DOE/SC-ARM/TR-076

> 183-GHz Radiometer (GVR) Handbook
>
> November 2006
>
> MP Cadeddu
>
> Work supported by the U.S. Department of Energy,
>
> Office of Science, Office of Biological and Environmental Research

# Contents {#contents .Heading-Front}

[1 General Overview 1](#general-overview)

[2 Contacts 1](#contacts)

[3 Deployment Locations and History
1](#deployment-locations-and-history)

[4 Near-Real-Time Data Plots 1](#near-real-time-data-plots)

[5 Data Description and Examples 2](#data-description-and-examples)

[6 Data Quality 5](#data-quality)

[7 Instrument Details 6](#instrument-details)

# Figures {#figures .Heading-Front}

[1 Examples of GVR Unfiltered and Filtered Brightness temperatures.
4](#__RefHeading___Toc152472272)

[2 GVR Simplified Component Level Block Diagram
7](#__RefHeading___Toc152472273)

[3 GVR-Measured Brightness Temperatures as a Function of PWV Retrieved
from the\
Co-Located MWR 8](#__RefHeading___Toc152472274)

# Tables {#tables .Heading-Front}

[1 Status and Location of the GVR 1](#__RefHeading___Toc152472355)

[2 Primary Variables 2](#__RefHeading___Toc152472356)

[3 Secondary Variables 2](#__RefHeading___Toc152472357)

[4 Diagnostic Variables 3](#__RefHeading___Toc152472358)

[5 Data Quality Thresholds 3](#__RefHeading___Toc152472359)

[6 Dimension Variables 3](#__RefHeading___Toc152472360)

[7 Instrument Specifications 7](#__RefHeading___Toc152472361)

# General Overview

The G-Band Vapor Radiometer (GVR) provides time-series measurements of
brightness temperatures from four double sideband channels centered at ±
1, ± 3, ± 7, and ± 14 GHz around the 183.31-GHz water vapor line.
Atmospheric emission in this spectral region is primarily due to water
vapor, with some influence from liquid water. The 183.31 ± 14-GHz
channel is particularly sensitive to the presence of liquid water. The
sensitivity to water vapor of the 183.31-GHz line is approximately 30
times higher than at the frequencies of the two-channel microwave
radiometer (MWR) for a precipitable water vapor (PWV) amount of less
than 2.5 mm. Measurements from this instrument are therefore especially
useful during low-humidity conditions (PWV \< 5 mm).

# Contacts

## Mentor

Maria Cadeddu

Decision and Information Sciences Division

Argonne National Laboratory, Bldg. 900

Argonne, Illinois 60439

630.252.7408

[mcadeddu@anl.gov](mailto:jcliljegren@anl.gov)

## Vendor/Instrument Developer

ProSensing, Inc.

107 Sunderland Road

Amherst, Massachusetts 01002

413.549.4402

[info@prosensing.com](mailto:info@radiometrics.com)

# Deployment Locations and History 

[]{#__RefHeading___Toc152472355 .anchor}**Table 1**. Status and Location
of the GVR

  ----------- ------------- -------------- ------------- ----------- -------------
  **Serial\   **Property\   **Location**   **Date\       **Date\     **Status**
  Number**    Number**                     Installed**   Removed**   

  001                       NSA/C1         2006/09/1                 Operational
  ----------- ------------- -------------- ------------- ----------- -------------

# Near-Real-Time Data Plots

Plots of near-real-time data can be viewed at the DQHands (Data Quality
Health and Status) system accessible through the web site:
<http://dq.arm.gov/>. Click on "QC Metrics and Plots" and select the
desired site and data stream. The GVR is located at the site "North
Slope of Alaska" (NSA), the data stream is "nsagvrC1.b1" or
"nsagvrC1.a0" and the facility is "C1."

# Data Description and Examples

## Data File Contents

Data streams available from the Atmospheric Radiation Measurement
Program Archives are:

-   **nsagvrC1.a0:** Raw signal counts at all frequencies

-   **nsagvrC1.b1:** Calibrated, unfiltered and filtered brightness
    temperatures

### Primary Variables and Expected Uncertainty[^1]

The primary variables measured by the GVR are brightness temperatures at
183.31 ± 1, 183.31 ± 3, 183.31 ± 7, and 183.31 ± 14-GHz. Because of some
interference caused by a near-by U.S. Air Force radar a filter is
routinely applied to all data. In the nsagvrC1.b1 datastream there are
two sets of brightness temperatures one is the original time-series with
interference spikes and the second one is the filtered time series.

[]{#__RefHeading___Toc152472356 .anchor}**Table 2**. Primary Variables

  ------------ --------------------------------------------- ---------- -----------------
  **Variable   **Quantity Measured**                         **Unit**   **Uncertainty**
  Name**                                                                

  tbsky1       183.3 ± 1 GHz sky brightness temperature      K          2 K
               (filtered)                                               

  tbsky3       183.3 ± 3 GHz sky brightness temperature      K          2 K
               (filtered)                                               

  tbsky7       183.3 ± 7 GHz sky brightness temperature      K          2 K
               (filtered)                                               

  tbsky14      183.3 ± 14 GHz sky brightness temperature     K          2 K
               (filtered)                                               

  tbsky1u      183.3 ± 1 GHz sky brightness temperature      K          2 K
               (unfiltered)                                             

  tbsky3u      183.3 ± 3 GHz sky brightness temperature      K          2 K
               (unfiltered)                                             

  tbsky7u      183.3 ± 7 GHz sky brightness temperature      K          2 K
               (unfiltered)                                             

  tbsky14u     183.3 ± 14 GHz sky brightness temperature     K          2 K
               (unfiltered)                                             
  ------------ --------------------------------------------- ---------- -----------------

### Secondary/Underlaying Variables

[]{#__RefHeading___Toc152472357 .anchor}**Table 3**. Secondary Variables

  ------------ ---------------------------------------------- ---------- -----------------
  **Variable   **Quantity Measured**                          **Unit**   **Uncertainty**
  Name**                                                                 

  time         Time offset from midnight                      s          

  warm1        183.3 ± 1 GHz warm load signal                 count      

  warm3        183.3 ± 3 GHz warm load signal                 count      

  warm7        183.3 ± 7 GHz warm load signal                 count      

  warm14       183.3 ± 14 GHz warm load signal                count      

  hot1         183.3 ± 1 GHz hot load signal                  count      

  hot3         183.3 ± 3 GHz hot load signal                  count      

  hot7         183.3 ± 7 GHz hot load signal                  count      

  sky1         183.3 ± 1 GHz sky signal                       Count      

  sky3         183.3 ± 3 GHz sky signal                       Count      
  ------------ ---------------------------------------------- ---------- -----------------

### Diagnostic Variables

[]{#__RefHeading___Toc152472358 .anchor}**Table 4**. Diagnostic
Variables

  ----------------------------------- ----------------------------- ---------- -----------------
  **Variable Name**                   **Quantity Measured**         **Unit**   **Uncertainty**

  temp_hot1                           Temperature of the hot        C          0.2 C
                                      absorber at the tip                      

  temp_hot2                           Temperature of the hot        C          0.2 C
                                      absorber at the center                   

  temp_warm                           Temperature of the warm       C          0.2 C
                                      absorber                                 

  temp_ext                            Temperature of external       C          0.5 C
                                      sensor                                   

  temp_component_plate                Physical temperature of       C          
                                      component plate                          

  temp_antenna_feed_horn              Physical temperature of       C          
                                      antenna feed horn                        

  temp_thermoelectric_cooling_plate   Physical temperature of       C          
                                      thermoelectric cooling plate             
  ----------------------------------- ----------------------------- ---------- -----------------

### Data Quality Flags

Data quality flags are named qc\_'fieldname' (i.e. qc_tbsky1). Possible
values for qc_flags are: 0 (value is within the specified range), 1
(missing value), 2 (value is less than the specified minimum), 4 (value
is greater than the specified maximum), and 8 (value failed the valid
"delta" check). Specified maximum and minimum values are shown in Table
5.

[]{#__RefHeading___Toc152472359 .anchor}**Table 5**. Data Quality
Thresholds

  --------------------------- --------------------- ---------------------
  **Field Name**              **Min**               **Max**

  tbsky1                      3                     310

  tbsky3                      3                     310

  tbsky7                      3                     310

  tbsky14                     3                     310

  temp_hot1                   45                    75

  temp_hot2                   45                    75

  temp_warm                   10                    20
  --------------------------- --------------------- ---------------------

### Dimension Variables

[]{#__RefHeading___Toc152472360 .anchor}**Table 6**. Dimension Variables

  -------------------- -------------------------- ------------------------
  **Field Name**       **Min**                    **Max**

  base_time            Base time in Epoch         seconds since 1970-1-1
                                                  0:00:00 0:00

  time_offset          Time offset from base_time s

  lat                  north latitude             degrees

  lon                  east longitude             degrees

  alt                  altitude                   meters above Mean Sea
                                                  Level
  -------------------- -------------------------- ------------------------

Some additional variables defined in the global fields are:

-   delta_time_warm = time difference between the observation of the sky
    and the warm target = 4 s

-   delta_time_hot = time difference between the observation of the sky
    and the hot target = 2 s

## Annotated Examples

In this section we show an example of unfiltered and filtered data the
user should expect. In Figure 1 we show unfiltered (black line) and
filtered (red line) brightness temperatures at the four GVR channels (±
1, ± 3, ± 7, and ± 14-GHz from top to bottom). Data were collected on
September 23, 2006.

![](media/image1.jpeg){width="5.0375in" height="3.6125in"}

[]{#__RefHeading___Toc152472272 .anchor}**Figure 1**. Examples of GVR
Unfiltered (black line) and Filtered (red line) Brightness Temperatures

## User Notes and Known Problems

Between 4/12/2004 and 9/1/2006 the instrument was working as a
prototype. Data collected during this time have segments of elevated
noise due to a slowly varying reduction of receiver sensitivity. This
effect was dependent on the system temperature and it is noticeable
during warmer days (surface temperatures \>\~ 255 K).

## Frequently Asked Questions

This section is not yet available.

# Data Quality

## Data Quality Health and Status

Daily quality check on this data stream can be found at the DQHands
page: <http://dq.arm.gov/>. Click on "QC Metrics and Plots" and select
the desired site and data stream. The GVR is located at the site "NSA,"
the data stream is "nsagvrC1.b1" or "nsagvrC1.a0" and the facility is
"C1."

## Data Reviews by Instrument Mentor

The instrument mentor submits a monthly summary report IMMS accessible
from the instrument web page. Some of the general checks performed by
the instrument mentor are shown below.

1\. In general the filtered brightness temperature time series should be
smooth and with low noise levels.

2\. Brightness temperatures should be greater than 2.75 K and less than
approximately 310 K (We do not expect the ambient temperature in Barrow
to be significantly higher than 30 C).

3\. External temperature readings (ext_temp) can be compared to tower
measurements. The agreement should be +/- 2 K.

4\. The difference between the tip and the center of the hot absorbet
(t_hot1-t_hot2) should be less than 2 C.

5\. The temperature of the hot target (temp_hot1 and T_hot2) should be
greater than 45 C and less than 75 C.

6\. The temperature of the warm target should be greater than 10 C and
less than 20 C.

7\. Measured brightness temperatures are also compared with model
computations as a general quality check.

## Data Assessments by Site Scientist/Data Quality Office

The Data Quality office daily data assessment can be view at the DQHands
web page.

## Value-Added Procedures and Quality Measurement Experiments

No Value-Added Procedures are available at this stage.

# Instrument Details

## Detailed Description

### List of Components

-   RF section: white plastic box with rectangular Mylar window on top.

-   Desktop PC host computer with monitor, keybord and mouse, and
    National Instrument break-out box mounted on top pf PC enclosure.

-   Temperature control box.

-   Radiometer cart with industrial blower on bottom shelf.

-   Cables.

### System Configuration and Measurement Methods

In this section we give a brief description of the GVR hardware
configuration. The material in this section can be found in \[1\]. Refer
to the same reference for further details on this instrument. A
simplified block diagram of the GVR receiver is shown in Figure 2. The
down-welling atmospheric radiation is captured by a 10 cm diameter, 1.7°
beam-width, 90-degree parabolic metal mirror and focused to a corrugated
feed horn. A sub-harmonically pumped mixer, using a 91.655 GHz LO
signal, down-converts the upper and lower sidebands to base-band, where
a broad-band low-noise amplifier increases the noise signal power and
sets the receiver noise temperature. A broadband power splitter divides
the amplified signal between four channels before filtering. The center
frequency and bandwidth of the filters are 1/0.5, 3/1, 7/1.4 and 14/2
GHz respectively. The band-limited noise signals are square-law
detected, converted to a TTL pulse-train using highly linear Analog
Devices AD650 voltage-to-frequency converters (20 ppm nonlinearity and
less than 0.3 ms response time to a step input as configured), and
frequency-counted using an FPGA processor. This frequency counting
effectively integrates and measures the square-law detector voltage, or
equivalently the noise power. The measured noise power from the four
channels together with the instrument temperature readings are time
stamped and transmitted to a data logger PC via an RS232 (or optional
RS422) bus.

![](media/image2.wmf)

[]{#__RefHeading___Toc152472273 .anchor}**Figure 2**. GVR Simplified
Component Level Block Diagram

To stabilize the temperature of the receiver components, a packaging
technique described in was employed, consisting of an insulated
cold-plate in a box, in a temperature controlled enclosure. The
resulting component plate temperature stability limited the effect of
receiver-gain rate-of-change to below 100 K/Hr, requiring system gain
and offset calibrations only a few times a minute to achieve a sub-K
measurement precision.

Since neither noise sources nor low-loss fast switches are readily
available at G-band, external (to the antenna horn) hot and warm
calibration absorbers are required to track the receiver gain and
offset. The metal mirror of the GVR is rotated with a stepper motor to
point the radiometer antenna beam to the calibration loads. The hot and
warm calibration absorbers are constructed using the Firam-160 absorber,
with the hot load packed in an insulated box covered with a 1 mil Mylar
window (\~0.03 dB loss factor) tilted by 4°. The Mylar window is tilted
by about twice the antenna beam-width to minimize the reflection of the
receiver emitted radiation back to the receiver. The hot load is
convection heated to a uniform temperature of 343 +/-- 0.5 K, while the
warm load is left to soak to the temperature of the outer enclosure,
which is heated with a PID controller to 293 K. The temperature of the
absorbers is monitored with Resistance Temperature Detectors (RTD)
temperature sensors and the readings are recorded along with the
receiver noise power data.

### Specifications

[]{#__RefHeading___Toc152472361 .anchor}**Table 7**. Instrument
Specifications

  ---------------------------------------- ------------------------------
  **Parameter**                            **Value**

  Receiver noise temperature 1 GHz         1750 K

  Receiver noise temperature 3 GHz         1610 K

  Receiver noise temperature 7 GHz         1600 K

  Receiver noise temperature 14 GHz        2170 K

  Measurement precision, NEDT (Noise       0.2 K (200 ms averaging time)
  Equivalent Delta-T)                      

  Measurement Stability (Allan STD)        \< 0.05 K over 1000 s

  Measurement rate                         10/min with continuous
                                           calibration

  Antenna aperture                         4"

  Antenna beamwidth                        1.5°

  Temperature range                        -40 to 10 C (Environmental
                                           Chamber tested)

  Output                                   ASCII data files
  ---------------------------------------- ------------------------------

## Theory of Operation

The GVR measures brightness temperatures from four double sideband
channels centered at ± 1, ± 3, ± 7, and ± 14 GHz around the 183.31-GHz
water-vapor line. Atmospheric emission in this spectral region is
primarily due to water vapor, with some influence from liquid water. The
183.31 ± 3 and 183.31 ± 14‑GHz channels are particularly sensitive to
the presence of liquid water.

The sensitivity of the GVR channels to the presence of water vapor is
much stronger than that of the 22‑GHz water vapor line. In Figure 3 the
dependence of GVR-measured clear-sky brightness temperatures on PWV is
shown \[2\]. The PWV is retrieved from the MWR measurements. The
circles, diamonds, triangles, and squares are model computations from
one year of radiosonde data. Figure 3 shows the nonlinear response of
the GVR to water vapor as well as the saturation of the channels close
to the line center. These results are consistent with those of \[3\] who
estimated the sensitivity to PWV at these frequencies to be
approximately 30 times higher than at the frequencies of the MWR for PWV
less than 2.5 mm. The 183.31 ± 14 GHz channel sensitivity to liquid
water path is about 3.5 times greater than the sensitivity of the
31.4-GHz channel of the MWR \[2\].

The sensitivity of this channel to ice scattering was estimated by \[4\]
and found to be negligible (\< 0.6 K) at the GVR frequencies for typical
winter ice clouds. Since the presence of Ice Water Path will increase
the brightness temperature observed by the 183.3 ± 14 GHz channel, the
presence of Ice Water Path would be interpreted as a contribution from
liquid clouds.

![](media/image3.wmf){width="5.504166666666666in" height="3.925in"}

[]{#__RefHeading___Toc152472274 .anchor}**Figure 3**. GVR-measured
brightness temperatures as a function of PWV retrieved from the
co-located MWR. Data are for non-cloudy conditions only. The circles,
diamonds, triangles, and squares are model computations from one year of
radiosonde data. The nonlinear response to PWV is evident in the ± 1 and
± 3 channels \[2\].

## Calibration

### Theory

The GVR is calibrated using a warm (\~293 K) and a hot (\~333 K)
FIRAM-160 absorber. The absorbers are tilted by about 10 deg to ensure
better than -60 dB reflectivity or 0.999999 emissivity. The resulting
absorber brightness temperatures are taken to be equal to their physical
temperature. The warm load "floats" at the internal temperature of the
GVR enclosure. The hot load, packaged in an insulated box with a Mylar
window, is convection heated: behind the absorber (opposite of the Mylar
window) is a heated metal plate and a row of fans. The fans circulate
the air around the load indirectly heating it. RTD placed at several
locations in and around the hot load indicate that the maximum
temperature differences are less than 0.5 K. The average temperature
reported by sensors is used as the hot load equivalent brightness
temperature. The accuracy of the temperature sensors is approximately
0.2 K, making them the most significant source of calibration error.
This is because a + 0.2 K error at the 333 K hot load temperature and a
-- 0.2 K error at the 293 K warm load temperature will amplify to a --
2.8 K error at a 33 K sky brightness temperature.  The metal mirror of
the radiometer continuously cycles between the warm load, hot load and
the sky, calibrating the instrument once every 10 seconds. This
calibration is sufficiently frequent as the Allan Deviation stability of
the GVR receiver was measured to be less than 0.05 K at a ∆T of 1000
sec. The operational calibration of the GVR was independently verified
in the spring of 2006 with an external calibration load. A larger
insulated hot load convection chamber was constructed with a higher
precision temperature meter and sensors, calibrated with a
NIST-traceable voltmeter and temperature probe with a 0.05 K absolute
accuracy. The measured absolute error was less than 2 K in the four
receiver channels over the brightness temperature measurement range. The
brightness temperatures used in this paper are the ones originally
obtained from the instrument. The revised calibration produced a slight
(\~ 1 K) increase in the brightness temperatures of the ±1 and ±3‑GHz
channel slightly improving the agreement with the model. In the next
section we will apply the calibration correction to a few cases to
determine the possible contribution of calibration uncertainties to the
measurement.

### Procedures

Brightness temperatures ![](media/image4.wmf){width="0.25in"
height="0.25in"} are produced for each channel according to:

![](media/image5.wmf){width="1.8611111111111112in"
height="0.2638888888888889in"} (1)

where *V~sky~* is the signal recorded when the reflector is oriented
towards the sky and *V~warm~* is the signal recorded from the warm
target at temperature *T~warm~*. G is the gain computed as:

![](media/image6.wmf){width="1.1111111111111112in"
height="0.4583333333333333in"} (2)

where *V~hot~* is the signal recorded from the hot target and
![](media/image7.wmf){width="0.25in" height="0.20833333333333334in"} is
computed by averaging the temperature of the hot target at the tip
(*T~hot1~*) and at the center (*T~hot2~*).

![](media/image8.wmf){width="1.4583333333333333in"
height="0.20833333333333334in"} (3)

Keeping into account the Mylar loss factor L = 1.0116, the final
brightness temperature is computed as:

![](media/image9.wmf){width="2.3194444444444446in" height="0.25in"}. (4)

A filter is applied to the raw data (counts) as well as to the final
brightness temperatures to produce the final brightness temperatures.
The filter acts to ensure that each data point is bounded within the
range of intensities defined by its neighbors. For each data point, the
maximum and minimum values of the first four neighbors are determined.
If the difference between the point and the maximum (or the minimum) is
greater than a fixed threshold (in this case set to 3 K) the point is
replaced with the average of the four neighbors. This filter is less
smoothing than a traditional median filter as it trades noise
suppression for temporal resolution.

### History

-   Deployed as SBIR prototype in Barrow, Alaska on April 14, 2005

-   Calibration verified with external calibration box on April 19, 2006

-   Factory hardware upgrades on May--August 2006

-   Reinstallation in Barrow on September 1, 2006

## Operation and Maintenance

### User Manual

Not available.

### Routine and Corrective Maintenance Documentation

Not available.

### Software Documentation

Not available.

### Additional Documentation

Not available.

## Glossary

**Uncertainty:** We define uncertainty as the range of probable maximum
deviation of a measured value from the true value within a 95%
confidence interval. Given a bias (mean) error *B* and uncorrelated
random errors characterized by a variance *σ^2^*, the root-mean-square
error (RMSE) is defined as the vector sum of these,

$\text{RMSE} = \left( B^{2} + s^{2} \right)^{\frac{1}{2}}$.

(*B* may be generalized to be the sum of the various contributors to the
bias and *σ^2^* the sum of the variances of the contributors to the
random errors). To determine the 95% confidence interval we use the
Student's *t* distribution: *t*~n;0.025~ ≈ 2, assuming the RMSE was
computed for a reasonably large ensemble. Then the *uncertainty* is
calculated as twice the RMSE.

See the ARM Glossary (<http://www.arm.gov/about/glossary.stm>)

## Acronyms

See the ARM Acronyms and Abbreviations.
(<http://www.arm.gov/about/aconyms.stm>)

## Citable References

\[1\] AL Pazmany. 2006. "Compact 183- GHz radiometer for water vapor and
liquid water sensing." Submitted to *IEEE Transactions of the
Geosciences Remote Sensing,* June.

\[2\] MP Cadeddu, JC Liljegren*,* and A Pazmany. 2006. "Measurements and
retrievals from a new183‑GHz water-vapor radiometer in the Arctic."
accepted by *IEEE Transactions of the Geosciences Remote Sensing,*
September.

\[3\] PE Racette, ER Westwater, Y Han, AJ Gasiewski, M Klein, D Cimini,
DC Jones, W Manning, EJ Kim, JR Wang, V Leuski, and P Kiedron. 2005.
"Measurements of low amounts of precipitable water vapor using
ground-based millimeterwave radiometry." *Journal of Atmospheric and
Oceanic Technology*, Vol. 22, no. 4, pp. 317-333, April.

\[4\] D Cimini, ER Westwater, AJ Gasiewski, M Klein, V Leuski, and J
Liljegren. 2006. "Ground-based millimeter- and submillimeter-wave
observations of low vapor and liquid water contents." Submitted to *IEEE
Transactions of the Geosciences Remote Sensing,* June.

[^1]: See section (6.5) for a definition of uncertainty.