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Group delay & Phase advance (GPS vs Ionosphere)
Posted by geodesist on August 20, 2021 at 9:20 amDear colleagues, I hope you are doing well.
1/. Please could I know what that means : (Group delay & Phase advance), when the GPS/GNSS radio wave crossing the Ionosphere.
2/. why the geodesists prefer to utilize the code (pseudo-distance) measurement instead of the carrier-phase measurement .
thank you very much for your help and support.
Cordially
OleManRiver replied 2 years, 1 month ago 10 Members · 11 Replies- 11 Replies
I’m a surveyor. All I really care about is “how long that quarter mile is”.
You are the geodist, I’d like you to tell me the answer to your question.
Not said snippy, just looking at our log in names! And with a big ???.
Are you a student?
The rain has quit. I’m off to feed ticks.
Nate
The ionosphere and troposphere contain ionized particles that interact with the electromagnetic waves of the GNSS signals and slow their passage. The waves thus travel at less than the speed of light or radio waves in a vacuum. The distance pseudorange appears longer than the true distance by some unknown amount.
The amount of delay is variable over time and location. If a base or permanent station (for example US CORS) has a very long time average, the variable delays tend to average out and give an accurate position.
The delay is similar over an area on the ground of perhaps 10’s of kilometers. That is why most survey work uses corrections or comparisons from another station simultaneously receiving the same signals, to solve for a new point’s coordinates.
The code bits on the GNSS signal are very long compared to the carrier cycles. There are fewer code bit transitions than carrier cycles. Thus in noise the receiver has fewer features to average in locating the time of code arrival than to find carrier phase and the code time uncertainty is larger.
Group delay versus phase delay is a difficult concept to explain. All I will say is that even in the absence of noise the iono-tropo delay makes the code bit edges appear with different carrier phase, and this is an additional uncertainty for the receiver.
I hope some of this ramble helps.
.You use the handle “Geodesist” which implies you should know the answers to the questions you are asking. Also you are making assumptions on what data “other” geodesists are using. You should select a handle that better indicates who you are and what you do.
It is hard to give simple answers on such a complicated subject. The ionosphere is not a single uniform entity, it includes various layers with characteristics that vary over time, some times the differences are like day or night. It varies with the direction from where you are to the equator, the North Pole, and the the corrections can vary based on the earth’s and ionosphere’s rotation relative to the speed of light.
“(Group delay & Phase advance)” are words from a technical manual, one of my GPS text uses them but never fully explains them. I cannot determine either in absolute values.
Paul in PA, PE, PLS
Your second question is wrong. Geodesists do not prefer code over carrier. The best vector reduction aims to solve for the integer bias ambiguity yielding an integer-fixed ion-free solution. A code solution is best used as a starting point for subsequent carrier phase processing.
In the early days the code was subject to selective availability requiring code-less processing. When it was set to zero, processing became much easier with code providing good starting values for the integer search.
As for your first question:
See: ftp://tai.bipm.org/iers/convupdt/chapter9/icc9.pdf for detailed formulas.
Review the following presentation (link on slide). Summary of an article of possible interest is also shown.
Do a web search. Lots of reputable sites on this and related issues.
Two additional references of possible interest:
https://gssc.esa.int/navipedia/index.php/Ionospheric_Delay
https://gssc.esa.int/navipedia/index.php/Phase_%26_Group_Velocity
@paul-in-pa I disagree with your admonishment. Most of my best reference material on geodesy far predates the first satellite. GPS has become an indispensable tool for us, but GPS does not equal geodesy.
By my observation, most who claim the title ‘geodesist’ are nothing more than barely adequate users of thier favorite brand of GPS equipment. They can’t give a correct definition of geodesy, or name the basic elements of any of the major transformations (much less use them).
Abd we wonder why nobidy wants to enter our wonderful world…
I’ve always found this to be a helpful explanation of code-based vs. carrier phase solutions, with enough graphics to visualize it. Click “Next” at the bottom of each page to step through the whole thing.
https://www.trimble.com/gps_tutorial/dgps-advanced2.aspx
I don’t care which has the most ionosphere distortion? I try to calibrate my rtk work per job and keep my static readings simultaneous; hopefully keeping ionospheric corrections to a minimum. I wonder how many bar fights have been started by geodesist discussing this in a smoky pub?
@oldpacer I wish I could write well enough to describe the visual you just gave me. ???
As for group delay and phase advance, I’m not able to give a full derivation, and an online forum isn’t a convenient place to write a scientific paper even if I could. But I can mention some of the phenomenon I’d be thinking about if I were going to learn about it.
We’re probably all familiar with a single frequency radio wave traveling through a vacuum. If we take a snapshot of it, and plot the amplitude of the electric field as a function of distance, we get more or less a sine wave; the only departure is the amplitude gets lower as you get further from the source.
We also know that if a radio wave is to carry information, it must be modulated, so the amplitude or the phase or both are changed a little to represent the information. If we look at the modulated wave on a spectrum analyzer, we probably see the carrier wave as a spike in the middle with sidebands on either side.
If the wave is propagating in a vacuum, the wave will maintain its shape no matter how far it goes. When you do the math you find that this is because the speed of light (and radio waves) is the same no matter the frequency. But in the ionosphere the speed of radio waves is different for different frequencies. This means that the various frequencies that combine to form the modulated radio wave are traveling at slightly different speeds, and as they travel along, the different speeds cause the shape of the wave to change.
It’s sort of like a bunch of cars stopped at a traffic light. Just after the light turn green, the overall pattern of the group looks the same as before they started. By the time they’ve gone five blocks, the group looks different because each car is going at a somewhat different speed.
Of course, there’s a lot of math between what I wrote and a finished formula for phase advance or group delay.
As for where this kind of stuff is taught, I learned about it in my Physics III class as an undergraduate in electrical engineering about 1979.
Lots of great info here and we could all get down in the weeds. But to use KISS. I will just state this. Geodesist care about size and shape of earth. Orbital scientist care about how the satellites orbit the earths mass. The signals from the satellite are used and understood by both to a degree they need them. Scientist also spend many hours collecting the time of both satellites clocks Atomic Frequency Standards and the Monitoring stations. The time is corrected both for long term clock drifts and also short term issues. Signals analysis from scientist or engineers and mathematicians study the issues of signal delays from ionosphere and troposphere. All codes ca and such are used for there strength and weaknesses. Basic L1 vs L2 One is stronger but yet has a weakness. The other is weaker but has a strength. I will go out on a limb and say no manufacturer adheres to the ICD xxx specs given to them by our government. Talking GPS only because in usa its the only one we can point a finger at that is monitored and those values needed by usa citizens can if they want research and see how good or bad it is. The same signals we get in our every day equipment are used differently by many different people that drive control and monitor the gps constellation and ground stations. I mean mechanical engineers signals engineers orbital scientist geodesist gravity time experts all and more go into the gps. Surveyors we use it but we are the low end percent wise of people who use gps. We are experts in how we use it. But its much much more. Weather people use what we model out for lets say RTK networks. So the above question is asked. Why do weather people want what our VRS or network rtk systems throw out. In that lies the answer to the question you could have asked. Your not far off you are close. Now read study but not just the geodesist side. And dont just think rtk. When looking at ionosphere delays. Or interruptions.
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