Worst earth curvature over water

Does anyone know how worst earth curvature is calculated?

I've run into a situation where the worst earth curvature over the ocean is a rather excessive 15m above the ocean.

Just wondering if I can safely ignore this recommendation.

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The worst earth curvature is calculated using Figure 2 in ITU-R P.530 and is dependent on the path length. It is calculated for a continental temperate climate and so may be a little different in other regions, however the concept of earth curvature and its impact on radio signals is not dependent on whether the path is over ocean or land.

If you are designing for availabilities above 99.9% then it should be taken into consideration. You can see the impact by looking at the "Excess Path Loss at ke" parameter which is available as a selectable column in the PTP links view (see Managing Lists for more information on managing the list views) or from the blue information icon against Lowest Mode Availability on PTP 820 links. If you have sufficient fade margin on your link to be able to tolerate this additional loss then it can be ignored.

In the image you have provided there is only a small incursion into the Fresnel zone and therefore unless your link is already very marginal it may well be able to tolerate the additional loss.


Hi, Amirul here. I would like to ask two questions:

a) by referring to the image that i have attached, my understanding on the Fresnel zone, worst earth
curvature and terrain at k=1.33 is correct or wrong?

b) my blue shaded region has small incursion into the Fresnel Zone (touched the grey line). If i set my
design criteria for the link to be 30dB or higher, and the result i get from this particular simulation is
30.60 dB (sufficient fade margin), will i be able to tolerate the aditional loss and ignored this issue?

Appreciate your kind assistance Sir/Madam. Thank you

Hi Amirul,

Unfortunately Rachel is out of the office until Tuesday so I will answer to the best of my knowledge.

a) The image that you posted has the correct labels.

b) As Rachel mentioned, the “Excess Path Loss at ke” parameter can be displayed in the PTP Links table view. For a similar length link over water:

I see these excess loss values:


As you can see there can be several dBs of additional loss which need to be accounted for in the link design. As a result, the 0.6 dB of additional fade margin that you have in your design will not be sufficient to cover the additional loss.



Thank you very much Andy sir for answering my previous questions.

If we changed our design criteria for the fade margin to be 20 dB, will the 30.60 dB be acceptable to cover the additional losses?

In our case before, we set the design of fade margin to be 30 dB or higher and the result from the simulation is 30.60 dB. So, there will be 0.6 dB difference which is I assume before, is sufficient to cover the loss but unfortunately it is not. Therefore, what is the minimum figure that is required in order for the link to be good even the 60% of its Fresnel zone touching the worst earth curvature?

Thank you for the explanation and I appreciate it very much.

Hi Amirul,

a) It depends on the Excess Path Loss at ke value for your particular link.If your link has a similar 5.1 dB excess loss then it should be fine.

b) As with (a) it will depend on your particular link. LINKPlanner will predict the values for you, but you need to decide if the results are acceptable for your requirements. I don’t have any real-world experience with links to know if there is a rule-of-thumb that you can apply. Someone else on the forum may be able to provide that guidance based on their experience.