Description of Monte Lema radar products

Document in postscript.

1. Product POLAR-Z

This product contains reflectivity data for all the antenna elevations of a full volume scan, updated each 5 minutes. One volume dataset is characterized by 20 image files, one for each elevation. POLAR-Z permits the access to the highest spatial resolution of the data which is accessible to the user (fine spatial resolution at short range, sparse at wide range).

The reflectivity assigned tooneimage pixel is representative for one cell with extension of 1 degree in azimuth and 1 km in range laying over the surface scanned during the given elevations. The range gates (the discrete points in range along a single radial beam) are placed between 1km and a maximum which is elevation dependent, for the lowest elevations being of 230 km. The cells reflectivity is coded in range 0 to 15 corresponding to a linear scale in dB_Z .

The figure shows an example of this product for the radar station Monte Lema at the first elevation of the volume scan which is -0.3 degrees below the horizon; the data was taken at 09:32 utc time on 1. March 1994. The maximal measured reflectivity in this image reaches the class 52-55 dB_Z which is coded with value 14.

2. Product POLAR-U

This product contains Doppler velocity data for all the antenna elevations of a full volume scan, updated each 5 minutes. One volume dataset is characterized by 20 image files, one for each elevation.

The Doppler velocity assigned to one image pixel is representative for one cell with extension of 1 degree in azi- muth and 1 km in range laying over the conical surface scanned during the given elevation. The range gates (the discrete points in range along a single radial beam) are placed between 1km and a maximum wich is elevation de- pendent, for the lowest elevations being of 130 km. The cells velocity is coded in range 0 to 15 corresponding to a linear scale in fractions of the Nyquist velocity dependent from the active pulse repetition frequency.

The figure shows an example of this product for the radar station Monte Lema at the first elevation of the volume scan which is -0.3 degrees below the horizon; the data was taken at 09:32 utc time on 1. March 1994 (day 060). The velocities coded 01 to 07 are negatives (against the radar), those coded 09 to 15 are positives (toward the radar), 08 is for zero (perpendicular to the beam) and 00 is for unknown velocity (clutter or shielding or noise).

3. POLAR PRODUCTS DESCRIPTION

Description form for product UYx:

• Product name: POLAR-U
• Product identifier: Uyx with x = {A or D or L},
  y={1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K}
  e.g. one for each of the 20 antenna elevations
  (see "Product identifiers","Antenna scan table,N")
• Physical data: radial velocity
• Image format on file: 360 lines (corresponding to azimuths 0 to 359 in steps of 1 degree) of "n" samples (corresponding to range 1 to "n" (see "Antenna scan table, D maxU ") in km) 8bit/sample.
• Byte order: lines clockwise starting from North, samples by increasing range
• Update interval: 5 minutes
• Projection: none (polar scan of the antenna)
• Pixel coding: values in range [0-15]
• Pixel coding: values in range [0-15] or in range [0-255]
• Physical pixel values: velocities (see "Product data coding")
• Pixel resolution in space: 1 [degree] by 1 [km]

Description form for product ZYx:

• Product name: POLAR-Z
• Product identifier: Zyx with x = {A or D or L},
  y={1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,G,H,I,J,K}
  e.g. one for each of the 20 antenna elevations
  (see "Product identifiers","Antenna scan table,N")
• Physical data: reflectivity
• Image format on file: 360 lines (corresponding to azimuths 0 to 359 in steps of 1 degree) of "n" samples (corresponding to range 1 to "n" (see "Antenna scan table, D maxR ") in km) with 8bit/sample.
• Byte order: lines clockwise starting from North, samples by increasing range
• Update interval: 5 minutes
• Projection: none (polar scan of the antenna)
• Pixel coding: values in range [0-15]
• Physical pixel values: reflectivities (see "Product data coding")
• Pixel resolution in space: 1[degree] by 1[km]
  

APPENDIX A . Antenna scan table

The chosen scan sequence allows an aviation product every 2.5 min (two Half-Volume-Scans, calculated alternatively from revolutions 1-10 and 11-20) and a full volume scan every 5 min (revolutions 1-20), without exceeding an acceleration of the antenna of 10 o /s 2 and a maximum speed of the antenna of 20 o /s in elevation (36 o /s in azimuth). The table gives for each revolu- tion with index N, the Elevation in degrees above the horizon, the change of the elevation DElev in degrees at the end of the revolution, the maximum distance in km D max R and D max U for analysing the data in reflectivity and Doppler applications, the revolutions per minute (RPM), the Time per revolution in seconds, the time needed to change the elevation (Step) in seconds and the pulse repetition frequency (PRF) in number of pulses per second. The calibra- tion takes place during the indicated elevations(*). The wind-profile over the radar station is estimated every 2.5 min from the highest elevation of each Half-Volume-Scan (+).

APPENDIX B . Product identifiers

Radar products are identified by 3 characters TFS with T -> Type, F -> Format, S -> Source:

Type                    Format               Source
----------------------------------------------------------------------
C: Calibration          A: 200x200x2km       A: Albis
O: Overview             D: 100x5             C: Composite (A+D+L)
P: Precipitation (Rain) G: 320x256x2km       D: La Dôle
R: Rapid                H: 400x400x1km       E: Composite (A+D)
S: Status               L: 305x269x2km       L: Lema
T: Today                P: 360xN (see notes) X: (experimental)
U: Doppler velocity     S: 305x269x1km
V: Visibility           V: various (see notes)
W: Wind                 Y: (see notes)
Z: Reflectivity

Notes:

• A 32 bytes header is present at the beginning of each product file, with exception of product TGx which has its own heading (see "Products and file names").
• Products PHx and PSx have the vertical Z-profile (12 values of 4 bytes) inserted before the product image data in the stream of information saved on file
• Wind data contains 5 arrays of 100 elements each: horizontal velocity, azimuthal direction, vertical velocity, rms velocity error and the number of effectively con- sidered azimuths.
• Products have different data types: nibbles for TODAY; bytes for all other image products (except for visibility images which have 16-bit data); 32 bit for wind- profile data.
• Format "P" is used for both polar products (reflectivity-Z and velocity-U) with 360 azimuth values and with the range values in units of 1-km indicated in the antenna scan table.
• Format "V" is used for message data in the "Calibration" and "Status" products. - Format "Y" is used for products of various formats, when different levels (heights or elevations) are available:
  
  

  Identifier    Format  Levels(name-of-each)
  ----------------------------------------------
  OYC           L       (1,2,...,9,A,B,C,M)
  OYL           A       (1,2,...,9,A,B,C)
  VYL           H       (1,2,...,9,A,B,C)
  UYL,ZYL       P       (1,2,..,9,A,B,C,D,E,F,G,H,I,J,K)
  

APPENDIX C . Product and file names

Operational products are stored in a directory tree resident on a disk of the local server. This tree is built on a directory with standard name /srn/data/. Each product type is located on one different subdirectory having his own standard name (= the product identifier).

Data files are present under the mentioned subdirectories and have a name following a conven- tion related to the product identification also contained in the file data self in form of a prefix- ing header.

APPENDIX C.1 . Subdirectory names for products

(Example for Composite and Lema; analog for Albis and La Dôle):

CVL, OYC, OYL, PAL, PHL, PLC, PSL, RHL, RLC, RSL, SVC, SVL, TGA, TGC, TGD, TGE, TGL, TGX, UYL, VYL, WDL, ZYL

APPENDIX C.2 . File name convention for products

TFS'YY'DDD'HH'MM'Q'C.prd with:

Example: SVL9332711407L.prd ->
Status Lema on day 327 of 1993 at 11 h 40 min, quality = 7, compressed

NOTE:
The quality factor is expressed in hexadecimal form (0,1,...,9,A,...,F) and takes into account the completeness of the data set (i.e., the number of used elevations), the con- dition of the radar transmitter (the radiated power), and the condition of the receiver (the current estimate of the receiver noise figure).

For products RAIN and VISIB (being composed by different single scan volume data) the quality factor is built as the average of the source data factors. For product TODAY-COMPOSITE the quality factor assumes another meaning: it denotes the presence of the data from the different stations summing up the following codes: 1 for Albis, 2 for La Dôle and 4 for Lema (e.g. the resultant quality code for TGC is 1 when just Lema is up and 7 when data from all 3 stations are present).

The time indication is related to the termination time of the full respectively half scan.

APPENDIX C.3 . Standard file header of products

CH'TFS'YY'DDD'HH'MM'Q'C'bb.bb.bbbb.bbbb.ss.bb (32 bytes) where:

NOTE 1: For the products RAIN and VISIB a "count" of the cumulated images is available in the header information.

NOTE 2: For the product TODAY the first 4 lines of the file contain a string (called DIGITAL ANNOTATION) with information on the product identification:

Digital annotation string: MRDDMOYYHHMMXX with:

APPENDIX D . Product data coding

The product data is not present in physical units (for example in floating point format) but a discretization and coding operation is applied to the measured data. This has the drawback to reduce the data granularity but it was made for reasons of efficiency (transmission time, storage size).

The reflectivity data having a large range of possible values is coded following a logarithmic scale and use 4bit/pixel; the velocities are coded with a linear scale taking in consideration the maximum permitted value (dependent from the pulse repetition frequency) and use either a 4bit/pixel or a 8bit/pixel scheme.
For reasons of compatibility with the past product TG uses 3 bit/pixel.

APPENDIX D.1 . Rain rate - 16 values table (OY*, RS*, PS*,ZY*)

APPENDIX D.2 . Rain rate - 7 values table (TG*)

...............................(code 7 is reserved for graphics overlay)

APPENDIX D.3 . 4-bit coding scheme for Doppler velocity : 16 values table (UY*)

Velocity for a coded value = k ® (Vn/16)*(2*k-15± 1) for k=0..15

Example: k = 10 -> the velocity interval is Vn*[4/16,6/16].

Vn is the Nyquist velocity in [m/s] for the current PRF (Pulse Repetition Frequency) in [Herz]
by a wavelength (l) in [m] given by the relationship:
Vn = l*PRF/4.

Considering a l of 5.5 cm and taking the appropriate PRF (see "Antenna scan table" )we
have:

N.B.: - a frequency shift of 40Hz corresponds to a radial speed of 1 m/s.
- a positive velocity is toward the radar (in the direction of the beam)

APPENDIX D.4 . 8-bit coding scheme for Doppler velocity : 256 values table (UY*)

Velocity for a coded value = k ® (Vn/255)*(2*k-256± 1) for k=1..255

Value = 0 ® missing value

NOTES:
For the interpretation of the Doppler data it is important to take note of following aspects related to the signal processing and data handling procedures of the equipment:

• In the coding scheme with 4-bit the samples with a "missing value" (corresponding to clutter or reflectivity below 13 dBZ) are coded with a value of zero and thus cells with a 0 value can be either assigned to a velocity in range [-16/16 , -14/16]*Vn or to an undefined value.
• There are no algorithms for dealiasing: targets with absolute velocities above Vn are found as having k*Vn units lower than the correct value (with k integer). For example for a velocity of 35 m/s and a Nyquist of 16.5 m/s we obtain a value of (35 - 2*16.5) = 2 m/s.
• UY data ~ before 1997 was coded with 4-bit; successively a 8-bit coding scheme has been
  adopted. Please check the maximal value present in the dataset to be sure of the adopted cod
  ing method.

APPENDIX E . Coordinates conversion

The Swiss National Coordinates result from a conformal mapping (true angles) with skew axis of the earth surface to a cylindrical surface.

The earth surface (approximated by a Bessel's rotation ellipsoid) is first projected onto a sphere with radius the earth radius in Berne (i.e. the distance between Berne and the ellipsoid's rota- tion centre) having as centre the ellipsoid's rotation centre. The sphere surface is successively projected onto a cylindrical surface tangent in Berne having the cross section correspondent to the great circle of the sphere perpendicular to the earth meridian passing across Berne.

Geographic coordinates of Berne: LAT = 46.952, LON = 7.440

The radar products OVERVIEW, RAIN, RAPID, TODAY, VISIB are georeferenced with respect to the Swiss National Coordinates system.

Following formulas are useful in association with the indications of the Swiss National Coordinates given in the various product layouts.

Swiss National (Y,X) to Geographic (LAT,LON) coordinate conversion:

LAT = 45.16 + 0.008967*X - 7.7E-7*(Y-600)**2 [dec.degr.,km]
LON = 07.44 + (0.008967*Y-5.38)/cos(LAT) [dec.degr., km]

precision better than 16 seconds (0.5km) within the Swiss TODAY-Composite (298/405, 862/-31).

Geographic (LAT,LON) to Swiss National (Y,X) coordinateconversion:

X = 111.526*LAT + 0.5*(LON-7.44)**2 - 5036.44 [km,dec.deg.]
Y = 111.526*(LON-7.44)*cos(LAT) + 600 [km, dec.degr.]

with precision better than 1 km within the Swiss TODAY-Composite (298/405, 862/-31).

APPENDIX F . Radar parameters

Table 1: Comparison of the main radar characteristics

Table 2: Location of weather radars in Switzerland

Table 3: Dates of radar equipment renewal