A S7 should be fine. Are you careful with your antenna position? If the XSP is directly above you, the antenna should be parallel to each other and parallel to the ground. If it 1000+m away, the antenna should be parallel to each other, equidistant from the aircraft and tilted slight back from vertical. If you're a long ways away and at a higher altitude, the tilt should be a little more. There's a diagram in the XSP manual.