diff --git a/dump1090.c b/dump1090.c
index 25b3737..ef0504d 100644
--- a/dump1090.c
+++ b/dump1090.c
@@ -243,6 +243,7 @@ void modesInit(void) {
     // Prepare error correction tables
     modesChecksumInit(Modes.nfix_crc);
     icaoFilterInit();
+    modeACInit();
 
     if (Modes.show_only)
         icaoFilterAdd(Modes.show_only);
diff --git a/dump1090.h b/dump1090.h
index 45f0829..aac2d1e 100644
--- a/dump1090.h
+++ b/dump1090.h
@@ -550,7 +550,9 @@ extern "C" {
 //
 int  detectModeA       (uint16_t *m, struct modesMessage *mm);
 void decodeModeAMessage(struct modesMessage *mm, int ModeA);
-int  ModeAToModeC      (unsigned int ModeA);
+void modeACInit();
+int modeAToModeC (unsigned int modeA);
+unsigned modeCToModeA (int modeC);
 
 //
 // Functions exported from mode_s.c
diff --git a/faup1090.c b/faup1090.c
index 8a29f95..3002f4b 100644
--- a/faup1090.c
+++ b/faup1090.c
@@ -95,6 +95,7 @@ static void faupInit(void) {
     // Prepare error correction tables
     modesChecksumInit(1);
     icaoFilterInit();
+    modeACInit();
 }
 
 //
diff --git a/mode_ac.c b/mode_ac.c
index baac314..c482fd7 100644
--- a/mode_ac.c
+++ b/mode_ac.c
@@ -29,15 +29,59 @@
 //
 
 #include "dump1090.h"
+#include <assert.h>
 //
 //=========================================================================
 //
 // Input format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
 //
-int ModeAToModeC(unsigned int ModeA) 
+
+static int modeAToCTable[4096];
+static unsigned modeCToATable[4096];
+static int internalModeAToModeC(unsigned int ModeA);
+
+void modeACInit()
 {
-  unsigned int FiveHundreds = 0;
-  unsigned int OneHundreds  = 0;
+    for (unsigned i = 0; i < 4096; ++i) {
+        unsigned modeA = indexToModeA(i);
+        int modeC = internalModeAToModeC(modeA);
+        modeAToCTable[i] = modeC;
+
+        modeC += 13;
+        if (modeC >= 0 && modeC < 4096) {
+            assert(modeCToATable[modeC] == 0);
+            modeCToATable[modeC] = modeA;
+        }
+    }
+}
+
+// Given a mode A value (hex-encoded, see above)
+// return the mode C value (signed multiple of 100s of feet)
+// or INVALID_ALITITUDE if not a valid mode C value
+int modeAToModeC(unsigned modeA)
+{
+    unsigned i = modeAToIndex(modeA);
+    if (i >= 4096)
+        return INVALID_ALTITUDE;
+
+    return modeAToCTable[i];
+}
+
+// Given a mode C value (signed multiple of 100s of feet)
+// return the mode A value, or 0 if not a valid mode C value
+unsigned modeCToModeA(int modeC)
+{
+    modeC += 13;
+    if (modeC < 0 || modeC >= 4096)
+        return 0;
+
+    return modeCToATable[modeC];
+}
+
+static int internalModeAToModeC(unsigned int ModeA)
+{
+    unsigned int FiveHundreds = 0;
+    unsigned int OneHundreds  = 0;
 
   if ((ModeA & 0xFFFF8889) != 0 ||         // check zero bits are zero, D1 set is illegal
       (ModeA & 0x000000F0) == 0) { // C1,,C4 cannot be Zero
@@ -100,7 +144,7 @@ void decodeModeAMessage(struct modesMessage *mm, int ModeA)
 
     // Decode an altitude if this looks like a possible mode C
     if (!mm->spi) {
-        int modeC = ModeAToModeC(ModeA);
+        int modeC = modeAToModeC(ModeA);
         if (modeC != INVALID_ALTITUDE) {
             mm->altitude = modeC * 100;
             mm->altitude_unit = UNIT_FEET;
diff --git a/mode_s.c b/mode_s.c
index e4ada73..0ea822c 100644
--- a/mode_s.c
+++ b/mode_s.c
@@ -134,7 +134,7 @@ static int decodeAC13Field(int AC13Field, altitude_unit_t *unit) {
             return ((n * 25) - 1000);
         } else {
             // N is an 11 bit Gillham coded altitude
-            int n = ModeAToModeC(decodeID13Field(AC13Field));
+            int n = modeAToModeC(decodeID13Field(AC13Field));
             if (n < -12) {
                 return INVALID_ALTITUDE;
             }
@@ -167,7 +167,7 @@ static int decodeAC12Field(int AC12Field, altitude_unit_t *unit) {
         // Make N a 13 bit Gillham coded altitude by inserting M=0 at bit 6
         int n = ((AC12Field & 0x0FC0) << 1) | 
                  (AC12Field & 0x003F);
-        n = ModeAToModeC(decodeID13Field(n));
+        n = modeAToModeC(decodeID13Field(n));
         if (n < -12) {
             return INVALID_ALTITUDE;
         }
diff --git a/track.h b/track.h
index 9326f10..5761621 100644
--- a/track.h
+++ b/track.h
@@ -200,4 +200,16 @@ struct aircraft *trackUpdateFromMessage(struct modesMessage *mm);
 /* Call periodically */
 void trackPeriodicUpdate();
 
+/* Convert from a (hex) mode A value to a 0-4095 index */
+static inline unsigned modeAToIndex(unsigned modeA)
+{
+    return (modeA & 0x0007) | ((modeA & 0x0070) >> 1) | ((modeA & 0x0700) >> 2) | ((modeA & 0x7000) >> 3);
+}
+
+/* Convert from a 0-4095 index to a (hex) mode A value */
+static inline unsigned indexToModeA(unsigned index)
+{
+    return (index & 0007) | ((index & 0070) << 1) | ((index & 0700) << 2) | ((index & 07000) << 3);
+}
+
 #endif
diff --git a/view1090.c b/view1090.c
index 3b26d05..38b2fc4 100644
--- a/view1090.c
+++ b/view1090.c
@@ -112,6 +112,7 @@ void view1090Init(void) {
     // Prepare error correction tables
     modesChecksumInit(Modes.nfix_crc);
     icaoFilterInit();
+    modeACInit();
 }
 
 //