Sovremennye problemy distantsionnogo zondirovaniya Zemli iz kosmosa, 2015, Vol. 12, No. 2, pp. 61-76
Potential of Earth limb navigation measurements in the visible and near-IR spectral range
B.S. Zhukov
1 , S.B. Zhukov
1 , A.A. Forsh
1 1 Space Research Institute RAS, Moscow, Russia
Horizon sensors can be used both for s/c attitude and position determination. Autonomous horizon navigation can be utilized to back-up the GLONASS/GPS systems in Earth orbits and to substitute them in orbits around other bodies of the Solar system. Present-day space-borne horizon sensors work mostly in the thermal IR spectral range. Horizon sensors of the visible and near infrared spectral range (VNIR: 0.4–1 μm) could be significantly simpler and cheaper. The Earth horizon is not visible from space in the VNIR spectral range due to large atmospheric optical thickness and is often covered by clouds. Therefore a stable definition of a conventional horizon on the Earth limb is required. MODTRAN atmospheric transfer simulations as well as processing of GOES-East images suggest a possible definition of the conventional horizon in the VNIR as the altitude of the half-maximal radiance of the limb. Though the conventional horizon is most stable in the blue spectral range, utilization of longer wavelength VNIR parts may be desirable, allowing simultaneous navigation by landmarks. The key parameter of space-borne horizon sensors, which has the strongest effect on s/c position estimation accuracy, is their field of view that should be selected as large as possible, ideally covering the whole Earth, even at the expense of resolution. The mean altitude of the conventional horizon in the 0.5–0.7 mcm spectral range was found to be equal to 19 km. The error (1σ) of VNIR horizon sensors in horizon altitude as well as in s/c position measurement in orbits below 1000 km was estimated as 2–3 km when the sensor resolution at the limb is in the range of 1 to 10 km. The altitude error in geostationary orbits increases to 20 km. Measurement accumulation along the orbit can provide a significantly better orbit determination accuracy.
Keywords: autonomous space navigation, horizon navigation, horizon sensors, Earth limb, conventional horizon
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