or CLASS. Calibration and observing techniques The receiver noise temperature is calibrated at the start of every observing shift by measuring the difference in receiver response to ambient temperature and liquid nitrogen temperature loads. The loads are made of Eccosorb, a carbon-impregnated foam highly absorbent to microwaves and cone-shaped to prevent direct reflection of LO back to the feed. Carbon monoxide line intensities are calibrated using the room-temperature chopper wheel method and the two-layer atmosphere model of Kutner (1978). At the carbon monoxide signal frequency the atmospheric opacity is appreciable, mostly due to molecular oxygen and water vapor, and corrections to the observed line intensities for signal attenuation must be applied. Kutner's two-layer model of the atmosphere parameterizes the elevation dependence of the correction factor in terms of only 3 parameters, each of which has a physical interpretation. Because oxygen has a much greater scale height than water vapor, the model assumes they can be considered separate layers, oxygen above water, with different characteristic temperatures and opacities. The temperature and opacity of oxygen in the upper atmosphere do not vary much seasonally and are assumed to be constant at 255 K and 0.378, respectively, at the signal frequency. The remaining parameters in the model, the temperature and opacity of water and the fraction of the received power from the sky, are determined through antenna tippings (measurements of the intensity of the sky signal as a function of elevation) at least once per six-hour observing shift, and more frequently if the weather is changing. Typical zenith water opacities ranged from 0.10 to 0.15, with values as low as about 0.05 in the coldest, driest weather. A 1-second calibration is performed at the start of each scan to correct for short term variations of the receiver gain and atmospheric opacity. The observing season for the 1.2 m telescope, like other millimeter-wave telescopes at temperate northern latitudes, generally runs from October to May, with the best conditions in November through March. Cold, dry days afford the best observations, because of the decreased atmospheric opacity due to water vapor and the colder sky in general. Overall, the weather permits operation of the telescope roughly half of the time between October and May. To obtain flat spectral baselines close to the galactic plane where emission typically covers a large range in velocity, spectra were acquired by position switching every 15 s between the source position