BIO-LOGGING SCIENCE
Memoirs of the National Institute of Polar Research


Memoirs of the NIPR 58: 210-226 (2004)
An advance in geolocation by light
PA 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 a firm theoretical basis for animal geolocation by template-fitting to irradiance data. The resulting approach matches a theoretical model of the irradiance vs. time-of-day 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, and the method can yield useful results whenever the sun makes a significant excursion into that elevation range. Early results based on data taken on land at 48N latitude confirm the predictions vs. season, and show promising performance when compared with earlier threshold-based methods.
Technical report



Memoirs of the National Institute of Polar Research