diff --git a/impl/src/sdr/sdr.c b/impl/src/sdr/sdr.c
index 00156ab..6c53af7 100644
--- a/impl/src/sdr/sdr.c
+++ b/impl/src/sdr/sdr.c
@@ -69,6 +69,8 @@ static int tx_callback(hackrf_transfer *transfer)
 	int result;
 	int retcode = 0;
 
+	fprintf(stderr, "tx_callback()\n");
+
 	sdr_ctx_t *sdr_ctx = (sdr_ctx_t*)transfer->tx_ctx;
 
 	if(transfer->valid_length % 2 != 0) {
@@ -96,11 +98,20 @@ static int tx_callback(hackrf_transfer *transfer)
 		sdr_ctx->tx_done = true;
 	}
 
+	if(sdr_ctx->tx_start_time == 0.0) {
+		sdr_ctx->tx_start_time = get_hires_time();
+		sdr_ctx->tx_duration = 0.0f;
+	}
+
 	for(size_t i = 0; i < samples_read; i++) {
 		transfer->buffer[2*i + 0] = clamp_float2int8(creal(samples[i]));
 		transfer->buffer[2*i + 1] = clamp_float2int8(cimag(samples[i]));
 	}
 
+	sdr_ctx->tx_duration += (double)samples_read / SDR_TX_SAMPLING_RATE;
+
+	fprintf(stderr, "copied %u samples to HackRF.\n", samples_read);
+
 	if(samples_read < samples_requested) {
 		// fill the rest with zeros
 		memset(&transfer->buffer[2*samples_read], 0, 2*(samples_requested-samples_read));
@@ -255,6 +266,7 @@ result_t sdr_start_tx(sdr_ctx_t *ctx, size_t burst_size)
 	CHECK_HACKRF_RESULT(result, "hackrf_start_tx");
 
 	ctx->status = SDR_STATUS_TX;
+	ctx->tx_start_time = 0.0f; // will be updated by tx_callback
 
 	return OK;
 }
diff --git a/impl/src/sdr/sdr.h b/impl/src/sdr/sdr.h
index 6a29228..b6debc2 100644
--- a/impl/src/sdr/sdr.h
+++ b/impl/src/sdr/sdr.h
@@ -32,6 +32,8 @@ typedef struct {
 
 	sdr_status_t   status;
 	bool           tx_done;
+	double         tx_duration;
+	double         tx_start_time; // time when tx_callback() was first called
 } sdr_ctx_t;
 
 result_t sdr_init(sdr_ctx_t *ctx);