measurement and analysis of weather phenomena systematic

Measurement and Analysis of Weather Phenomena with K-Band Rain Radar

Jun-Hyeong Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

[email protected]

Ki-Bok Kong Development team Kukdong Telecom

Nonsan, Republic of Korea [email protected]

Seong-Ook Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

[email protected]

Abstract—To overcome blind spots of an ordinary weather radar which scans horizontally at a high altitude, a weather radar which operates vertically, so called an atmospheric profiler, is needed. In this paper, a K-band radar for observing rainfall vertically is introduced, and measurement results of rainfall are shown and discussed. For better performance of the atmospheric profiler, the radar which has high resolution even with low transmitted power is designed. With this radar, a melting layer is detected and some results that show characteristics of the meting layer are measured well.

Keywords—K-band; FMCW; rain radar; low transmitted power; high resolution; rainfall; melting layer

I. INTRODUCTION A weather radar usually measures meteorological

conditions of over a wide area at a high altitude. Because it observes weather phenomena in the area, it is mainly used for weather forecasting. However, blind spots exist because an ordinary weather radar scans horizontally, which results in difficulties in obtaining information on rainfall at higher and lower altitudes than the specific altitude. Therefore, a weather radar that covers the blind spots is required.

A weather radar that scans vertically could solve the problem. This kind of weather radar, so called an atmospheric profiler, points towards the sky and observes meteorological conditions according to the height [1]. Also, because the atmospheric profiler usually operates continuously at a fixed position, it could catch the sudden change of weather in the specific area.

In this paper, K-band rain radar which has low transmitted power and high resolutions of the range and the velocity is introduced. The frequency modulated continuous wave (FMCW) technique is used to achieve high sensitivity and reduce the cost of the system. In addition, meteorological results are discussed. Reflectivity, a fall speed of raindrops and Doppler spectrum measured when it rained are described, and characteristics of the melting layer are analyzed as well.

II. DEVELOPMENT OF K-BAND RAIN RADAR SYSTEM

A. Antenna To suppress side-lobe levels and increase an antenna gain,

offset dual reflector antennas are used [2]. Also, separation

wall exists between the transmitter (Tx) and receiver (Rx) antennas to improve isolation between them. With these methods, leakage power between Tx and Rx could be reduced. Fig. 1 shows manufactured antennas and the separation wall.

B. Design of Tranceiver Fig. 2 shows a block diagram of the K-band rain radar.

Reference signals for all PLLs in the system and clock signals for every digital chip in baseband are generated by four frequency synthesizers. In the Tx baseband module, a field programmable gate array (FPGA) controls a direct digital synthesizer (DDS) to generate an FMCW signal which decreases with time (down-chirp) and has a center frequency of 670 MHz. The sweep bandwidth is 50 MHz which gives the high range resolution of 3 m. Considering the cost, 2.4 GHz signal used as a reference clock input of the DDS is split and used for a local oscillator (LO). the FMCW signal is transmitted toward raindrops with the power of only 100 mW. Beat frequency which has data of the range and the radial velocity of raindrops is carried by 60 MHz and applied to the input of the Rx baseband module. In the Rx baseband module, quadrature demodulation is performed by a digital down converter (DDC). Thus, detectable range can be doubled than usual. Two Dimensional-Fast Fourier Transform (2D-FFT) is performed by two FPGAs. Because the 2D FFT is performed with 1024 beat signals, the radar can have high resolution of the radial velocity. Finally, data of raindrops are transferred to a PC with local LAN via the an UDP protocol. TABLE I. shows main specification of the system.

Fig. 1. Manufactured antenna and separation wall.

2016 URSI Asia-Pacific Radio Science Conference August 21-25, 2016 / Seoul, Korea

1

Systematic Shifts in the Frequency of NPLI-CsF1

Aishik Acharya

Time and Frequency Division,

CSIR National Physical Laboratory,

Dr. K. S. Krishnan Marg, New Delhi 110012, India

Acedamy of Council of Scientific and Innovative Research,

CSIR road, Taramani, Chennai-600113, India

Email- [email protected]

Poonam Arora




Vattikonda Bharath




Acedamy of Council of Scientific and Innovative Research,

CSIR road, Taramani, Chennai-600113, India

Shuchi Yadav




Ashish Agarwal




Amitava Sen Gupta

The NORTHCAP UNIVERSITY,

Sector 23A, Gurgaon- 122017, India

Formerly Time and Frequency Division,



Abstract— At CSIR-National Physical Laboratory, India

(NPLI) we have developed first generation of Cesium atomic

fountain (NPLI-CsF1) primary frequency standard (PFS). NPLI-

CsF1 is now fully operational and its frequency is now being

evaluated along with all systematic and statistical uncertainties.

The present experimental data conforms that fountain frequency

is stable to few parts in 1015 at less than one day averaging time.

The fountain frequency is compared with the Hydrogen MASER

(H-MASER) which is contributing to TAI already and hence

fountain frequency is traceable to TAI. Recent evaluation results

were communicated to BIPM and published in circular T. Typical

stability of NPLI CsF1 is ~7 x 10-13 τ-1/2. The present performance

of fountain is limited by the measurement uncertainty of cold

collisional shift.

Keywords— Laser Cooling, Cold Atoms, Systematic Uncertainty,

Time Scale, NPLI-CsF1 .

I. INTRODUCTION

In 1967, the SI time unit, second, was defined as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133atom” [1].

According to the definition Cesium atomic fountains are now the primary frequency standard with highest measurement accuracy and minimum uncertainty that can reproduce the

length of the second. Different metrological institutes all over the world already has developed the cesium atomic fountain frequency standard with greater accuracy that already take part in evaluation of TAI (International Atomic Time) [2-11]. At National Physical Laboratory India we also have developed our first generation of atomic fountain clock with typical stability of few parts in 1015 and put into operation [12]. Recently the performance of NPLI CsF1 has been evaluated with respect to TAI and the results are published in the circular T of BIPM [13-14]. During last two years continuous rigorous frequency evaluation with respect to TAI has been performed. During each evaluation cycle all the major frequency biases with their measurement uncertainties are evaluated. Typical stability of NPLI-CsF1 is about 7 x 10-13 τ-1/2. The performance of NPLI CsF1 presently is limited by the measurement uncertainty of cold collisional shift because of the poor signal to noise ratio of the fountain detection signal and we are trying to improve the uncertainty of the measurement. A detail description and short comings regarding this has been discussed in the following sections.

II. FREQUENCY EVALUATION OF NPLI CSF1

NPLI CsF1 is basically consists of three major parts viz. a)

Physics package, b) optical setup and c) electronic controls. In

figure 1 and 2 fountain physics package and detailed optics

layout is described.

974

Fountain operation is pulsed and sequential. Cesium atoms

are first laser cooled at a temperature of 6.5µK and trapped in

magneto optical trap (MOT) and molasses. Then they tossed up

through the ultra-high vacuum at a height of 65.5 cm from the

center of the MOT. During the ballistic flight on the way up and

down atom cloud interact with microwave frequency. The

microwave interrogation is performed using a TE011 cylindrical

cavity (Ramsey cavity) made of oxygen free high thermal

conductivity (OFHC) copper inside the drift region. The quality

factor of the cavity is 2000. The Ramsey cavity is mounted

inside the drift region and sits 52.5cm above the MOT center.

Another TE011 cavity made of OFHC copper is used for state

selection and is placed below the Ramsey cavity. Typical

linewidth of the central resonance of NPLI-CsF1 ~1.5Hz for an

interaction time of 325ms. Acousto Optic Modulators (AOM)s

and indigenously developed mechanical shutters are used to

block the laser light during the flight time. After interaction

with the microwave the atoms are then interrogated for their

states at the detection zone and the detection signals, in the form

of voltage signal from the photodiodes, are then acquired using

an application based on National instrument’s LabVIEW

platform using data acquisition hardware. The entire fountain

operation is autonomous and controlled by indigenously

developed fountain sequence controller.

It is now possible to run the fountain continuously without

any major interruption provided the ambient temperature is

stable. During frequency evaluation center frequency is

operated in square wave modulation across the central

resonance. During evaluation fountain frequency is compared

with hydrogen MASER which is one of the active clock already

contributing to TAI. Therefore fountain frequency is traceable

to TAI. During each of evaluation period careful estimation of

frequency biases with their uncertainty is necessary. For our

case major contributing biases like 2nd order Zeeman shift,

Black body radiation shift, cold collisional shift, gravitational

shift are carefully estimated before and after of each evaluation

cycle. A detailed evaluation scheme is shown in figure 3.

III. EVALUATION OF SYSTEMATICS (TYPE B UNCERTAINTY)

Since last two years rigorous frequency evaluation of NPLI

CsF1 has performed. Each evaluation cycle lasts for 10-20 days.

Before starting the evaluation cycle critical parameters related

with laser powers, positions of the mechanical shutters,

microwave amplitude etc. are thoroughly checked and

optimized. International inter-comparison of frequency during

May 2013 with other fountains from four different countries

already confirms all the frequency differences between the

fountains agreed within the 1-sigma uncertainty in the low 10-

15 level [15]. Figure 4 shows the stability of NPLI CsF1 with Figure 3: Optics Layout

Figure 2: Physics Package

Figure 1: Evaluation Scheme

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respect to H MASER. Typical stability is around 7 x 10-13 τ-1/2.

In the next sections we will discuss the major contributing

biases in detail.

A. 2nd Order Zeeman Shift:

The dominant frequency correction of the NPLI-CsF1 is associated with the second-order Zeeman shift. A long solenoid coil around the flight tube produces a static magnetic field in order to define the quantization axis and to remove the degeneracy of the Zeeman hyperfine levels of the ground state. Three compensation coils are used at the top and bottom of the flight tube to create highly homogeneous magnetic field in the flight region. During the measurement a thorough magnetic field mapping is performed using cold Cs atoms as probe and the non-zero mf (mf=±1, ±2, ±3…) transitions are observed. Then the bias can easily be de-convoluted using Breit Rabi formula [16]. Typical frequency bias of NPLI CsF1 due to 2nd order Zeeman Shift is around+50.5 × 10−15.

B. Black Body Radiation Shift

During Ramsey interaction atoms are constantly exposed to the electromagnetic radiation of the vacuum assembly. This leads to AC stark effect which further causes frequency shift [18]. The shift thus can be calculated as: (where T is the temperature of the vacuum enclosure)

∆𝜗𝐵𝐵𝑅

𝜗0

= −1.711 × 10−14 (𝑇

300)

4

× [1 + 0.014 (𝑇

300)

2

] … … … … … … . . (1)

Temperature of the flight tube is measured by the help of three calibrated platinum resistor thermistor. Typical value of the bias is −17.3 × 10−15 with an uncertainty of 2 × 10−16.

C. Gravitational Red Shift

In order to compare different clocks located at different

heights all over the world, a correction should be applied as the

fountain frequency is shifted by gravitational potential [17-18].

The height of the microwave cavity of NPLI CsF1 is 179.86m

from geoid. This calculation was done by the help of Septentrio

receiver as the antenna height of the receiver was precisely

calibrated. Thus the bias due to the gravitational potential

is +19.6 × 10−15. And the uncertainty due to the determination

of the height is ±0.1 × 10−15.

D. Cold collisional Shift

The collisions between cold Cs atoms shift the clock transition frequency. The shift has a linear dependence on the atomic number density. In order to estimate the collisional shift, the fountain frequency is measured at different atomic densities and extrapolating to zero density. The atom density is changed by varying the state selection microwave amplitude. The collision coefficient was calculated as:

𝐾𝑐𝑜𝑙𝑙 =𝜗𝐻𝐷 − 𝜗𝐿𝐷

𝑁𝐻𝐷 − 𝑁𝐿𝐷

… … … … … … … … … … … … … … … … . (2)

Where 𝜗𝐻𝐷,𝜗𝐿𝐷, and 𝑁𝐻𝐷, 𝑁𝐿𝐷are the frequency offset and

atomic density in high density and low density, respectively.

The co-efficient is then multiplied with the average atom count

during the evaluation cycle to obtain the bias.

To change the atomic cloud density the measurement was also

performed by changing the MOT loading time initially. But as

the detection zone is situated just above the MOT in the NPLI

CsF1 physics package, due to high background of Cs vapor the

signal to noise suffers during atom density switching. Also due

to the higher cloud temperature it is also difficult to switch atom

density between high and low other that 2:1 ratio in the present

setup. Which further includes uncertainty in the measurement.

Thus in our case the shift is estimated with a large uncertainty

of 20% of the bias value [18]. This is the dominant uncertainty

in the uncertainty budget of the fountain.

E. Other Systematic Uncertainties

Since the major contributor in uncertainty budget of the

fountain is cold collisional shift, in the frequency evaluation we

have not included the other biases like light shift, background

gas collisional, majorana transition, distributed cavity phase

shift, cavity pulling which contribute to 10-16 level. Only the

corresponding uncertainties are taken into account.

In the following table no 1 the uncertainty budget of NPLI

CsF1 is described:

Figure 4: The Allan deviation of the frequency difference between

NPLI CsF1 and H-MASER as a function of the averaging time (τ).

The typical short-term stability is 7 x 10-13 τ-1/2over measurement

period of 1day. Maser instability is dominant after around one day

averaging time.

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Table 1: Uncertainty budget of NPLI CsF1

Effect Bias

( × 𝟏𝟎−𝟏𝟓)

Uncertainty

(× 𝟏𝟎−𝟏𝟓)

2nd Order Zeeman Shift 50.5 0.06

Black Body Radiation -17.3 0.23

Gravitational Red Shift 19.6 0.11

Cold Collisional Shift -12.0 2.4

Light shift 0.0 0.2

Background gas collisions

Cavity pulling

Rabi, Ramsey Pulling

Majorana transitions

0.0

0.0

0.0

0.0

0.1

0.01

0.1

0.1

Spectral impurity 0.0 0.2

Microwave leakage 0.0 0.1

DCP 0.0 0.2

Total(UB)

40.8

2.45

IV. CONCLUSION

NPLI has developed its first Cs fountain frequency standard NPLI-CsF1. The fountain is fully operational and its frequency is being periodically evaluated over last two years. NPLI-CsF1 has successfully contributed live to BIPM for TAI evaluation and the results are reported on Circular T no 335. The achieved frequency stability is 7 x 10-13/ τ1/2, which is limited by the phase noise of the local oscillator (H-Maser). Most of systematic uncertainties have been evaluated. Major contributor among these because of cold collisional shift. Presently the total type B uncertainty of NPLI CsF1 is evaluated as 2.45 x 10-15 which is still large compared to the other contributing fountains. We are now improving the bias estimation and measurement procedure.

Acknowledgment The authors gratefully acknowledge funding from Council

of Scientific and Industrial Research.

References

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measurement and analysis of weather phenomena systematic

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