Monitoring and attributing changes in Earth climate requires high-radiometric-accuracy spectral measurements of sunlight reflected from the Earth's surface. The HyperSpectral Imager for Climate Science (HySICS) is a new imaging spectrometer intended to ultimately achieve ~ 0.2 % radiometric accuracies, being ~ 10 × better than existing spaceflight instruments. We describe the results from a high-altitude balloon flight demonstrating techniques intended to meet these high-accuracy requirements.