Estimate one location per stationary period from twilight times

For each stationary period (stap_id) with usable twilight events, estimate a single longitude, latitude, and effective zenith angle by minimizing the squared spherical distance to the corresponding iso-zenith small circles.

Each twilight defines a small circle on the Earth: the set of locations at a fixed angular distance (zenith angle) from the subsolar point at that time. The fitted location is the point that is, in a least-squares sense, closest to all those circles simultaneously.

Usage

geolight_fit_location(
  tag,
  fitted_location_duration = Inf,
  extent = NULL,
  zenith_init = 96,
  zenith_bounds = c(60, 120),
  compute_known = FALSE,
  quiet = FALSE
)

Arguments

tag

A GeoPressureR tag object containing tag$twilight and tag$stap.

fitted_location_duration

Minimum duration (in days) of stationary period(s) eligible to be used as a fitted location from twilight times. Default is Inf (disabled).

extent

Numeric vector c(W, E, S, N) in degrees (longitude west/east, latitude south/north).

zenith_init

Initial zenith angle (degrees).

zenith_bounds

Numeric vector of length 2 giving zenith bounds (degrees).

compute_known

Logical; if FALSE, known stationary periods are copied from tag$stap (rather than being estimated).

quiet

Logical; if TRUE, suppress informative messages.

Value

A path data.frame derived from tag$stap with added columns: lon, lat, zenith. Rows where fitting was not possible remain NA.

See also

Other geolight: geolight_map(), geolight_solar(), plot_twl_calib(), twilight_create(), twilight_label_read(), twilight_label_write()

Examples

withr::with_dir(system.file("extdata", package = "GeoPressureR"), {
  tag <- tag_create("18LX", quiet = TRUE) |>
    tag_label(quiet = TRUE) |>
    twilight_create() |>
    twilight_label_read()
})
tag <- tag_stap_daily(tag, quiet = TRUE)
path <- geolight_fit_location(tag, fitted_location_duration = 5, quiet = TRUE)