sub gps_week_seconds_day {
  my $self          = shift;
  my $epoch_seconds = $self->gps_seconds_since_epoch;
  my $week          = int($epoch_seconds/Secs_per_week);
  my $seconds       = $epoch_seconds - $week*Secs_per_week;
  my $day           = int($seconds/Secs_per_day);
  ($week, $seconds, $day);
}