first commit

backend/doppler/calculator.go

@@ -0,0 +1,92 @@
+package doppler
+
+import (
+	"fmt"
+	"math"
+	"sync"
+	"time"
+
+	"SatMaster/backend/propagator"
+)
+
+const (
+	SpeedOfLight = 299792.458 // km/s
+)
+
+// Calculator computes Doppler-shifted frequencies.
+type Calculator struct {
+	mu          sync.RWMutex
+	nominalDown float64 // Hz
+	nominalUp   float64 // Hz
+	obsLat      float64
+	obsLon      float64
+	obsAlt      float64
+}
+
+func NewCalculator() *Calculator {
+	return &Calculator{}
+}
+
+func (c *Calculator) SetObserver(lat, lon, altM float64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.obsLat = lat
+	c.obsLon = lon
+	c.obsAlt = altM
+}
+
+func (c *Calculator) SetNominal(downHz, upHz float64) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.nominalDown = downHz
+	c.nominalUp = upHz
+}
+
+// Correct computes Doppler-corrected downlink and uplink frequencies.
+// Returns (downlinkHz, uplinkHz).
+func (c *Calculator) Correct(pos *propagator.SatPosition, obs propagator.Observer, _ time.Time) (float64, float64) {
+	c.mu.RLock()
+	nomDown := c.nominalDown
+	nomUp := c.nominalUp
+	c.mu.RUnlock()
+
+	if nomDown == 0 && nomUp == 0 {
+		return 0, 0
+	}
+	if pos == nil {
+		return nomDown, nomUp
+	}
+
+	// Range rate in km/s (positive = receding, negative = approaching)
+	rr := pos.RangeRate
+
+	// Doppler factor: f_received = f_nominal * (1 - v/c)
+	// For downlink: satellite is the transmitter
+	dopplerFactor := 1.0 - rr/SpeedOfLight
+
+	correctedDown := nomDown * dopplerFactor
+	// For uplink: we pre-correct in reverse so the satellite receives nominal
+	correctedUp := nomUp / dopplerFactor
+
+	return correctedDown, correctedUp
+}
+
+// ShiftHz returns the Doppler shift in Hz for a given nominal frequency.
+func ShiftHz(nominalHz, rangeRateKmS float64) float64 {
+	return nominalHz * (-rangeRateKmS / SpeedOfLight)
+}
+
+// RangeRateFromPositions computes range rate from two consecutive positions.
+func RangeRateFromPositions(prev, curr *propagator.SatPosition, dt float64) float64 {
+	prevRange := prev.Range
+	currRange := curr.Range
+	return (currRange - prevRange) / dt
+}
+
+// FormatShift formats a Doppler shift in Hz for display.
+func FormatShift(shiftHz float64) string {
+	if math.Abs(shiftHz) >= 1000 {
+		return fmt.Sprintf("%+.2f kHz", shiftHz/1000)
+	}
+	return fmt.Sprintf("%+.0f Hz", shiftHz)
+}