Understanding the PPP output report

I didn't get any responses to this post, but I just wanted to update and say that ChatGPT has been a good starting point for my questions and I'm making progress! 

There are a lot of different makes and models of GPS, and supporting software, capable of measuring and analysing PPP. All have there own formats. It really enhances your chances of getting a response if you specify which you are inquiring about.  

As Norman_Oklahoma pointed out there are a wide array of software and hardware out there that do more or less the same thing using different formats. You would probably be better served to purchase a good book on the subject like this one. 

Unfortunately there isn't a one stop and shop cook book that covers everything you're asking about but this would be a good start. Once you enter the world of GPS, whether you want to or not, you'll need to have some kind of foundational working knowledge of geodesy as you're working with global earth centered coordinates with WGS84 and how you choose to reproject them is largely determined by what you're trying to accomplish, which you haven't exactly made clear.

PPP is a great tool that keeps getting better. I've looked at these reports for quite a few years, not saying that I know the answers to all your questions but will give my understanding of it.

Posted by: @islandoffset

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Hello, from scanning a few posts about PPP on here, I can tell that I have a lot to learn. 

I'm looking at the pdf output report and trying to define the meaning of each value on the first page. I have attached the report I'm currently looking at. I found the "report description - static" resource online (here) but it doesn't give me enough information. 

• Data Start: Self-explanatory.
• Data End: Self-explanatory.
• Duration of Observations: Self-explanatory.
• Processing Time: Would this be when I submitted the Rinex file to the PPP website?

Yes, that's the time when PPP processed the file.

• Product Type: Final, Rapid, or Ultra-Rapid based on how much time elapsed between gathering the data and submitting it. I found a table (below) explaining the differences on this website (here)

Orbit type	When the orbit is ready for use	Accuracy
ULTRA-RAPID	about 60 minutes after the end of each hour	~15 cm
RAPID	about 12 hours after the end of each day	~5 cm
FINAL	12 days after the end of each week	∼2 cm

• Observations: Phase and Code. What does this mean?

Tells you which signals were used for processing. PPP only uses GPS and GLONASS, which of course the graphs confirm.

• Frequency: Double. I know we use dual-frequency antennas (Hemisphere S631s). What does that mean exactly? That the antenna 'speaks' to the satellites in two radio frequencies at the same time to somehow improve accuracy?

I believe this just confirms that both L1 and L2 are processed, in other words no L5.

• Mode: Static. self-explanatory
• Elevation Cut-Off: 7.5 degrees. We set up our elevation mask at 10 degrees when gathering data, so how does this apply?

PPP filters noisy signals, but it will attempt to make use of signals as low as 7.5 degrees. If you have 10 degree cut-off set on your receiver then of course nothing below 10 degrees gets recorded. It does tell you that you could reduce your setting to 7.5 degrees and PPP would evaluate those signals. It's a trade-off at the lower elevations, yes those signals help with height, but I suspect they are weighted lower in the solution due to tropospheric delay, just a hunch.

• Rejected Epochs: Is that based on the elevation cut-off?

I believe this is a measure of epochs as a percentage of all epochs (which are at 30s interval) where the processor completely rejects all signals due to poor quality. 

• Fixed Ambiguities: The online document describes this as "the % of carrier-phase observations (?) with fixed ambiguities. What does that mean?

Larger topic that I'm not an expert in, refers to fixing integer number of wavelenghts, float being when there are fractional parts. Higher is definitely better 😀 . If you have a low percentage, that usually means you have some bad obstructions 

• Estimation Steps: 30.00 sec. I believe that PPP decimates our data into 30 second intervals so that it can process efficiently, but still stay accurate. Is that correct? We gather our data every 1 second - is that too much?

30 seconds is what is typically used for geodetic work. Any less than that and the geometry of the satellites hasn't changed enough for the solution to have realistic statistics. You'll get overly optimistic results with lower intervals.

• Antenna Model: Self-explanatory.
• APC to ARP: A default offset value based on the antennal model. 
• ARP to Marker: I assume this is the HI that we enter during station set up.
• Estimated Position in LLH
  • NAD83 (CSRS) (2002.0): Is this the estimated position based on the reference frame (NAD83(CSRS)) and epoch (2002.0) that I selected on the PPP website when I submitted the Rinex file? So is this the A priori position after it has been translated to these models/datums?
  • SIG_PPP (95%): The uncertainty expressed in standard deviation (95%).
  • SIG_TOT (95%): The uncertainty expressed in total standard deviation (95%) including both PPP and epoch transformation uncertainties.
  • A priori: This is the A prior position. I see a footnote on the pdf that the "Coordinates from RINEX header used as a priori position". Would this be the 'raw' position gathered by my ARP? Would it be relative to the datums I selected in my data collector at the start of the project - UTM Zone10 NAD83 and WGS84?
  • Estimated - A priori: I believe this is the difference between the two.

The estimated position is the solution, and the sigmas indicate the confidence. PPP sigma is just the processing uncertainty, while the total sigma combines the processing uncertainty with the epoch transformation based on the velocity model used. 2002 was a few years ago. The velocity model is derived from observations at active control stations and some passive control stations, but like any model there are uncertainties for locations elsewhere.

• Orthometric Height: CGVD2013 (CGG2013a): I selected the vertical datum model of CGVD2013 when I submitted it to the PPP website. This is the height I use as part of my final XYZ position to say that position is "geolocated". How does this work exactly? Are the lat and long values projected onto the geoid model to get the height? If so, does the process use the estimated lat and long or the a priori lat and long?

The lat and long values are used to determine the separation between the ellipsoid and the geoid at that location. The ellipsoid height of the solution is therefore critical, so no point calculating this before the final solution is available.

 

• 95% PPP Error Ellipse: Some kind of uncertainty, but what exactly is it?
• 95% TOT Error Ellipse: Some kind of uncertainty, but what exactly is it?
• UTM (North): Zone 10: Self-explanatory.
• Northing: The value I use in my final XYZ. How does this work exactly? Is the estimated (or a priori?) latitude projected onto the UTM Zone 10 grid? 
• Easting: The value I use in my final XYZ. How does this work exactly? Is the estimated (or a priori?) longitude projected onto the UTM Zone 10 grid? 
• Scale Factors: Self-explanatory.

The error ellipses are just a different representation of the uncertainties shown in the lat/long estimated positions, this time referenced to the grid projection. The grid coordinates shown are the estimated position, which is the final solution. You can easily check the math on this with the TRX tool that NRCan provides as well.

The next few pages of graphs have good enough explanations for me on that online resource. I rarely look at those to be honest!

Thank you for any insight!

 

IO

@islandoffset

https://drive.google.com/drive/folders/1LhfYOWium1xYhpYMVtwWv93gNlyRDhSW?usp=sharing

@jacob-wall My respects. You have outlined the subject thoroughly it was an engaging read. Even after 15 years, I still from time to time employ static measurements using dual GPS receivers (L1 + L2+ L5 frequencies, GPS, GLONASS, Galileo) to establish precise control points with fixed coordinates on-site. Reference stations occasionally yield anomalous data.