diff --git a/apps/spacer/README.md b/apps/spacer/README.md
index e9f645320..5a8725a96 100644
--- a/apps/spacer/README.md
+++ b/apps/spacer/README.md
@@ -2,3 +2,7 @@
 
 Compare GPS with Baido and Glonass
 
+This turns GNSS receiver into mode with all three systems enabled, and
+displays debug info from all of them. Click into top left corner to
+switch navigation systems.
+