Mathias Roehring

Tip of the Week #16 - GNSS Planning Tool: Get a Sense

Blog Post created by Mathias Roehring on Sep 9, 2020

"Good old days"

Some of you may remember the days of TerraSync showing a few GPS satellites in the skyplot. See below a GeoExplorer 2008 happily tracking 7 GPS satellites:

Geo2008, TerraSync

Now, about 12 years later the sky has changed dramatically. 

See below the skyplot at the Christchurch office from the Trimble Planning tool (http://www.gnssplanningonline.com/#/settings) :

Skyplot Christchurch, New Zealand

What a change! 

Easily 4 times more satellites of different constellations. And by the way: TerraFlex does not even have a skyplot anymore. Do we still need a skyplot? 

 

The skyplot was an important tool for planning. With only a few satellites above the horizon it was essential to know how many are left after taking canopy or buildings into account. Out of the 7 satellites from the example above another few are lost quickly behind buildings or other obstacles, ending up with too few to track any position.

 

Today, having plenty of satellites available it’s almost certain that you’ll see sufficient satellites to track a good quality and accurate position. For most applications there is no need to check the sky or plan for satellite visibility. If you are in a really bad urban canyon and you are not getting enough satellites you may be lost anyway and even the best planning may still not be of great help. 

 

So, what’s the point of a GNSS Planning Online tool?

 

Mind the gap, use the gap

It may still be useful for planning extreme environments. Start with the Settings and locate yourself.

However, the best part of it is the educational benefit. 

If you in Chicago for example, you may have a skyplot like this:

Skyplot Chicago

You can instantly see that the satellites are somewhat opposite to our NZ office in the Southern Hemisphere. But practically, how can this skyplot be helpful?

 

To get most satellites in a tricky canopy location the user of a handheld receiver should position himself or herself north (in the Northern Hemisphere) of the antenna to block less satellites with their own body. Simply because there is the gap of satellites above the North pole anyway. 

User with skyplot

The same applies for mapping features like a tree. What’s the best location for you to map a tree? South of the tree (Northern hemisphere). Then the tree will be north and therefore in “the gap”. 

 

On the move

Indeed not everyone is aware that the GNSS satellites are moving. A GPS satellite is moving 3.6 kilometers per second or 2.2 miles per second. In the GNSS Planning Tool you can play the satellites movement (upper left, red circle):

Skyplot play

Now you can imagine how the constellation of satellites is changing and there may still be situations when the arrangement of visible satellites is less optimal. in general: The wider the satellites are spread the better it is. However, as said, under normal circumstances you don't need to worry to much about that anymore.

 

Space weather

In the World View tab or the Charts tab you also get some information about the current space weather. Most of the time it’s not causing issues but sometimes scintillation may occur and that can cause outages of GNSS signals. For information on TEC and Scintillation see here: http://www.gnssplanningonline.com/#/ionosphericeffects

 

In the Satellite Library tab you can also see the status of satellites and if they are healthy or not. 

 

You can use the tool for planning but also for education (yourself or others) to get a better sense of where the satellites are and what they are doing. It’s not a big deal nowadays and because the tool is easy to use it may take you just 5 minutes to get a sense. 

 

Enjoy it!

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