astrodynamics-gnss 0.9.0

//! Broadcast-store selection and SPP source adapter.

use crate::broadcast::satellite_state;
use crate::constants::{
    BDS_EPOCH_MINUS_GPS_EPOCH_S, GPST_MINUS_BDT_S, GPS_EPOCH_TO_J2000_S, SECONDS_PER_WEEK,
};
use crate::glonass;
use crate::id::{GnssSatelliteId, GnssSystem};
use crate::spp::EphemerisSource;

use super::{
    is_beidou_geo, parse_glonass, parse_iono_corrections, parse_leap_seconds, parse_nav,
    BroadcastRecord, GlonassRecord, IonoCorrections, NavMessage, NavParseError, GLONASS_MAX_AGE_S,
    MAX_EPHEMERIS_AGE_S,
};

/// A queryable set of parsed broadcast records, usable as an SPP
/// [`EphemerisSource`].
///
/// For a satellite and epoch it selects the record whose reference time `toe` is
/// nearest in **continuous** GPS time (week number times the week length plus
/// the seconds of week, not the seconds of week alone), and rejects the query as
/// having no ephemeris if it falls outside that record's validity window — half
/// the broadcast GPS curve-fit interval, or the coarse [`MAX_EPHEMERIS_AGE_S`]
/// fallback for systems that do not broadcast one — so a stale or wrong-week
/// product cannot silently produce a position.
///
/// [`from_nav`](BroadcastStore::from_nav) applies a default usability policy:
/// only healthy GPS LNAV and healthy Galileo I/NAV records are kept (F/NAV and
/// unhealthy satellites are dropped). [`new`](BroadcastStore::new) keeps records
/// verbatim for callers that want their own policy.
pub struct BroadcastStore {
    records: Vec<BroadcastRecord>,
    glonass: Vec<GlonassRecord>,
    leap_seconds: Option<f64>,
    iono: IonoCorrections,
}

impl BroadcastStore {
    /// Build a store from already-parsed Keplerian records, verbatim (no policy
    /// filter, no GLONASS records, no leap-second offset, and no ionosphere
    /// coefficients; use [`from_nav`](Self::from_nav) to capture those).
    pub fn new(records: Vec<BroadcastRecord>) -> Self {
        Self {
            records,
            glonass: Vec::new(),
            leap_seconds: None,
            iono: IonoCorrections::default(),
        }
    }

    /// Parse a RINEX 3.x/4.xx navigation file and keep the records usable for
    /// single-frequency positioning: healthy GPS LNAV, Galileo I/NAV, BeiDou
    /// D1/D2, and healthy GLONASS. The header's broadcast ionosphere coefficients
    /// (see [`iono_corrections`](Self::iono_corrections)) and leap-second offset
    /// are captured.
    pub fn from_nav(text: &str) -> Result<Self, NavParseError> {
        let records = parse_nav(text)?
            .into_iter()
            .filter(Self::is_default_usable)
            .collect();
        let glonass = parse_glonass(text)?
            .into_iter()
            .filter(|r| r.sv_health == 0.0)
            .collect();
        Ok(Self {
            records,
            glonass,
            leap_seconds: parse_leap_seconds(text),
            iono: parse_iono_corrections(text),
        })
    }

    /// The broadcast ionosphere coefficients parsed from the navigation header
    /// (GPS `GPSA`/`GPSB` and BeiDou `BDSA`/`BDSB`). Empty for a store built with
    /// [`new`](Self::new).
    pub fn iono_corrections(&self) -> IonoCorrections {
        self.iono
    }

    /// The held GLONASS records.
    pub fn glonass_records(&self) -> &[GlonassRecord] {
        &self.glonass
    }

    /// The default usability policy: healthy and a single-frequency-appropriate
    /// message — GPS LNAV, Galileo I/NAV (the E1 message), and BeiDou D1/D2;
    /// Galileo F/NAV and unhealthy satellites are excluded.
    fn is_default_usable(r: &BroadcastRecord) -> bool {
        r.sv_health == 0.0
            && matches!(
                r.message,
                NavMessage::GpsLnav
                    | NavMessage::GalileoInav
                    | NavMessage::BeidouD1
                    | NavMessage::BeidouD2
            )
    }

    /// The held records.
    pub fn records(&self) -> &[BroadcastRecord] {
        &self.records
    }

    /// Keep only the records matching a predicate (e.g. a custom message/health
    /// policy on a store built with [`new`](BroadcastStore::new)).
    pub fn retain(&mut self, keep: impl FnMut(&BroadcastRecord) -> bool) {
        self.records.retain(keep);
    }

    /// Continuous GPS time (seconds since the GPS epoch) of a record's `toe`.
    fn toe_continuous_s(rec: &BroadcastRecord) -> f64 {
        f64::from(rec.week) * SECONDS_PER_WEEK + rec.elements.toe_sow
    }

    /// The half-validity window (seconds either side of `toe`) for a record: half
    /// the broadcast GPS fit interval, or [`MAX_EPHEMERIS_AGE_S`] when no fit
    /// interval is broadcast (Galileo, BeiDou).
    fn half_window_s(rec: &BroadcastRecord) -> f64 {
        match rec.fit_interval_s {
            Some(fit) => fit / 2.0,
            None => MAX_EPHEMERIS_AGE_S,
        }
    }

    /// The record for `sat` whose `toe` is nearest `t_continuous_s` (seconds
    /// since the GPS epoch) **among those whose validity window covers the
    /// query** (see [`half_window_s`](Self::half_window_s)). Filtering by
    /// validity before choosing the nearest means a query just past one record's
    /// fit interval can still be served by a farther record whose own window is
    /// wide enough, rather than being rejected outright.
    fn select(&self, sat: GnssSatelliteId, t_continuous_s: f64) -> Option<&BroadcastRecord> {
        self.records
            .iter()
            .filter(|r| r.satellite_id == sat)
            .filter(|r| {
                (t_continuous_s - Self::toe_continuous_s(r)).abs() <= Self::half_window_s(r)
            })
            .min_by(|a, b| {
                let da = (t_continuous_s - Self::toe_continuous_s(a)).abs();
                let db = (t_continuous_s - Self::toe_continuous_s(b)).abs();
                da.partial_cmp(&db).unwrap_or(core::cmp::Ordering::Equal)
            })
    }

    /// The GLONASS record for `sat` nearest the GPST-aligned query `t_j2000_s`
    /// (within [`GLONASS_MAX_AGE_S`]), with `tk` = query − the record's reference
    /// epoch in GPS time. Returns `None` if no leap-second offset was parsed (the
    /// GLONASS UTC epoch then cannot be placed on the GPST timeline).
    fn select_glonass(
        &self,
        sat: GnssSatelliteId,
        t_j2000_s: f64,
    ) -> Option<(&GlonassRecord, f64)> {
        let leap = self.leap_seconds?;
        let toe_gpst = |r: &GlonassRecord| r.toe_utc_j2000_s + leap;
        let rec = self
            .glonass
            .iter()
            .filter(|r| r.satellite_id == sat)
            .min_by(|a, b| {
                let da = (t_j2000_s - toe_gpst(a)).abs();
                let db = (t_j2000_s - toe_gpst(b)).abs();
                da.partial_cmp(&db).unwrap_or(core::cmp::Ordering::Equal)
            })?;
        let tk = t_j2000_s - toe_gpst(rec);
        if tk.abs() <= GLONASS_MAX_AGE_S {
            Some((rec, tk))
        } else {
            None
        }
    }
}

impl core::str::FromStr for BroadcastStore {
    type Err = NavParseError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        Self::from_nav(s)
    }
}

impl EphemerisSource for BroadcastStore {
    fn position_clock_at_j2000_s(
        &self,
        sat: GnssSatelliteId,
        t_j2000_s: f64,
    ) -> Option<([f64; 3], f64)> {
        // GLONASS is not Keplerian: integrate its broadcast state vector with the
        // RK4 propagator. Its reference epoch is UTC, mapped onto the GPST-aligned
        // query via the parsed leap-second offset.
        if sat.system == GnssSystem::Glonass {
            let (rec, tk) = self.select_glonass(sat, t_j2000_s)?;
            let state0 = [
                rec.pos_m[0],
                rec.pos_m[1],
                rec.pos_m[2],
                rec.vel_m_s[0],
                rec.vel_m_s[1],
                rec.vel_m_s[2],
            ];
            let state = glonass::propagate(state0, rec.acc_m_s2, tk);
            let clock = glonass::clock_offset_s(rec.clk_bias, rec.gamma_n, tk);
            return Some(([state[0], state[1], state[2]], clock));
        }

        // Supported Keplerian systems only; a record from any other system (e.g.
        // QZSS/SBAS) reports no ephemeris rather than being evaluated with the
        // wrong model. (`from_nav` already restricts records, but `new` accepts
        // arbitrary ones.)
        if !matches!(
            sat.system,
            GnssSystem::Gps | GnssSystem::Galileo | GnssSystem::BeiDou
        ) {
            return None;
        }

        // Map the receive instant (J2000, GPST-aligned) onto the satellite
        // system's continuous time and seconds of week. BeiDou runs on BDT
        // (= GPST - 14 s) with its week epoch 1356 weeks after the GPS epoch, and
        // its geostationary satellites take the GEO orbit branch.
        let gpst_continuous = t_j2000_s + GPS_EPOCH_TO_J2000_S;
        let (t_continuous, is_geo) = if sat.system == GnssSystem::BeiDou {
            (
                gpst_continuous - GPST_MINUS_BDT_S - BDS_EPOCH_MINUS_GPS_EPOCH_S,
                is_beidou_geo(sat),
            )
        } else {
            (gpst_continuous, false)
        };

        let rec = self.select(sat, t_continuous)?;
        let sow = t_continuous.rem_euclid(SECONDS_PER_WEEK);
        let state = satellite_state(
            &rec.elements,
            &rec.clock,
            &rec.constants(),
            sow,
            rec.group_delay_s,
            is_geo,
        );
        Some((
            state.orbit.position().as_array(),
            state.clock.dt_clock_total_s,
        ))
    }
}