All US SmartNet stations are positioned to ensure an unprecedented level of internal precision and consistency between themselves, while at the same time providing an accurate tie to the new NAD83(NA2011) National Spatial Reference System. By implementing the draft NGS RTN Guidelines we positioned all of our stations to provide the requisite 2 cm horizontal and 4 cm vertical accuracy to the NSRS, while at the same time achieving the 1 cm level internal consistency needed to provide the high-precision and high-quality network corrections our users have come to expect.
This internal precision and consistency and external tie is really quite a challenge. And in many ways it fundamentally changes everyone's perception of "what is right". From the NGS RTN Guidelines:
For each reference station contained in the RTN, adopt values for its 3-dimensional positional coordinates (at a selected reference date) and a velocity that are consistent, with corresponding values adopted by NGS for reference stations in the CORS network, to within 2 cm in each horizontal dimension (north-south and east-west) and 4 cm in ellipsoid height.
D. Implementing Recommendation 2: Adopt RTN station coordinates and velocities that are consistent with CORS coordinates and velocities
For each RTN reference station contained in the CORS network, NGS encourages the RTN administrator to adopt values for the station’s 3-dimensional positional coordinates (at an administrator-selected reference date) and velocity which will agree with the corresponding NGS-adopted values for this station’s positional coordinates (at the NGS-selected reference date) and velocity in the following sense:
The reference station’s coordinates for any given day of RTN operation, as computed from administrator-adopted values, should differ by no more than 2 cm in each horizontal dimension (north-south and east-west) and/or by no more than 4 cm in ellipsoid height from corresponding coordinates computed from NGS adopted values.
As mentioned previously, a procedure for computing positional coordinates at one reference date using positional coordinates for a different reference date is presented in Appendix IV.A.
It would be convenient if the administrator-adopted values were identical to the NGS-adopted values, but the administrator may have more available resources than NGS to monitor the positional coordinates of his/her RTN reference stations. Hence, the administrator may be the first to detect when positional coordinates and/or velocities need to be revised. The administrator should then advise NGS at firstname.lastname@example.org as to the discrepancy, for it may be the case that the administrator-adopted values are more accurate than the NGS-adopted values, whereupon NGS would consider revising its
The RTN administrator may want to adopt values that differ (within the above tolerances) from the NGS-adopted values because RTN technology requires a higher level of internal consistency among positional coordinates than what is required for standard CORS applications. This internal consistency enhances the ability to determine accurate corrections to the GNSS data for the systematic errors associated with orbits, clocks, atmospheric refraction, and other phenomena. These accurate corrections will better enable rapid and reliable resolution of the integer ambiguities, as is needed for cm level positioning. In many cases, due to the different scale of their respective missions, the RTN administrator will have more available resources than NGS to determine
internally consistent positional coordinates for his/her RTN reference stations.
The RTN administrator may use any procedure that he/she deems appropriate to determine positional coordinates (at a selected reference date) and velocities for all reference stations contained in his/her RTN. For RTN reference stations that are not contained in the CORS network, the administrator-adopted values should be consistent with those yielded by OPUS in the following sense:
If--for any period of time spanning 60 consecutive days--a person submits daily (24-hour) GPS data files from a RTN reference station to OPUS, then the average coordinates from these 60 OPUS solutions should differ by no more than 2 cm in each horizontal dimension nor by no more than 4 cm in ellipsoid height from the average positional coordinates for these 60 days, as computed using administrator-adopted values.
Appendices IV.B and IV.C provide some suggestions as to how a RTN administrator may determine positional coordinates (at a selected reference date) and velocities for his/her RTN reference stations.
Full document can be found here - NGS RTN Guidelines - Draft
To achieve these internal and external requirements, we performed a rigorous adjustment of all stations in SmartNet using Bernese software to produce IGS08 coordinates. These IGS08 coordinates were then transformed to NAD83(NA2011) and NAD83(CSRS v6) coordinates for the United States and Canada respectively, using transformation parameters provided by the NGS in the US and the GSD in Canada. The epoch dates for the transformations used differ depending on the location of the stations themselves.
In areas having low horizontal or vertical velocities, the common NSRS epoch 2010.00 was used. In areas having high horizontal or vertical velocities, an alternate epoch was applied to assure best fit over time. To understand what we are doing where, please watch our SmartNet North American Adjustment Webinar
By adopting a standard reference frame and only having to shift epochs, we are able to meet the internal and external demands on the systems. To find out more about the new NAD83(NA2011) NSRS, please visit http://www.ngs.noaa.gov/web/surveys/NA2011/. Also as part of this shift to a new reference frame, customers will need to make sure to use the new Geoid 2012 Revision A, to assure consistent and accurate elevations are obtained. For more information about the new Geoid and to find the the new Geoid 2012A bin files please visit the link below:
* All SmartNet Published Coordinates are to the ARP of the antenna.