Application of a North America reference frame to the Pacific

Northwest Geodetic Array(PANGA)

M M Miller, V M Santillan, Geodesy Laboratory, Central Washington UniversityD J Johnson, Earth and Space Science, University of Washington

PBO stated requirements:• PBO needs

– “…that plate boundary deformation be adequately characterized over the maximum ranges of spatial and temporal scales common to active continental tectonic processes.”

[ES Facility Proposal]

• How broad is the plate boundary?• Is there a “stable plate interior”?

– to within potential GPS accuracy ~ 0.1 mm/yr– would require accurate modeling of non-tectonic

deformation– If so, where is this stable plate interior?

• PBO will address these questions by– Network design including broad GPS spatial

coverage across North America– Research (SNARF WG)

Frame versus System

• Reference System– Set of conventions

• Reference Frame– Set of coordinates of physical points (stations, quasars…)

consistent with conventions

Reference Frame

• A specific realization of a reference system– consistent with its conventions– based on physical observations

And, in our case

• Selected set of GPS stations– Specified parameters of the station motion model

• Position coordinates at some conventional epoch• Velocity coordinates• Instantaneous coordinate offsets (e.g., co-seismic,…)• or more generally – set of coordinates at many epochs

• Note that the frame depends on– Definition of the reference system, particularly the models– Adopted set of stations– Adopted set of observations leading to parameter estimates

International GPS Service Network

Reference system issues:

• Current reference systems:– Comprise a set of station

positions and linear velocities

– Rely on global minimization of all included velocities

– Stations eliminated as they are compromised

• Compromises abound:– What stations are available

during a given time interval– Trade offs between quality and

quantity of time series– Non-linearity of time series

• Time correlated noise• Periodic events at well understood

frequency (annual and semi-annual effects)

• Periodic events discovered from time series

• Episodic events (earthquake offsets, etc.)

• Changes in rate

Data analysis approach:

• Working assumptions:– ITRF2000 is the point of

entry

– Stabilize to a regionally meaningful reference frame

– To get at North America, GPS definition internal to solution is needed

– Use topography as proxy for deformation (cf Sella’s stable plate)

• Pragmatic considerations:– What stations are available

during a given time interval– Trade offs between quality and

quantity of time series– Non-linearity of time series

• Correlated noise• Episodic events• Other events

– Evolution of analysis strategy– Imperfect error models– Ignore post-glacial effects( and

other non-rigid plate effects)– Until recently, the length/quality

of time series and our ability to manipulate the time series has been the first order control on uncertainty

More on analysis:

• Implementation:– Analysis of sub-networks with

ambiguity resolved, JPL fiducial free products and daily frame files

– Combination by common subset of stations

– Daily solutions stabilized to a set of 42 stations (regional and NA)

– Annual and semi-annual effects removed on case-by-case basis

– Velocity solution derived from this for T>2.3

– Rotation to North America zero motion explicitly solved for each solution

• Results:– A stable, internally consistent

frame approaching tenths of mm resolution

– Useful to regional tectonics– Lots of compromises

Defining North America• Non-equivalence with set of stations included in reference frame

– Stations included vs. stations whose velocity should be minimized.

• North America plate– Presumed rigid

• Reality– Deforming plate margins– Glacial isostatic adjustment– Seasonal jittering within itrf

• Geometric compromises– Glacial models are controversial and differences matter– No land/plate west of the PBO network– Plate affinity of Siberian sites is not certain

• Rays of light– We have a relatively good set of stations with long history to use as a starting point– We can design choices to help constrain glacial models– Knowledgeable investigators can use reference frame for different results– Proposed approach will create powerful tools and a common standard for regional stabilization

Reference system improvements:

• Pragmatically:– Data analysts are well served

by this approach if sufficiently long time series are used and velocities are the goal

– Most investigators have broader goals, especially structure of time series

• Yields:– Position and velocity– Known non-linearity of time series

are either ignored or solved for independent of reference frame

• Time correlated noise• Periodic events at well understood

frequency (annual and semi-annual effects)• Periodic events discovered from time series• Episodic events (earthquake offsets, etc.)• Changes in rate

• PBO/modern requirements:– PBO is aimed to reveal non linear earth

deformation - in this case North America-centric

– Scientific community aims to coordinate with surveying standards (such as NAREF)

– Investigators desire a physically meaningful reference frame

– The absolute vertical is still problematical

• Reality of what’s needed:– Position and linear velocity– Amplitude and phase of annual and

semi-annual components– Conventions for handling other periodic

events, epoch updates, and stations with non-linear velocities

– Agreement on the physically meaningful reference frame

Towards SNARF:

• Frame depends on– Definition of the reference system,

particularly the models– Adopted set of stations– Adopted set of observations leading

to parameter estimates

• Frame definition– Selected set of GPS stations– Specified parameters of the station

motion model• position coordinates at some conventional

epoch• velocity coordinates• Instantaneous coordinate offsets (e.g., co-

seismic,…)• Explicit inclusion of annual and semi-annual

(in the out years)

– or more generally – set of coordinates at many epochs

• Optimized to meet PBO science requirements

– Create a regional stabilization• Plate scale• Regional nets

– Solve for periodic effects• Annual and semi annual• Known periodic and episodic effects

– Reveal the structure of time series

• Transformation to NAREF– Surveying standard in

development– Develop standard tools for

transformation