diff --git a/fft.h b/fft.h
new file mode 100644
index 0000000..bf5cb11
--- /dev/null
+++ b/fft.h
@@ -0,0 +1,78 @@
+/* 
+ * fft.h is Based on
+ * Free FFT and convolution (C)
+ * 
+ * Copyright (c) 2019 Project Nayuki. (MIT License)
+ * https://www.nayuki.io/page/free-small-fft-in-multiple-languages
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ * - The above copyright notice and this permission notice shall be included in
+ *   all copies or substantial portions of the Software.
+ * - The Software is provided "as is", without warranty of any kind, express or
+ *   implied, including but not limited to the warranties of merchantability,
+ *   fitness for a particular purpose and noninfringement. In no event shall the
+ *   authors or copyright holders be liable for any claim, damages or other
+ *   liability, whether in an action of contract, tort or otherwise, arising from,
+ *   out of or in connection with the Software or the use or other dealings in the
+ *   Software.
+ */
+
+
+#include <math.h>
+#include <stdint.h>
+
+static uint8_t reverse_bits(uint8_t x, int n) {
+	uint8_t result = 0;
+	for (int i = 0; i < n; i++, x >>= 1)
+		result = (result << 1) | (x & 1U);
+	return result;
+}
+
+/***
+ * dir = forward: 0, inverse: 1
+ */
+void fft(float array[][2], uint8_t n, uint8_t dir) {
+	int levels = 0;  // Compute levels = floor(log2(n))
+	for (uint8_t temp = n; temp > 1U; temp >>= 1)
+		levels++;
+
+	uint8_t real = dir & 1;
+	uint8_t imag = ~real & 1;
+
+	for (uint8_t i = 0; i < n; i++) {
+		uint8_t j = reverse_bits(i, levels);
+		if (j > i) {
+			float temp = array[i][real];
+			array[i][real] = array[j][real];
+			array[j][real] = temp;
+			temp = array[i][imag];
+			array[i][imag] = array[j][imag];
+			array[j][imag] = temp;
+		}
+	}
+
+	// Cooley-Tukey decimation-in-time radix-2 FFT
+	for (uint8_t size = 2; size <= n; size *= 2) {
+		uint8_t halfsize = size / 2;
+		uint8_t tablestep = n / size;
+		for (uint8_t i = 0; i < n; i += size) {
+			for (uint8_t j = i, k = 0; j < i + halfsize; j++, k += tablestep) {
+				uint8_t l = j + halfsize;
+				float tpre =  array[l][real] * cos(2 * M_PI * k / n) + array[l][imag] * sin(2 * M_PI * k / n);
+				float tpim = -array[l][real] * sin(2 * M_PI * k / n) + array[l][imag] * cos(2 * M_PI * k / n);
+				array[l][real] = array[j][real] - tpre;
+				array[l][imag] = array[j][imag] - tpim;
+				array[j][real] += tpre;
+				array[j][imag] += tpim;
+			}
+		}
+		if (size == n)  // Prevent overflow in 'size *= 2'
+			break;
+	}
+}
+
diff --git a/main.c b/main.c
index 4bafd45..23feb52 100644
--- a/main.c
+++ b/main.c
@@ -23,6 +23,7 @@
 #include "usbcfg.h"
 #include "si5351.h"
 #include "nanovna.h"
+#include "fft.h"
 
 #include <chprintf.h>
 #include <shell.h>
@@ -36,6 +37,7 @@
 static void apply_error_term(void);
 static void apply_error_term_at(int i);
 static void cal_interpolate(int s);
+static void transform_domain(void);
 
 void sweep(void);
 
@@ -54,6 +56,8 @@ int8_t redraw_requested = FALSE;
 int8_t stop_the_world = FALSE;
 int16_t vbat = 0;
 
+uint8_t domain = DOMAIN_TIME;
+uint8_t tdrfunc = TDR_IMPULSE;
 static THD_WORKING_AREA(waThread1, 640);
 static THD_FUNCTION(Thread1, arg)
 {
@@ -82,6 +86,7 @@ static THD_FUNCTION(Thread1, arg)
           draw_battery_status();
       }
 
+      transform_domain();
       /* calculate trace coordinates */
       plot_into_index(measured);
       /* plot trace as raster */
@@ -107,6 +112,35 @@ toggle_sweep(void)
   sweep_enabled = !sweep_enabled;
 }
 
+static
+void
+transform_domain(void)
+{
+  if (domain != DOMAIN_TIME) return; // nothing to do for freq domain
+  // use spi_buffer as temporary buffer
+  // and calculate ifft for time domain
+  float* tmp = (float*)spi_buffer;
+  for (int ch = 0; ch < 2; ch++) {
+      for (int i = 0; i < 128; i++) {
+          tmp[i*2+0] = 0.0;
+          tmp[i*2+1] = 0.0;
+      }
+      memcpy(spi_buffer, measured[ch], sizeof(measured[0]));
+      fft((float(*)[2])tmp, 128, 1);
+      memcpy(measured[ch], spi_buffer, sizeof(measured[0]));
+      for (int i = 0; i < 101; i++) {
+          measured[ch][i][0] /= 128.0;
+          measured[ch][i][1] /= 128.0;
+      }
+      if (tdrfunc == TDR_STEP) {
+          for (int i = 1; i < 101; i++) {
+              measured[ch][i][0] += measured[ch][i-1][0];
+              measured[ch][i][1] += measured[ch][i-1][1];
+          }
+      }
+  }
+}
+
 static void cmd_pause(BaseSequentialStream *chp, int argc, char *argv[])
 {
     (void)chp;
diff --git a/nanovna.h b/nanovna.h
index 655e78e..a33cf5e 100644
--- a/nanovna.h
+++ b/nanovna.h
@@ -22,8 +22,20 @@
 /*
  * main.c
  */
+
 extern float measured[2][101][2];
 
+enum {
+  DOMAIN_FREQ, DOMAIN_TIME
+};
+
+enum {
+  TDR_IMPULSE, TDR_STEP
+};
+
+extern uint8_t domain;
+extern uint8_t tdrfunc;
+
 #define CAL_LOAD 0
 #define CAL_OPEN 1
 #define CAL_SHORT 2
diff --git a/plot.c b/plot.c
index 2367372..91ef03a 100644
--- a/plot.c
+++ b/plot.c
@@ -1367,8 +1367,15 @@ cell_draw_marker_info(int m, int n, int w, int h)
   chsnprintf(buf, sizeof buf, "%d:", active_marker + 1);
   cell_drawstring_5x7(w, h, buf, xpos, ypos, 0xffff);
   xpos += 16;
-  frequency_string(buf, sizeof buf, frequencies[idx]);
-  cell_drawstring_5x7(w, h, buf, xpos, ypos, 0xffff);
+  if (domain == DOMAIN_FREQ) {
+      frequency_string(buf, sizeof buf, frequencies[idx]);
+      cell_drawstring_5x7(w, h, buf, xpos, ypos, 0xffff);
+  } else {
+#define SPEED_OF_LIGHT 299792458
+      float distance = ((float)idx * (float)SPEED_OF_LIGHT) / ( (float)(frequencies[1] - frequencies[0]) * 128.0 * 2.0);
+      chsnprintf(buf, sizeof buf, "%f m", distance);
+      cell_drawstring_5x7(w, h, buf, xpos, ypos, 0xffff);
+  }
 
   // draw marker delta
   if (previous_marker >= 0 && active_marker != previous_marker && markers[previous_marker].enabled) {
diff --git a/ui.c b/ui.c
index 0012db5..59f2255 100644
--- a/ui.c
+++ b/ui.c
@@ -664,6 +664,24 @@ menu_channel_cb(int item)
   ui_mode_normal();
 }
 
+static void
+menu_tdr_cb(int item)
+{
+  switch (item) {
+    case 0:
+      domain = (domain == DOMAIN_FREQ) ? DOMAIN_TIME : DOMAIN_FREQ;
+      break;
+    case 1:
+      tdrfunc = TDR_IMPULSE;
+      break;
+    case 2:
+      tdrfunc = TDR_STEP;
+      break;
+  }
+
+  ui_mode_normal();
+}
+
 static void 
 choose_active_marker(void)
 {
@@ -874,11 +892,20 @@ const menuitem_t menu_channel[] = {
   { MT_NONE, NULL, NULL } // sentinel
 };
 
+const menuitem_t menu_tdr[] = {
+  { MT_CALLBACK, "TDR MODE", menu_tdr_cb },
+  { MT_CALLBACK, "IMPULSE", menu_tdr_cb },
+  { MT_CALLBACK, "STEP", menu_tdr_cb },
+  { MT_CANCEL, S_LARROW" BACK", NULL },
+  { MT_NONE, NULL, NULL } // sentinel
+};
+
 const menuitem_t menu_display[] = {
   { MT_SUBMENU, "TRACE", menu_trace },
   { MT_SUBMENU, "FORMAT", menu_format },
   { MT_SUBMENU, "SCALE", menu_scale },
   { MT_SUBMENU, "CHANNEL", menu_channel },
+  { MT_SUBMENU, "TDR", menu_tdr },
   { MT_CANCEL, S_LARROW" BACK", NULL },
   { MT_NONE, NULL, NULL } // sentinel
 };
@@ -1235,6 +1262,14 @@ menu_item_modify_attribute(const menuitem_t *menu, int item,
       *bg = 0x0000;
       *fg = 0xffff;
     }
+  } else if (menu == menu_tdr) {
+      if ((item == 0 && domain == DOMAIN_TIME)
+       || (item == 1 && tdrfunc == TDR_IMPULSE)
+       || (item == 2 && tdrfunc == TDR_STEP)
+       ) {
+        *bg = 0x0000;
+        *fg = 0xffff;
+      }
   }
 }