Does quasi-decadal periodicity affect GPS observations?

GPS coordinates on ground points are a moving target, constantly changing.

About the only thing I understood in that article was the authors names.
And that took two try's.

It's called gravity; which is measured according to mass....

Very few understand the mass of water; it's heavy. Ever try to pick up a 10 gallon aquarium? Now ask yourself how many 10 gallon aquariums are in the polar ice caps? How many 10 gallon aquariums are in the 7 oceans of the world? What is the effect of ice changing to water and moving to the 7 seas?

The satellites are tiny little specks driven mostly by the earths gravity; they are in a constant relationship with earth; where ever it wobbles. therefore we are able to maintain a repeatable position on the earths surface for a relatively short period of time.

At least that's my guess.....

MightyMoe, post: 366865, member: 700 wrote: GPS coordinates on ground points are a moving target, constantly changing.

Is that what Peter Ehlert was talking about when he suggested that it was important to check the Epochs that NGS stations were adjusted? How often does that happen?

rfc, post: 366878, member: 8882 wrote: Is that what Peter Ehlert was talking about when he suggested that it was important to check the Epochs that NGS stations were adjusted? How often does that happen?

Yes, they update data sheets every so often and list older values, less than every ten years

Next big adjustment is 2020, its mainly a vertical shift, so we are told, but no doubt xy will slide some more.

rfc, post: 366860, member: 8882 wrote: I know it's not Friday, the day I usually ponder the larger ponderables before heading out into the field, but came across this ominous sounding advisory in Science Magazine, and couldn't resist sharing:

http://advances.sciencemag.org/content/2/4/e1501693

Doesn't this impact the very ECEF reference system that GNSS uses? It sure seems that if mass is being re-distributed around the earth, it would. If so, how do they keep track of such decadal oscillations?

With respect to your last paragraph: yes, yes and multiple geodetic techniques including but not only GNSS. Much of the work related to this issue is coordinated by the IGS.

You may want to look at GRACE (Gravity Recovery and Climate Experiment) a space-based sensor studying the earth's gravity field and temporal changes due to water transport (including that in the form of clouds). They have a lot of good presentations and animations. GRACE has been superseded by GOCE. One of the two research institutes responsible for GRACE is the CSR (Center for Space Research) at UT-Austin.

The impact of these processes on products like GNSS orbits is reflected in the determination of ERP (Earth Rotation Parameters). Info on this product can be found at the following links. The UNB document page 61 has a nice explanation.

http://www.acc.igs.org/trf/sinex-combi_ferland_jog09.pdf

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4367342/#!po=4.83871

http://www2.unb.ca/gge/Pubs/TR171.pdf

https://igscb.jpl.nasa.gov/components/latest_igu.html

HTH,

DMM

Actually the nice explanation to which I referred is section 1.2 on page 5 (printed page). The maths starting on 61 are nice in a different way...

GeeOddMike, post: 366948, member: 677 wrote: Actually the nice explanation to which I referred is section 1.2 on page 5 (printed page). The maths starting on 61 are nice in a different way...

You know, I checked it out and at first I thought...."He's joking! He's just funnin' with the Grasshopper."...Then I thought, "No, GeeOddMike wouldn't do that; he's a nice guy"...finally I just sucked it up and said to myself "Wow, there are some smart folks here, and he must be one of the smartest!" I got a little beyond Scott Zelenak's understanding but I gotta admit it: The partial derivative of the station vector in the space-fixed frame? Uhhh...if I ever get a chance to play with GPS, I think I might understand why some become "button pushers".

Thanks for the follow up post! You definitely had me worried.:excruciating::excruciating:

Sorry about that.

BTW, a more accessible article on the issue of Earths tipping 's here:

http://www.scientificamerican.com/article/earth-is-tipping-because-of-climate-change1/

It was linked from the GRACE site: http://www.csr.utexas.edu/grace/

It affects the transformation from terrestrial frame (ECEF) to celestial frame (ECI), which involves the Earth orientation parameters (EOP). So in principle it makes the GPS satellite orbits a bit less predictable and the their ephemerides a little less accurate. Now, GPS satellites are monitored pretty frequently, we don't have to extrapolate too far ahead in time, and these error sources wouldn't grow significantly.
My two cents...
-FGN.

Dear rfc,

>>>Does quasi-decadal periodicity affect GPS observations?
>>>Doesn't this impact the very ECEF reference system that GNSS uses?
>>>If so, how do they keep track of such decadal oscillations?

Let me try to break down the jargon for you, and directly
address your questions. I‰Ûªll explain some things you know to
help our other readers.

The key item is that satellite motions are best modeled in an
inertial frame. Informally, an inertial frame is a coordinate
system where the only "forces" one feels are directly attributable
to a physical cause (such as a gravitational or magnetic field, for
example). An inertial frame may have uniform (unaccelerated)
motion. An inertial frame‰Ûªs orientation is fixed relative to the
stars. This is the kind of frame used for orbit computations.

If you don‰Ûªt use an inertial frame for orbits, then you need to
account for "fictitious forces". These are things like centrifugal
force, Coriolis force, etc... One can argue philosophy on if
these forces are "real" or not. Bottom line, one can use any
coordinate system, inertial or not, as long as one accounts for the
effects appropriate to the selected coordinate system.

For day-to-day stuff on Planet Earth, we like an ECEF (Earth
Centered, Earth Fixed) coordinate system. This is not an inertial
frame. This is because the Earth‰Ûªs motion is not uniform
relative to an inertial frame. The principal motion is Earth axial
rotation, which causes day and night. Next Earth has a set of
orientation motions that are gyroscopic: precession, nutation,
wobble. Some of these motions are well modeled, and others
are not. These are the components of an ECEF-ECI
transformation (ECI = Earth Centered, Inertial).

Orbit computation codes will computationally migrate
observations to an inertial frame at a chosen epoch of time prior
to solution. Earth axial rotation is a major element here, but all
the elements are used. The IGS (International GNSS Service)
takes an extra step, and transforms orbital results back from ECI
(at an epoch) into ECEF (at specified time steps).

Now, let‰Ûªs parse that ECEF-ECI transformation.

The precession and nutation parameters are well-modeled and
are documented as part of the J2000 ECI system. These are
power series with an impressive number of coefficients. The
series are updated every 50 years. The three wobble terms are
x1, x2, and delta UT1. The first two are polar motion, aka
Chandler wobble. The last is variation in Earth rotation, and
leads to the introduction of leap seconds in our civil time
system. These three terms are not well-modeled.

Since the Earth‰Ûªs wobble and rotation variation are not well-
modeled, it is necessary to observe and monitor these departures
from uniform motion. This has been done for over a century
now - - optical astronomy in the past, new systems today
(GNSS, Doris, SLR, VLBI). The IERS (The International Earth
Rotation and Reference Systems Service) is the umbrella
organization that oversees this.

The understanding of Earth wobble has been getting better,
particularly with space-based observation systems. This allows
one to pull out various components, such as annual wobble,
Chandler wobble, Markowitz wobble, and quasi-decadal
wobble. However, the source mechanism of the last two is not
clear, since the observations vs. the model do not "add up". The
Science paper demonstrates continent-ocean water transport is,
in fact, the source mechanism when one accounts for climate
change effects. This is an important step along the way to
understanding how the Earth wobbles.

Now, let‰Ûªs review your questions.

>>>Does quasi-decadal periodicity affect GPS observations?

Depends on the word "affect". We "cheat" if we use an ECEF
frame as inertial in a GNSS solution. However, we apply a
correction (the Sagnac Effect) that accounts for the relative
motion of the Earth to the GNSS satellite. Signal transit time is
approximately 75 milliseconds, and relative motion is about 4
km/sec. This is documented in the DoD IS-GPS-200 manual.
This is a correction due to choice of the ECEF coordinate frame.
The other components: precession, nutation, wobble (including
quasi-decadal), and delta UT1 are negligible over the signal
transit time. However, when it comes to transforming a
coordinate between frames, then none of those elements are
negligible.

If, in the broad sense of "affect", we include the influence on a
GNSS position in space, then yes, water transport affects
gravitational force that will, minutely, influence a GNSS.

>>>Doesn't this impact the very ECEF reference system that GNSS uses?

Yes. As described above, quasi-decadal periodicity is a
component of total wobble, which is a component of the
complete transformation between inertial (ECI) and ECEF
frames.

>>>If so, how do they keep track of such decadal oscillations?

As mentioned above, a global network of space-type observation
systems (GNSS, Doris, SLR, VLBI), that monitor the Earth over
decades of time. Codes are written that apply the standard
models, as described in the IERS Conventions, and extract
estimates of x1, x2, and UT1. These results are periodically
published. For inspection of the quasi-decadal oscillations, one
then estimates and extracts the annual, Chandler, and Markowitz
components, leaving quasi-decadal and drift.

Very best wishes.

Just.....wow.
Thank you Sir.