CORS PRODUCTS

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CORS PRODUCTS SERVICES Coordinates and
Velocities

• Richard Snay Mike Cline
• NGS Convocation
• Silver Spring, MD
• October 27, 2005

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INITIAL CORS COORDINATES

• NGS computes adopted positional coordinates for a
  new CORS using at least 10 days of GPS data.
• NGS predicts the adopted velocity for a new CORS
  with HTDP. (The predicted vertical velocity is
  zero.) This velocity is revised (if necessary)
  only after at least 2 years of GPS data have been
  collected at the site.

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REVISING CORS POSITIONS VELOCITIES

• NGS estimates positional coordinates for each
  CORS every day via a rigorous network adjustment.
• NGS will revise the adopted positional
  coordinates and velocity of a CORS, if daily
  coordinates differ systematically from the
  adopted coordinates for several consecutive days.
  Such differences are displayed on the CORS web
  site in the 60-day plots.
• ITRF tolerance 1 cm horizontal and 2 cm
  vertical.
• NAD 83 tolerance 2 cm horizontal and 4 cm
  vertical.

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CORS MULTIYEAR SOLUTION

• Each year or so, NGS computes a multiyear
  solution that incorporates as much GPS data as
  practical, plus the latest software and
  procedures, to estimate positional coordinates
  and velocities for all CORS.
• The latest multiyear solution uses data spanning
  the
• 1994 2003 time period.
• NGS will begin computing a new solution within
  the next few months. (We are waiting on IGS to
  adopt new antenna calibration results.) Once
  started, this solution will require about 6
  months to complete.
• The results of this solution will be used to
  evaluate currently adopted CORS positional
  coordinates and velocities. (Results may impact
  the readjustment of the traditional NSRS network.)

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ITRF2004 COMING

• The International Earth Rotation and Reference
  System Service will soon release ITRF2004,
  probably in early 2006.
• NGS will need to compute ITRF2004 positional
  coordinates and velocities for all CORS.
• NGS, in collaboration with NRCan, will need to
  adopt a transformation from ITRF2004 to
  NAD_83(CORS96).

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Seasonal variations in height
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North American Vertical Velocities
Vertical Velocities from CORS
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Tide gauges located within 40 km of a CORS having
an accurate ITRF00 velocity
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• Absolute sea level rise as a function of latitude

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HOW SHOULD NAD_83(NSRS) BE DEFINED ?

• NGS is currently readjusting the coordinates for
  the traditional GPS stations that comprise the
  NSRS. The positional coordinates will be
  published in a reference frame to be called
  NAD_83(NSRS).
• Should NAD_83(NSRS) NAD_83(CORS96)? OR
• Should the scale of NAD_83(NSRS) equal the scale
  of ITRF2004?
• Should the rotation rates in the transformation
  from ITRF2004 to NAD_83(NSRS) represent a more
  accurate plate tectonic model than NUVEL-1A?
• Should the translation rates in the
  transformation between NAD_83(NSRS) and ITRF2004
  be set equal to zero?