As a promising motion-adaptive tumor treatment, respiratory-gated radiotherapy carries its merits of minimizing incidence and severity of normal tissue complication by precisely adapting the radiation beam to the targeted lung tumor. Accurate measurement of respiration motion is of vital importance in respiratory-gated radiotherapy. However, conventional gating techniques, relying on implanted internal markers or external markers placed on patients' body, are either invasive to the patient's body or bring insufficient accuracy and discomfort to the patients. In this paper, we present an accurate noncontact means of deriving respiratory gating signals for the use in gated lung cancer radiotherapy. Two 5.8 GHz radar sensors are used to monitor the chest wall and abdominal movements simultaneously to get high resolution and enhanced parameter identification. The radar sensors can also be used in structural health monitoring (SHM). It is shown that our multiple radar system can supply reliable gating signals for accurate mobile tumor tracking in radiotherapy.