## 3 Instrument Description  
The LAnTeRN proof-of-concept instrument is composed of two assemblies, the transmitter (Tx) and the receiver (Rx). The construction of each is very similar, consisting of a rigid plastic enclosure with an attached mast pole. The mast pole is topped with Tx/Rx optics and Wi-Fi antennas. The system block diagram is shown in Figure 5, and the proof-of-concept instrument specifications are given in Table 1. The Tx consists of two stable DFB laser wavelength sources (online and offline) with precise current and temperature control, capable of maintaining the wavelengths to \(<\)\(\pm\)0.2 pm. Each wavelength is amplified and individually modulated by semiconductor optical amplifiers (SOA). These sources are combined and then transmitted through a single fiber to the mast. An in-line tap couples a portion of the transmitted signal to a reference detector. That electrical signal is amplified,  
Figure 4: Modeled spectral density of noise components and total noise from photodetector and transimpedance amplifier. Figure 5: System block diagram of the prototype LAnTeRN system. filtered, and then digitized by the processor. A software lock-in function decodes the transmitted amplitudes by detecting their respected modulation tones. The Tx combined signal is sent toward the Rx from a simple open fiber as a broad-angle beam. Using a wider beam angle minimizes the aiming accuracy required. After passing through the atmosphere the light is received by the Rx, which has a field-of-view approximately equal to that of the Tx. Using commercial optics and a narrow-band optical filter (BPF), the Rx focuses the signal on an InGaAs detector whose electrical output is then amplified and filtered. The Rx processor digitizes the amplified signal, decodes the digitized signal using a lock-in function with a stable frequency reference, and saves the resulting output data from the lock-in to file. Both Tx/Rx, shown in Figure 6, have a GPS receiver and Wi-Fi communication. The GPS receiver allows accurate time-stamping and precise geo-location. The Wi-Fi communication enables remote control and monitoring of the receiver functions from the transmitter location. \begin{table}
\begin{tabular}{c c} \hline Manufacturer & Exelis, Inc. \\ First Model Completed & December 2012 \\ Dimensions (Tx \& Rx) & 52 \(\times\) 81 \(\times\) 30 cm, 132 cm mast \\  & (20.5\({}^{\mathrm{o}}\)\(  
The LAnTeRN transmitter software was designed to control the laser wavelengths, the modulation intensity, and the modulation frequency while monitoring the wavelength of the online laser and controlling the relative intensity of the modulated laser signals. An algorithm was developed to actively maintain a fixed ratio of transmitted optical power between the two laser wavelengths in order to avoid introducing transmitter fluctuations into the received signal and to allow the receiver to operate without direct communication with the transmitter. In the case where the transmitter and receiver are co-located, the transmitter fluctuations can be divided out of the received signal using the transmitted beam as a reference. When the transmitter and receiver are separated, the software must maintain a fixed, known transmitted ratio of online to offline laser power. The latter case does introduce some noise in the system and limits the SNR that can be achieved. This limitation is discussed further in Section 3. The receiver processor software utilizes a digital lock-in processing algorithm that allows for very long lock-in integration times. One challenge of long-integration lock-in processing is saving the high-sample-rate data for the duration of the lock-in period. The LAnTeRN receiver software overcomes this necessity with a near-real-time continuous lock-in algorithm that allows integration times of hours without using large blocks of memory. This capability, coupled with the stability of the frequency standards, allows the LAnTeRN system to make measurements at extremely low signal levels.