This Monday I will get my first experience with static GPS. I will occupy an NGS point for at least 4:10:00, the boss
likes a ten minute buffer, then move on to another point. The data will be used by NGS for the 2022 datum. The boss
showed me the logging data from a short static session he ran at the office. Does anyone have a description of what all
the data means?
If you viewed the file itself in a text viewer then I would say you were looking at RINEX file.
NGS requires The Receiver Independent Exchange Format (RINEX) files.?ÿ Depending on how your receiver collects data, each occupation set creates at 3 files (The observation data file , the navigation message file and the Meteorological data file).
The observation file header contains the basic information about the setup (antenna type & height, time, point name etc.).?ÿ The main body of the file grouped by individual observation and contains information collected from each satellite.
Wiki has a link to the IGS documentation on the RINEX format if you want to really delve into it.
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Sounds like you are very new at GPS. Things you want to make sure you have recorded, most of which should be in the RINEX file.
Site IPD number, Receiver type and serial number, antenna type and serial number, antenna height to ARP (Antenna Reference Point), Company name.
Record the antenna height twice at the beginning and twice at the end of the observation. Measure in feet and remeasure in meters, do not measure in one and convert to other. You can add information on the "comment" lines. The RINEX file should automatically include the RINEX converter type and RINEX file type.
Of interest in the data for basic GPS, the L1 and L2 are wave length specific information and are dependent on the receiver algorithms. i.e. two different receivers can give two different values for the exact same signal.
C1, P1, C2 or P2 are actual metric measurements from the satellite antenna to the receiver antenna, (20,000,000m to 24,000,000m typical)(a value like 19,730,000m means the satellite is almost directly overhead). They vary because the speed of light varies with the signal wave length and varies in different directions from the receiver antenna due to atmospheric conditions. C1 varies from P1 because of algorithmic differences, etc.
D1 and D2 are Doppler differences and the sign varies whether the satellite is approaching or departing from an overhead position.
The L1 and L2 values are to 3 decimal points and the 4th and 5th digits after the decimal point refer to other values of the quality of that signal. L1 is geater than L2 by a factor of 1.283333, the difference in wavelength. If you divide L1 by L2 and get a value near 1.283+/- that tells you the receiver is calculating the observations independently. Some receivers base both values on a intertwined calculation and the differences can be in the order of 1.2833xx to 1.2833yy.
Every epoch of data is preceded by the Julian date and GPS time of the signal, the number of satellites reported and the satellite PRN numbers in order of data, plus there is number included as to the number of GPS seconds added to the UTC clock.
A RINEX file from any manufacturers receiver can be used with any manufacturers post processing software, hence the term RINEX, Receiver INdependent EXchange format.
That is the quick version.
Paul in PA
Site IPD number, Receiver type and serial number, antenna type and serial number, antenna height to ARP (Antenna Reference Point), Company name.
Typo: that's PID, the NGS unique 2-letter plus 4-digit identifier for each disk or other mark in their data base.
It's good to have all that info Paul listed, but the only items on that list you submit to OPUS besides the file are antenna type and height of the Anrenna Reference Point (ARP) above the mark. Those are critical to getting good results.
If you are curious about the RINEX contents, you may learn something, but that is not necessary to do the job.?ÿ There are better things to study about GNSS, particularly how to identify locations that have bad multipath.
Does anyone have a description of what all
the data means?
It means that you are going to have 4 hrs and 10 minutes of free time on your hands. Bring some reading material.
You'll also need to take photos of your setup.?ÿ Digital photos are part of the required submittals.
To spare the OP the need to search, here is a presentation including RINEX file samples: http://geodesyattamucc.pbworks.com/w/file/fetch/65589625/lecture_GPS_2013.pdf
the RINEX file format is completely described here: https://www.ngs.noaa.gov/CORS/RINEX211.txt
there is also a later version 3.0.
Having been ??sheltered in place? too long, I annoyingly provide the following excruciating details:
1. The design of the NAVSTAR GPS included the provision for two frequencies to allow the elimination of the effect of the ionosphere. The group and phase effects are frequency dependent. The speed of light does not change. Early GPS included the L1 and L2 frequencies there are now provisions for an L5 frequency. All frequencies are multiples of the 10.23MHz fundamental frequency (relying on memory the multipliers are 154, 120 and 115). We get the wavelength of the frequency ?ÿas c / frequency (c = speed of light = 299,792,458 m/s).
2. GPS time is continuous and provided from the SV in GPS week and GPS seconds of week. The year, date, time and day of year are derived from the SVs broadcast. The GPS toolkit on the NGS site has the C code for these conversions.
3. The nominal SV orbit radius of the US GPS system is 26,560,000 m with the origin at the geo-center. The actual orbit radius for each SV is encoded in the NAV message. As we observe from the surface of the Earth the PR varies as a function of the relative locations of the receiver and individual SV as well as the height of the receiver. As indicated in another reply the closest range is at zenith.
4. The PRs pseudoranges ((transmit time - receipt time) * c) ?ÿin the RINEX observation file as the name indicates are ??false? because they are contaminated by timing and other errors removed during data combinations (differencing) or by additional data (precise point positioning).?ÿ
5. The best graphics and explanation of how to compute a SVs position from the data in the NAV message is in the University of New Brunswick Lecture Note 58 (aka ??Guide to GPS Positioning) by Wells, et al. The MIT Open Course Ware project includes Prof Herring??s excellent ??Modern Navigation? 12.215 course and is highly recommended.
Cheers,
?ÿ
DMM
Wiki has a link to the IGS documentation on the RINEX format if you want to really delve into it.
Now on my laptop. Thanks!
The MIT Open Course Ware project includes Prof Herring??s excellent ??Modern Navigation? 12.215 course and is highly recommended.
Now on my laptop. Thanks!
Record the antenna height twice at the beginning and twice at the end of the observation.
I will be using a GPS pole and a bipod. I suggested using a sandbag on each foot in case a large truck passes by. My first setup is in the grass median of a fairly large road. We have a wood tripod with a centering rod, but it is in a state of disrepair. A new one will be ordered.
It means that you are going to have 4 hrs and 10 minutes of free time on your hands.
Will it really be free time, or will I have to constantly monitor the positions of the satellites and adjust the antennae mask accordingly? The boss wants the mask set at ten.
Once you set the thing running you leave it alone for the duration.
With regard to masking - a very typical strategy is to collect down to 10°, then mask out above that in the office processing. Although in this case the office processing goes on in Silver Spring.
I think it's very important to note that you're not "collecting data for NGS."?ÿ You and hopefully multitudes of others around the country are collecting GPS on Bench Marks -- https://noaa.maps.arcgis.com/apps/webappviewer/index.html?id=6093dd81e9e94f7a9062e2fe5fb2f7f5 ?ÿso that NGS can compute and publish an accurate datum transformation tool that will related the North American Datum of 1988 (NAVD 88) and the North American-Pacific Geopotential Datum of 2022 (NAPGD 2022).?ÿ This will include the island vertical datums as well. While NGS cares about the accuracy they have nowhere near the resources to collect these data.?ÿ They compute them as a service to the surveying and mapping community - they do not really use them.?ÿ The accuracy of the tool will be totally dependent on the number and distribution of those GPsonBM observations.?ÿ All of these data must be into NGS by the end of December '21.?ÿ Whatever is available to them at that time will dictate the integrity of the transformation.?ÿ It's highly unlikely there will be a second shot at adding more data after that time.?ÿ This will be part of the legacy that the current surveying community will leave to those coming behind.
I suggest you do 1, no more than 2 of these and get them submitted so that you know exactly what is needed when you submit them to NGS.?ÿ That way you are not having to go back and re-do a bunch of fieldwork.
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Yes, you will be sitting for the 4 hours, no need to mess with things.?ÿ Actually, you can setup several receivers and let them all run at the same time.
You should check level at the beginning and end of each session.?ÿ You will need 1 closeup pic of the monument, and 1 horizon with the tripod setup too.
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There is some more to familiarize yourself with, check out the links the other guys posted.
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Remember to come back and let us know how it went for you after you submit one.
Until you get into a routine, I would suggest you do an OPUS run without sharing, to see if it looks good and makes sense relative to the data sheet. It would be embarrassing to share and then find a blunder.
Don"t rush the submission. Wait a day or two until OPUS can give you "rapid orbit" results, not "ultra-rapid".
Then have your pictures and any update for the to-reach description handy and resubmit the file with sharing selected. The web site will ask for this information. An email will let you know when the submission is ready for your final approval.
Look at some shared solutions to get a feel for what the result will look like.
the speed of light in vacuum does not change. The signal passes thru the vacuum of space and the variable density of atmosphere, which does effect its speed. And each frequency has different propagation delay. And the path is effected by refraction. Which is why signals below 10 degree, or even 15, are ignored. Near horizon signals aren’t just weaker, the path (time delay, speed) is less certain.
As for mask, the mask is set in the post processing. So recording to zero degrees has no detrimental effects. I routinely collect to 5 or zero. In post I set 10-15. But may entertain 5 if there are obstructions on 1-2 sats in one direction. It??s depends on the specs. But with 4 hr data, and knowing that 10 deg is the lowest that??ll be used, enter all the settings in advance correctly in the office, and save as default. Power cycle, check the settings, record an hour in the parking lot and submit to OPUS, and check the recorded data to know that the correct settings are the active default. Have another person check as well. (Maybe post a file here if your worried.) Leave nothing to chance, and then there are fewer things to do, which is fewer opportunities for mistakes.
Parking lot test is your best defense. Make a check list. Bring spare power and cables.?ÿ
