ITM model propagation settings
Most of the parameters that can be configured are part of the ITM model that is implemented in Radio Mobile.
To be able to do proper predictions on the RF propagation a number of parameters have to be set. These parameters are directed by the environment in which the network will perform and by the demands and objectives of the evaluated network.
The settings for the model are configured in 'Network settings > Parameters'
The ITM model has 2 modes of operation:
- Area Mode
- Point – To – Point Mode
Area mode is used in cases where the exact terrain is not known. Environmental parameters and statistical parameters are used in calculating path loss. Terrain roughness is approximated from a user-entered value of delta h (change in terrain height).
Point–To–Point Mode (PTP mode)
When using PTP mode the model retrieves a terrain profile based on the user entered latitude and longitude values for the transmitter and receiver. Statistical and environmental parameters are used with the terrain profile in calculating path loss.
Radio Mobile is using the ITM model in PTP-mode because of the easy to access SRTM data on the internet.
|The PTP model relates the statistical variance of terrain elevations to classical diffraction theory, and predictions made by the model agree closely with measured data. Classical diffraction theory itself cannot make precise calculations of radio fields over irregular terrain because descriptions of the irregularities in sufficiently high resolution are not feasible.
While other prediction models construct simple approximations to the terrain as combinations of classical diffraction edges, the PTP modl constructs an equivalent rounded obstacle in terms of terrain elevation statistics.
The relative accuracy of the PTP model is established by its agreement with measurement data rather than by diffraction theory. Neither approach is perfect. Therefore predictions of the PTP model should be checked by other methods if they are to support critical decisions.
The propgation model is generally configured in 'Network properties > Parameters' This chapter wil only describe the parameters that are involved in the propagation model. For all other parameters go to 'Network properties > Parameters'.
The settings for the sites (units) are configured in the 'Network properties > systems' and 'Unit properties'. These are settings as antenna type, height, direction, site coordinates, Tx Eirp, RX sensitivity.
Surface and ground related parameters
Together these properties determine the nature of the radio wave reflection on the ground in a Line-Of-Sight radio link. In general, the more conductive the terrain is, the greater is the risk to have important attenuation or fluctuations of the radio signal. The worst case is the “picket fence” type. In the absence of any specific data, default values should be used.
The terrain surface refractivity is a measure of the air refractivity above/near the ground. In general, the average refractivity would decrease with altitude, being a maximum at sea level.
It is because of refractivity of atmosphere that the index of refractivity is continually changing (This is probably because of density and temperature of air) So the electromagnetic wave doesn't travels straight but a bit curved. K=4/3 is a mean value, but it could also be a bigger or a lower value.
The default value is 301 N-units and that corresponds to K=4/3 which means that is assumed that the Earth radius is 8500 km. The use of K factor is a simplification and was popular in the earlier model using Abacus. But the formal scientific parameter is the Surface Refractivity.
Ground conductivity is expressed in N-Units (parts per million). In the absence of any specific data, the default value of 301 N-units should be used.
Ground conductivity is an extremely important factor in determining the field strength and propagation of surface wave (ground wave) radio transmissions. Low frequency (30-300 kHz) and medium frequency (300-3000 kHz) radio transmissions are particularly reliant on good ground conductivity as their primary propagation is by surface wave.
Ground conductivity is expressed in Siemens per meter.
Relative ground permittivity
The Relative ground permittivity is expressed in Farads per meter.
Suggested values for surface parameters:
||Relative ground Permittivity
To represent the RF environment conditions have to be selected for terrain surface refractivity, dielectric constant of ground, and conductivity of ground.
Radio Climate settings
These options set some of the calculation parameters in the ITS algorithm used in the program. The atmospheric conditions like climate and weather vary in the different areas of the world, and affect both the refractive index of free air and play an important role in determining the strength and fading properties of radio signals. For instance, the refractive index gradient of air near the surface of the earth determines the way a radio ray is bent or refracted as it passes through the atmosphere.
The following Radio Climates can be selected:
- Equatorial (Congo),
- Continental Subtropical (Sudan),
- Maritime Subtropical (West Coast of Africa),
- Desert (Sahara),
- Continental Temperate, common to large landmasses in the Temperate Zone,
- Maritime Temperate, over land (United Kingdom and Continental West Coasts),
- Maritime Temperate, over sea.
in accordance to the usage in the network the polarisation has to be set 'horizontal' or 'vertical'.
Mode of variability.
Longley-Rice defines four modes of variability. The mode selected determines the meaning of the reliability and confidence values used in the model. The mode of variability can be considered the "point of view" for considering the meaning of "reliability" and "confidence" in the calculations.
The modes of variability defined by Longley-Rice are:
- Single message mode,
- Individual mode,
- Mobile mode,
- Broadcast mode.
These modes are defined in more detail in NTIA, p. 37.
The 'Spot mode' (single message mode) is for a one-try message.
The 'Accidental mode' (individual mode) is for interference evaluation.
The 'Mobile mode' is for units that are moving while communicating.
The 'Broadcast mode' is for stationary units.
The effect of percentage of time, locations, and situations depends on the mode selected.
The percentages for Reliability and Confidence are entered when not greyed-out.
|Time variability accounts for variations of hourly median values of attenuation due to, for example, slow changes in atmospheric refraction or in the intensity of atmospheric turbulence. The computed field strength value is an hourly median value; the actual field strength at the receiver location would be expected to be above that value during half of each hour and below that value for half of each hour. Time variability describes the effects of these changes over time. The time variability for the calculation is expressed as a percentage from 0.1% to 99.9%. This value gives the fraction of time during which actual received field strength is expected to be equal to or higher than the hourly median field computed by the program. This variable allows you to specify how you want to deal with the time variability of changing atmospheric (and other) effects as described above. Entering higher percentage reliability values effectively reduces the variability resulting from these factors. The resulting field strength predicted by the program will be lower, but with increased reliability that the actual field that could be measured would equal or exceed the computed value at any given time
|Location variability accounts for variations in long-term statistics that occur from path to path due to, for example, differences in the terrain profiles or environmental differences between the paths. The location variability for the calculation is expressed as a percentage from 0.1% to 99.9%. This value gives the fraction of locations where actual received field strength is expected to be equal to or higher than the median field computed by the program. This variable allows you to specify how you want to deal with the location variability. Entering higher percentage reliability values effectively reduces the variability resulting from these factors. The resulting field strength predicted by the program will be lower, but with increased reliability that the actual field that could be measured would equal or exceed the computed value at any given time.
|Situation variability accounts for variations between "like appearing" (NTIA, p. 30) systems with the same system parameters and environmental conditions, including differences in the ability of individuals to accurately take field strength readings. "It is at this point that 'hidden variables' enter, variables whose effects we do not understand or which we simply have not chosen to control. The values of these variables are at the whim of nature and differ between what would otherwise be identical situations. The effects of these differences produce the changes in observed statistics" (NTIA, p.30). Situation variability describes the effects of the changing conditions resulting from these "hidden variables." The situation variability for the calculation is expressed as a percentage from 0.1% to 99.9%. This value gives the fraction of "identical" paths on which actual received field strength is expected to be equal to or higher than the field computed by the program. This variable allows you to specify how you want to deal with the "hidden variables" that are "at the whim of nature" as described above. Entering higher percentage confidence values effectively reduces the variability resulting from these factors. The resulting field strength predicted by the program will be lower, but with increased confidence that the actual field that could be measured would equal or exceed the computed value
Toggle between Normal or Interference propagation mode. For interference studies, the model is optimistic.
Voice networks are expected to transmit messages so that others can hear them the first time. Repeat transmissions are used in the real world, simply because a user requests a repeat voice transmission, it is normally expected that
a voice message is transmitted only once.
Voice communication is similar to being in a very noisy room, trying to carry on a conversation. If a person constantly has to repeat a statement because it cannot be heard above the noise in the room, communication is not reliable.
Computers, on the other hand, do not care about repeat message transmission. Until a message is received with no errors, a computer can ask for a message re transmission as many time as are required to “get the message through”.
Use this option for a net where need-lines from command posts to subordinate units are required, but not between subordinates. Rebroadcast units can be used to increase the communication range.
Data net, star topology
Data Net, Star Topology has 1 master station communicating with several distant outstations. Outstations are “slaves” and only respond when information is requested from them. They cannot initiate a message on their own. Some data transmission protocols have available a Report By Exception (RBE) mode, that allows a slave station to originate a message. Communication is usually polled by the master station.
The biggest thing to keep in mind is that there is no collision detection or “traffic cop” maintaining order among message traffic. If a collision occurs, a message will be corrupted and the Master station has to poll all the outstations to find out any missed message traffic.
Use this option for a data net where a master unit polls slave units, with no links between slave units.
Data net, cluster
Data Net, Cluster (Node/Terminal) is similar to an Ethernet network. Essentially, any radio unit is assumed to be capable of communicating with any other radio unit. Messages are addressed and rely on a network to get a message through using any available nodes.
Radio Mobile will calculate paths, using as much iteration as necessary, to find the shortest successful path between units. If no path is found after the maximum number of iterations is reached, a link will be shown in red.
Use this option for a data net with nodes that can retransmit datagrams (rebroadcast, digipeating).
Maximum number of rebroadcasts.
If set to zero, this parameter will inhibit retransmission.
(Maximum number of rebroadcasts) = (Time to live) - 1
Portions of this text are provided by Brian Henderson and can also be found in his Program Operating Guide.