This dissertation documents the development, calibration, and field testing of the Atmospheric Ionizing Radiation Tissue Equivalent Dosimeter (AirTED), an instrument designed to measure cosmic radiation exposure at aviation altitudes. AirTED measures energy deposition spectra from both high and low linear energy transfer (LET) ionizing radiation using a tissue equivalent proportional counter (TEPC) and a silicon PIN diode, respectively. A novel TEPC detector head was developed to minimize microphonic noise, alongside custom analog signal processing circuits and PCBs. A Red Pitaya single-board computer was fully implemented as a spectrometer. AirTED was characterized at the Los Alamos Neutron Science Center (LANSCE) using a neutron beam with an energy spectrum similar to that at aviation altitudes. AirTED was deployed on a NASA WB-57 high-altitude research aircraft during two flight campaigns. Measurements of absorbed dose showed an average disagreement of 17% (TEPC) and 5% (silicon diode) when compared to estimates from the CARI-7 computer model. SpaceTED, a modified version of AirTED, was launched to the International Space Station in November 2023 and made continuous measurements for over 10 months. Preliminary results indicate measurements from the TEPC and silicon diode can be combined to produce a single comprehensive LET spectrum. Field testing has demonstrated that AirTED meets its design objectives and operates as intended on aircraft and spacecraft.