Publication Abstract

An Advance in geolocation by light

P.A. Ekstrom

A new analysis of twilight predicts that for observations made in narrow-band blue light, the shape of the light curve (irradiance vs. sun elevation angle) between +3 and -5 degrees (87 to 95 degrees zenith angle) has a particular rigid shape not significantly affected by cloudiness, horizon details, atmospheric refraction or atmospheric dust loading. This shape is distinctive, can be located reliably in measured data, and provides the basis for a new approach to animal geolocation.

The resulting "template fit" approach matches a theoretical model of the irradiance vs. time-ofday to the relevant portion of a given day's data, adjusting parameters for latitude, longitude, and cloudiness. In favorable cases, there is only one parameter choice that will fit well, and that choice becomes the position estimate. The entire process can proceed automatically in a tag.Theoretical estimates predict good accuracy over most of the year and most of the earth, with difficulties just on the winter side of equinox and near the equator. Polar regions are favorable whenever the sun crosses -5° to +3° elevation. Early results based on data taken on land at 48°N latitude confirm the predictions vs. season, and show performance significantly improved over

Reference:

P.A. Ekstrom. 2004. An Advance in geolocation by light. Memoirs of the National Institute for Polar Research, Special Issue, 58: 211-227p