The Tomorrow.io Pathfinder Mission: Software-Defined Ka-band Precipitation Radar in Space

As the first stage in the developmentof a precipitation mapping constellation of microwave radars and sounders,Tomorrow.io is launching two Pathfinder spacecraft, Tomorrow-R1 andTomorrow-R2, each outfitted with a novel Ka-band precipitation radar payloadbased on a software-defined radar (SDR) intermediate frequency (IF)transceiver. The radar transmitter is tunable throughout the allocated 35.5-36GHz active transmission band, and features an all-solid-state transmittercapable of 42 dBm saturated power output at 30% duty cycle, which is coupled toa solid 1.2 m Cassegrain reflector antenna system. Capable of 500 MHz ofinstantaneous bandwidth, the SDR allows for pulse-to-pulse reconfigurabilityof the transmit waveform and digital receiver parameters, offering uniquesampling capabilities for precipitation sensing as well as the ability tooperate in a wideband altimetry mode. Here we provide an overview of thePathfinder mission and development approach, placing it in the context of thebroader spaceborne cloud and precipitation radar program of record, as well asrelevant details around the instrument design, capabilities, sampling approach,and estimated sensitivity. For the planned nominal precipitation samplingparameters, the system will feature a range resolution of 240 m, a horizontalsampling resolution of 2.4 km, and a high-SNR reflectivity precision of 0.1 dB,while the absolute calibration accuracy goal is 1 dB. The so-called minimumdetectable reflectivity $(\mathbf{dB}Z_{\mathbf{min}})$ metric is argued here to be dependent on themaximum allowable random measurement error, with a calculated value of 8.2 dB $Z$ for a maximum reflectivity error of 2.2 dB.