Within this function the distance is obtained in meters, but a piece of the calculation should be done in kilometers, as hinted by the function GetAttenDbKm (). 


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HINTED SOLUTION: 
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double
UanPropModelThorp::GetPathLossDb (Ptr<MobilityModel> a, Ptr<MobilityModel> b, UanTxMode mode)
{
  double dist = a->GetDistanceFrom (b);
  
  return m_SpreadCoef * 10.0 * std::log10 (dist)
         + dist/1000.0 * GetAttenDbKm (mode.GetCenterFreqHz () / 1000.0);
}