2.1. Measurement Technique
 WVMS4 detects the 616-523 H2O transition line at 22.235 GHz. The principle of operation of millimeter wave receivers is well documented by Parrish et al. , among others. The instrument uses a flat mirror positioned at 45° to the axis of rotation, which is defined by the center of the antenna beam, to direct the incoming signal from the desired direction.
 The measurement cycle begins with a “tipping measurement,” during which the sky is scanned at 29 discrete angles. The 11 angles from 45° to 75° above the horizon are used in the calculation of the tropospheric optical depth near 22 GHz. The additional 18 angles are used to diagnose pointing issues, antenna issues, absorber bar problems, and other parameters that are helpful to determine failure points remotely.
 Following the tipping measurement, the instrument enters a noise-balancing routine. The mirror alternates between two positions so that the beam is first pointed at a predetermined reference angle near zenith containing both sky and a reference absorber bar at ambient temperature. The ambient temperature reference absorber bar, developed in 2002, consists of a 12 × 65 cm piece of AN74 mounted on a 9 × 61 cm plank of 1.2 cm thick poplar wood. It is positioned 1 m from the antenna at 45° to the incident radiation in two axes to reduce reflections, as shown in Figure 1. The instrument then determines a low-elevation (signal) angle at which the total power of the measurement is similar to the total power of the measurement at the reference angle. The elevation angle of the reference position is chosen to give a signal angle of at least ∼20° above the horizon when the tropospheric opacity is low, ensuring that the antenna beam does not intersect the ground in the signal position. We note that, while it is possible to balance the system by fixing the signal angle and varying the reference angle instead, this has proven to be unsatisfactory because variations in the position of the reference absorber in the antenna beam can result in changes to the instrumental baseline.
Figure 1. WVMS4 as installed at Table Mountain, California. The instrument has outdoor and indoor components. The waveguide and upper rails pass through an acrylic window.
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 The measurement scan then begins using this combination of signal and reference angles. Ten signal-reference pairs are collected (a subscan) before the balancing routine is run again. Five subscans constitute a full scan, after which the data are recorded. The total integration time of a full scan is about 18 min. The measurement cycle consists of four full scans, each taken at slightly different combinations of two antenna positions and two reference angle positions. Between full scans the antenna is moved along its axis by 1/4 wavelength, i.e., 3.378 mm at 22.2 GHz, in order to both monitor and allow for the cancelation of any standing waves which may occur between the antenna and any external structure in the beam (most importantly the reference absorber) as shown in Figure 2. In addition, the reference angle is generally alternated between two angles after each pair of 1/4λ antenna movements. This allows us to monitor the sensitivity of the measurement to the position of the reference absorber bar in the beam.
Figure 2. Block diagram of WVMS4. As shown here, the antenna, RF circuit, and spectrometer ride on a carriage that is periodically moved 1/4λ relative to the mirror.
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2.2. Front End
 The instrument stand is constructed of 6 mm aluminum square tubing and is rugged enough to withstand mild earthquakes and the very high winds which are common at Table Mountain. Since the primary goal of these measurements is to detect trends in water vapor, it is extremely important that the positions and pointing of the antenna and mirror remain stable over a period of years. If the true pointing is ∼1° closer to the horizon than is assumed in the retrieval, then retrieved mixing ratios at all altitudes will be ∼7% too large at a typical angle of measurement due to the resultant error in the assumed air mass. Therefore, the instrument has been designed to be free standing, bolted only to the floor, and not in any way connected to the wall. Any variation in this stability can be easily detected with a precision inclinometer placed anywhere on the stand. The stand is 1.2 m tall and cantilevers the center of the mirror 80 cm from the building to keep the edge of the roof out of the antenna beam.
 The mirror is a 56 × 38 cm ellipse constructed of a lightweight honeycomb core aluminum panel and mounted at a 45° angle to the axis of rotation. The mirror is rotated by an Empire Magnetics U42 stainless steel waterproof stepping motor, which is required to hold the mirror stationary to within 0.1° in 110 km/h wind. This motor has been designed to survive in the harshest of weather conditions including being buried in snow and ice at Table Mountain. It is operated by a Parker Gemini 8-A motor driver which is set to 20,000 counts per revolution, giving an angular resolution of 0.018°. A Parker 6K2 two-axis controller interfaces the motor to the computer and provides feedback from the encoder and home sensor. The mirror is brought to the home position marked by the motor's internal home sensor every four scans to ensure precise movement. The antenna and rotating mirror are aligned using a helium neon laser mounted on the rear of the horn with a custom fixture, and alignment is verified through 360° of rotation. During some seasons it is possible to use the sun as a target to check the alignment of the system, but the utility of these measurements is limited since they can only be used to infer errors along the axis of rotation of the mirror.
 The antenna is a corrugated feed horn with a 15 cm aperture and 43 cm in length producing a gain of ∼25 dB and a FWHM, or 3 dB, beam width of ∼8°. It was manufactured by hand in the early 1990s, and due to its length and conical nature consists of five segments. These segments pose a problem for long-term measurements, since good electrical contact between them is essential to maintain the beam shape [DeWachter et al., 2009]. Water can work its way into the seams between the segments and leave behind dirt and mineral deposits. These deposits can build up over time and separate the segments, causing artifacts in the data. Care is taken to cover each segment with silicon tape, and they are periodically taken apart and cleaned. Heat tape is used to heat the entire horn above the ambient temperature. The horn is also insulated with fiberglass and covered with heavy aluminum foil.
 In previous generations of WVMS instruments the interior of the horn has been protected by using a resonant window made of two sheets of Dow Chemical's polyvinylidene chloride microwave plastic placed at an angle to the face of the horn and separated by 1/4λ. The interior of the horn was then placed under positive pressure with warm dry air passing between the plastic sheets. However, this plastic is flexible and moves slightly with atmospheric pressure, causing traveling baseline waves. While retrievals in the upper stratosphere and mesosphere were possible with such a covered feed horn, retrievals in the midstratosphere and lower stratosphere, which use bandwidths as large as 400 MHz, were found to be significantly affected by the resonant window. At this time, both the instrument at Table Mountain and the instruments at Mauna Loa are operated without the cover; however because of insects it may not be possible to operate an instrument without a cover at Lauder.
 The antenna, RF chain, programmable attenuator, and spectrometer all ride on a carriage mounted on cross roller bearing Micro Slides. This carriage is coupled to the frame via a Parker 404 motion table with home and limits proximity sensors. The table is driven by a Parker OS series U23 stepper motor giving the carriage motion control to better than 25 μm. The carriage is moved 3.378 mm (1/4λ) between each full scan. This motor is driven by a Parker Gemini 5-A driver and controlled by the same Parker 6K2 controller as the outdoor mirror motor.
 The instrument is controlled using custom software and a custom built industrial PC. In addition there is an auxiliary box containing the following three USB devices. The noise diodes are operated via relay by a Measurement Computing (MC) Switch and Sense USB device. The indoor, outdoor, bar, and hot load temperatures are measured with YSI 46040 glass thermistors. These are attached to an MC USB Temperature device which converts resistance to temperature. The third USB device, an MC 1608, records voltages produced by the Macro Sensor's Linear Variable Differential Transformer (LVDT) that precisely measures the position of the translation table. The WVMS4 instrument can be accessed, powered on/off, and operated remotely. An external Internet-accessible camera equipped with motion detection monitors the viewing angles and mirror positions.
 The analog signal received by the horn travels through an RF/IF front end with two down conversions before being digitized by the spectrometer, as shown in Figure 3. The instrument is calibrated after each subscan (five times per scan) using a Noise Com 5142 noise diode as a reference. This noise diode injects 15 dB (ENR) noise at 22.235 GHz. The noise output variation with temperature is less than 0.01 dB/°C and the noise output variation with voltage is less than 0.1 dB/1% ΔV. The noise diode is coupled into the signal using a Flann Microwave 20 dB broadwall coupler. There is a second noise diode for redundancy, and the two diodes are compared every four scans. The signal travels though the couplers into a Miteq AMFWW-SF-21982248-16-10P room temperature amplifier with a noise figure of 1.3 dB at 22.235 GHz, a passband of 500 MHz, and gain of 35 dB, as shown in Figure 3. This is followed by a 500 MHz band-pass filter which removes the lower sideband. The RF signal then passes to the first IF mixer and preamp, Spacek Labs P22-1.25 with another 25 dB of gain. The first LO frequency is 21.26 GHz which is provided by a Miteq PLDRO with a stability of ±0.01 ppm. This mixes the line center frequency down to 975 MHz. The cabling between the amplifier, oscillator, and the first IF mixer is critical for stable measurements.
Figure 3. The RF signal chain and the two intermediate frequency bands as the signal is mixed down from 22.235 GHz to 252 MHz.
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 The 975 MHz signal then passes through another 500 MHz band-pass filter, through a Mini-Circuits ZFL-2000 20dB amplifier, and into a Mini-Circuits ZFM-2 mixer. The second LO frequency is 723 MHz, provided by a Nova Engineering NovaSource G6 which also utilizes a 10 MHz reference stable to ±1.0 ppm.
 Although the back end spectrometer measures from 0 to 500 MHz there is an artifact, further discussed in section 2.3, that occurs precisely in the middle of this range (at 250 MHz). Hence the second IF is centered at 252 MHz in order to avoid placing this artifact precisely at the center frequency of the molecular emission. The signal then passes through an antialiasing low-pass filter and into the final gain stage provided by a Mini-Circuits ZFL-2000 amplifier with 20 dB of gain. The RF front end is contained in an aluminum enclosure with power connections, providing EM shielding. The components heat the interior of the box above ambient temperature, but the box is not temperature controlled. The SMA connector on the output of the box passes the 500 MHz bandwidth signal from the antenna centered at 252 MHz with a total power of ∼6 dBm.
2.3. Back End
 The Acqiris AC240, now sold by Agilent as the U1080A-001, is an 8-bit high-speed 6U compact PCI Digitizer with onboard signal processing utilizing the Xilinx Vertex II PRO FPGA. This device is installed in an Adlink 6130R 1U compact PCI two slot chassis. An Adlink PCI to PCIX bridge occupies the second slot which allows communication with the PC. For use as a 16,384 channel spectrometer, the AC240 requires 32 Kpoint Fast Fourier Transform analyzer firmware also sold by Agilent. For this application we use the entire dynamic range of the spectrometer at a 1 Gs/s sampling rate. In this setup the spectrometer has a bandwidth of 500 MHz and a resolution of 30.52 KHz. 20,000 samples are accumulated by the unit for each mirror position. With communication, transfer time, and screen refresh, this takes less than three seconds. At the 2285 m altitude of the Table Mountain site, the FPGA temperature is maintained at about 40°C. To account for variations in room temperature, the firmware provides an internal calibration of the unit, which is performed with each measurement.
 The spectrometer is very sensitive to input power. The manufacturer suggests that care should be taken to select an input voltage range that will allow the signal to be recorded using as much of the dynamic range of the digitizer as possible. A nominal total input power of 1 dBm is preferable. To accommodate this requirement the RF signal is passed through an Aeroflex-Weinschel programmable attenuator between the IF box and the AC240. The attenuation is set to provide a total power of 1 dBm to the AC240 from the front end analog components when the instrument is installed. The attenuation is increased by 2 dB during tipping measurements, noise diode comparisons, and manual calibrations. Depending on pointing, sky conditions, and calibration sources, the total input power to the spectrometer varies slightly.
 The output of the AC240 contains several periodic artifacts. Some of these artifacts become more or less prominent depending on input power. The unit operates on a 62.5 MHz clock that is doubled to 125 MHz which produces harmonic spikes. The 16th subharmonic 7.8125 MHz is prominent, and most of the larger spikes are multiples of this value with the largest spikes at 125 MHz and 250 MHz. With the input of the unit terminated, the zero measurement produces a spectrum in several pieces with six major segments. Each of these segments has a slightly different zero level, as shown in Figure 4. Most of these patterns can be accounted for by recording the noise floor and subtracting it from the measured spectrum. A zero level is recorded during observations by increasing the attenuation on the variable attenuator to 64 dB. However, there is a step 885 kHz wide centered at 250 MHz. The size of this step changes with input power, as shown in Figure 5. Another prominent feature at the zero input level is a 122.0703125 kHz pattern which is the 1024th subharmonic of 125 MHz. This pattern persists throughout the raw data measurements. Since these patterns are nearly identical in the signal and reference positions, they should not affect the spectra used in the retrievals. Nevertheless, we have centered the spectrometer at 252 MHz so that none of these features falls near the center of the emission, since measurements from these channels very near the line center provide most of the information for the upper stratospheric and mesospheric retrievals.
Figure 4. Two different Acqiris AC240 units with the inputs terminated. Although the spikes differ in magnitude, the pattern is very similar.
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Figure 5. The 885 kHz feature found at the center of the Acqiris AC240 with (left) the input terminated and with (right) the signal from the sky.
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