Satellite Geolocation for Animal Tracking

Description Usage Arguments Details Value See Also

Estimate location from consecutive twilights

thresholdEstimate(trise, tset, zenith = 96, tol = 0)

thresholdLocation(twilight, rise, zenith = 96, tol = 0.08)

thresholdPath(twilight, rise, time = twilight, zenith = 96, tol = 0.08,
  unfold = TRUE)

`trise`	vector of sunrise times.
`tset`	vector of sunset times.
`zenith`	the solar zenith angle that defines twilight.
`tol`	tolerance on the sine of the solar declination.
`twilight`	the observed times of twilight as POSIXct.
`rise`	logical vector indicating which twilights are sunrise.
`time`	times for which locations are required.
`unfold`	if `TRUE`, unfold longitudes across the dateline.

These functions estimate the location of a stationary observer given the times at which the observer sees two successive twilights. thresholdEstimate estimates locations given pairs of times of sunrise and sunset. thresholdLocation is a wrapper for thresholdEstimate that estimates locations given a sequence twilight times and rise indicators, while thresholdPath interpolates the estimates generated by thresholdLocation to give locations at a sequence of arbitrary times.

Longitude is estimated by computing apparent time of local noon from sunrise and sunset, and determining the longitude for which this is noon. Latitude is estimated from the required zenith and the sun's hour angle for both sunrise and sunset, and averaged.

When the solar declination is near zero (at the equinoxes) latitude estimates are extremely sensitive to errors. Where the sine of the solar declination is less than tol, the latitude estimates are returned as NA.

The thresholdPath function interpolates the estimates generated by thresholdLocation to produce estimates at the arbitrary set of times specified the by the time argument. If time is NULL, thresholdPath returns the estimates generated by thresholdLocation. If unfold=TRUE, thresholdPath attempts to construct a continuous path that does not wrap longitudes into (-180,180]. However, this process can fail if the observer crosses the dateline near an equinox, and it may be necessary to make manual adjustments.

These functions provides the same basic functionality of the coord function from GeoLight, but are based on different astronomical approximations.

thresholdEstimate returns estimated locations as a two column (lon,lat) matrix. thresholdLocation and thresholdPath return a list with components