Passive ranging using atmospheric oxygen absorption spectra

The depth of absorption bands in observed spectra of distant, bright sources can be used to estimate range to the source. A novel approach is presented and demonstrated using observations of the oxygen absorption band near 762 nm. Range is estimated by comparing observed values of band-average absorption against curves derived from either historical data or model predictions. Curves are based on fitting a random band model to the data, which reduces average range error by 67% compared to the Beer's Law model used in previous work. A new modification to existing band models for long, inhomogeneous paths is presented and shown to reduces error 50% in short-range experiments. A static rocket motor test was observed using a Fourier transform spectrometer at a range of 2.8 km. The range estimated from this data was accurate to within 0.5% (14 m). Similar accuracy was also achieved at shorter ranges using a lamp as a surrogate target. Long-range performance is predicted by using FASCODE and theoretical models to extrapolate observed short-range performance. Range error of 5% or less is predicted at ranges up to 400 km for a representative target.