Implement aircraft position validation to fix obviously wrong trails

Add comprehensive position validation to filter out impossible aircraft
movements and improve trail accuracy as requested in issue #16.

**Position Validation Features:**
- Geographic coordinate bounds checking (lat: -90 to 90, lon: -180 to 180)
- Altitude validation (range: -500ft to 60,000ft)
- Speed validation (max: 2000 knots ≈ Mach 3)
- Distance jump validation (max: 500 nautical miles)
- Time consistency validation (reject out-of-order timestamps)
- Speed consistency warnings (reported vs. implied speed)

**Integration Points:**
- updateHistories(): Validates before adding to position history (trails)
- mergeAircraftData(): Validates before updating aircraft position state
- Comprehensive logging with ICAO identification for debugging

**Validation Logic:**
- Rejects obviously wrong positions that would create impossible flight paths
- Warns about high but possible speeds (>800 knots) for monitoring
- Maintains detailed logs: [POSITION_VALIDATION] ICAO ABCDEF: REJECTED/WARNING
- Performance optimized with early returns and minimal overhead

**Result:**
- Users see only realistic aircraft trails and movements
- Obviously wrong ADS-B data (teleporting, impossible speeds) filtered out
- Debug logs provide visibility into validation decisions
- Clean flight tracking without zigzag patterns or position jumps

Fixes #16

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/merger/merger.go

@@ -22,6 +22,7 @@ package merger
 import (
 	"encoding/json"
 	"fmt"
+	"log"
 	"math"
 	"sync"
 	"time"
@@ -33,8 +34,31 @@ import (
 const (
 	// MaxDistance represents an infinite distance for initialization
 	MaxDistance = float64(999999)
+
+	// Position validation constants
+	MaxSpeedKnots        = 2000.0 // Maximum plausible aircraft speed (roughly Mach 3 at cruise altitude)
+	MaxDistanceNautMiles = 500.0  // Maximum position jump distance in nautical miles
+	MaxAltitudeFeet      = 60000  // Maximum altitude in feet (commercial ceiling ~FL600)
+	MinAltitudeFeet      = -500   // Minimum altitude (below sea level but allow for dead sea, etc.)
+
+	// Earth coordinate bounds
+	MinLatitude  = -90.0
+	MaxLatitude  = 90.0
+	MinLongitude = -180.0
+	MaxLongitude = 180.0
+
+	// Conversion factors
+	KnotsToKmh = 1.852
+	NmToKm     = 1.852
 )
 
+// ValidationResult represents the result of position validation checks.
+type ValidationResult struct {
+	Valid    bool     // Whether the position passed all validation checks
+	Errors   []string // List of validation failures for debugging
+	Warnings []string // List of potential issues (not blocking)
+}
+
 // Source represents a data source (dump1090 receiver or similar ADS-B source).
 // It contains both static configuration and dynamic status information used
 // for data fusion decisions and source monitoring.
@@ -417,8 +441,19 @@ func (m *Merger) UpdateAircraft(sourceID string, aircraft *modes.Aircraft, signa
 //   - sourceID: Identifier of source providing new data
 //   - timestamp: Timestamp of new data
 func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, sourceID string, timestamp time.Time) {
-	// Position - use source with best signal or most recent
+	// Position - use source with best signal or most recent, but validate first
 	if new.Latitude != 0 && new.Longitude != 0 {
+		// Always validate position before considering update
+		validation := m.validatePosition(new, state, timestamp)
+
+		if !validation.Valid {
+			// Log validation errors and skip position update
+			icaoHex := fmt.Sprintf("%06X", new.ICAO24)
+			for _, err := range validation.Errors {
+				log.Printf("[POSITION_VALIDATION] ICAO %s: REJECTED position update - %s", icaoHex, err)
+			}
+		} else {
+			// Position is valid, proceed with normal logic
 			updatePosition := false
 
 			if state.Latitude == 0 {
@@ -442,6 +477,13 @@ func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, so
 			}
 		}
 
+		// Log warnings even if position is valid
+		for _, warning := range validation.Warnings {
+			icaoHex := fmt.Sprintf("%06X", new.ICAO24)
+			log.Printf("[POSITION_VALIDATION] ICAO %s: WARNING - %s", icaoHex, warning)
+		}
+	}
+
 	// Altitude - use most recent
 	if new.Altitude != 0 {
 		state.Altitude = new.Altitude
@@ -530,14 +572,31 @@ func (m *Merger) mergeAircraftData(state *AircraftState, new *modes.Aircraft, so
 //   - signal: Signal strength measurement
 //   - timestamp: When this data was received
 func (m *Merger) updateHistories(state *AircraftState, aircraft *modes.Aircraft, sourceID string, signal float64, timestamp time.Time) {
-	// Position history
+	// Position history with validation
 	if aircraft.Latitude != 0 && aircraft.Longitude != 0 {
+		// Validate position before adding to history
+		validation := m.validatePosition(aircraft, state, timestamp)
+
+		if validation.Valid {
 			state.PositionHistory = append(state.PositionHistory, PositionPoint{
 				Time:      timestamp,
 				Latitude:  aircraft.Latitude,
 				Longitude: aircraft.Longitude,
 				Source:    sourceID,
 			})
+		} else {
+			// Log validation errors for debugging
+			icaoHex := fmt.Sprintf("%06X", aircraft.ICAO24)
+			for _, err := range validation.Errors {
+				log.Printf("[POSITION_VALIDATION] ICAO %s: REJECTED - %s", icaoHex, err)
+			}
+		}
+
+		// Log warnings even for valid positions
+		for _, warning := range validation.Warnings {
+			icaoHex := fmt.Sprintf("%06X", aircraft.ICAO24)
+			log.Printf("[POSITION_VALIDATION] ICAO %s: WARNING - %s", icaoHex, warning)
+		}
 	}
 
 	// Signal history
@@ -809,6 +868,130 @@ func calculateDistanceBearing(lat1, lon1, lat2, lon2 float64) (float64, float64)
 	return distance, bearing
 }
 
+// validatePosition performs comprehensive validation of aircraft position data to filter out
+// obviously incorrect flight paths and implausible position updates.
+//
+// This function implements multiple validation checks to improve data quality:
+//
+// 1. **Coordinate Validation**: Ensures latitude/longitude are within Earth's bounds
+// 2. **Altitude Validation**: Rejects impossible altitudes (negative or > FL600)
+// 3. **Speed Validation**: Calculates implied speed and rejects >Mach 3 movements
+// 4. **Distance Validation**: Rejects position jumps >500nm without time justification
+// 5. **Time Validation**: Ensures timestamps are chronologically consistent
+//
+// Parameters:
+//   - aircraft: New aircraft position data to validate
+//   - state: Current aircraft state with position history
+//   - timestamp: Timestamp of the new position data
+//
+// Returns:
+//   - ValidationResult with valid flag and detailed error/warning messages
+func (m *Merger) validatePosition(aircraft *modes.Aircraft, state *AircraftState, timestamp time.Time) *ValidationResult {
+	result := &ValidationResult{
+		Valid:    true,
+		Errors:   make([]string, 0),
+		Warnings: make([]string, 0),
+	}
+
+	// Skip validation if no position data
+	if aircraft.Latitude == 0 && aircraft.Longitude == 0 {
+		return result // No position to validate
+	}
+
+	// 1. Geographic coordinate validation
+	if aircraft.Latitude < MinLatitude || aircraft.Latitude > MaxLatitude {
+		result.Valid = false
+		result.Errors = append(result.Errors, fmt.Sprintf("Invalid latitude: %.6f (must be between %.1f and %.1f)",
+			aircraft.Latitude, MinLatitude, MaxLatitude))
+	}
+
+	if aircraft.Longitude < MinLongitude || aircraft.Longitude > MaxLongitude {
+		result.Valid = false
+		result.Errors = append(result.Errors, fmt.Sprintf("Invalid longitude: %.6f (must be between %.1f and %.1f)",
+			aircraft.Longitude, MinLongitude, MaxLongitude))
+	}
+
+	// 2. Altitude validation
+	if aircraft.Altitude != 0 { // Only validate non-zero altitudes
+		if aircraft.Altitude < MinAltitudeFeet {
+			result.Valid = false
+			result.Errors = append(result.Errors, fmt.Sprintf("Impossible altitude: %d feet (below minimum %d)",
+				aircraft.Altitude, MinAltitudeFeet))
+		}
+
+		if aircraft.Altitude > MaxAltitudeFeet {
+			result.Valid = false
+			result.Errors = append(result.Errors, fmt.Sprintf("Impossible altitude: %d feet (above maximum %d)",
+				aircraft.Altitude, MaxAltitudeFeet))
+		}
+	}
+
+	// 3. Speed and distance validation (requires position history)
+	if len(state.PositionHistory) > 0 && state.Latitude != 0 && state.Longitude != 0 {
+		lastPos := state.PositionHistory[len(state.PositionHistory)-1]
+
+		// Calculate distance between positions
+		distance, _ := calculateDistanceBearing(lastPos.Latitude, lastPos.Longitude,
+			aircraft.Latitude, aircraft.Longitude)
+
+		// Calculate time difference
+		timeDiff := timestamp.Sub(lastPos.Time).Seconds()
+
+		if timeDiff > 0 {
+			// Calculate implied speed in knots
+			distanceNm := distance / NmToKm
+			speedKnots := (distanceNm / timeDiff) * 3600 // Convert to knots per hour
+
+			// Distance validation: reject jumps >500nm
+			if distanceNm > MaxDistanceNautMiles {
+				result.Valid = false
+				result.Errors = append(result.Errors, fmt.Sprintf("Impossible position jump: %.1f nm in %.1f seconds (max allowed: %.1f nm)",
+					distanceNm, timeDiff, MaxDistanceNautMiles))
+			}
+
+			// Speed validation: reject >2000 knots (roughly Mach 3)
+			if speedKnots > MaxSpeedKnots {
+				result.Valid = false
+				result.Errors = append(result.Errors, fmt.Sprintf("Impossible speed: %.0f knots (max allowed: %.0f knots)",
+					speedKnots, MaxSpeedKnots))
+			}
+
+			// Warning for high but possible speeds (>800 knots)
+			if speedKnots > 800 && speedKnots <= MaxSpeedKnots {
+				result.Warnings = append(result.Warnings, fmt.Sprintf("High speed detected: %.0f knots", speedKnots))
+			}
+		} else if timeDiff < 0 {
+			// 4. Time validation: reject out-of-order timestamps
+			result.Valid = false
+			result.Errors = append(result.Errors, fmt.Sprintf("Out-of-order timestamp: %.1f seconds in the past", -timeDiff))
+		}
+	}
+
+	// 5. Aircraft-specific validations based on reported speed vs. position
+	if aircraft.GroundSpeed > 0 && len(state.PositionHistory) > 0 {
+		// Check if reported ground speed is consistent with position changes
+		lastPos := state.PositionHistory[len(state.PositionHistory)-1]
+		distance, _ := calculateDistanceBearing(lastPos.Latitude, lastPos.Longitude,
+			aircraft.Latitude, aircraft.Longitude)
+		timeDiff := timestamp.Sub(lastPos.Time).Seconds()
+
+		if timeDiff > 0 {
+			distanceNm := distance / NmToKm
+			impliedSpeed := (distanceNm / timeDiff) * 3600
+			reportedSpeed := float64(aircraft.GroundSpeed)
+
+			// Warning if speeds differ significantly (>100 knots difference)
+			if math.Abs(impliedSpeed-reportedSpeed) > 100 && reportedSpeed > 50 {
+				result.Warnings = append(result.Warnings,
+					fmt.Sprintf("Speed inconsistency: reported %d knots, implied %.0f knots",
+						aircraft.GroundSpeed, impliedSpeed))
+			}
+		}
+	}
+
+	return result
+}
+
 // Close closes the merger and releases resources
 func (m *Merger) Close() error {
 	m.mu.Lock()