A system for determining a concentration of an analyte in a medium of a living animal, the system comprising: an optical sensor, the sensor including: a first graft covering at least a first portion of the sensor housing; first indicator molecules embedded in the first graft and having a first optical characteristic responsive to the concentration of the analyte; a second graft covering at least a second portion of the sensor housing; second indicator molecules embedded in the second graft and having a second optical characteristic responsive to the concentration of the analyte; a substrate; a first light source fabricated in or mounted on the substrate and configured to emit first excitation light to the first indicator molecules in the first graft; a first signal photodetector fabricated in or mounted on the substrate and configured to receive first emission light emitted by the first indicator molecules and output a signal indicative thereof; a first reference photodetector fabricated in or mounted on the substrate and configured to receive first reference light and output a signal indicative thereof; a second light source fabricated in or mounted on the substrate and configured to emit second excitation light to the second indicator molecules in the second graft; a second signal photodetector fabricated in or mounted on the substrate and configured to receive second emission light emitted by the second indicator molecules and output a signal indicative thereof; and a second reference photodetector fabricated in or mounted on the substrate and configured to receive second reference light and output a signal indicative thereof; and a transceiver configured to: receive the signals indicative of the first signal light, the first reference light, the second signal light, and the second reference light; determine whether the first graft is damaged or degraded using one or more of the received signals; and if the first graft is determined to be damaged or degraded, calculate a concentration of the analyte in the medium of the living animal based on the received signals indicative of the second signal light and the second reference light but not the received signals indicative of the first signal light and the first reference light. a first graft covering at least a first portion of the sensor housing; first indicator molecules embedded in the first graft and having a first optical characteristic responsive to the concentration of the analyte; a second graft covering at least a second portion of the sensor housing; second indicator molecules embedded in the second graft and having a second optical characteristic responsive to the concentration of the analyte; a substrate; a first light source fabricated in or mounted on the substrate and configured to emit first excitation light to the first indicator molecules in the first graft; a first signal photodetector fabricated in or mounted on the substrate and configured to receive first emission light emitted by the first indicator molecules and output a signal indicative thereof; a first reference photodetector fabricated in or mounted on the substrate and configured to receive first reference light and output a signal indicative thereof; a second light source fabricated in or mounted on the substrate and configured to emit second excitation light to the second indicator molecules in the second graft; a second signal photodetector fabricated in or mounted on the substrate and configured to receive second emission light emitted by the second indicator molecules and output a signal indicative thereof; and a second reference photodetector fabricated in or mounted on the substrate and configured to receive second reference light and output a signal indicative thereof; and a transceiver configured to: receive the signals indicative of the first signal light, the first reference light, the second signal light, and the second reference light; determine whether the first graft is damaged or degraded using one or more of the received signals; and if the first graft is determined to be damaged or degraded, calculate a concentration of the analyte in the medium of the living animal based on the received signals indicative of the second signal light and the second reference light but not the received signals indicative of the first signal light and the first reference light. receive the signals indicative of the first signal light, the first reference light, the second signal light, and the second reference light; determine whether the first graft is damaged or degraded using one or more of the received signals; and if the first graft is determined to be damaged or degraded, calculate a concentration of the analyte in the medium of the living animal based on the received signals indicative of the second signal light and the second reference light but not the received signals indicative of the first signal light and the first reference light.