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//! Observer state at an instant.
mod light_time;
use crate::{C_SQUARED, Dt, Position, Real, Spacetime, Velocity};
/// An observer at a specific instant.
///
/// Combines time, position, velocity, and local gravitational
/// information. It is the main input type used by relativistic light-time
/// methods in this library.
#[derive(Clone, Copy, Debug, PartialEq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "tsify", derive(tsify::Tsify))]
pub struct Observer {
/// The time of this observer.
///
/// Any [`Scale`](crate::Scale) is accepted. This time is treated as coordinate time.
pub time: Dt,
/// Position of the observer in meters.
///
/// Typically expressed in a barycentric (solar-system barycenter) or
/// heliocentric frame, depending on the application.
pub position: Position,
/// Velocity of the observer in meters per second.
pub velocity: Velocity,
/// Newtonian gravitational potential Φ at the observer’s location
/// (in m² s⁻²).
///
/// This value is usually negative for bound orbits. It should normally
/// include contributions from the Sun and all relevant planets.
pub grav_potential_m2_s2: Real,
/// Characteristic length scale (in meters) over which the gravitational
/// field varies significantly at this location.
///
/// - Use `0.0` (the default) for all solar-system, GNSS, and weak-field
/// applications.
/// - Provide a non-zero value only when working in strong gravitational
/// fields (e.g. near neutron stars or black holes), where the library’s
/// higher-order curvature terms become relevant.
pub characteristic_length_scale: Real,
}
impl Observer {
/// Creates a new `Observer` for typical solar-system, GNSS,
/// or weak-field use.
///
/// This is the recommended constructor for most applications.
/// It sets the `characteristic_length_scale` to `0.0`, which disables
/// higher-order curvature terms in the proper-time model.
///
/// ## Parameters
///
/// - `time`: The time of the observer.
/// - `position`: Position in meters (usually barycentric or heliocentric).
/// - `velocity`: Velocity in m/s.
/// - `grav_potential_m2_s2`: Newtonian gravitational potential Φ
/// at the location (in m²/s²).
#[inline]
pub const fn new(
time: Dt,
position: Position,
velocity: Velocity,
grav_potential_m2_s2: Real,
) -> Observer {
Self {
time,
position,
velocity,
grav_potential_m2_s2,
characteristic_length_scale: 0.0,
}
}
/// Returns the instantaneous proper-time rate `dτ/dt` for this observer.
///
/// This value indicates how fast a physical clock located at this observer
/// would advance relative to the time used by this `Observer`.
/// A returned value of `1.0` means the clock advances at the same rate
/// as the observer's time coordinate. Values are typically slightly different
/// from `1.0` due to the effects of velocity and gravitational potential.
///
/// This rate is computed using the library’s unified proper-time model.
/// It is used internally for light-time corrections and Doppler calculations.
#[inline]
pub const fn proper_time_rate(&self) -> Real {
Spacetime::from_potential_velocity_and_scale(
self.grav_potential_m2_s2 / C_SQUARED,
self.velocity,
self.characteristic_length_scale,
)
.proper_time_rate()
}
/// Returns the ratio of proper time rates between the receiver and transmitter
/// for a one-way signal.
///
/// This method computes:
///
/// ```text
/// ratio = rx.proper_time_rate() / self.proper_time_rate()
/// ```
///
/// ### Interpretation
///
/// - A value of `1.0` indicates that both clocks run at the same rate.
/// - A value **less than `1.0`** means the receiver’s clock runs slower than
/// the transmitter’s clock. The receiver will observe a lower frequency
/// than was emitted.
/// - A value **greater than `1.0`** means the receiver’s clock runs faster
/// than the transmitter’s clock. The receiver will observe a higher frequency
/// than was emitted.
///
/// The ratio captures the combined effect of special-relativistic time dilation
/// (due to velocity) and general-relativistic gravitational time dilation.
///
/// ### Typical Usage (One-Way)
///
/// This ratio is often combined with the classical kinematic Doppler term
/// to estimate the total one-way frequency shift:
///
/// ```text
/// approximate_frequency_shift ≈ ratio * (1 - v_radial / C)
/// ```
///
/// where `v_radial` is the radial velocity (positive when the receiver is
/// receding).
///
/// ### Two-Way Usage
///
/// For round-trip (two-way) measurements, square the one-way ratio:
///
/// ```rust
/// use deep_time::{Dt, Observer, Position, Spacetime, Velocity};
///
/// let bodies = [
/// (Position::from_au(0.0, 0.0, 0.0), 1.3271244e20), // Sun
/// (Position::from_au(1.0, 0.0, 0.0), 3.9860044e14), // Earth
/// ];
///
/// let tx_pos = Position::from_au(1.0, 0.0, 0.0);
/// let rx_pos = Position::from_au(1.00257, 0.0, 0.0);
///
/// let grav_potential_tx = Spacetime::grav_potential_from_point_masses(tx_pos, bodies.iter().copied());
/// let grav_potential_rx = Spacetime::grav_potential_from_point_masses(rx_pos, bodies.iter().copied());
///
/// let transmitter = Observer::new(
/// Dt::span_f(0.0),
/// tx_pos,
/// Velocity::ZERO,
/// grav_potential_tx,
/// );
///
/// let receiver = Observer::new(
/// Dt::span_f(0.0),
/// rx_pos,
/// Velocity::from_speed(800.0),
/// grav_potential_rx,
/// );
///
/// let one_way_ratio = transmitter.relativistic_clock_rate_ratio(receiver);
/// let two_way_ratio = one_way_ratio * one_way_ratio;
/// ```
///
/// **Note:** Squaring the one-way ratio is a common first-order approximation.
/// For higher precision (especially during flybys or when uplink and downlink
/// geometries differ significantly), consider using
/// [`round_trip_light_time_correction`](Self::round_trip_light_time_correction)
/// instead.
///
/// This pattern is commonly used when correcting two-way Doppler (range-rate)
/// data for relativistic clock effects.
///
/// ### Limitations
///
/// - This method only accounts for the **difference in clock rates** between
/// the two ends.
/// - It does **not** include Shapiro delay or higher-order relativistic effects
/// on signal propagation.
/// - The combination with classical Doppler shown above is a first-order
/// approximation.
///
/// ## Parameters
///
/// - `self` — Transmitter state at the time of transmission.
/// - `rx` — Receiver state at the approximate time of reception.
#[inline]
pub const fn relativistic_clock_rate_ratio(&self, rx: Observer) -> Real {
rx.proper_time_rate() / self.proper_time_rate()
}
}
#[cfg(feature = "wire")]
impl Observer {
/// Current wire format version.
pub const WIRE_VERSION: u8 = 1;
/// Size of the canonical wire representation in bytes.
pub const WIRE_SIZE: usize = 1 + Dt::WIRE_SIZE + Position::WIRE_SIZE + Velocity::WIRE_SIZE + 16;
/// Serializes this [`Observer`] into a fixed buffer.
///
/// Layout:
/// - Byte 0: Version
/// - Bytes [1..]: time (Dt wire) + position (24) + velocity (24) + grav_potential (8) + char_length_scale (8)
pub fn to_wire_bytes(&self) -> [u8; Self::WIRE_SIZE] {
let mut buf = [0u8; Self::WIRE_SIZE];
buf[0] = Self::WIRE_VERSION;
let mut offset = 1usize;
let time = self.time.to_wire_bytes();
buf[offset..offset + Dt::WIRE_SIZE].copy_from_slice(&time);
offset += Dt::WIRE_SIZE;
let pos = self.position.to_wire_bytes();
buf[offset..offset + Position::WIRE_SIZE].copy_from_slice(&pos);
offset += Position::WIRE_SIZE;
let vel = self.velocity.to_wire_bytes();
buf[offset..offset + Velocity::WIRE_SIZE].copy_from_slice(&vel);
offset += Velocity::WIRE_SIZE;
buf[offset..offset + 8].copy_from_slice(&self.grav_potential_m2_s2.to_le_bytes());
offset += 8;
buf[offset..offset + 8].copy_from_slice(&self.characteristic_length_scale.to_le_bytes());
buf
}
/// Deserializes an [`Observer`] from exactly `WIRE_SIZE` bytes.
///
/// Returns `None` if the version is unknown or any component is invalid.
///
/// ## Security
///
/// Safe for untrusted input. Fixed size with layered validation of inner types.
pub fn from_wire_bytes(bytes: &[u8]) -> Option<Self> {
if bytes.len() != Self::WIRE_SIZE {
return None;
}
if bytes[0] != Self::WIRE_VERSION {
return None;
}
let mut offset = 1usize;
let time = Dt::from_wire_bytes(&bytes[offset..offset + Dt::WIRE_SIZE])?;
offset += Dt::WIRE_SIZE;
let position = Position::from_wire_bytes(&bytes[offset..offset + Position::WIRE_SIZE])?;
offset += Position::WIRE_SIZE;
let velocity = Velocity::from_wire_bytes(&bytes[offset..offset + Velocity::WIRE_SIZE])?;
offset += Velocity::WIRE_SIZE;
let grav_potential_m2_s2 = Real::from_le_bytes([
bytes[offset],
bytes[offset + 1],
bytes[offset + 2],
bytes[offset + 3],
bytes[offset + 4],
bytes[offset + 5],
bytes[offset + 6],
bytes[offset + 7],
]);
offset += 8;
let characteristic_length_scale = Real::from_le_bytes([
bytes[offset],
bytes[offset + 1],
bytes[offset + 2],
bytes[offset + 3],
bytes[offset + 4],
bytes[offset + 5],
bytes[offset + 6],
bytes[offset + 7],
]);
Some(Self {
time,
position,
velocity,
grav_potential_m2_s2,
characteristic_length_scale,
})
}
}