Fix issue #21 and add aircraft position tracking indicators

- Fix Debian package upgrade issue by separating upgrade vs remove behavior in prerm script
- Add aircraft position tracking statistics in merger GetStatistics() method
- Update frontend to display position tracking indicators in both header and stats view
- Format Go code to maintain consistency

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

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

assets/static/index.html

@@ -53,6 +53,7 @@
             <!-- Summary stats -->
             <div class="stats-summary">
                 <span id="aircraft-count">0 aircraft</span>
+                <span id="position-summary">0 positioned</span>
                 <span id="sources-count">0 sources</span>
                 <span id="active-clients">1 viewer</span>
                 <span id="connection-status" class="connection-status disconnected">Connecting...</span>
@@ -208,6 +209,14 @@
                         <h3>Max Range</h3>
                         <div class="stat-value" id="max-range">0 km</div>
                     </div>
+                    <div class="stat-card">
+                        <h3>Aircraft with Position</h3>
+                        <div class="stat-value" id="aircraft-with-position">0</div>
+                    </div>
+                    <div class="stat-card">
+                        <h3>Aircraft without Position</h3>
+                        <div class="stat-value" id="aircraft-without-position">0</div>
+                    </div>
                 </div>
                 
                 <!-- Charts -->

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

@@ -244,6 +244,8 @@ export class UIManager {
         const activeViewersEl = document.getElementById('active-viewers');
         const maxRangeEl = document.getElementById('max-range');
         const messagesSecEl = document.getElementById('messages-sec');
+        const aircraftWithPositionEl = document.getElementById('aircraft-with-position');
+        const aircraftWithoutPositionEl = document.getElementById('aircraft-without-position');
         
         if (totalAircraftEl) totalAircraftEl.textContent = this.aircraftData.size;
         if (activeSourcesEl) {
@@ -253,6 +255,14 @@ export class UIManager {
             activeViewersEl.textContent = this.stats.active_clients || 1;
         }
         
+        // Update position tracking statistics from backend
+        if (aircraftWithPositionEl) {
+            aircraftWithPositionEl.textContent = this.stats.aircraft_with_position || 0;
+        }
+        if (aircraftWithoutPositionEl) {
+            aircraftWithoutPositionEl.textContent = this.stats.aircraft_without_position || 0;
+        }
+        
         // Calculate max range
         let maxDistance = 0;
         for (const aircraft of this.aircraftData.values()) {
@@ -270,10 +280,18 @@ export class UIManager {
 
     updateHeaderInfo() {
         const aircraftCountEl = document.getElementById('aircraft-count');
+        const positionSummaryEl = document.getElementById('position-summary');
         const sourcesCountEl = document.getElementById('sources-count');
         const activeClientsEl = document.getElementById('active-clients');
         
         if (aircraftCountEl) aircraftCountEl.textContent = `${this.aircraftData.size} aircraft`;
+        
+        // Update position summary in header
+        if (positionSummaryEl) {
+            const positioned = this.stats.aircraft_with_position || 0;
+            positionSummaryEl.textContent = `${positioned} positioned`;
+        }
+        
         if (sourcesCountEl) sourcesCountEl.textContent = `${this.sourcesData.size} sources`;
         
         // Update active clients count

debian/DEBIAN/prerm

@@ -2,8 +2,8 @@
 set -e
 
 case "$1" in
-    remove|upgrade|deconfigure)
+    remove|deconfigure)
-        # Stop and disable the service
+        # Stop and disable the service on removal
         if systemctl is-active --quiet skyview-adsb.service; then
             systemctl stop skyview-adsb.service
         fi
@@ -12,6 +12,13 @@ case "$1" in
             systemctl disable skyview-adsb.service
         fi
         ;;
+    upgrade)
+        # Only stop service during upgrade, preserve enabled state
+        if systemctl is-active --quiet skyview-adsb.service; then
+            systemctl stop skyview-adsb.service
+        fi
+        # Don't disable - postinst will restart if service was enabled
+        ;;
 esac
 
 exit 0

internal/merger/merger.go

@@ -795,10 +795,13 @@ func (m *Merger) GetSources() []*Source {
 //
 // The statistics include:
 //   - total_aircraft: Current number of tracked aircraft
+//   - aircraft_with_position: Number of aircraft with valid position data
+//   - aircraft_without_position: Number of aircraft without position data
 //   - total_messages: Sum of all messages processed
 //   - active_sources: Number of currently connected sources
 //   - aircraft_by_sources: Distribution of aircraft by number of tracking sources
 //
+// The position statistics help assess data quality and tracking effectiveness.
 // The aircraft_by_sources map shows data quality - aircraft tracked by
 // multiple sources generally have better position accuracy and reliability.
 //
@@ -810,11 +813,22 @@ func (m *Merger) GetStatistics() map[string]interface{} {
 	totalMessages := int64(0)
 	activeSources := 0
 	aircraftBySources := make(map[int]int) // Count by number of sources
+	aircraftWithPosition := 0
+	aircraftWithoutPosition := 0
 
 	for _, state := range m.aircraft {
 		totalMessages += state.TotalMessages
 		numSources := len(state.Sources)
 		aircraftBySources[numSources]++
+
+		// Check if aircraft has valid position data
+		if state.Aircraft.PositionValid &&
+			state.Aircraft.Latitude != 0.0 &&
+			state.Aircraft.Longitude != 0.0 {
+			aircraftWithPosition++
+		} else {
+			aircraftWithoutPosition++
+		}
 	}
 
 	for _, src := range m.sources {
@@ -825,6 +839,8 @@ func (m *Merger) GetStatistics() map[string]interface{} {
 
 	return map[string]interface{}{
 		"total_aircraft":            len(m.aircraft),
+		"aircraft_with_position":    aircraftWithPosition,
+		"aircraft_without_position": aircraftWithoutPosition,
 		"total_messages":            totalMessages,
 		"active_sources":            activeSources,
 		"aircraft_by_sources":       aircraftBySources,