diff -r f774ff724ee4 src/mobility/position-allocator.cc
--- a/src/mobility/position-allocator.cc	Mon Sep 07 11:59:10 2009 +0100
+++ b/src/mobility/position-allocator.cc	Tue Sep 08 09:58:07 2009 +0200
@@ -315,4 +315,72 @@
 }
 
 
+
+NS_OBJECT_ENSURE_REGISTERED (UniformDiscPositionAllocator);
+
+TypeId
+UniformDiscPositionAllocator::GetTypeId (void)
+{
+  static TypeId tid = TypeId ("ns3::UniformDiscPositionAllocator")
+    .SetParent<PositionAllocator> ()
+    .SetGroupName ("Mobility")
+    .AddConstructor<UniformDiscPositionAllocator> ()
+    .AddAttribute ("rho",
+                   "The radius of the disc",
+                   DoubleValue (0.0),
+                   MakeDoubleAccessor (&UniformDiscPositionAllocator::m_rho),
+                   MakeDoubleChecker<double> ())
+    .AddAttribute ("X",
+                   "The x coordinate of the center of the  disc.",
+                   DoubleValue (0.0),
+                   MakeDoubleAccessor (&UniformDiscPositionAllocator::m_x),
+                   MakeDoubleChecker<double> ())
+    .AddAttribute ("Y",
+                   "The y coordinate of the center of the  disc.",
+                   DoubleValue (0.0),
+                   MakeDoubleAccessor (&UniformDiscPositionAllocator::m_y),
+                   MakeDoubleChecker<double> ())
+    ;
+  return tid;
+}   
+
+UniformDiscPositionAllocator::UniformDiscPositionAllocator ()
+{}
+UniformDiscPositionAllocator::~UniformDiscPositionAllocator ()
+{}
+
+void 
+UniformDiscPositionAllocator::SetRho (double rho)
+{
+  m_rho = rho;
+}
+void 
+UniformDiscPositionAllocator::SetX (double x)
+{
+  m_x = x;
+}
+void 
+UniformDiscPositionAllocator::SetY (double y)
+{
+  m_y = y;
+}
+Vector
+UniformDiscPositionAllocator::GetNext (void) const
+{
+  UniformVariable r (-m_rho, m_rho);
+  double x,y;
+  do
+    {
+      x = r.GetValue ();
+      y = r.GetValue ();
+    }
+  while (sqrt(x*x + y*y) > m_rho);
+  
+  x += m_x;
+  y += m_y;
+  NS_LOG_DEBUG ("Disc position x=" << x << ", y=" << y);
+  return Vector (x, y, 0.0);
+}
+
+
 } // namespace ns3 
diff -r f774ff724ee4 src/mobility/position-allocator.h
--- a/src/mobility/position-allocator.h	Mon Sep 07 11:59:10 2009 +0100
+++ b/src/mobility/position-allocator.h	Tue Sep 08 09:58:07 2009 +0200
@@ -182,7 +182,8 @@
 
 /**
  * \brief allocate random positions within a disc
- * according to a pair of random variables.
+ * according to a given distribution for the polar coordinates of each
+ * node with respect to the provided center of the disc 
  */
 class RandomDiscPositionAllocator : public PositionAllocator
 {
@@ -204,6 +205,30 @@
   double m_y;
 };
 
+
+/**
+ * \brief allocate positions which are uniformly distributed within a
+ * disc of a given radius.  Note that this is NOT equivalent to using
+ * a RandomDiscPositionAllocator with a uniformly-distributed rho.
+ */
+class UniformDiscPositionAllocator : public PositionAllocator
+{
+public:
+  static TypeId GetTypeId (void);
+  UniformDiscPositionAllocator ();
+  virtual ~UniformDiscPositionAllocator ();
+
+  void SetRho (double rho);
+  void SetX (double x);
+  void SetY (double y);
+
+  virtual Vector GetNext (void) const;
+private:
+  double m_rho;
+  double m_x;
+  double m_y;
+};
+
 } // namespace ns3
 
 #endif /* RANDOM_POSITION_H */