Specifying Antenna Array Configuration in Station UDD file

Release: Feb 16, 2010

1. Station UDD file

Station User-Defined Data (UDD) file contains description of the digisonde location and hardware that is necessary to correctly display and process data by the following UMLCAR software:

• SAO Explorer: Interactive ionogram analysis
• Drift Explorer: Interactive drift data analysis
• ARTIST-5: ionogram autoscaler
  
• DCART: Digisonde 4D commanding/processing software
  
• Dispatcher WWW and Dispatcher MP: data managers
  
• Ion2PNG,Sky2PNG,DVL2PNG,DRG2PNG: image generating software
• DIDBase and DriftBase ingestion software

UDD files include description of the digisonde antenna array.

2. Digisonde Antenna Arrays

Two different types of antenna arrays can be implemented in the Digisonde system. 7-antenna array is used for the DGS256 (DISS) system while 4-antenna array is used with the DPS system.

2.1. Standard Antenna Arrays

Figure 1 shows two types of standard-per-manual antenna layouts.

(a)	(b)

Figure 1. a) Standard Digisonde 7-antenna array layout. b) Standard Digisonde 4-antenna array layout

2.2. Mirror Antenna Arrays

Figure 2 shows two types of mirror antenna layouts that are different from the standard ones in the direction of antenna enumeration: in a standard array, antennas are enumerated counter-clockwise, and in a mirror array, antennas are enumerated clockwise.

(a)	(b)

Figure 2. a) Mirror Digisonde 7-antenna array layout. b) Mirror Digisonde 4-antenna array layout

2.3. Antenna Coordinate System

Figure 3 defines the system of antenna coordinates:

Figure 3. Coordinate system used in UDD for antenna positions

Here X is the X-axis of coordinate system, Z is the local vertical pointing up, and Y forms the right-hand system. Although selection of X axis for antenna layouts can be arbitrary, we recommend aligning X-axis to one of the array sides as in Figure 1 and 2. For many existing digisonde locations, X axis pointed to the Compass North at the time of array installation.

3. Specification of Antenna Array Orientation

Digisonde antenna orientation is described by providing two values:

    (1) X-31: angle between the X-axis and the line connecting antenna 3 and 1 (Line 3-1)
    (2) X-DEC: Declintation angle of the X-axis from the Geographic North

For most practical cases, X-axis for antenna array is selected as shown in Figure 1 and 2. In this case, the X-31 angle:

X-31 = 0 for both standard and mirror 7-antenna arrays
X-31 = -30° for standard 4-antenna array
X-31 = +30° for mirror 4-antenna array

The X-axis of antenna array generally corresponds to the North direction; however, antenna array can be rotated to better fit the field site. If space is not a limiting factor, it is best to orient antenna as shown in Figure 1 with X-axis pointing to the Geographic North. Orientation to Compass North has been commonly used in the past as a technically simpler solution. 

When X-axis points to the Geographic North, X-DEC is zero. When X-axis points to Geomagnetic North (Compass North). X-DEC is equal to the magnetic declination of the digisonde site at the time of the sounder installation (as shown in Figure 4).

Figure 4. Definition of the X-DEC parameter

The UDD file stores X-31 parameter as question *091 and X-DEC parameter as question *079, for example:

X-DEC: DECLINATION (angle) OF THE ANTENNA ARRAY X-AXIS
from the Geographic North Pole.
Values from 0 to 180 degrees mean clockwise declination and values
from 0 to -180 degrees mean anti-clockwise declination

*079 <24.5>

X-31 PARAMETER = deviation in degrees of direction 3-1 from X-axis.
               Values from -180 to 180. Positive means counter-clockwise.
               This parameter has a sense when <090> != 0
*091 < -30 >

4. Antenna Array Specification for Station UDD file

The UDD file contains separate antenna array descriptions for ionogram and drift data. Such distinction is made to reflect the difference in data content between the ionogram and drift operating modes. In the ionogram mode, signal directions are already calculated as a part of digisonde operations and made available in the data. In the drift mode, raw data contain all receive channels for directional analysis.

4.1. ANTENNA PATTERN: Array Specification for Ionogram Mode

The following ionogram formats contain directional information:

• MMM by Digisonde 256
• BEM by DPS-4
• RSF by DPS-4 and Digisonde 4D
• Generic by Digisonde 4D
  

Echo direction data in three commonly used data formats, MMM-256, BEM, and RSF, are stored as oblique beam codes that have to be translated by the visualization software to the standard azimuth codes 1-12 (see Figure 5).

Figure 5. Standard Digisonde Azimuith Codes for ionogram measurements

Correct translation of the beam codes to the azimuth codes requires knowledge whether ANTENNA_PATTERN is one of the following four cases:

0 - Standard per manual (as in Figure 1)
1 - Rotated by 180 degree
2 - Mirror Image (as in Figure 2)
3 - Non-standard

For all digisonde systems prior to Digisonde 4D, the azimuth codes are relative to the X axis of the antenna array. For most of the fielded systems, X axis is aligned to the Compass North at the time of installation and migration of the magnetic poles has little effect. With a few notable exceptions, therefore, directions in MMM, BEM, and RSF ionograms are given in the Geomagnetic coordinates. Interpretation of the azimuth codes becomes more intricate for locations where drift of magnetic declination is substantial or antenna array layout is far from the standard. For detailed explanations please contact Ivan Galkin at Ivan_Galkin@uml.edu.

RSF ionograms by Digisonde 4D use new beamforming algorithms to always produce azimuth codes in the Geographic coordinates, regardless of the actual antenna array layout, so that it is free from the artifacts of magnetic declination drift.

Generic format developed for Digisonde 4D systems does not store beam or azimuth codes and instead directly reports  asimuth angle of the echoes in degrees (using Geographic coordinates).

4.2. Array Specification for Drift Mode

4.2.1. LAYOUT, MAXDIST, X-DEC and X-31: one of standard or mirror layouts

For measurements in the drift mode and subsequent calculation of the Doppler skymaps and ionospheric drift velocities, post-processing software requires an accurate specification of the antenna array.

The array specification include the following 4 components:

ANTENNA LAYOUT
  0  non-standard layout
  1  seven antennas standard layout
  2  seven antennas mirrored layout
  3  four antennas standard layout
  4  four antennas mirrored layout
*090 < 3 >

LAYOUT options 1 and 3 are shown in Figure 1, and options 2 and 4 are shown in Figure 2. Non-standard layout is discussed below in section 4.2.2.

ANTENNA MAXDIST - The max distance between antennas in meters.
                  This parameter is not used when <090> = 0
                  As all standard layouts (when <090> != 0) assumed equilateral
                  triangles than MAXDIST for 4-antennas standard layout is
                  equal to the length of triangle side and for 7-antennas
                  standard layout is equal to the outer triangle side length
*092 < 60.00 >

Two parameters LAYOUT and MAXDIST are sufficient to describe geometry of antenna placement in the antenna array coordinates. The other two parameters, X-DEC and X-31, describe orientation of the antenna relative to the Geographic coordinates as explained above in section 3.

4.2.2. Examples of array specifications, one of standard or mirror layouts

Commonly used in Digisonde 256 and DISS LAYOUT = 1 MAXDIST = 100.0 X-31 = 0 X-DEC = magnetic declination at the time of array installation
Known installations: Millstone Hill Goose Bay DISS LAYOUT = 2 MAXDIST = 100.0 X-31 = 0 X-DEC = magnetic declination at the time of array installation

Commonly used in DPS. LAYOUT = 3 MAXDIST = 60.0 X-31 = -30 X-DEC = magnetic declination at the time of array installation
Known installations: Rome LaTrobe U. LAYOUT = 4 MAXDIST = 60.0 X-31 = 30 X-DEC = magnetic declination at the time of array installation

Other known cases

Here's more examples of existing Digisonde antenna arrays configurations in the new encoding scheme:

Formerly "ROTATED DGS-256".  Known installations: Karachi Kokubunji  Beijing LAYOUT = 1 MAXDIST = 100.0 X-31 = 180 X-DEC = magnetic declination at the time of array installation
DPS working on the internal loop of the array configuration "MIRRORED 7 ANTENNA DEVN=0" Millstone Hill LAYOUT = 4 MAXDIST = 60.0 X-31 = 0 X-DEC = magnetic declination at the time of array installation

DPS working on the internal loop of the array configuration "STANDARD 7 ANTENNA DEVN=0" Sondestrom Ramey AFB LAYOUT = 3 MAXDIST = 60.0 X-31 = 0 X-DEC = magnetic declination at the time of array installation
Formerly known as "DPS ROTATED" Juliusruh? LAYOUT = 3 MAXDIST = 60.0 X-31 = 150 X-DEC = magnetic declination at the time of array installation
Orientation of array to Geographic North (recommended option) Hermanus LAYOUT = 3 MAXDIST = 60.0 X-31 = -30 X-DEC = 0

4.2.3. ANTENNA POSITIONS: Calculated coodinates of antennas

Positions of individual antennas in the array are always defined in UDD files. Drift Explorer libraries always recalculate positions from LAYOUT and MAXDIST parameters and report inconsistencies.

ANTENNA POSITIONS (X Y Z)

Assume the (central) antenna 1 at (0, 0, 0)
All length are in meters relative to antenna 1
The coordinate system has to be right hand with Z pointing up.
Select X along one of the triangle sides closest to North direction
Then X points roughly North and Y points roughly West
Don't specify seven antennas for four antenna configuration

        Ant1    Ant2    Ant3    Ant4 
*080 <  0.00,  30.00, -30.00,   0.00  > X
*081 <  0.00,  17.32,  17.32, -34.64  > Y
*082 <  0.00,   0.00,   0.00,   0.00  > Z

4.2.4. Non-standard Antenna Arrays

LAYOUT parameter is set to 0 for non-standard antenna arrays that do not fall in any of the mentioned examples.  ANTENNA POSITIONS definitions are used for all directional analysis.

Note: For non-standard antenna configurations, MAXDIST and X-31 parameters are undefined.