1 A conical scan type spaceborne precipitation radar K. Okamoto 1),S. Shige 2), T. Manabe 3) 1: Tottori University of Environmental Studies, 2: Kyoto University 3: Osaka Prefecture University 6 October th Conference on Radar Meteorology

2 Purpose of Study TRMM Precipitation Radar(PR): Active phased array radar: Very heavy weight (465kg) Only limited large-size satellite can mount it. Study the feasibility of lightweight rain radar which can be mounted on many satellites. Study the conical scan type rain radar. Many satelliteborne microwave radiometers employ the conical scan type. Mechanical scan ・・・ Simple system …Light weight Electrical scan ・・・ Complex system…Heavy weight y z Scan angle x Satellite AMSR-E antenna on the Aqua

3 Conical Scan a b rlrl x y  y H V z x 00 Study conical scan type rain radar parameters. Antenna diameter D and scan angle(=cone angle)  are two varible parameters. Side view Top view rsrs tt V  ： Scan angle =cone angle Along track resolution Cross track resolution Angular velocity Velocity of ground track : Antenna beam width Satellite altitude

4 Ground track of the conical scan radar 2b= km Altitude H = 407 km Cone angle  = 11 deg. Frequency =13.6 GHz Antenna diameter D = 1.1 m Antenna beam width  0 =1. 459deg Velocity of ground track V=7.205 km/s Along track resolution  r l [km] =10.76 km Swath width 2b= km Angular velocity  = deg/s Along track resolution  r l =10.76 km Moving direction To observe a raining area without any gaps between adjacent swaths, a satellite must move the same distance as the along track resolution while the antenna beam goes round a circle. Example Swath width

5 PRF （ Pulse Repetition Frequency ） T ex TmTm  TmTm  H/(Ccos  ) T1T1 T2T2 t=0 1 n n+1 Echo of the first transmitted pulse will be received between the n-th and n+1 th transmitted pulse.  min  max Martin time for the satellite altitude variation Margin time for the transmit-receive switching T m ： Margin time for the transmit-receive switching Margin time for the transmit-receive switching T ex ： double of pulse width, frequency agility is applied.

6 Independent sample number N a : Angle-bin number T a : Dwell time per an angle-bin N d : Independent sample number rlrl y rsrs x Angle-bin a （ Required value ） Independent sample number: Independent sample number of more than 64 is required to average fluctuated rain echoes. Frequency agility: Independent sample number is doubled by transmitting a pair of slightly different frequency pulses.

7 Transmitted peak power Radar equation S min : Minimum detectable power k B : Boltzmann constant (= × J/K ) T e :Ambient temperature(=290 K) B : Receiver band width(=0.78 MHz) k-R relation Z-R relation L : System loss(=2.4 dB)  :Transmit-receive beam overlap rate P r :Received power P t :Transmitted peak power G 0 :Antenna Gain  0 :Antenan beam width C :Velocity of light  : Pulse width(=1.67  s) H B : Thickness of bright band （ =0.5 km) H ru : Thickness of uniform rain layer （ =5 km) Ｎ Ｆ : Receiver noise figure(=3.0 dB)  :Complex permittivity of water S min = dBm : Wavelength(=2.204 cm) Distance between satellite and rain top

8 An example of the conical scan radar parameter Frequency f [GHz]13.6 Antenna diameter D [m]1.0 Scan angle  [deg] 11 Range resolution c  /2 [m] 250 System loss L [dB]2.4 Receiver noise figure N F [dB]3.0 Ambient temperature T e [km]290 Receiver band width B [MHz]0.78 Thickness of bright band H B [km] 0.5 Height of rain layer H ru [km]5 Satellite altitude H [km]407 Variation of satellite  H [km] 11 Maximum height of rain H r [km]15 Minimum detectable rain rate R [mm/h] 1.0 Velocity of ground track V [km/s] Conditions precedent Determined parameters ItemsValues Antenna beam width  0 [deg] Along track resolution  r l [km] Cross track resolution  r s [km] Scan width（internal） 2a [km] Scan width（External） 2b [km] Scan period T [s]1.493 Angular velocity  [deg/s] Antenna gain G 0 [dB]42.17 S min [dBm] Beam overlap rate0.89 Transmitted peak power P t [W] Angle-bin number N a 48 n4 PRF1289 Independent number N d X 280

9 Independent sample number N d Transmitted peak power P t [W] Scan angle  (deg) Independent sample number N d Transmitted peak power Pt [W] Independent sample number N d and transmitted peak power P t [W] of rain radar as a function of scan angle , where antenna diameter D is the variable parameter.

10 Trajectory of the conical scan superimposed on the cloud resolving model Antenna diameter D=0.8 m ， Scan angle  =5 deg. Antenna diameter D=1.8 m ， Scan angle  =29 deg.

11 Antenna diameter D(m) 0.8, 1, 1.4 ， 1.6, 1.8 Scan angle  (deg) 5, 8, 11, 14, 17, 20, 23, 26, 29 Offset parabolic antenna Parameters of antenna diameter D and scan angle  used in the calculation. Radiation pattern of the offset parabolic antenna

12 Approximation of the antenna pattern Approximated antenna pattern used to calculate received power (Scan angle  =17 deg. ， Antenna diameter D=1.6 m) f() Gain function G(  )=G 0 ・ f(  ) f(  ) [dB] Gaussian Envelope f  ) [dB] Minimum -50 dB Gaussian Envelope Minimum -50 dB Calculated antenna pattern

13 Simulation flow of rain observation by radar Radar reflectivity factor Z e (True value) Received power from rain P r Received power from clutter P s Z value including rain attenuation: Z m Total received power P=P r + P s Ocean scattering coefficients  0 Rain rate R Attenuation corrected Z value Z e-r No rain Yes No Cloud Resolving Model Correlation Coefficient

14 Calculation of rain scattered received power  0 ： Beam width × ×  ： Scan angle  ： Azimuth angle  ： Zenith angle 20km Satellite Grid No. 3 × 407km Rain Scattering Volume × True value Z: Z e Rain scattered received power P r Z e ( ,  ) True value rain rate : R Z-R, k-R relation Cloud Resolving Model Range-bin, 250 m

15 Calculation of received power from surface clutter area Ocean scattering coefficients  0 Received power from clutter: P s Calculate received power from surface clutter area at each rain scattering volume. Surface clutter area Rain scattering volume Scan angle Satellite Incidence angle

16 Results of simulation (Correlation coefficients between the true Z e and the calculated Z e-r ） 相関係数 Correlation coefficients Scan angle (deg)

17 Examples of system parameters of the conical scan rain radar Radar #1Radar # 2TRMM PR Antenna diameter D [m] Scan angle  [deg] Satellite altitude H [km] Pulse width  [  s] 1.67×2 Antenna beam width  0 [deg] Along track resolution  r l [km] Swath width （ External ） 2b [km] PRF [Hz] Independent sample number N d Antenna gain G 0 [dB] S min [dBm] Transmitted peak power P t [W] Weight W [kg] Power consumption P [W]

18 Summary Calculation of system parameters of the satelliteborne conical scan type rain radar, where antenna diameter D and scan angle(=cone angle)  are two varible parameters. Simulation of rain observation by the conical scan type rain radar, where offset parabolic antenna pattern and cloud resolving model are used. Calculation of correlation coefficients between the true Z values and simulated Z values. Even if the antenna diameter is small, correlation coefficients are large when the scan angle is small. System study of spaceborne conical scan type radar including estimation of weight and power consumption. Estimated weight of the conical scan type rain radar becomes much smaller than TRMM precipitation radar.