estimation is as following:

y*=x'b+e, e follows N(0,sigma^2) , normal distribution

y*<a1,  y<0

a1=<y*=<a2,  y=0

y*<a2,  y>0

How could I get the Ln[G(yj |  Xj,a,b)] (the log likelyhood)in order to implement the Maximum Lilelyhood estimation
for a1 and a2?

Could anyone please help me ?