diff --git a/src/gnss.c b/src/gnss.c
index 64fcbcf..2a1b38a 100644
--- a/src/gnss.c
+++ b/src/gnss.c
@@ -8,8 +8,12 @@
 #include "manager.h"
 #include "at.h"
 
+#include <sys/sendfile.h>
+#include <sys/stat.h>
+#include <errno.h>
+
 #define BUFFER_SIZE 256
-#define UPLOAD_DELAY 100000
+#define UPLOAD_DELAY_US 10000
 #define RESCHEDULE_IN_SECS 30
 
 static void gnss_step(struct EG25Manager *manager);
@@ -297,39 +301,34 @@ static void init_assistance_data_upload(struct EG25Manager *manager)
 
 static void upload_assistance_data(struct EG25Manager *manager)
 {
-    char buffer[2*BUFFER_SIZE];
-    gint len;
-    gboolean success = TRUE;
+    gint error;
+    glong written_total = 0;
+    gint ret;
+    struct stat sb;
 
-    /* Copy downloaded XTRA assistance data to the modem over serial */
-    while((len = read(manager->gnss_assistance_fd, buffer, 2*BUFFER_SIZE)) > 0)
-    {
-        len = write(manager->at_fd, buffer, len);
-        if (len < 0) {
-            success = FALSE;
-            g_error("Writing GNSS assistance data failed: %d", len);
-            break;
-        }
-        usleep(UPLOAD_DELAY);
-        g_message("Uploaded %d bytes", len);
+    if (fstat(manager->gnss_assistance_fd, &sb) != 0) {
+        g_error("Unable to upload xtra data: %s", g_strerror(errno));
     }
 
+    do {
+        errno = 0;
+        /* Copy downloaded XTRA assistance data to the modem over serial */
+        ret = sendfile(manager->at_fd, manager->gnss_assistance_fd, &written_total, sb.st_size);
+        error = errno;
+        usleep(UPLOAD_DELAY_US);
+    } while ((!error && written_total < sb.st_size) || (ret == -1 && error == EAGAIN));
+
     /* Clear QFUPL AT command and process next */
     at_next_command(manager);
 
     /* Go to the next step if successful */
-    if (success) {
+    if (!error) {
+        g_message("Successfully uploaded %ld bytes to the modem", written_total);
         manager->gnss_assistance_step++;
         gnss_step(manager);
-    }
-    /* Restart upload */
-    else {
-        g_message ("Rescheduling upload because of failure in %ds",
-                   RESCHEDULE_IN_SECS);
+    } else {
+        g_critical("Unable to upload xtra data: %s", g_strerror(error));
         manager->gnss_assistance_step = EG25_GNSS_STEP_LAST;
-        g_timeout_add_seconds(RESCHEDULE_IN_SECS,
-                              G_SOURCE_FUNC(gnss_upload_assistance_data),
-                              manager);
     }
 }