. Iris V2.1 assembled flight transponder stack. SEPTEMBER 2019 Target destination Max range LunaH-Map Lunar $1 Mkm Lunar IceCube Lunar $1 Mkm Lunar Flashlight Lunar $1 Mkm ArgoMoon Heliocentric N/Ay CubeSat for Solar Particles Heliocentric $15 Mkm BioSentinel Heliocentric $84 Mkm Near Earth Asteroid Scout Asteroid $180 Mkm y Mission details not available. IEEE A&E SYSTEMS MAGAZINE 35