Double_t shift;  // with B-field scaled down with beam momentum
                     // exept: PSD_Zpos = 6 & (En = 2 || En = 4): B-field = 0.6T

    if (PSD_Zpos == 6.)
    {
	if (En == 2) shift = 19.6;
	else if (En == 4) shift = 11.2;
        else shift = 7.9;
    }
    else if (PSD_Zpos == 8.) shift = 10.9;
    else if (PSD_Zpos == 10.) shift = 13.9;
    else if (PSD_Zpos == 12.) shift = 16.9;
    else if (PSD_Zpos == 15.) shift = 21.4;
    else shift = 0.;

    shift /= module_pitch; // in unit of module pitch

    for (Int_t il=1; il<=49; il++)
    {
	fraw[il]   = (il - 1)/7 + 1; //change if shift: -0.5 +0.5 (25AGeV Au, FieldMuonMagnet, 15m), -0.5 +0.65 (idem, idem, 19m)
	fcolmn[il] = (il - 1)%7 + 1;
	fraw[il]   -= 4;
	fcolmn[il] -= 4;
	fcolmn[il] += shift;
	//cout << "module " << il << ": raw, colmn = " << raw << ", " << colmn << endl;
	fphi_mod[il] = TMath::ATan2(fraw[il], fcolmn[il]);
	fpt_mod[il] = TMath::Sqrt(fraw[il] * fraw[il] + fcolmn[il] * fcolmn[il]);
    }