Satellite Availability

Shawn Billings

Shawn Billings
5PLS
I've noticed that around 2pm Central time that RTK gets very challenging in canopy. I've not used mission planning in years, but I finally took a look today. GPS from 9am to 6pm in my area is between 7 and 8 sv's above a 10° mask, and drops to 6 briefly at 2pm. At 10pm there are 12 GPS satellites available above 10° and then again about 5am there are 10+.

Interestingly, Glonass also drops considerably along with GPS.



This is a very handy and easy to use mission planning site if you don't already have one available:

http://gnssmissionplanning.com/
 

James Suttles

Active Member
Have noticed the same thing here in NC, From 2pm on it gets very difficult to get a shot in Canopy, and the leaves this year seem to be thicker than normal. Guess all the rain and no killing frost in the first part of spring, made for thicker cover
 

Shawn Billings

Shawn Billings
5PLS
I don't have the ability to test it. However, when the current low spot occurs with GPS and Glonass, Galileo adds 4 and Beidou adds 4, which is pretty low for them as well. It's pretty incredible that all of these constellations have poor availability at the same time of day. We'll have to see how testing from others works out.
 

Darren Clemons

Well-Known Member
I don't have the ability to test it. However, when the current low spot occurs with GPS and Glonass, Galileo adds 4 and Beidou adds 4, which is pretty low for them as well. It's pretty incredible that all of these constellations have poor availability at the same time of day. We'll have to see how testing from others works out.
That always seemed strange to me too Shawn. This “mid day doldrums” has affected us for years.....it’s always been this way. If you have a particular “tough” spot to get, you better be in there as early after sunrise as you can get or very, very late in the day. Just a random guess based on 20+ years of using RTK, I’d say your chances of getting a particular rough point at 8:30 am are about 3/1 compared to getting it at 12:30/2:00 pm.
While the number of satellites are always a bit less during this time, I’ve always though the atmosphere and other “signals” in the airwaves have always had a bit to do with it also.
First thing of any morning, the sky (even if cloudy) will seem a bit “cleaner” and clearer, especially in mid summer before the haze and humidity get so, so sticky.
Also, not many other waves of transmissions from cell phones and countless other electronic pieces of equipment are jamming the airwaves yet.
So, so, so many times, we’re all thinking it’s “just” the satellites (or lack thereof) for us not getting the fix we need, where many, many times it’s our UHF or other transmission signal trying to get the data transferred from the base to the rover that’s the problem
 

Sdrake14

Active Member
Thank you for this post. I have noticed the same anomaly here on the west coast.
A huge difference between morning and afternoon in heavy redwood cover.
 

Glen Yasharian

New Member
Shawn: Thanks for the reference to the planning site. I've used the Trimble site for years at https://www.gnssplanning.com/; it would be interesting to compare the two. Here in NE PA, there is also a dip in the number of satellites around mid-day. The planning software seems to be realistic as to the number of satellites at specific times. However, as Nate pointed out, the location of the SV's in relation to potential obstacles at your specific site seems to be equally important, so, time of day is important for both factors (number and orientation). I find those charts more challenging to interpret.
 
Shawn: Thanks for the reference to the planning site. I've used the Trimble site for years at https://www.gnssplanning.com/; it would be interesting to compare the two. Here in NE PA, there is also a dip in the number of satellites around mid-day. The planning software seems to be realistic as to the number of satellites at specific times. However, as Nate pointed out, the location of the SV's in relation to potential obstacles at your specific site seems to be equally important, so, time of day is important for both factors (number and orientation). I find those charts more challenging to interpret.
I also use a app. on my Iphone called GPS Plan, together with Trimble planning in the office. It provides number of SV, DOPS, and Sky View. It can help ease my mind when I am having that impatient moment!!
 

Shawn Billings

Shawn Billings
5PLS
That always seemed strange to me too Shawn. This “mid day doldrums” has affected us for years.....it’s always been this way. If you have a particular “tough” spot to get, you better be in there as early after sunrise as you can get or very, very late in the day. Just a random guess based on 20+ years of using RTK, I’d say your chances of getting a particular rough point at 8:30 am are about 3/1 compared to getting it at 12:30/2:00 pm.
While the number of satellites are always a bit less during this time, I’ve always though the atmosphere and other “signals” in the airwaves have always had a bit to do with it also.
First thing of any morning, the sky (even if cloudy) will seem a bit “cleaner” and clearer, especially in mid summer before the haze and humidity get so, so sticky.
Also, not many other waves of transmissions from cell phones and countless other electronic pieces of equipment are jamming the airwaves yet.
So, so, so many times, we’re all thinking it’s “just” the satellites (or lack thereof) for us not getting the fix we need, where many, many times it’s our UHF or other transmission signal trying to get the data transferred from the base to the rover that’s the problem
I believe the most significant atmospheric issue is the ionosphere. It protects us against radiation from the Sun. During that double hump peak solar cycle we experienced around 2001 (as memory serves), the ionosphere could pose real problems for differential GPS. During solar events such as coronal mass ejections (CME) that were Earth pointed or near Earth pointed, the ionosphere would become charged up, interfering with GNSS signals. The Total Electron Count (TEC) models were post-developed (so you couldn't use it for planning necessarily) but it was pretty good for troubleshooting today's failures and might be helpful to know what to expect tomorrow. The TEC would peak just before sunrise and drop just after sunset. During the night, when the observer was pointed away from the Sun, the observer could almost work as normal. Today's equipment seems to track much better. I recall a few years ago using the Triumph-LS on a 12-mile baseline during a pretty high Kp-index and could not see any difference to normal operation. In those early 2000's, a high Kp-index meant you should probably just keep the GPS in the truck or plan to observe for a significantly longer period of time. Even in non-peak solar cycles, daytime atmospheric conditions will have more interference than nighttime, but it is probably not terribly perceptible for us.

As for satellite availability, the weak and strong periods are always changing. I've heard people suggest that 1-2pm is always bad, but I don't believe that is the case. The constellation changes by a few minutes every day and the Air Force moves them around a bit from their scheduled orbits from time to time. With Glonass and GPS and now Beidou and Galileo, the low availability is more and more trivial, but as one can see from the current availability charts, it isn't always trivial.
 

Sean Joyce

Active Member
That always seemed strange to me too Shawn. This “mid day doldrums” has affected us for years.....it’s always been this way. If you have a particular “tough” spot to get, you better be in there as early after sunrise as you can get or very, very late in the day. Just a random guess based on 20+ years of using RTK, I’d say your chances of getting a particular rough point at 8:30 am are about 3/1 compared to getting it at 12:30/2:00 pm.
While the number of satellites are always a bit less during this time, I’ve always though the atmosphere and other “signals” in the airwaves have always had a bit to do with it also.
First thing of any morning, the sky (even if cloudy) will seem a bit “cleaner” and clearer, especially in mid summer before the haze and humidity get so, so sticky.
Also, not many other waves of transmissions from cell phones and countless other electronic pieces of equipment are jamming the airwaves yet.
So, so, so many times, we’re all thinking it’s “just” the satellites (or lack thereof) for us not getting the fix we need, where many, many times it’s our UHF or other transmission signal trying to get the data transferred from the base to the rover that’s the problem
Amen Darren , Mission planning has been the difference for me between getting garbage on a tough point or getting a verified RTK shot on the same point at a different time.
It really helps to pinpoint the exact spot and put in the obstructions (terrain) on the mission planner.
Still hoping for mission planning incorporated into the L.S. as I mentioned here a few years back.
 

Shawn Billings

Shawn Billings
5PLS
Performance seemed much improved yesterday over the previous week or two in the afternoon. I did not notice a significant drop in performance or satellite count. The availability chart shows a brief period around 3:40 central in my location that has a dip to 11 total GPS and Glonass satellites, but it is very brief and with the exception of a brief period of 12 total GPS and Glonass is entirely 13+ for the rest of the day. (10° mask).
 
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