Elevation Mask – The Surveyors’ Solution to some (but not all) DGPS problems

by Umberto on 18 October, 2020 in Online

In the last few years in the training world the buzz around the Elevation Training Mask went sky high. And it was simply a mask that reduced your oxygen intake.

But they claimed it may give you astonishing results. I don't know if it is true, but I for sure know a mask that can give your DGPS better results.

Elevation Mask.

Have you ever modified it?

Most of the online surveyors I worked with surprisingly didn’t know the benefit of an accurate elevation mask setting, and, to be honest, it can come quite handy in certain situations.

But let’s go in order and start with the theory first and, after that, I will tell you the two times the elevation mask saved my back and how you can leverage that too.

DGPS Elevation Mask: Definition

The Elevation Mask is that setting that makes the DGPS receiver exclude the satellites that are low on the horizon from the position calculation. It is an angular value and indicates that, at any given time, all the satellites that are between 0° (the horizon) and the selected value will be rejected for the calculation.

In the image below I tried to summarize when a satellite is excluded or accepted from the receiver, based on the elevation mask setting.

  • The ones below the red line are below the horizon, therefore the DGPS antenna shouldn’t be able to track them.
  • The ones between the red and the yellow lines are those who fall in the elevation mask. So? What happens? The antenna can track them and sends the signals captured to the receiver, but the elevation mask flags them as “Not Usable” and therefore are discarded.
  • Lastly, those above the yellow lines are the satellites that are above the mask and consequently will be used for the position calculation.
Elevation Mask - Satellites Calculation

In this picture we can see how a DGPS determines which satellites to use or exclude for its calculations based on the Elevation Mask value.

Why shall we use the elevation mask, and discard information, instead of using all the satellites tracked?

Good question.

It can be simply answered with one word: Precision.

Exactly my friend, because having more info doesn’t mean that you will have better data.

Let me use an example.

Imagine that you are going on a trip to a city you have never visited before, and you are looking for a specific location: its majestic and ancient tower.

While you look for it, you stop to ask for directions to every single person you meet. Every single one. You listen up also to those who tell you as the first answer that they are not from there or don’t know the area very well.

After 20 minutes you have collected a huge group of info but unfortunately, some of them are opposite of the others.

Result? You are nowhere nearer to the majestic and ancient tower than you were 20 minutes before.

So you decide to change strategy and to filter that info.

You ask again for directions, but this time you collect only the information from people who actually know the area and can truly show you the right path to your destination.

Result? You reach the majestic and ancient tower in 7 and a half minutes. Not bad right?

The very same thing happens with the DGPS.

If you let the DGPS accept also information from satellites that are low on the horizon (that have a higher degraded signal), its accuracy will decrease.

Why is the signal from the satellites that are low on the horizon bad for our accuracy?

Every signal gets degraded while traveling in its medium. The same happens also to satellite signals.

In this specific case, since we are talking of electromagnetic signals, the main cause of degradation comes from the Ionosphere.

The Ionosphere is the outer part of the Earth’s atmosphere that, when it’s reached by the Sun’s rays, gets filled with ions and electrons.

Or, as Wikipedia would say it:

The ionosphere (/aɪˈɒnəˌsfɪər/) is the ionized part of Earth's upper atmosphere, from about 60 km (37 mi) to 1,000 km (620 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. The region below the ionosphere is called neutral atmosphere, or neutrosphere[ 

Source: https://en.wikipedia.org/wiki/Ionosphere

Coming back to our signals from the satellites, what happens to a signal that travels more in that mix of electrons and ions?

Exactly, it gets degraded more and becomes unusable.

That is literally what happens to the signal from the satellites that are low on the horizon as you can see in the image below.

Elevation Mask - Ionosphere horizon

Here is shown how the signal of satellites low on the horizon is more affected by the ionosphere.

Which problems can be solved with a good elevation mask setting? And which cannot be avoided?

For sure a good elevation mask can avoid you to have a dilution or you overall precision, since it cuts out all the unwanted signals from satellites being low on the horizon, but not much more.

Once it helped me a lot on TMS (Tug Management System) Job.

During a night a TUG was heavily rolling and was constantly disappearing from our screens (actually it wasn’t disappearing but jumping at kilometers away from our location) due to the deteriorated position by low satellites.

I couldn’t do much from remote (the survey was based on the FPSO, but we had access through SeaMesh net) since all the systems were up and running correctly.

I couldn’t do much except playing with the elevation mask. And so, I did. I increased it to 18°.

I know it is a damn high value but when you have the client behind your shoulder asking you why that boat keeps disappearing you are open to any possible solution.  😉

It is a really high value, even because it reduces the whole sky to 144°.

But you know what happened? It worked.

And, once the night passed, I put the values back to the default as the vessel wasn’t struggling anymore.

ATTENTION - Be careful when you apply this.

For the sake of your knowledge, on 5 TUGs on that job, that was the only one experiencing that problem and only during that night.

Don’t ask me why. You know better than me what happens offshore when “everything should work seamlessly”.

Am I telling you to use the same trick? Not at all…or at least you could try…and come back with the default settings if it does not help you.

What for sure the elevation mask couldn’t do for you is helping you with other errors, like multipath for example.

Multipath error

This images shows how the Multipath error can be caused.

In case you don’t know what multipath is, Wikipedia has you covered.

In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.

Multipath propagation causes multipath interference, including constructive and destructive interference, and phase shifting of the signal; destructive interference causes fading. This may cause a radio signal to become too weak in certain areas to be received adequately […].

Source: https://en.wikipedia.org/wiki/Multipath_propagation

Basically, it is the dilution of precision of a DGPS caused by signals that do not reach the antenna directly instead, during the travel, they bounce on one or more surfaces, like buildings, mountains, and so on.

The only solution to this is a choke ring antenna, an antenna (shown in the image below) that could cut out the noise coming from the multipath signals.

Choke Ring Antenna

This is the Choke Ring antenna. An antenna purposely built to reduce the multipath interferences.

How should the Elevation Mask be set? 

So basically at the end...how this value should be set?

As we can agree it for sure doesn’t have to be 0 (zero).

But basically, it DEPENDS.

You will have to try it and see the best fit for the job you are doing and the area you are working in.

Anyhow, I can give you some advice:

  • Hemisphere DGPS usually have a default value of 7°
  • Trimble DGPS usually have a default value of 10°

A value between 7 and 10 seems like a safe investment. Test out what works better for you.

Moreover, during alongside calibrations, I would increase it a bit…to have a cleaner signal.

For this topic, I think it’s all.

Glad to have you on board with us fellow Smartest Surveyor. If you have any question or any subject you want to se explored here feel free to comment below.

Chat soon,
Umberto

P.S. Did you find this article useful? If yes, share it so it can reach more offshore workers like us!


Tags

Instruments, Tips & Tricks


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