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NASA Technical Memorandum 104611 



NASA-TM-104611 19940033279 



NASA ER-2 Doppler Radar 
Reflectivity Calibration for the 
CAMEX Project 



I. J. Caylor, G. M. Heymsfield, S. W. Bidwell, and S. Ameen 



AUGUST 1994 



p — -V 

I SEP I 3 1994 | 



LAKGL^ fe/.nci! CENTER 

HAMPTUa. VifiGif-';.', 




NA SA Technical Library 

3 1176 01403 7585 



NASA Technical Memorandum 104611 



NASA ER-2 Doppler Radar 
Reflectivity Calibration for the 
CAMEX Project 



I. J. Caylor 

S. Ameen 

Science Systems and Applications, Inc. 

Lanham, Maryland 



G. M. Heymsfield 

S. W. Bidwell 

Goddard Space Flight Center 

Greenbelt, Maryland 



National Aeronautics and 
Space Administration 

Goddard Space Flight Center 

Greenbelt, Maryland 20771 
1994 



This publication is available from the NASA Center for AeroSpace Information, 
800 Elkridge Landing Road, Linthicum Heights, MD 21090-2934, (301) 621-0390. 



Table of Contents 



Page 

1. Introduction 1 

2. EDOP Hardware 1 

3. Receiver Calibration 3 

3.1 Receiver Losses 4 

3.2 External Source Calibration 4 

3.3 Internal Source Calibration 5 

4. Antenna Gain 6 

5. Transmitted Power 7 

6. Radar Constants 7 

7. Long-Term Stability 9 

8. Range Equation 10 

9. Ground Truth Comparison 11 

9.1 WSR-88D Processing 11 

9.2 EDOP Processing 12 

9.3 Comparison of EDOP and WSR-88D 12 

10. Ocean Surface Analysis 13 

11. Forward Beam 14 

12. Summary 14 

13. References 15 



1. INTRODUCTION 

The NASA ER-2 Doppler radar (EDOP) participated in 
the Convection and Atmospheric Moisture Experiment 
(CAMEX) based at the Wallops Flight Facility during 
September and October 1993. The data obtained 
during these flights represent the first reliable 
reflectivity measurements from the EDOP instrument. 
This report describes the calibration of the CAMEX 
reflectivity data. 

Because the configuration of the EDOP microwave and 
signal processing systems has been altered since the 
conclusion of CAMEX, the calibrations presented in 
this report should not be considered definitive in 
terms of future campaigns. However, the calibration 
procedure as described can be adapted easily. 

The weather radar equation relates radar hardware 
characteristics and received power to a physically 
meaningful parameter called the effective reflectivity 
factor (Z ( ,). The equation is 



Z € - R% 



A 2 / 



10241n2 



P t G 2 <\> 2 \k? CT7T 3 



(1) 



where the quantity in brackets is termed the radar 
constant and contains the radar system parameters. 
These parameters are the wavelength (k), the antenna 
gain (G), the beamwidth (<()), the pulse width (x), 
receiver system losses (/ r ), and a dielectric constant for 
water, |fc| 2 ~0.93. The peak transmit power is (P f ), the 
received power (P r ), and range (R). 

Therefore, the radar equation can be rewritten in 
logarithmic units as 

dBZ e - RC[dB]+P r [dBm] + 20log(K) , (2) 

where RC is the radar constant in units of decibels 
(dB) and contains the hardware-dependent parameters. 
A receiver calibration is used to convert the power (P r ) 
from measured engineering units (A/D counts) into 
dBm. These values, along with a range vector (gate 
spacing), can be used to compute dBZ from Equation 
2. Zero dBZ is the reflectivity factor in dB for a 1-mm 
diameter drop per cubic meter. 

The primary purpose of this report is to evaluate the 
three terms in Equation 2. In the following sections, 
the EDOP radar will be briefly described followed by 
discussions pertaining to each of the primary terms in 
Equation 1: P t , P r , the antenna gain (G), and the losses 



(/). Using these results, the radar constants are 
computed for each of the EDOP receivers. 

Data obtained during the CAMEX flight on 5 October 
1993 are used to illustrate the results of the calibration 
analysis for precipitation echoes. In particular, the 
calibrated reflectivity is compared with data from a 
ground-based radar, and the scattering cross section 
for the ocean surface is also estimated. The ranging 
performance is evaluated, as is the long-term stability 
of the radar hardware. A summary, along with 
recommendations for calibrations in future field 
campaigns, is contained in the final section. 

Drs. Lee Miller and Robert Meneghini are thanked for 
their many helpful comments and discussions. Mr. 
Marshall Shepherd acquired the weather buoy and 
Melbourne WSR-88D data and the authors appreciate 
his diligent efforts installing the WSR-88D formatting 
software. 



2. EDOP HARDWARE 

This section is not intended to be a comprehensive 
discussion of the EDOP hardware but rather a brief 
description of hardware related to calibration issues. 
Additional details of the radar hardware and software 
can be found in NASA documentation and overviews 
by Heymsfield et al. (1989, 1991, 1993). 

EDOP is a dual antenna X band multiparameter radar 
housed in the nose of the NASA ER-2 high-altitude 
aircraft. One antenna is fixed in the nadir direction, 
while the forward antenna is oriented to stare along 
the aircraft flight track 30° forward of nadir. Table 1 
lists the primary EDOP specifications. Note that 
although the hardware for Doppler measurements was 
in place during CAMEX, the Doppler parameters were 
not provided by the real-time data system. 

A block diagram of the transmitter and receiver is 
shown in Figure 1. The two antennas share a coherent 
transmitter, the power being divided equally, as well 
as the associated oscillator and timing components. 
The nadir receiver detects vertically polarized copolar 
(W) returns while the forward receiver is a dual 
channel system that can simultaneously process the 
vertical copolar (W) and cross-polar (VH) returned 
powers. Thus, there are three logarithmic receiver 
chains from two antennas which require calibration. 
Although only reflectivity processing was available for 
CAMEX, the Doppler signal processor software is 
implemented at Goddard Space Flight Center. 




Figure 1. Simplified block diagram for EDOP. 



TABLE 1. EDOP Radar Specifications 


Transmitter 




Frequency 


9.72 GHz 


Peak Power (nominal) 


25 kw 


Duty Cycle 


0.0044 max. 


Pulse Width 


0.25, 0.5, 1.0 us 


PRF 


2200, 4400 Hz 


Antenna 




Antennas 


2 


Antenna Diameter 


0.76 m 


Antenna Beamwidth 


2.9° 


Type 


Offset paraboloid 


Gain (nominal) 


36 dB 


First Sidelobe Level 


< -26 dB 


Cross-polarization Level 


< -30 dB 


Transmit Polarization 




Nadir 


Vertical 


Forward 


Vertical 


Receive Polarization 




Nadir 


Copolar 


Forward 


Co and cross-polar 


Receiver 




Noise Figure 


1.79 dB 


Intermediate Frequency 


60 MHz 


Linear Doppler Channels 


2 


Log Reflectivity Channels 


3 


Dynamic Range with AGC 


110 dB 


Data System 




A/D Converters 


7x12 bits, 10 MHz 


Signal Processors 


24xAT&T DSP32C 


Gate Spacing 


150, 75, 37.5 m 


Gates 


872 (max) 


Integration Cycle 


0.25 to 1.0 s 


Products 




Nadir 


Z, u, a, SNR 


Forward 


Z, LDR, \), a, SNR 



Figure 1 illustrates several features that are relevant to 
the EDOP calibration. EDOP contains an internal 
calibration system routes a continuous wave (CW) 
signal into the receiver. In addition, the receiver and 
transmitter chassis are physically separated in the nose 
of the ER-2 and are connected by lengths of semirigid 
coax cable which have a significant loss. 

Although a standard target sphere calibration is highly 
desirable, the physical arrangement of EDOP poses 
problems for such a calibration. The microwave and 



data systems, along with the antennas, are fixed into 
the ER-2 nose structure pointing downwards through 
a radome. In addition, the antennas are nonsteerable. 
Therefore, performing an end-to-end calibration on an 
external target with the system installed in flight 
configuration is rather impractical. 

There are a number of operating parameters which can 
be configured at flight time. 

• Pulse repetition frequency (PRF) 

• Transmitted pulse width (x) 

• IF bandwidth 

• Integration cycle 

• Sampling frequency and number of gates 

These parameters, which may vary from flight to 
flight, depending on observational objectives, must be 
accounted for in order to properly calibrate the 
received power into effective reflectivity factor. 

In addition, the radar signal processor can operate in 
one of two modes. One mode, termed raw, provides 
a subset of the time series at each gate, typically 16 
independent samples per second. The second 
processor configuration is termed -processed mode in 
which the complete time series are integrated in real 
time at each gate over a specified time interval (typical 
integration cycles are 0.5 and 1.0 s). Since the 
averaging is done with samples from a logarithmic 
amplifier, a bias is introduced in the estimated mean. 
This bias depends on the number of independent 
samples and, hence, the integration mode of the 
processor. 



3. RECEIVER CALIBRATION 

No rigorous calibration was performed prior to 
CAMEX, but upon the completion of the experiment, 
the radar was removed from the ER-2 nose and set up 
at Goddard Space Flight Center (GSFC). The GSFC 
configuration does not include the antennas and 
involves a different set of waveguide and connecting 
radio frequency (RF) cables. As will be explained 
later, this requires some correction to the resulting 
receiver calibration in order to be applicable to the 
data recorded during CAMEX flights. 

A receiver calibration is used to convert the returned 
echo power (P r ) from engineering units into units of 
power through a function called the calibration curve. 
In general, the engineering units are binary A/D 
counts, while power is expressed logarithmically as 



dBm. Although the digitizer counts (C D ) are directly 
related to the power collected by the antenna, there 
are gains, losses and nonlinearities in the receiver 
chain which must be taken into account. 

Received power (P r ) is estimated by adding the 
receiver loss (/ r ) to the power measured at each 
calibration step (PJ, in units of dB: 



where P m is a function of the digital counts. 
3.1 Receiver Losses 



(3) 



ER-2 nose (Dicaudo, 1970). The radome loss estimate 
assumes a normal incidence angle against a plane 
radome surface. This is approximately the case for the 
nadir beam, while the loss for the forward beam may 
be slightly higher than listed in Table 2 because of the 
more complicated geometry of the radome. 

The IF filter insertion loss only needs to be considered 
for the case where the calibration measurements were 
performed with a filter different from that used to 
acquire reflectivity data. In such a case, only the 
differential insertion loss between the two filters 
should be used in Equation 4 (see Table 9). 



Received power is the echo power present at the 
antenna port and can be estimated by using the 
calibration curve and accounting for losses associated 
within the receiver itself. These losses can be divided 
into two general groups. Losses which are constant, 
such as waveguide and insertion losses, can be termed 
fixed losses. Losses which depend on the variable 
flight configuration of the radar and data processing 
(Section 2), such as length of integration and IF 
bandwidth, are termed configuration losses. It should 
be noted that configuration losses are constant for any 
given flight. 

The receiver loss for EDOP is divided into three 
components. There is a fixed loss (I) associated with 
the radome and waveguide. A second term gives the 
difference between the insertion loss of the IF filter 
(A/,) used in flight and that used for the calibration. 
The third component is a loss, or more accurately a 
correction factor, A/ c , for differences between 
laboratory and flight configurations in the semirigid 
RF cable connecting the transmitter and receiver 
enclosures. 



Table 2. EDOP Receiver Fixed Losses (/ ) 



+ l g + A/. + A/ c 



(4) 



The fixed and variable losses are listed in Tables 2 and 
3, respectively. It should be noted that the above 
values do not include the logarithmic integration bias 
which will be accounted for in the final computation 
of the radar constant. 





Nadir 


Forward 
W 


Forward 
VH 


Radome 


0.11 


0.11 


0.18 


Rotary joint 


0.1 


N/A 


N/A 


Waveguide 


0.15 


0.30 


0.24 


T/R circulator 


0.2 


0.2 


0.2 



Table 3. Correction (/ c ) for EDOP RF Cables for 
External Calibration 





Nadir 


Forward 
W 


Forward 
VH 


Flight cables 


3.55 


3.17 


3.41 


Bench cables 


1.00 


1.00 


1.00 


Correction 


2.55 


2.17 


2.41 



3.2 External Source Calibration 



In general, the fixed losses are those specified by the 
manufacturer and are assumed to be accurate. The 
waveguide loss listed in Table 2 was measured by 
personnel at the NASA Wallops Flight Facility. 

The radome loss is a theoretical estimate for an A- 
sandwich material with the dimensions as used in the 



A typical technique to calibrate the receiver chain is to 
insert a known pulsed radio frequency (RF) power 
level at the top of the chain in the best case at the 
antenna port. The RF power is stepped through a 
number of levels covering the receiver's dynamic 
range while the resulting A/D binary count values are 
recorded. In this manner, a function can be 



determined which relates A/D counts to dBm. This 
procedure effectively incorporates most, if not all, of 
the gains and losses inherent in the receiver system. 

A vector network analyzer (Hewlett Packard 8510C) 
was used to inject a single frequency RF pulse 
(t=2 us) into the nadir antenna port. The analyzer 
was stepped through the dynamic range of the 
receiver front-end low-noise amplifier, from 
approximately to -110 dBm by increments of 5 dBm. 
Separate tests were also performed for the forward 
copolar and cross-polar receivers. 

The total receiver loss for each of the three channels 
was computed from Tables 2 and 3 using Equation 4. 



Nadir W Loss: 
Forward W Loss: 
Forward VH Loss: 


2.21 dB 
1.78 dB 
2.43 dB 



In this case, A/, is not zero and is given by the 
insertion loss difference between the 8-MHz and 2- 
MHz filters (see Table 9). The circulator insertion loss 
is neglected, since it is in the calibration path. 

Using the receiver losses and calibration test data, the 
received power can be estimated from Equation 3. 
The results of the calibration for the three receivers is 
shown in Figure 2. It is clear that there is saturation 
at the top of the dynamic range at approximately 
-20 dBm. The maximum count level of 2047 is 
attained by the nadir channel, so it is likely that the 



§ 

o 
U 



1800 

1400 

1000 

600 

200 

-200 

-600 

-1000 

-1400 

-1800 



; ' 1 ..■■■ 1 ' 1 ■ L ...'. ■ t '.. I.... 1 - 1 . • ..! .. . V .J 'J l_«l L_» ..:.. 


</&—'-&- — <> 


X -'_■ - - a m 


/*' 


/ f 


J f 


rf 


/> 


yp 


*F 


— •— Nadir 




& 


— o--- Forward W 


" 


Jf^ 


-» - Forward VH 










■: i -i 1 ^ - i .- i ■_ r ■ , l - ■■. ■-- 1 




■ 



-120 



-100 



-80 -60 -40 

Received Power (dBm) 



-20 



Figure 2. Results of the three channel receiver calibration using 
an external pulsed RF source. 



A/D itself is saturating. However, the two forward 
channels saturate at lower count values, which 
indicates that the amplifiers on the signal conditioning 
card are being saturated. 

3.3 Internal Source Calibration 

EDOP provides an internal calibration chain so that 
performance of the radar can be tracked during flight. 
The RF calibration source is a continuous wave (CW) 
signal from the field-effect transistor amplifier in the 
travelling wave tube (TWT) exciter (see Figure 1). 
During calibration, the exciter signal is routed from 
the TWT with a directional coupler into a series of 
variable attenuators located in the transmitter chassis. 
The attenuator output is split with a 4-way power 
divider to feed the three receiver chains via PIN diode 
switches just after the transmit/receive circulators. 
During laboratory tests, a self-calibrating power meter 
(Hewlett Packard 437B) was connected to the fourth 
port of the power splitter to monitor the absolute 
calibration power level. 

It is important to note that losses after the switches are 
folded into the calibration curve, whereas losses ahead 
of the switches, and hence not part of the calibration 
path, must be explicitly accounted for in the analysis. 

There are two straightforward methods to measure the 
power at each attenuator setting. If the attenuation at 
a given setting is well known, then the input power to 
the attenuator can be measured once and the value at 
each step computed from the attenuation. The second 
method involves measuring the output power directly 
from the attenuator at each step; in which case, 
knowledge of the attenuation value at each setting is 
not required. Because of limitations on the dynamic 
range of the power meter, the first method was 
selected for use with EDOP calibration. 

The EDOP radar uses a pair of 6-bit digitally 
programmable RF attenuators in series which can 
provide nominal attenuation from to 126 dB. The 
specification for the digital attenuator indicates that 
the deviation from the digital setting is proportional 
(±3%) to the attenuation, and so the actual attenuation 
can vary significantly from that which is selected. In 
order to provide a greater accuracy during calibration, 
the attenuation for each of the two units was 
measured and is listed in Table 4. Attenuator B is 
generally within the manufacturer's specification, 
while the values for attenuator A deviate quite 
significantly (19.4%). 



For the calibration measurement at GSFC, short 
lengths (approximately 2.25 ft) of semirigid coax were 
used to connect the transmitter to the receiver, while 
the length of the cables used for flight operations is 
about 7.25 ft. The transmission loss of a 0.141-inch- 
diameter semirigid coax is approximately 0.38 dB ft" 1 
at a frequency of 9.72 GHz. There is an additional 
attenuation which is attributed to the SMA connectors. 
The measured attenuation for the flight cables and the 
estimated attenuation for the cables used on the bench 
are listed in Table 5 and the difference between these 
two values is the correction factor (A/ c ) in Equation 4. 

Summing the losses in Tables 2 and 5 gives the total 
receiver loss (/,) for each channel. 



TABLE 4. Measured Attenuations for the EDOP 
Digital Attenuators 



Nadir W Loss: 2.63 dB 
Forward W Loss: 2.30 dB 
Forward VH Loss: 2.55 dB 



For the internal calibration, the bandpass filter is 
identical to that used in flight so A/ f = 0. 

The resulting calibration curves from the internal 
calibration test are shown in Figure 3. These curves 
have very similar slopes to the curves in Figure 2 and 
also saturate in a similar manner. However, there are 
some obvious nonlinearities which are attributed to 
instability of the digital attenuators. 



Setting (dB) 


Attenuator A 


Attenuator B 


1 


0.93 


0.61 


2 


1.79 


1.74 


4 


3.87 


4.00 


8 


6.76 


8.16 


16 


13.41 


16.11 


32 


25.78 


31.17 


63 


52.54 


61.79 



TABLE 5. Correction (l c ) for EDOP RF Cables for 
Internal Calibration 





Nadir 


Forward 
W 


Forward 
VH 


Flight cables 


3.55 


3.17 


3.41 


Bench cables 


1.48 


1.48 


1.48 


Correction 


2.07 


1.69 


1.93 



4. ANTENNA GAIN 



The antennas used for EDOP are of an offset 
paraboloid type. The manufacturer's measured gains 
for the antennas are 



Nadir W Gain: 
Forward W Gain: 
Forward VH Gain: 


36.1 dB 

36.3 dB 

36.4 dB 



at a frequency of 9.625 GHz (Davis, 1991). The beam 
widths are approximately 2.9° in both the E and H 
planes. 

A fact that is not accounted for is that the center 
frequency for EDOP is actually 9.72 GHz, which may 
change the gains listed above because of the narrow 



u 



1800 






.'....[...'... 1. .. 




l» » • • * k A 

,P £>--0-0-©-0-0-<S> 
-■-»-■-■-»—•-■ 


- 


1400 

1000 

600 








ft 




- 


200 












- 


-200 






/* 






- 


-600 
















" 


-1000 
-1400 








- -■- - 


Nadir 

Forward W 
Forward VH 




- 


-1800 


- 








.,... ....................... 


■ 



-80 -60 -40 

Received Power (dBm) 



-20 



FIGURE 3. Three channel receiver calibration using the internal 
CW calibration source. 



design bandwidth of the antennas. However, it is 
believed that any deviations are small. 

Unfortunately, because of complications which arose 
during initial installation into the ER-2, the forward 
antenna was moved out of focus. While realignment 
of the forward antenna has subsequently been 
completed, the above antenna gains will be used for 
the CAMEX calibration. Adjustment of the radar 
constant for the forward beam is discussed in Section 
11. 



5. TRANSMITTED POWER 

The peak transmitted power for the forward and nadir 
channels were measured at GSFC. 



Nadir: 
Forward: 



68.0 dBm 

68.1 dBm 



The measurement was made at the output port of the 
transmitter chassis and includes losses for an arc 
detector, 50-dB coupler, and power splitter. However, 
in the flight configuration, there are a number of 
further losses which reduce the transmitted power as 
shown in Table 6. Note that the waveguide loss (l g ) 
from Table 2 is also listed since the same antenna is 
used for transmission and reception. 

The transmitted power is found by subtracting the 
losses in Table 6 from the measured value for the peak 
power. 



TABLE 6. Losses (l g ) for EDOP 
Transmit Power 





Nadir 


Forward 


Waveguide 


0.15 


0.30 


Rotary joint 


0.1 


0.1 


Radome 


0.11 


0.11 


Total 


0.36 


0.41 



6. RADAR CONSTANTS 

Four flights were achieved during the CAMEX project, 
and the radar configuration for each is shown in Table 

7. On the first flight only qualitative data were 
obtained because of a faulty power regulator on the 
signal conditioning card. The regulator was replaced 
and, for the remaining three flights, the processor was 
configured to operate in raw mode with an acquisition 
time of one sample per range gate every 0.06 s. The 
radar constants will be computed for the configuration 
of the final three flights. 

The radar constant, in units of dB, is given by 

2G 



RC - 169.14 + l int + l bw 



- (P t -l 8 ) + lOlog 



( X^ 



U 2 T 



(5) 



The first four terms are in units of dB and, for the 
CAMEX flights, the last term of Equation 5 evaluates 
to 61.44 dB (x=0.25ps). The effective reflectivity factor 
is defined with units of mm 6 -m" 3 , and the range is 
usually defined in units of kilometers. These two unit 
conversions introduce an additional factor of 240 dB 
(10 24 ) in the radar constant. 

The integration loss is a numerical effect caused by 
integrating the received signal in logarithmic units 
(dB) rather than linear units (mW) where the mean of 
a sum of logarithmic values is biased from the true 
mean. For randomly varying radar signals, the bias is 
a function of the number of independent samples used 
in the average (Zrnic, 1975). As the number of 
independent samples increases to about 32, the bias 
rapidly approaches a limit of 2.5 dB (Table 8). At an 
interval of 0.06 s, each sample is effectively 
independent, and, if 32 or more samples are used to 
compute the mean power, then l int = 2.5 dB. 

The loss related to the IF filter bandwidth is described 
by Doviak and Zrnic (1979). This loss results from 
distortion of the received pulse by the IF bandpass 
filter. Therefore, this loss factor can vary for EDOP, 
depending on the width of the transmitted pulse and 
the width of the selected IF filter (2 or 8 MHz). Table 
9 lists losses for the four possible combinations of 
pulse width and IF filter bandwidth and for the 
CAMEX flights i to = 3.99 dB. 

The EDOP radar constants for the CAMEX project are 
computed from Equation 5 and shown below. Note 
that for the forward VH (cross-polar) channel, the 



TABLE 7. EDOP Configuration for CAMEX 





12 September 


25 September 


3 October 


5 October 


Mode 


Processed 


Raw 


Raw 


Raw 


Pulse width (us) 


0.25 


0.25 


0.25 


0.25 


IF bandwidth (MHz) 


2.0 


2.0 


2.0 


2.0 


PRF (Hz) 


2200 


2200 


2200 


2200 


Gate spacing (m) 


75 


150 


150 


150 


Integration (s) 


1.0 


N/A 


N/A 


N/A 


Sample interval (s) 


N/A 


0.06 


0.06 


0.06 



TABLE 8. EDOP Losses (/,„,) for Logarithmic 
Integration Bias 



Independent Samples 


Integration Bias (dB) 


8 


2.1 


16 


2.3 


>32 


2.5 



TABLE 9. EDOP Losses (l lno and /,) for IF Filter Bandwidth 



3dB Bandwidth 
(MHz) 


Filter Loss, l^ (dB) 


Insertion Loss, /, (dB) 


x=0.25us 


x=1.0us 


Nadir 


Forward W 


Forward VH 


2.0 


3.99 


0.90 


4.6 


4.6 


5.0 


8.0 


0.90 


0.21 


5.3 


5.4 


5.4 



factor of 2G is actually the vertical copolar gain times 
the cross-polar gain (Section 4). 



Nadir W: 
Forward W: 
Forward VH: 


97.51 dB 
97.06 dB 
96.96 dB 



in 



7. LONG-TERM STABILITY 

The long-term stability of EDOP, particularly 
relation to thermal conditions, was investigated by 
examining two parameters extracted from the flight 
data of 5 October 1993: the receiver background noise 
level and the peak level of the transmitted pulse. 

The high power transmitted pulse, although 
significantly attenuated, leaks through a circulator and 
a T/R switch into the receiver. Since the data system 
begins digitizing prior to each PRT, the transmitted 
pulse is sampled, in a qualitative manner, at all times 
throughout the flight. Figure 4 shows the transmitted 
pulse peak level for the three receivers over a period 
of approximately 2.5 hours. After an initial period for 
the radar to reach equilibrium temperature, the 
leakage level is very constant to better than 0.25 dB. 

The receiver background noise level was computed by 
averaging data from range gates in which no cloud 
echo appeared. The gates selected for this analysis 
were those approximately 0.825 to 1.05 km below the 
ER-2, which was above cloud top for the 5 October 
1993 flight. An analysis which produced similar 
results was also performed for gates well beyond the 
surface echo. 

The background noise level (Figure 5) shows several 
short-term excursions of about 2 dB. Analysis of 
surface return echoes shows these excursions are 
associated with passage of the ER-2 over water rather 
than land. The NASA AMPR 10-GHz radiometer data 
were examined, and the excursions in the radar 
background level are highly correlated with deviations 
of >100° K in the brightness temperature. In terms of 
radar stability, however, the noise level exhibits no 
long-term trends over the course of the flight. 

The signal processor was programmed to make 
periodic internal calibrations (Section 3.3) during the 
5 October 1993 flight. During the calibration, an 
attenuator was stepped through a range of settings. 



o 



-38 
-40 
-42 
-44 
-46 
-48 
-50 
-52 
-54 











- 


Nadir 

Forward VV 

Forward VH 


- 








: , : 



500 



1000 
TIME (x5 sec) 



1500 



2000 



FIGURE 4. Peak value of the transmit pulse for the nadir (solid), 
forward copolar (dashed) and cross-polar (dot-dash) receivers. 





-1UU 

-105 


. 










ft 







Nadir 


* 


3 










Forward VV 




c* 


-110 









Forward VH 




5 






■ 


O 










S 


-115 
-120 


/-^"•--•^-X-y 


, > v 


/'V* T''/ V> "''V : 






— Land — »— Ocean — 


■ Land ~— 


— Ocean ~ 




-125 
5( 






■ 




)0 1000 


1500 


2000 








TIME (x5 sec) 







Figure 5. Background noise level for the three receivers, as in 
Figure 4. 



Data from each calibration performed during flight 
was extracted for the 63-dB setting. These data (Figure 
6) show a large and long-term trend of over 1 dB 
during the course of the flight, while data at a higher 
attenuation setting of 95 dB exhibit a long-term drift of 
nearly 5 dB (see Figure 7). 

The above results indicate that the only serious long- 
term stability problems are associated with the 
calibration hardware chain. An examination of 
engineering data for the 5 October 1993 flight (Figure 
8) shows that the air temperature of the transmitter 
chassis varied by nearly 10° C during the flight. The 
large deviation of the transmitter base temperature is 
caused by instrument heating until a thermostat 



3 



o 



-55 



-60 



-65 



-70 





-._ --., -....,■ 


- 




Nadir 

Forward W 

Forward VH 











500 



1000 
TIME (x5 sec) 



1500 



2000 



Figure 6. 5 October 1993 flight calibration data for the 63 dB 
attenuator setting. 




10 



500 



1000 
TIME (x5 sec) 



1500 



2000 



Figure 8. Measured temperature of the transmitter and receiver for 
the 5 October 1993 flight. 



3 



o 

Oh 

s 



-90 



-95 



-100 



-105 




500 



1000 
TIME (x5 sec) 



1500 



2000 



FIGURE 7. Flight calibration data for the 95 dB attenuator setting, 
as in Figure 6. 



500 



400 - 



300 



200 



100 




1000 
TIME (x5 sec) 



2000 



Figure 9. Difference between the ER-2 pressure altimeter and the 
derived height from EDOP reflectivity. The duration is 2.5 hours. 



activates internal cooling fans. 

After contacting the vendors of the various 
components in the calibration chain, the manufacturer 
of the digital attenuators acknowledged that there 
were known problems with temperature stability that 
had eventually required the units in question to be 
redesigned. 



8. RANGE EQUATION 

No highly accurate measurements have been made on 
a test range with EDOP, but analysis of the CAMEX 
data shows that there are no significant problems. 



The timing signal which clocks the A/D converters 
has been measured in the laboratory. This clock, 
typically running at 1 MHz, has been measured to 
have an accuracy of better than 10 kHz. This 
measurement implies a gate spacing of better than 1%. 
Thus, for an example with a selected spacing of 150 m, 
each gate center is positioned to better than 1.5 m. 
The total cumulative error which could be introduced 
at a 20-km range, the nominal distance to the surface, 
is approximately 200 m. 

The radar-derived height from the surface has also 
been examined. The height is determined with the 
following algorithm. The gates in the nadir beam 
containing the transmitter leakage are located. The 
first range gate immediately following the leakage 



10 



signal is defined as being at range zero. The gate 
containing the maximum surface return is then found, 
and the difference between gate zero and the surface 
gate multiplied by the gate spacing is the radar- 
determined aircraft altitude. 

Figure 9 shows the difference in height between the 
radar-derived height and the ER-2 pressure altimeter. 
The radar-determined absolute height typically agrees 
to better than 400 m with the ER-2 navigation altitude. 
However, the magnitude of the height variations is 
typically less than 150 m, which is the width of one 
gate. It must be stressed that the ER-2 navigation 
system in operation during CAMEX used a pressure 
altitude. There is indication from other ER-2 
investigators that the pressure altitude can be in error 
by several hundred meters and generally is an 
underestimate of the true aircraft height above the 
surface. 

The height of. the peak bright band signal was also 
determined from EDOP data for 5 October 1993. This 
altitude was determined by first detecting the range 
gate containing the maximum surface return and 
locating the gate with the peak bright band echo in the 



nadir channel (Figure 10). The difference in the two 
gates is defined as the radar-determined bright band 
height, which for 5 October 1993 at 19:10 UTC was 
found to be approximately 4.35 km. 

The 6 October hr UTC radiosonde ascent from the 
Palm Beach National Weather Service (NWS) station 
showed the 0° C level to be at 4.48-km height, which 
is a deviation of 130 m from the EDOP observations. 
The radar bright band is typically found at or below 
the 0° C isotherm with the height depression being 
dependent on environmental conditions and cloud 
microphysics. So the EDOP ranging is consistent with 
external measurements and ground truth observations. 

Thus, for CAMEX, the range R k to the kth gate is 

R k - kAR , (6) 

where k is the gate number and AR is the gate spacing. 
During CAMEX, the gate spacing was 150 m (Table 7) 
and from the data set available it was not possible to 
verify Equation 6 for other gate spacings. It is 
important to emphasize that gate number k=0 is 
defined as the first gate following the transmit pulse. 



13 



14 



15 



* 16 

W 

U 

< 

^ 17 

Q 



18 



19 



20 









--_, Bright Band 


Stratiform/ 


Convective- 


./; 


^i Surface 


:-- "Z^~ 



10 



20 30 40 50 

REFLECTIVITY (dBZ) 



60 



Figure 10. Reflectivity height profiles from the nadir antenna 
on 5 October 1993, showing stratiform and convective 

signatures. 



9. GROUND TRUTH COMPARISON 

The ER-2 flew a straight-line flight track near 
Melbourne, Florida, on 5 October 1993. At the 
beginning of the track, the aircraft was approximately 
280 km south of the National Weather Service (NWS) 
WSR-88D radar, headed north-northeast. The flight 
track lasted approximately 12 minutes, during which 
the ER-2 approached to within about 140 km of 
Melbourne. 

As the ER-2 progressed along the track, it overflew 
several convective systems along a loosely arranged 
squall line. These observations allow for direct 
comparison of EDOP nadir reflectivity against a 
calibrated, ground-based radar. Although the large 
distance from Melbourne makes the WSR-88D data 
less than optimum, these data do provide a significant 
check of the EDOP calibration. 

9.1 WSR-88D Processing 

The WSR-88D is an S band coherent radar with 
approximately a 1° beamwidth and a range gate width 
of 1 km for the reflectivity scans (see Federal 
Meteorological Handbook No. 11). The Melbourne, 
Florida, WSR-88D is located at latitude 28.113056°N 



11 



and longitude 80.6544°W. 

During the time of the flight track, the Melbourne 
radar performed three reflectivity volume scans in PPI 
mode. The scans were performed at 19:02:58, 19:08:54, 
and 19:14:51 UTC and, because of the large distances 
involved, only the lowest sweeps at approximately 
0.44° elevation, were selected for analysis. 

A special software package was written to remap the 
PPI scan data into geocentric coordinates of latitude 
and longitude. The resolution volumes (range gates) 
that intersected the ER-2 subtrack position were 
determined with an algorithm that accounts for WSR- 
88D beamwidth, earth curvature, and refractive 
bending of the beam. For each of the intersection 
points along the flight track, the reflectivity, 
coordinates and the height above the Earth's surface 
were determined. It is worth noting that, for a PPI 
scan, these data do not lie along a constant height but 
rather are at increasing heights as the range increases. 
No smoothing or averaging of the WSR-88D data was 
performed. A 4/3 Earth approximation was used to 
account for refractive effects (Doviak and Zrnic, 1984) 
and the radar to geocentric coordinate conversion was 
based upon algorithms presented by Heymsfield et ah 
(1983). 

9.2 EDOP Processing 

A comparison of EDOP and ground-based radar data 
presents several problems. One of those problems, as 
noted above, is the registration in terms of latitude 
and longitude of the two sets of radar data, one from 
a scanning ground-based radar and the other from a 
nadir-staring radar on a high-altitude moving 
platform. The coordinates for a beam of EDOP data 
are determined by using the latitude and longitude of 
the ER-2 at the point in time the EDOP dwell was 
collected and computing the height above ground level 
for each range gate in the dwell (see Section 8). 

A second problem involves the differences in 
resolution volumes between EDOP and the WSR-88D. 
The WSR-88D volume is a horizontally oriented 
conical section 1 km long and, at the ranges of interest, 
several kilometers in height. In contrast, the EDOP 
resolution volumes are vertically oriented conical 
sections 150 m deep by, at most, 1 km wide at the 
ground. Thus, the EDOP volumes can be viewed as 
thin horizontal disks compared to the rather large 
block-shaped volumes of the WSR-88D. 

The EDOP radar data were processed in such a way 



that the EDOP resolution volume was roughly 
equivalent to the resolution volume of the WSR-88D. 
This was accomplished by averaging the EDOP dwells 
for a distance of 768 m along track. Furthermore, the 
vertical resolution of the EDOP data was decreased by 
averaging a variable number of range gates along each 
dwell, with the number increasing at greater distances 
from Melbourne. The number of gates was chosen so 
that the vertical-height resolution roughly matched the 
WSR-88D beam width at a given range. 

Finally, for each of the vertically and horizontally 
averaged EDOP dwells, the range element was 
selected that was located at the height corresponding 
to the WSR-88D beam height above ground level. 

9.3 Comparison of EDOP and WSR-88D 

Figure 11 shows the along-track profile of the WSR- 
88D reflectivity (dashed line) and the EDOP 
reflectivity profile from the nadir antenna (solid line). 
During the 12 minutes required for the ER-2 to 
traverse the 140-km track, the Melbourne radar 
performed three volume scans. In order to 
compensate for storm movement and evolution, the 
WSR-88D reflectivity profile was broken into three 
sections, one for each of the scans. The first section at 
track distances 0-45 km was earliest in time (19:02:58 
UTC), while the last section at 95-140 km was from the 
final scan at 19:14:51. 

The agreement between the two profiles is slightly 
worse at smaller track distances, which corresponds to 
greater range from the Melbourne radar. This 
discrepancy is possibly attributed to Earth-curvature 



60 



50 



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3 40 

H 

£ 30 

H 

U 

w 

uJ 20 



10 





1 




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EDOP 










WSR-88D 




k- M- 






* 




'i 


"f'Vr v\ 


V 


* — 


— 19:02:58- — —++ 


19:08:54 


-■«• 19:14:51 


— *■ 



20 40 60 80 100 120 

ALONG-TRACK DISTANCE (km) 



140 



Figure 11, Comparison of EDOP nadir reflectivity and 
Melbourne, Florida, NWS WSR-88D for 5 October 1993. 



12 



and beam-bending effects that were not completely 
corrected with the standard atmosphere approximation 
(4/3 Earth-radius algorithm). Small initial errors can 
create relatively large displacements in the position at 
significant distances from the WSR-88D. 

The profiles have an rms error deviation of ±6.9 dB 
and there does not appear to be significant systematic 
bias. 



10. OCEAN SURFACE ANALYSIS 

The ocean surface presents a standard target at most 
microwave frequencies with the value of the scattering 
cross section being heavily dependent on wind speed 
and incidence angle of the microwaves. The wind 
direction and microwave polarization are smaller 
effects. Because the ocean surface is not a randomly 
distributed target which fills the complete resolution 
volume, a variation of the radar equation is used 



10 



P f (GA) 2 6<l>/o 
r " 2 9 iz 2 R 2 ln2 



(7) 



where o° is the backscatter coefficient for the surface 
(Meneghini and Kozu, 1990). The losses are the same 
as those described in the previous sections. 

Equation 7 describes the beam-limited case which 
relates the return power to the backscatter cross 
section when the footprint on the ocean surface is 
limited by the beam width. This is, in essence, a result 
of the pulse width (75 m for CAMEX) being relatively 
large for the 3° beam width (Nathanson, 1969; Ulaby 
et al, 1982). 

During the 5 October 1993 flight, the ER-2 performed 
a 360° turn, involving several bank angles, above a 
precipitation-free region of the ocean south of Florida. 
This turn allowed surface backscatter cross section 
data to be collected from the nadir beam at incidence 
angles of 18°, 23°, and 30°. The mean and standard 
deviation for o° at each incidence angle were 
computed along a flight track distance of 12 km. 
Figure 12 shows the results from this surface analysis 
and includes a o° value for zero incidence angle which 
was collected when the aircraft was in level flight. 

Weather buoy and ship meteorological data show that 
the surface winds during the 5 October 1993 flight 
were 3 to 7 m s" 1 , generally in the 60°-70° direction. 
Representative scattering coefficients for these wind 



s 

% 

55 
O 



w 

CO 
CO 

O 

u 



-10 



-20 



-30 



RADSCAT: 3.0 ra/s 
RADSCAT: 6.5 m/s 
EDOP 




10 20 30 40 

INCIDENCE ANGLE (dcg) 



50 



FIGURE 12. Ocean surface backscatter coefficient derived from 
EDOP data on 5 October 1993 (■) and NASA RADSCAT (Jones, 
1977). 

speeds measured with the RADSCAT K u band 
scatterometer (Jones, 1977) are shown in Figure 12. In 
addition to wind speed, there is a lesser dependence 
of o° on microwave polarization and aspect angle with 
respect to wind direction (e.g. upwind, downwind or 
crosswind). 

The only EDOP data available for nonzero incidence 
angles are from the 360° turn. As the <f data are 
computed along the 12 km segment of the turn, the 
aspect angle with respect to the surface wind is 
continuously changing. Therefore the EDOP results 
can be considered as an approximate mean value over 
aspect angle, with variation from wind direction 
contributing to the error bars. The deviation in o° for 
horizontal polarization and 30° incidence is 
approximately ±3 dB between upwind and crosswind 
with the variation decreasing as the incidence angle 
decreases (Jones, 1977; Masuko et al, 1986). 

The polarization of the RADSCAT results used in 
Figure 12 is horizontal although there is negligible 
difference in the observations for polarization with 
incidence angles less than 30°. Theoretical estimates 
indicate that polarization differences may start to occur 
at incidence angles of 20° to 25° with the copolar 
vertical polarized returns several dB larger than for 
horizontal polarization (Brown, 1978). Though the 
polarization for EDOP is vertical with reference to the 
forward direction of the aircraft, as the ER-2 banks the 
antenna is pointed to one side of the flight track and 
the polarization in the plane of incidence becomes 
horizontal. 



13 



In general, the EDOP values are several decibels too 
small if the higher wind speeds are considered. Given 
the uncertainty in the surface wind speed at the time 
of observation and problems with processing the data 
for a single wind direction aspect angle, an adjustment 
to the EDOP radar constants based on these results is 
not considered appropriate. 



11. FORWARD BEAM 

As noted in Section 4, the forward antenna was 
misaligned during installation into the ER-2 for the 
CAMEX project, and subsequent examination of the 
data first showed a problem with the antenna. 
Although patterns of the forward antenna as used 
during CAMEX show degraded sidelobes and main 
beam, it is possible to estimate the magnitude of the 
problem. The results of the analysis can produce 
somewhat degraded reflectivity data from the forward 
antenna. The usefulness of the CAMEX forward 
antenna data will depend on the particular application. 

Figure 13 shows a comparison of the nadir and 
forward reflectivity profiles along a 140-km track at 
2.25-km height above ground (Section 9). The 
calibration constants derived in Section 6 were used to 
produce the data shown in the figure. The mean 
difference between the two profiles is 5.5 ± 3.0 dBZ. 
Therefore, an increase of 5.5 dB to the forward radar 
constant is suggested as a suitable correction. 

If the discrepancy lies solely in the antenna gain this 
corresponds to a 2.75-dB decrease in the gain. 



However, there is 
beamwidth as well. 



likely a degradation in the 





50 


■ ■ T - — ■ | 


' 1 ' 1 i 


• i 


_ 




Nidir 






Forward 










40 


Difference 






- 






3 












%*• 












►> 
H 


30 








\ 


g 
























H 












U 


20 








'. 


w 












_l 












u< 










■■j 


8 


10 




\ '\_ 


( \ 


;~ 






i , 


f\ J v '._■' ' 


' '' •.■■* 


, 


















"' * -\ r - J \A 














' *„'-. '"-■^i ^\ 


■ '"\' 











' i 


'. 





20 40 60 80 100 

ALONG TRACK DISTANCE (km) 



120 



140 



FIGURE 13. Comparison of nadir and forward reflectivity during a 
140-km flight track on 5 October 1993. Height is 2.25 km above 
ground. 



Examination of the forward antenna cross-polar data 
from CAMEX show that they are corrupted most 
probably because of significant degradation in the 
cross-polar isolation of the misaligned forward 
antenna. As a result, no further effort has been made 
at this time to analyze the linear depolarization ratio 
(LDR) signatures obtained during CAMEX. 



12. SUMMARY 

This report presents an analysis of the calibration of 
the first EDOP flight data which were acquired during 
the 1993 CAMEX project. Previously, the radar system 
had been examined in the laboratory and a thermal- 
vacuum chamber, but during flight, the hardware 
experiences significantly greater thermal changes than 
in the laboratory. Therefore, flight data were required 
for a comprehensive calibration analysis. 

The calibration of the EDOP receivers was 
accomplished from laboratory measurements, and the 
radar constants were computed. Using this 
calibration, the radar reflectivity was estimated for the 
CAMEX data set. Comparison of these data with 
ground truth provided by a NWS WSR-88D radar 
show good agreement with EDOP nadir reflectivity 
and an rms error of approximately ±6.9 dBZ. The 
results of an analysis of the backscatter cross section 
from the ocean surface also agree to better than 3 dB 
with published scatterometer results. Although these 
two numbers are moderately large, they represent 
deviations from other observations rather than errors 
in EDOP calibration. The EDOP absolute calibration 
is considered to be significantly better than 3 dB. 

Analysis of height data shows no problems with 
ranging of the EDOP data. After an initial period of 
30 to 40 minutes, which is required to reach thermal 
equilibrium, the microwave transmitter and receiver, 
excluding the calibration chain, appear to be stable to 
better than 0.25 dB during flight. 

Through analysis of the CAMEX data and subsequent 
laboratory tests, several problems were pinpointed 
with the hardware. Results indicated that the radar 
constant for the forward antenna should be increased 
by 5.5 dB for the CAMEX observations to compensate 
for problems in the antenna alignment. A realignment 
of the forward antenna has been successfully 
completed by GSFC personnel, and the resulting 



14 



patterns closely match the original specification. GSFC 
engineering staff are modifying the calibration chain 
for improved temperature stability and will tune the 
signal conditioning electronics prior to the next 
campaign in order to bring the IF amplifier into 
dynamic range of the digital converter. 

Although difficult, at some stage a complete end-to- 
end calibration including antennas should be 
performed. Given the difficulties of using a balloon- 
borne sphere target, another option would be to 
operate EDOP from the ground near a calibrated 
ground-based radar, which would allow comparison 
of reflectivity profiles. 

For future campaigns, a number of recommendations 
are put forward which will aid the accuracy of the 
reflectivity calibration and its ease of implementation. 

• A standard procedure for calibration should 
be defined. In particular, the procedure 
should account for hardware configuration 
such as variable PRF, pulse width, and IF 
filters. 

• A standard calibration should be performed 
immediately before and after a campaign to 
provide a performance base line. The 
calibration should preferably be performed 
with the hardware in flight configuration. 

• Efforts should be made to obtain ground truth 
for precipitation reflectivity, over a calibrated 
radar, at least once during each campaign. 

• A series of banking maneuvers should be 
performed over a cloud-free area of the ocean 
to collect surface backscatter data for 
calibration comparisons. This procedure 
should be done at the end of a flight so that 
the system has thermally stabilized. 



13. 



REFERENCES 



Brown, G.S., 1978: Backscattering from a Gaussian- 
distributed perfectly conducting rough surface. IEEE Trans. 
Antennas Propagat., AP-26, 472-482. 

Davis, L, 1991: ER-2 Aircraft Doppler Radar (EDOP) Antenna 
System, ERA Report No. 91-0261, ERA Technology Ltd., 
Cleeve Road Leatherhead, Surrey KT22 7SA, England. 



Doviak, R.J., and D.S. Zrnic, 1979: Receiver bandwidth effect 
on reflectivity and Doppler velocity estimates. /. AppL 
Meteor., 18, 69-76. 

Doviak, R.J., and D.S. Zrnic, 1984: Doppler Radar and Weather 
Observations. Academic Press: New York, Chap. 2. 

Heymsfield, G.M., W. Boncyk, S. Bidwell, D. Vandemark, S. 
Ameen, S. Nicholson, and L. Miller, 1993: Status of the 
NASA/EDOP airborne radar system. 26th International Conf 
Radar Meteorology, Norman, Amer. Meteor. Soc, 374-375. 

Heymsfield, G.M., L.R. Dod, L. Miller, M. Craner, and D. 
Vandemark, 1991: Update on the NASA ER-2 Doppler radar 
system (EDOP). 25th International Conf. Radar Meteorology, 
Paris, Amer. Meteor. Soc, 855-858. 

Heymsfield, G.M., K.K. Ghosh, and L.C. Chen, 1983: An 
interactive system for compositing digital radar and satellite 
data. /. Climate AppL Meteor., 22, 705-713. 

Heymsfield, G.M., C. Parsons, L.R. Dod, and L. Miller, 1989: 
Planned ER-2 Doppler radar (EDOP) for studying convective 
storms and mesoscale phenomena. 24th International Conf. 
Radar Meteorology, Tallahassee, Amer. Meteor. Soc, 581-584. 

Jones, W.L., L.C. Schroeder, and J.L. Mitchell, 1977: Aircraft 
measurements of the microwave scattering signature of the 
ocean. IEEE Trans. Antennas Propagat., AP-25, 52-61. 

Masuko, H., K. Okamoto, M. Shimada, S. Niwa, 1986: 
Measurement of microwave backscattering signatures of the 
ocean surface using X band and K a band airborne 
scatterometers. /. Geophys. Res., 91 (Cll), 13065-13083. 

Meneghini, R., and T. Kozu, 1990: Spaceborne Weather Radar. 
Artech House: Norwood, MA, 199 pp. 

Nathanson, F.E., 1969: Radar Design Principles. McGraw-Hill: 
New York. 

Office of the Federal Coordinator for Meteorological Services 
and Supporting Research, 1991: Federal Meteorological 
Handbook No. 11: Doppler Radar Meteorological Observations. 
U.S. Department of Commerce, FCM-H11. 

Ulaby, F.T., R.K. Moore, and A.K. Fung: 1982: Microwave 
Remote Sensing. Addison-Wesley: Reading, MA, Volume II. 

Zrnic, D.S., 1975: Moments of estimated input power for 
finite sample averages of radar receiver outputs. IEEE Trans. 
Aerospace Elec. Syst., AES-11, 109-113. 



Dicaudo, V.J., 1970: Radomes. Radar Handbook, 
Skolnik, ed., McGraw-Hill: New York., 14.1-14.15. 



M.L 



15 



REPORT DOCUMENTATION PAGE 



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1. AGENCY USE ONLY (Leave blank) 



2. REPORT DATE 

August 1994 



3. REPORT TYPE AND DATES COVERED 

Technical Memorandum 



4. TITLE AND SUBTITLE 

NASA ER-2 Doppler Radar Reflectivity Calibration for the 
CAMEX Project 



6. AUTHOR(S) 

I. J. Caylor, G. M. Heymsfield, S. W. Bidwell, and S. Ameen 



5. FUNDING NUMBERS 



912 



7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES) 

Goddard Space Flight Center 
Greenbelt, Maryland 20771 



8. PEFORMING ORGANIZATION 
REPORT NUMBER 

94B00110 

Code 912 



9. SPONSORING / MONITORING ADGENCY NAME(S) AND ADDRESS (ES) 

National Aeronautics and Space Administration 
Washington, DC 20546-0001 



10. SPONSORING / MONITORING 
ADGENCY REPORT NUMBER 

NASATM-104611 



11. SUPPLEMENTARY NOTES 



J. Caylor and S. Ameen: Science Systems and Applications, Inc., Lanham, Maryland (located at 
Goddard Space Flight Center, Greenbelt, Maryland); G. Heymsfield and S. Bidwell, Goddard Space 
Flight Center, Greenbelt, Maryland 



12a. DISTRIBUTION /AVAILABILITY STATMENT 

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21090-2934, (301)621-0390. 



12b. DISTRIBUTION CODE 



13. ABSTRACT (Maximum 200 words) 

The NASA ER-2 Doppler radar (EDOP) was flown aboard the ER-2 high-altitude aircraft in September and 
October 1993 for the Convection and Moisture Experiment. During these flights, the first reliable reflectivity 
observations were performed with the EDOP instalment. This report details the procedure used to convert real-time 
engineering data into calibrated radar reflectivity. Application of the calibration results produces good agreement 
between the EDOP nadir pointing reflectivity and ground truth provided by a National Weather Service WSR-88D 
radar. The rms deviation between WSR-88D and EDOP is 6.9 dB, while measurements of the ocean surface 
backscatter coefficient are less than 3 dB from reported scatterometer coefficients. After an initial 30-minute period 
required for the instrument to reach thermal equilibrium, the radar is stable to better than 0.25 dB during flight. The 
range performance of EDOP shows excellent agreement with aircraft altimeter and meteorological sounding data. 



14. SUBJECT TERMS 

Precipitation radars; radar calibration; CAMEX; EDOP, radar equation; surface refer- 
ence, ground truth - WSR-88D; remote sensing; and precipitation 



15. NUMBER OF PAGES 

24 



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