Need to know the WGS84 realization. I have seen in several posts that folks use G1150 - out of use since 2012. I would like a definite answer for post-processing and for my own knowledge.
Thanks!
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I personally think that that is not connected to REACH but to the GPS system itself.
But I’m also interested in the answers.
The other question would be what happens if we use GLONASS and GPS together?
And finally is there a difference between Reach View and RTKpost?
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well to start, this is one of my favorite sites with some explanation of WGS84 realizations.
https://confluence.qps.nl/qinsy/en/world-geodetic-system-1984-wgs84-29855173.html
next i would like to add in that if you use the NGS tools HTDP you can take a given coordinate and convert it from one realization to another. i experimented with this once and found that if i took a point in g1762 and at most if i went back to g730 i would see less than 10cm difference. i may be doing this wrong, you tell me, but currently that is what i think the maximum consequence is for getting this question wrong in my area.
so my working theory is that the reference frame is not dependent on the receiver. if im wrong and it is hard coded into the firmware and someone takes a old GPS from the late 1990’s out and gathers a point with it (no WAAS) then that point will be in a older reference frame and at most systemically 10cm off.
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Thanks for the input. Having looked over a lot of documentation, it appears that the hardware has nothing to do with the WGS iteration. It is ‘dumb’ and only receives at the L1 phase center (for the Reach RS+ at least). So, the date that it is collected will determine the WGS84 iteration. So for data collected after Oct 10, 2016, the WGS84 - G1762 (iteration indicated by the GPS week 1762 epoch). Until there is an update…
As for the GLONASS question, I found that most receivers use the L1 phase center for both GPS and GLONASS so no difference. It would be nice to have this kind of documentation from emlid directly.
I found this site particularly helpful for the transformation equations:
World Geodetic System 1984 (WGS84)
The L1 phase center description was from looking at a lot of survey-grade antennae definitions. Having them at separate phase centers would mean processing them separately… that is something beyond me and RTKLib.
I hope emlid will register their receivers with NGS so that we can use a published pcv file for the units…
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Hi @tobias-dahms. I have not looked very much into the ReachView and RTKpost solutions. Are you referring to the base solution that is broadcast vs one derived from RTKpost?
Hi,
I refere to the solution which is calculated by Reachview and by RTKpost. Both are based on RTKlib, so it should be the same.
If you are talking about the raw stream from the receiver I can hardly imagine why that should differ from the current GPS implementation.
Hi,
I agree, the raw stream doesn’t differ. I don’t know about the ReachView solutions to the base station. I calculate basestation solutions in RTKPost.
cheers
Tatiana
when you say original you mean pre g730?
as listed here:
https://confluence.qps.nl/qinsy/en/world-geodetic-system-1984-wgs84-29855173.html
“First realization established by DoD in 1987 using Doppler observations.
Also known as WGS84 (1987), WGS84 (original), WGS84 (TRANSIT).
For surveying purposes, original WGS84 is identical to NAD83 (1986).
WGS84 is connected to ITRF90 by a 7-parameter Helmert transformation.”
How do you know this for certain?
Now I am confused. Is this a firmware setting?
The original WGS84 (with EPSG code 4326 added in the description here) is used for almost all gps/gnss device.
Quote:
_The original WGS 84 reference frame is still used by most consumer-grade GPS devices (smartphones, vehicle navigation, etc.). It has retained the original major-axis value to eliminate the need for various updates and modifications for these devices and mapping software. This allows existing collections of geospatial data to retain its values and not be subject to transformation or additional computation. _
This might explain it better.
This is also a good time to say, " this is why you dont save your coordinates in e.g Lat/long WGS84).
It will degrade over time.
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Thanks all for clearing this up. In summary, all data logged by the ReachRs+ is in WGS84 (Original).
Now I can move with confidence through the transformations. Anyone want to talk about accuracy next?
Cheers.
yeah I have read that article, it is a bit of an outlier. so if that is true then think about the consequences. where then is the decades more modern realization being used? IF the raw autonomous readings are based on wgs84(original) then they will be off by over a meter in my area.
a simple test with HTDP and INVERSE tells us this…
HTDP (VERSION v3.2.5 ) OUTPUT
TRANSFORMING POSITIONS FROM ITRF2008 or IGS08/IGB08 (EPOCH = 03-12-2017 (2017.1918))
TO WGS_84(transit) (EPOCH = 03-12-2017 (2017.1918))
LATITUDE 38 17 36.86824 N 38 17 36.85867 N 0.00 mm/yr north
LONGITUDE 122 18 40.43174 W 122 18 40.37260 W 0.00 mm/yr east
ELLIP. HT. -15.562 -15.058 m 0.00 mm/yr up
X -2679158.889 -2679157.984 m 0.00 mm/yr
Y -4236171.610 -4236172.867 m 0.00 mm/yr
Z 3931061.368 3931061.449 m 0.00 mm/yr
and then
https://www.ngs.noaa.gov/cgi-bin/Inv_Fwd/inverse2.prl
Output from INVERSE
Ellipsoid : GRS80 / WGS84 (NAD83)
Equatorial axis, a = 6378137.0000
Polar axis, b = 6356752.3141
Inverse flattening, 1/f = 298.25722210088
First Station : ITRF2008
LAT = 38 17 36.86824 North
LON = 122 18 40.43174 West
Second Station : WGS_84(transit)
LAT = 38 17 36.85867 North
LON = 122 18 40.37260 West
Forward azimuth FAZ = 101 36 11.2511 From North
Back azimuth BAZ = 281 36 11.2878 From North
Ellipsoidal distance S = 1.4671 m
also i would like to add about that article and the statement; " used by most consumer-grade GPS devices…"
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is he just referring to GPS only devices or does this refer to multi constellation, GNSS?
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the author does not cite any evidence for this or give any examples
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So then "professional’ devices like Trimble receivers are doing it right and everyone else is doing it wrong? WGS84(original) upgraded to g730 upgraded to (etc etc) on to… g1762 were changes made for a reason…why again would UBLOX use a out of date model of WGS84?
side note…
EPSG 4326 does not refer to any specific realization of WGS84 or epoch, its just a generic container space so to speak. QGIS/Proj lists epsg 104015 as the EPSG code for G1762.
Well, that depends on how you use the datum. For most users of gps it doesnt matter if its wgs84 or the latest realization, because the difference is to small and gps is to inaccurate.
For those who use wgs84 and transform/convert or whatnot, it highly depends on the tool you use to convert between datum.
If Emlids says they use WGS84 (the original one) i guess its true. They could easily have used the latest wgs84 realization but what good would that do? It would still be “wrong” in my part of the world.
You and possible few other would be happy to be a bit closer to the “true” position, but for the rest of us its still wrong and we need to apply local adjustment/datum which i think is the key.
If one is not sure about the WGS84 datum, go back a step and use ECEF.
I would guess most of the recivers with accuracy less then the accuracy of the different version of WGS84
True. But i dont think he have to. Most of the basic undertanding of how GPS or GNSS work is out there (the scary internet thingy).
So then "professional’ receivers are doing it more right and everyone else is doing it less right WGS84(original) upgraded to g730 upgraded to (etc etc) on to… g1762 were changes made for a reason (US army did the changes. Link)…
I could change the question, why would ublox use latest version of wgs84? They had to pick one and at some point they all would get outdated. Hence my first GPS had only WGS72…
Without local correction or propper transformation/convertion, all datums are “wrong” at some point. At least that is how i see it.
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This youtube series is nice for those who run into this topic. Covers much of what happens in a gnss situation.
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Hi,
In my undestanding, the realization must be take in consideration in cases where you need to convert/compatibilize networks/points taken in diferent ephochs or realization of ITRF… mainly in PPP processing of old GNSS data.
If you are processing you data in the relative mode, with a base/rover schem, the coordinates will be related to the realization/definition of the base coordinates used.
Example: Here um Brazil, we have a active network realized under Sirgas2000, in the epoch 2000.4, referenced to ITRF2000( IBGE / RBMC).
All data colected today (under G1762), when processed relativally to this cited network, will be in the sirgas2000, in the epoch 2000.4
But, if I need to process a data in PPP mode, to determine the coordinates Sirgas2000, some transformations are need to compatibilize the systems.
In the other words, in cases of relative processing, the realization does’t matter.
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ok here is my understanding at this point, feel free to find your own.
The WGS84 ellipsoid doesn’t change or rather has not changed since first laid out. what does change is the "realization’ of the WGS84 coordinate system which defines a “reference frame” (my wording here could be better). All that is defined in the receiver module is the WGS84 ellipsoid. the reference frame is delivered inherently in the satellite location portion of the navigation message, the ephemeris. I think this is why in RTK/WAAS/DGPS the correction source defines the product of the rover.
The satellites do not of themselves know their location characteristics, this is provided through them (the navigation message) from the control segment as the ephemeris. For GPS, the ground control segment ascertains the location of the satellites based on the “known” location of the ground control stations within the latest realization of the wgs84 reference frame. The navigation message and the ephemeris are used by the users receiver to determine the location (at a given time) of the the satellite and from there, with ranging, the user’s location.
so my conclusion is that the output of raw autonomous data from a ublox 8t/reach rs+ is currently based on WGS84(g1762)
references (with some paraphrasing)…
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interesting statements on: 1-1, 2-2, 2-3, 2-8, 2-9, 3-2,
http://earth-info.nga.mil/GandG/update/wgs84/NGA.STND.0036_1.0.0_WGS84.pdf -
“User access to the WGS 84 reference system is enabled by the transmission of data to the user on the orbits and atomic clock times of the GPS satellites. These data are transmitted in the “navigation message ” and can be used to compute the satellite positions in WGS 84 by the “ephemeris algorithm ” defined by GPS Interface Specification (Hofmann-Wellenhof et al., 2008). The navigation message is uploaded to the GPS satellites by the GPS control segment. Data in the navigation message is computed by the GPS control segment by least-squares estimation of the GPS orbit trajectories and atomic clock times while holding fixed the WGS 84 reference frame coordinates of official GPS tracking stations.”
http://www.nbmg.unr.edu/staff/pdfs/Blewitt_Encyclopedia_of_Geodesy.html -
“by (ground segment) tracking them, they are able to do several things. For example, since the stations already know their positions on the surface of the Earth, they are able to collect the signals from the satellites and know how much they have been delayed or attenuated by the atmosphere. They’re also able to track the satellites and determine their path, their orbit. From this, their ephemerides can be calculated…Those corrections go back to the upload stations and are uploaded back to the satellites. That’s where the information that is in the Navigation Message comes from.”
https://www.e-education.psu.edu/geog862/node/1735 -
"found in subframes 2 and 3 of the NAV Message. They contain information about the position of the satellite, with respect to time. This is called the satellite’s ephemeris…These parameters along with the argument of the perigee, ω, and the description of the position of the satellite on the orbit, known as the true anomaly, provides all the information the user’s computer needs to calculate earth-centered, earth-fixed, World Geodetic System 1984, GPS Week 1762 (WGS84 [G1762]) coordinates of the satellite at any moment."
https://www.e-education.psu.edu/geog862/node/1737
5. “While WGS84 has always been the basis for the GPS Navigation message computations, the particular version of the datum has changed. As of this writing the latest version of WGS84 is WGS84 (G1762). The number following the letter G is the number of the GPS week during which the coordinates first were used in the National Geospatial Intelligence Agency, NGA precise ephemeris estimations. Therefore, coordinates provided today by GPS receivers are based in WGS84 (G1762) which is the sixth update to the realization of the WGS84 Reference Frame.”
https://www.e-education.psu.edu/geog862/node/1801
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The newer realizations are adjusted occasionally in order to update the tracking station coordinates for plate velocity. These updates are identified by the GPS week, i.e. WGS84(G730, G873 and G1150).
https://d1rkab7tlqy5f1.cloudfront.net/CiTG/Over%20faculteit/Afdelingen/Geoscience%20%26%20Remote%20sensing/Study/CTB3310_TerrestrialReferenceSystems_TRS_2-1a.pdf -
"the WGS84 coordinates from a GPS receiver are in WGS84 (G1762) but with a epoch corresponding to the mid-point of the observation window during the period the GPS data was collected. "
www.ngs.noaa.gov/CORS/Articles/SolerWGS84(G1674)-to-NAD83(2011).pdf
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Perfect, great revision and citation.
The sattelites just transmit what the control station wants… including the definition of the system, small corrections, etc…
Thus, all recorded data by a citzen receiver, will be in the same realization of the gps system in a taken epoch