NASA has a need for airborne or space-based observatories and remote sensors in order to penetrate the opaque atmosphere between 1 and 10 THz. For observations >2 THz, technologically mature microwave sources typically have microwatt power levels which are insufficient to act as LOs for a heterodyne receiver. LongWave Photonics is proposing to develop a compact, frequency agile, frequency locked, single mode quantum cascade laser (QCL) system. The distributed feedback grating (DFB) QCL arrays pack multiple devices on a single semiconductor die with individual devices lasing at different frequencies. The source will be frequency agile over 150 GHz with center frequencies ranging from 2 to 5 THz range. The DFB QCL array will be packaged in a high-reliability Stirling cycle cooler. The source will be frequency locked to a gas reference cell which has multiple absorption lines. The lines are much more closely spaced than the IF bandwidth of the detector, allowing continuous frequency coverage over the tunable range. Phase I LO power is expected to be > 1 mW with > 10 mW in Phase II. Methods for amplitude stabilization will be investigated.