diff --git a/coordinates/sat.py b/coordinates/sat.py
index 2fc8285..18a6a5c 100644
--- a/coordinates/sat.py
+++ b/coordinates/sat.py
@@ -214,7 +214,7 @@ def gps_sat_xyz(ephemeris, sec):
     u0k = tettak + w0
     uk = u0k + cuc * cos(2 * u0k) + cus * sin(2 * u0k)
 
-    rk = a0 * (1 + e0 * cos(ek)) + crc * cos(2 * u0k) + crs * sin(2 * u0k)
+    rk = a0 * (1 - e0 * cos(ek)) + crc * cos(2 * u0k) + crs * sin(2 * u0k)
 
     ik = i0 + (cic * cos(2 * u0k) + cis * sin(2 * u0k)) + i_dot * tk