Structured state space model estimation

A=[-0.4809 0 -9.87 -9.87;0 0 0 0;0 1 0 0;0 -1 0 -5.316] B=[0;0;0;0] C=eye(4) D=[0;0;0;0] inimodal=idss(A,B,C,D,'StateName',{'u' 'q' 'theta' 'a1'},'InputName','input','OutputName',{'udot' 'qdot' 'thetadot' 'a1dot'}) %K=[0 0 0 0;0 0 0 0;0 0 0 0;0 0 0 0] %inimodal=idss(A,B,C,D) inimodal.Structure.A.Free(1,2)=0 inimodal.Structure.A.Free(1,3)=0 inimodal.Structure.A.Free(1,4)=0 inimodal.Structure.A.Free(2,2)=0 inimodal.Structure.A.Free(2,3)=0 inimodal.Structure.A.Free(3,:)=0 inimodal.Structure.A.Free(4,1)=0 inimodal.Structure.A.Free(4,2)=0 inimodal.Structure.A.Free(4,3)=0 inimodal.Structure.B.Free(1,1)=0 inimodal.Structure.B.Free(2,1)=0 inimodal.Structure.B.Free(3,1)=0 inimodal.Structure.C.Free=0 inimodal.Structure.D.Free=0 inimodal.Structure.K.Free=0 opt = ssestOptions; opt.InitialState = 'zero' data=iddata(ssout,input,0.002) sys=ssest(data,inimodal,'InitialState','zero','N4Weight','MOESP','Focus','simulation'); %opt=ssestOptions('InitialState','estimate','Focus','simulation') %modal=ssest(data,inimodal,opt);

This is the code im using to estimate the unknown parameters in a structured matrix. But,could not get any model with a good fit until now. The fit i get out of this is not even 1 %. please help with estimation of the parameters