diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3af637d..0fd0321 100755
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,3 +8,5 @@ and uses [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased](https://github.com/andrewplayer3/back_projection/compare/v0.0.0...develop) -- likely 0.0.1
 
+### Added
+-- Added validation for gpu existance (defaults to cpu if none exists)
diff --git a/back_projection/process.py b/back_projection/process.py
index 7a3313c..f0c6068 100755
--- a/back_projection/process.py
+++ b/back_projection/process.py
@@ -4,6 +4,7 @@
 import argparse
 import logging
 import os
+import subprocess
 from pathlib import Path
 from zipfile import ZipFile
 
@@ -30,9 +31,19 @@ def back_projection(granule: str, username: str, password: str, use_gpu: bool) -
     file.write(output_string)
     file.close()
 
+    gpu_exists = False
+    if use_gpu:
+        #  grab gpu number, -1 if there isn't one
+        proc = subprocess.Popen(HOME+"/sentinel/bestgpu.sh", stdout=subprocess.PIPE, shell=True)
+        (bestGPU, err) = proc.communicate()
+        bestGPU = str(int(bestGPU.strip()))
+        print("GPU: ", bestGPU)
+        if bestGPU != "-1":
+            gpu_exists = True
+
     # Run the back_projection through sentinel_cpu.py or sentinel_gpu.py VV
     run_backprojection = ""
-    if use_gpu:
+    if use_gpu and gpu_exists:
         print("RUNNING WITH GPU")
         print("----------------")
         run_backprojection = HOME + "/sentinel/sentinel_gpu.py"
@@ -45,8 +56,8 @@ def back_projection(granule: str, username: str, password: str, use_gpu: bool) -
     print("Command: ", run_backprojection)
     ret = os.system(run_backprojection)
 
-    # Convert each gslc to a multiband tiff
-    # (band 1: complex, band 2: real, band 3: amplitude)
+    # create a amplitude tiff
+    # band 1: amplitude
     make_tiff = "python3 " + HOME + "/make_tiff.py " + granule + ".geo "
     make_tiff += "elevation.dem.rsc " + granule
     print(make_tiff + "_VV.tiff")
@@ -60,8 +71,8 @@ def back_projection(granule: str, username: str, password: str, use_gpu: bool) -
     print("Command: ", run_backprojection)
     ret = os.system(run_backprojection)
 
-    # Convert each gslc to a multiband tiff
-    # (band 1: complex, band 2: real, band 3: amplitude)
+    # create amplitude tiff
+    # band 1: amplitude
     print(make_tiff + "_VH.tiff")
     ret = os.system(make_tiff + "_VH.tiff")