Improve transponder display and add combined signal quality indicator

**Transponder Display Improvements:**
- Simplified transponder capability labels to be more concise
- "Level 2+ Transponder" → "Level 2+"
- "Enhanced Transponder" → "Enhanced"
- "Alert/Emergency Status" → "Alert/Emergency"
- Removes redundant "Transponder" suffix for cleaner popup display

**New Signal Quality Feature:**
- Added SignalQuality field combining NACp, NACv, and SIL into human-readable assessment
- Quality levels: "Excellent", "Good", "Fair", "Poor" based on aviation standards
- Prioritizes surveillance integrity (SIL) then position accuracy (NACp)
- Automatically calculated when operational status messages (TC 31) are received
- Displayed in aircraft popup when available

**Signal Quality Algorithm:**
- **Excellent**: High integrity (SIL ≥ 2) with precise position (NACp ≥ 9)
- **Good**: Good integrity with moderate accuracy OR very high accuracy
- **Fair**: Some integrity with basic accuracy OR high accuracy without integrity
- **Poor**: Low but usable quality indicators
- **Empty**: No quality data available

**Frontend Enhancements:**
- Added "Signal Quality" row to aircraft popup
- Only displays when quality data is available
- Clean integration with existing popup layout
- Shows alongside transponder information

**Data Quality Context:**
- NACp: Position accuracy (0-11, higher = more precise location)
- NACv: Velocity accuracy (0-4, higher = more precise speed/heading)
- SIL: Surveillance integrity (0-3, higher = more reliable data)

This gives users a quick understanding of data reliability without needing
to interpret technical NACp/NACv/SIL values directly.

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

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

assets/static/js/modules/aircraft-manager.js

@@ -366,6 +366,10 @@ export class AircraftManager {
                     <div class="detail-row">
                         <strong>Transponder:</strong> ${aircraft.TransponderCapability}
                     </div>` : ''}
+                    ${aircraft.SignalQuality ? `
+                    <div class="detail-row">
+                        <strong>Signal Quality:</strong> ${aircraft.SignalQuality}
+                    </div>` : ''}
                     
                     <div class="detail-grid">
                         <div class="detail-item">

internal/merger/merger.go

@@ -134,6 +134,7 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		SIL                   uint8   `json:"SIL"`
 		TransponderCapability string  `json:"TransponderCapability"`
 		TransponderLevel      uint8   `json:"TransponderLevel"`
+		SignalQuality         string  `json:"SignalQuality"`
 		SelectedAltitude      int     `json:"SelectedAltitude"`
 		SelectedHeading       float64 `json:"SelectedHeading"`
 		BaroSetting           float64 `json:"BaroSetting"`
@@ -180,6 +181,7 @@ func (a *AircraftState) MarshalJSON() ([]byte, error) {
 		SIL:                   a.Aircraft.SIL,
 		TransponderCapability: a.Aircraft.TransponderCapability,
 		TransponderLevel:      a.Aircraft.TransponderLevel,
+		SignalQuality:         a.Aircraft.SignalQuality,
 		SelectedAltitude:      a.Aircraft.SelectedAltitude,
 		SelectedHeading:       a.Aircraft.SelectedHeading,
 		BaroSetting:           a.Aircraft.BaroSetting,

internal/modes/decoder.go

@@ -142,6 +142,9 @@ type Aircraft struct {
 	TransponderCapability string // Transponder capability level (from DF11 messages)
 	TransponderLevel      uint8  // Transponder level (0-7 from capability field)
 
+	// Combined Data Quality Assessment
+	SignalQuality string // Combined assessment of position/velocity accuracy and integrity
+
 	// Autopilot/Flight Management
 	SelectedAltitude int     // MCP/FCU selected altitude in feet
 	SelectedHeading  float64 // MCP/FCU selected heading in degrees
@@ -870,6 +873,9 @@ func (d *Decoder) decodeOperationalStatus(data []byte, aircraft *Aircraft) {
 	aircraft.NACp = (data[7] >> 4) & 0x0F
 	aircraft.NACv = data[7] & 0x0F
 	aircraft.SIL = (data[8] >> 6) & 0x03
+
+	// Calculate combined signal quality from NACp, NACv, and SIL
+	d.calculateSignalQuality(aircraft)
 }
 
 // decodeSurfacePosition extracts position and movement data for aircraft on the ground.
@@ -997,15 +1003,15 @@ func (d *Decoder) decodeAllCallReply(data []byte, aircraft *Aircraft) {
 	aircraft.TransponderLevel = capability
 	switch capability {
 	case 0:
-		aircraft.TransponderCapability = "Level 1 Transponder"
+		aircraft.TransponderCapability = "Level 1"
 	case 1:
-		aircraft.TransponderCapability = "Level 2 Transponder"
+		aircraft.TransponderCapability = "Level 2"
 	case 2, 3:
-		aircraft.TransponderCapability = "Level 2+ Transponder"
+		aircraft.TransponderCapability = "Level 2+"
 	case 4, 5, 6:
-		aircraft.TransponderCapability = "Enhanced Transponder"
+		aircraft.TransponderCapability = "Enhanced"
 	case 7:
-		aircraft.TransponderCapability = "Alert/Emergency Status"
+		aircraft.TransponderCapability = "Alert/Emergency"
 	}
 }
 
@@ -1064,3 +1070,63 @@ func (d *Decoder) decodeCommD(data []byte, aircraft *Aircraft) {
 	//
 	// This could be extended based on specific requirements and available documentation
 }
+
+// calculateSignalQuality combines NACp, NACv, and SIL into an overall data quality assessment.
+//
+// This function provides a human-readable quality indicator that considers:
+//   - Position accuracy (NACp): How precise the aircraft's position data is
+//   - Velocity accuracy (NACv): How precise the speed/heading data is
+//   - Surveillance integrity (SIL): How reliable/trustworthy the data is
+//
+// The algorithm prioritizes integrity first (SIL), then position accuracy (NACp),
+// then velocity accuracy (NACv) to provide a meaningful overall assessment.
+//
+// Quality levels:
+//   - "Excellent": High integrity with very precise position/velocity
+//   - "Good": Good integrity with reasonable precision
+//   - "Fair": Moderate quality suitable for tracking
+//   - "Poor": Low quality but still usable
+//   - "Unknown": No quality indicators available
+//
+// Parameters:
+//   - aircraft: Aircraft struct containing NACp, NACv, and SIL values
+func (d *Decoder) calculateSignalQuality(aircraft *Aircraft) {
+	nacp := aircraft.NACp
+	nacv := aircraft.NACv
+	sil := aircraft.SIL
+
+	// If no quality indicators are available
+	if nacp == 0 && nacv == 0 && sil == 0 {
+		aircraft.SignalQuality = ""
+		return
+	}
+
+	// Excellent: High integrity (SIL >= 2) with high accuracy (NACp >= 9)
+	if sil >= 2 && nacp >= 9 {
+		aircraft.SignalQuality = "Excellent"
+		return
+	}
+
+	// Good: Good integrity (SIL >= 2) with moderate accuracy (NACp >= 6) OR
+	//       moderate integrity with very high accuracy
+	if (sil >= 2 && nacp >= 6) || (sil >= 1 && nacp >= 10) {
+		aircraft.SignalQuality = "Good"
+		return
+	}
+
+	// Fair: Some integrity (SIL >= 1) with basic accuracy (NACp >= 3) OR
+	//       high accuracy without integrity info
+	if (sil >= 1 && nacp >= 3) || nacp >= 8 {
+		aircraft.SignalQuality = "Fair"
+		return
+	}
+
+	// Poor: Low values but still has some quality indicators
+	if sil > 0 || nacp > 0 || nacv > 0 {
+		aircraft.SignalQuality = "Poor"
+		return
+	}
+
+	// Default fallback
+	aircraft.SignalQuality = ""
+}