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% radar_eq.m: radar equation for monostatic and bistatic cases

% GUISDAP v.1.60 96-05-27 Copyright Asko Huuskonen and Markku Lehtinen

% 

% function radar_eq



  function ad_coeff=radar_eq(eff)



  global ch_fradar  ch_gain  ch_Pt ch_az ch_el p_XMITloc p_RECloc

  global v_lightspeed v_electronradius v_Boltzmann p_dtau p_R0 p_N0 

  global ad_range ad_w a_Magic_const

  global lpg_h lpg_w lpg_code lpg_bcs lp_vc vc_ch

  global ADDR_SHIFT name_site 

  global sc_angle sc_R0 sc_R1 ch_range k_radar k_radar0

  global a_Offsetppd lpg_wr GUP_iniver



  scale=(p_dtau*1e-6*v_lightspeed/2); % Scale factor from p_dtau units to meters



  Cbeam=0.460; % This factor depends on the beam geometry

  if nargin==0, eff=0.651; end

  lambda=v_lightspeed./ch_fradar;

% antenna efficiency is the ratio of the true gain and

% the theoretical gain 4*pi*A/lambda^2



% First find the ranges and the scattering angles from the common volume

% For monostatic case quite simply 

% Check that distance from transmitter to receiver less than 100 m

if max(abs(gg2gc(p_XMITloc)-gg2gc(p_RECloc)))<.1

  sc_angle=pi;

  sc_R0=p_R0*scale;

  sc_R1=p_R0*scale;

  k_radar=k_radar0;



  % Effective beam cross section

    Aeff=Cbeam*eff*(4*pi*sc_R0^2)./ch_gain;



  % Volume for unit pulse length 

    Veff=Aeff*scale; 

  % Comment: Pulse lengths are also expressed in units of p_dtau

  %    Multiplication by scale is included here so that

  %    is is not needed in functions dirthe and power_prof

elseif GUP_iniver<1.71

  fprintf('Obsolete, please reinitialise\n');

  [gg_sp,angle,ranges]=loc2gg(p_RECloc,[ch_el(1),ch_az(1),ch_range(1)],p_XMITloc);

  sc_angle=angle; sc_R0=ranges(2)*1E3; sc_R1=ranges(1)*1E3;

% Effective scattering volume in m^3 for the remotes

  P=(32*pi/0.3215)^2/3.63;

  Veff=(pi/P)^1.5*(sc_R0^2*sc_R1^2)/sqrt(sc_R0^2+sc_R1^2)/sin(sc_angle);

% fprintf(' Veff is %.4g\n',Veff)

  % Update range variables for the new geometry

  ind=find(lpg_bcs=='s'); range=sc_R1/scale;

  lpg_h(ind)=range*ones(size(ind));

  lpg_w(ind)=(range/100)*ones(size(ind));

  ad=ADDR_SHIFT+lpg_addr(ind);

  ad_range(ad)=range*ones(size(ad));

  ad_w(ad)=(range/100)*ones(size(ad));

  % Update now the radar k 

  k_radar=k_radar0*sin(sc_angle/2);

else

  ch=1;

  [gg_sp,angle,ranges]=loc2gg(p_RECloc,[ch_el(ch),ch_az(ch),ch_range(ch)],p_XMITloc);

  sc_angle=angle;

  sc_R0=ranges(2)*1E3;

  sc_R1=ranges(1)*1E3;

% IH: Since we now use monostatic setups, use that Veff

%     corrected later by intersection range.

%     Note: the following assumes that we do not cut the volume in pieces

  Aeff=Cbeam*eff*(4*pi*sc_R0^2)./ch_gain;

  Veff=Aeff*scale;%*1.9853; 

  opening_angle=0.6*pi/180*(1+sin(sc_angle)^2);

  %rx beam width   opening_angle*range*sc_angle_factor/pulse_speed

  range_cover=round(opening_angle*sc_R1/sin(sc_angle)/scale);

  %rise time: intersection + squewing

  rise_time=round(opening_angle*sc_R0*(tan(sc_angle-pi/2)+tan(pi/2-sc_angle/2))/scale);

  fprintf('Intersection: [Width Rise]=[%.0f %.0f] us',[range_cover rise_time]*p_dtau)

  fprintf(' Veff=%.4g\n',Veff(1))

  a11=[1:rise_time]/rise_time;

  if range_cover>rise_time

   a1=[a11 ones(1,range_cover-rise_time-1) fliplr(a11)];

  else

   a1=[a11(1:range_cover) fliplr(a11(1:range_cover-1))];

   rise_time=range_cover-1;

  end

  % Now go from square beam to a round one

  a1=.5+.5*sin((a1-.5)*pi);

  % Update range variables for the new geometry

  range=sc_R1/scale;

  % Edges do not fill the beam, correct for range

  range_factor=(1+rise_time/range*(sin((1:rise_time)/rise_time*pi/2)-1)).^(1-cos(sc_angle));

  a1(1:rise_time)=a1(1:rise_time).*range_factor;

  a1((end-rise_time+1):end)=a1((end-rise_time+1):end)./fliplr(range_factor);

  % Update now the radar k 

  k_radar=k_radar0*sin(sc_angle/2);

  if issparse(lpg_wr), lpg_wr=full(lpg_wr); end

  if isempty(a_Offsetppd)

    ppdoff=0;

  else

    ppdoff=a_Offsetppd/p_dtau;

  end

end



% Single electron cross section at the beam intersection for unit power

  polfac=1-0.5*sin(sc_angle)^2;

%fprintf(' Polarization factor is %.2f\n',polfac)

  P0perPt=4*pi*(v_electronradius^2)*polfac*...

    (ch_gain./(4*pi*sc_R0^2)).*(ch_gain./(4*pi*sc_R1^2)).*(lambda.^2/(4*pi));



  p_coeff0=P0perPt.*Veff*p_N0; 

% Calculate now the factor for virtual channels

  vc=find(vc_ch>0); % These virtual channels in use

  p_coeff0=p_coeff0(vc_ch(vc))./(v_Boltzmann*Ap(vc,0)/(p_dtau*1e-6));



for sig=find(lpg_bcs=='s')

  % Must find the virtual channel now

  lp=lpg_lp(sig);

  vc=lp_vc(lp(1));

  addr=ADDR_SHIFT+lpg_addr(sig);

  if max(abs(gg2gc(p_XMITloc)-gg2gc(p_RECloc)))<.1 | GUP_iniver<1.71

% For monostatic case calculate for each point the range factor

    ad_coeff(addr)=p_coeff0(vc)*(sc_R1./(scale*ad_range(addr))).^2;

  else

% For bistatic case calculate for each point the antenna beam factor

%  a=conv(a1,ones(1,round(lpg_w(sig))))/lpg_w(sig);

%  a=[0 a 0]; at=dummyrange-ppdoff+(1:length(a))-(length(a)-1)/2;

%  at=[0 at 100000/p_dtau]; a=[0 a 0];

   a=conv(lpg_wr(:,sig),a1)/sum(lpg_wr(:,sig));

   at=(1:length(a))-(length(a1)-1)/2-lpg_h(sig)-ppdoff+p_R0/p_dtau;

   b=interp1(at,a,ad_range(addr))'; c=sign(b*lpg_w(sig)-1);

   ad_range(addr)=range*c; ad_w(addr)=range_cover*c;

   ad_coeff(addr)=p_coeff0(vc)*b;

   lpg_w(sig)=range_cover;

   lpg_h(sig)=range;

  end

end