Modification Matrix validity Impact on Calculate Force calculation
4.3 Path Loss Calculations
4.3.1 Ground Altitude Determination
Atoll determines reception and transmission site altitude from Digital Terrain Model map. The method used to evaluate site altitude is based on a bilinear interpolation. It is described below.
Let us suppose a site S located inside a bin. Atoll knows the altitudes of four bin vertices, S’1, S’’1, S’2 and S’’2, from the DTM file (Centre of each DTM pixel).
1st step: Atoll draws a vertical line through S. This line respectively intersects (S’1,S’’1) and (S’2, S’’2) lines at S1 and S2.
2nd step: Atoll determines the S1 and S2 altitudes using a linear interpolation method.
3rd step: Atoll performs a second linear interpolation to evaluate the S altitude.
4.3.2 Clutter Determination
Some propagation models need clutter class and clutter height as information at receiver or along a transmitter-receiver profile.
Tip 2
Calculation area management
When performing prediction studies, it is recommended to follow this methodology to minimise recalculations:
1st step: Calculate without computation zone.
2nd step: Draw a computation zone and calculate.
3rd step: Decrease the calculation radius and calculate.
Figure 4.3Ground Altitude Determination - 1
Figure 4.4Ground Altitude Determination - 2
Figure 4.5Ground Altitude Determination - 3
Figure 4.6Ground Altitude Determination - 4
4.3.2.1 Clutter Class
Atoll uses clutter classes file to determine the clutter class.
4.3.2.2 Clutter Height
To evaluate the clutter height, Atoll uses clutter heights file if available in the .atl document; clutter height of a site is the height of the nearest point in the file.
Example: Let us suppose a site S. In the clutter heights file, Atoll reads clutter heights of four points around the site, S’1, S’’1, S’2 and S’’2. Here, the nearest point to S is S”2; therefore Atoll takes the S”2 clutter height as clutter height of S.
If you do not have any clutter height file, Atoll takes clutter height information in clutter classes file. In this case, clutter height is an average height related to a clutter class.
4.3.3 Geographic Profile Extraction
Geographic profile extraction is needed in order to calculate diffraction losses. Profiles can be based on DTM only or on DTM and clutter both. In fact, it depends on the selected propagation model.
4.3.3.1 Extraction Methods
4.3.3.1.1 Radial Extraction
Atoll draws radials from the site (where transmitter is located) to each calculation bin located along the transmitter calculation area border. In other words, Atoll determines a geographic profile between site and each bin centre.
Figure 4.7Clutter Height
Figure 4.8Radial calculation method
The receiver may be located either anywhere within a calculation bin (Point prediction) or at the centre of a calculation bin (Coverage study). Therefore, according to the receiver position, Atoll chooses the nearest profile and uses it (receiver is considered as located on the profile) to perform prediction study at the receiver.
4.3.3.1.2 Systematic Extraction
In this case, Atoll systematically extracts a geographic profile between the site (where transmitter resides) and the receiver.
Transmitter
Radial: Atoll will extract a geographic profile for each radial Centre of a bin located on the calculation border
Receiver: it may be anywhere in point analysis or at the centre of each calculation bin in coverage studies
Figure 4.9Site-bin centre profile
Figure 4.10Radial calculation method
Transmitter Geographic profiles
Receiver: it may be anywhere in point analysis or at the centre of each calculation bin in coverage studies
4.3.3.2 Profile Resolution: Multi-Resolution Management
Geographic profile resolution depends on resolution of geographic data used by the propagation model (DTM and/or clutter).
1. 1st case: If the chosen propagation model considers both DTM and clutter heights along the profile, the profile resolution will be the highest of the two.
Example 1: Standard Propagation Model is used to perform predictions. A DTM map with a 40 m resolution and a clutter heights map with a 20 m resolution are available.
Both DTM and clutter maps are considered when using the Standard propagation model. Therefore, here, the profile resolution will be 20 m. It means that Atoll will extract geographic information, ground altitude and clutter height, every 20 m. To get ground altitude every 20m, Atoll uses the bilinear interpolation method described in
"Ground Altitude Determination" on page 77. Clutter heights are read from the clutter heights map. Atoll takes the clutter height of the nearest point every 20m (see Path loss calculations: Clutter determination).
Example 2: Standard Propagation Model is used to perform predictions. A DTM map with a 40 m resolution and a clutter classes map with a 20 m resolution are available. No clutter height file has been imported in .atl document.
Both DTM and clutter maps are considered when using the Standard propagation model. Therefore, here, the profile resolution will be 20 m. It means that Atoll will extract geographic information, ground altitude and clutter height, every 20 m. To get ground altitude every 20 m, Atoll uses the bilinear interpolation method described in
"Ground Altitude Determination" on page 77. Atoll uses the clutter classes map to determine clutter height. Every 20 m, it determines clutter class and takes associated average height.
2. 2nd case: If the chosen propagation model takes into account only DTM map along the profile, profile resolution will be the highest resolution among the DTM files.
Example: Cost-Hata is used to perform predictions. Both DTM maps with 40 m and 25 m resolutions and a clutter map with a 20 m resolution are available.
Only DTM maps are considered along the whole profile when using Cost-Hata model. Therefore, here, the profile resolution will be 25 m. It means that Atoll will extract geographic information, only the ground altitude, every 25 m. DTM 1 is on the top of DTM 2. Thus, Atoll will consider ground elevation read from DTM 1 in the definition area of DTM 1 and DTM 2 elsewhere. To get ground altitude every 25 m, Atoll uses the bilinear interpolation method described in "Ground Altitude Determination" on page 77.
Explorer window Work space
DTM
•DTM 1 (25m)
•DTM 2 (40m) Clutter
•Clutter (20m)
Notes:
• The selected profile resolution does not depend on the geographic layer order. In the last example, whatever the DTM file order you choose, profile resolution will always be 25m.
On the other hand, the geographic layer order will influence the usage of data to establish the profile.
• The calculation bin of path loss matrices defined by the grid resolution is independent of geographic file resolution.