diff --git a/model/isl-propagation-loss-model.cc b/model/isl-propagation-loss-model.cc
index 86b8cb7..2d123ef 100644
--- a/model/isl-propagation-loss-model.cc
+++ b/model/isl-propagation-loss-model.cc
@@ -24,6 +24,7 @@
 
 #include "ns3/log.h"
 #include "ns3/mobility-model.h"
+#include "math.h"
 
 #include "isl-propagation-loss-model.h"
 
@@ -52,6 +53,31 @@ IslPropagationLossModel::~IslPropagationLossModel ()
 {
 }
 
+
+const double
+IslPropagationLossModel::EARTH_RAD = 6.3781E6;
+
+bool
+IslPropagationLossModel::GetLos (Ptr<MobilityModel> a, Ptr<MobilityModel> b)
+{
+  // origin of LOS
+  Vector3D o = a->GetPosition ();
+  double ol = o.GetLength ();
+
+  // second point of LOS
+  Vector3D bp = b->GetPosition ();
+  double bl = bp.GetLength ();
+
+  // unit vector
+  Vector3D u = Vector3D ((o.x-bp.x)/bl, (o.y-bp.y)/bl, (o.z-bp.z)/bl);
+
+  // center point of sphere is 0
+  double uo = (u.x*o.x) + (u.y*o.y) + (u.z*o.z);
+  double delta = (uo*uo) - (ol*ol - (EARTH_RAD*EARTH_RAD));
+
+  return delta < 0;
+}
+
 double
 IslPropagationLossModel::DoCalcRxPower (double txPowerDbm,
                                         Ptr<MobilityModel> a,
@@ -62,7 +88,7 @@ IslPropagationLossModel::DoCalcRxPower (double txPowerDbm,
   // curvature + distance)
 
   // primitivec cut-of at 1000 km
-  if (a->GetDistanceFrom (b) > 1000000.0)
+  if (!GetLos (a, b))
     {
       return 0;
     }
diff --git a/model/isl-propagation-loss-model.h b/model/isl-propagation-loss-model.h
index f98bbea..3eba95c 100644
--- a/model/isl-propagation-loss-model.h
+++ b/model/isl-propagation-loss-model.h
@@ -33,9 +33,22 @@ namespace ns3 {
 class IslPropagationLossModel : public PropagationLossModel
 {
 public:
+  static const double EARTH_RAD;
+
   static TypeId GetTypeId (void);
   IslPropagationLossModel ();
   virtual ~IslPropagationLossModel ();
+
+  /**
+   * true if there is at least one intersection of ISL line-of-sight with earth
+   * (LOS is blocked by earth)
+   *
+   * Assumes earth is sperical.
+   *
+   * \param a first node
+   * \param b second node
+   */
+  static bool GetLos (Ptr<MobilityModel> a, Ptr<MobilityModel> b);
 private:
   /**
    * Returns the Rx Power taking into account only the particular
diff --git a/test/isl-propagation-test-suite.cc b/test/isl-propagation-test-suite.cc
new file mode 100644
index 0000000..d17152b
--- /dev/null
+++ b/test/isl-propagation-test-suite.cc
@@ -0,0 +1,85 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+
+#include "ns3/core-module.h"
+#include "ns3/test.h"
+
+#include "ns3/leo-module.h"
+
+using namespace ns3;
+
+class IslPropagationAngleTestCase1 : public TestCase
+{
+public:
+  IslPropagationAngleTestCase1 ();
+  virtual ~IslPropagationAngleTestCase1 ();
+
+private:
+  virtual void DoRun (void);
+};
+
+IslPropagationAngleTestCase1::IslPropagationAngleTestCase1 ()
+  : TestCase ("Test LOS computation neighboring sats")
+{
+}
+
+IslPropagationAngleTestCase1::~IslPropagationAngleTestCase1 ()
+{
+}
+
+void
+IslPropagationAngleTestCase1::DoRun (void)
+{
+  Ptr<ConstantPositionMobilityModel> a = CreateObject<ConstantPositionMobilityModel> ();
+  a->SetPosition (Vector3D (IslPropagationLossModel::EARTH_RAD + 1000.0, 10, 0));
+  Ptr<ConstantPositionMobilityModel> b = CreateObject<ConstantPositionMobilityModel> ();
+  b->SetPosition (Vector3D (IslPropagationLossModel::EARTH_RAD + 1000.0, 0, 0));
+
+  NS_TEST_ASSERT_MSG_EQ (IslPropagationLossModel::GetLos (a, b), true, "LOS of neighboring satellites");
+}
+
+class IslPropagationAngleTestCase2 : public TestCase
+{
+public:
+  IslPropagationAngleTestCase2 ();
+  virtual ~IslPropagationAngleTestCase2 ();
+
+private:
+  virtual void DoRun (void);
+};
+
+IslPropagationAngleTestCase2::IslPropagationAngleTestCase2 ()
+  : TestCase ("Test LOS of sats without LOS")
+{
+}
+
+IslPropagationAngleTestCase2::~IslPropagationAngleTestCase2 ()
+{
+}
+
+void
+IslPropagationAngleTestCase2::DoRun (void)
+{
+  Ptr<ConstantPositionMobilityModel> a = CreateObject<ConstantPositionMobilityModel> ();
+  a->SetPosition (Vector3D (IslPropagationLossModel::EARTH_RAD + 1000.0, 0, 0));
+  Ptr<ConstantPositionMobilityModel> b = CreateObject<ConstantPositionMobilityModel> ();
+  b->SetPosition (Vector3D (- (IslPropagationLossModel::EARTH_RAD + 1000.0), 0, 0));
+
+  NS_TEST_ASSERT_MSG_EQ (IslPropagationLossModel::GetLos (a, b), false, "LOS of opposing");
+}
+
+class IslPropagationTestSuite : public TestSuite
+{
+public:
+  IslPropagationTestSuite ();
+};
+
+IslPropagationTestSuite::IslPropagationTestSuite ()
+  : TestSuite ("isl-propagation", UNIT)
+{
+  // TestDuration for TestCase can be QUICK, EXTENSIVE or TAKES_FOREVER
+  AddTestCase (new IslPropagationAngleTestCase1, TestCase::QUICK);
+  AddTestCase (new IslPropagationAngleTestCase2, TestCase::QUICK);
+}
+
+// Do not forget to allocate an instance of this TestSuite
+static IslPropagationTestSuite leoTestSuite;
diff --git a/wscript b/wscript
index 4b2b619..f122895 100644
--- a/wscript
+++ b/wscript
@@ -28,16 +28,17 @@ def build(bld):
 
     module_test = bld.create_ns3_module_test_library('leo')
     module_test.source = [
-        'test/leo-test-suite.cc',
-        'test/leo-propagation-test-suite.cc',
-        'test/isl-test-suite.cc',
+        'test/ground-node-helper-test-suite.cc',
         'test/isl-mock-channel-test-suite.cc',
-        'test/leo-mock-channel-test-suite.cc',
+        'test/isl-propagation-test-suite.cc',
+        'test/isl-test-suite.cc',
         'test/leo-input-fstream-container-test-suite.cc',
         'test/leo-mobility-test-suite.cc',
+        'test/leo-mock-channel-test-suite.cc',
+        'test/leo-propagation-test-suite.cc',
+        'test/leo-test-suite.cc',
         'test/satellite-node-helper-test-suite.cc',
-        'test/ground-node-helper-test-suite.cc',
-        ]
+    ]
 
     headers = bld(features='ns3header')
     headers.module = 'leo'