function s = h3d_scan_add_noise(s, sigma)

	% s = h3d_scan_add_noise(s, sigma)
	% adds noise to the readings
	
	nrays = numel(s.thetas_deg);
	nscans = numel(s.tilt_deg);
	for k=1:nscans
	for i=1:nrays
		rho = s.readings(k,i);

		if not(rho > 0) 
			continue
		end
		
		noise = randn * sigma;
		s.readings(k,i) = s.readings(k,i) + noise;
	end
	end