## 1 Introduction  
The authors have been exploring a new approach for measuring greenhouse gases (GHG) using Laser Absorption Spectroscopy (LAS) in an open-path configuration [1]. This new approach has the potential to enable GHG measurement from a transmitter in a geostationary orbit to a network of ground receivers. This approach complements other existing and planned approaches by providing high accuracy near-continuous measurements along a fixed column, which would enable a thorough evaluation of potential bias. This new approach also can be implemented for ground-to-ground applications which are currently unaddressed by available _in situ_ and remote methods, specifically where measurements are desired over a long path and in the presence of high aerosol content. Used in conjunction with existing methods, these new measurement approaches will provide the precisions required for distinguishing anthropogenic sources from biogenic sources and sinks on regional and local scales. The Laser Atmospheric Transmitter Receiver-Network (LAnTeRN) concept is an extension of the Exelis-developed Multifunctional Fiber Laser Lidar (MFLL) [2; 3; 4; 5]. MFLL was developed in 2004 as an airborne demonstration unit for a unique intensity-modulated continuous wave (IM-CW) lidar to actively measure integrated column amounts of atmospheric CO\({}_{2}\) and O\({}_{2}\). The MFLL system relies on low-power, high-reliability telecom laser components to implement a robust and flexible instrument. The MFLL has been extensively evaluated over 13 flight campaigns on three different aircraft by NASA LaRC and our partners at Atmospheric and Environmental Research Inc. (AER). The results show the instrument's ability to measure CO\({}_{2}\) and O\({}_{2}\) to accuracies approaching those required for the National Research Council's Active Sensing of CO\({}_{2}\) Emissions over Nights, Days and Seasons (ASCENDS) mission [6]. LAnTeRN has several fundamental characteristics in common with the MFLL instrument (e.g., an all fiber-coupled, high efficiency transmitter and utilization of the IM-CW approach), but is a fundamentally different implementation with distinct capability. The key difference is that LAnTeRN nominally operates in transmission rather than in the traditional backscatter lidar configuration, which has several distinct advantages. Operating in transmission offers the following benefits: (1) Eliminates partial path returns from clouds and aerosols as a source of bias which is a key requirement for climate data and other applications, e.g., volcano or forest fire monitoring, where large aerosol concentrations are present; (2) Significantly reduces instrument complexity and data volumes through the use of simple pure sine wave modulation and lock-in amplifier techniques; (3) Enables lower powers and smaller apertures because there is no loss of signal due to reflectance and broad scattering angles; (4) Eliminates the need for companion estimates of surface pressure derived either from model data or remotely sensed measurements to estimate dry air mole fractions, which is a big challenge for ASCENDS, due to the fact that surface pressure and geo-location information can be collected at the receiver, and (5) Reduces cost and risk for a space implementation by keeping the receivers and data collection on the ground, and thus eliminates the need to launch a large telescope and multiple high power transmitters required for ASCENDS.