Most SCIGN sites are constructed with a drilled-braced monument, designed at the Scripps Institution
of Oceanography by Frank Wyatt and Duncan Agnew. A SCIGN D3 antenna adaptor is then welded to the top
of the monument, upon which an antenna is mounted (see picture at left). To determine the monument type
for a SCIGN/PBO site, check the site log.
ARP - antenna reference point
This is the point the site's antenna height is measured to.
Most SCIGN sites use an Ashtech Dorne Margolin with chokerings model
(e.g., antenna model type ASH701945B_M; see the IGS Official Antenna and
Receiver Model Code List for model descriptions). The IGS Antenna Dimensions
page shows that the antenna reference point for this model is the bottom of
the pre-amplifier, or BPA. This is the code that appears in the Antenna Reference
Point field in the site's log.
GRP - geodetic reference point
This is the point from which the antenna height is measured. This is also the point
to which site coordinates are published. For SCIGN sites fitted with a SCIGN D3 adaptor
(again, available in the Monument section of the site log), this is always the top of
the center support bolt in this adaptor. See below for a diagram displaying this
and other features of SCIGN monuments.
ERP - eccentric reference point
This is a drilled mark on the north-facing monument leg, and is used for two purposes
to provide a secondary measurement if the adaptor is removed from the monument
to allow surveyors a point to which to easily level to on the monument
It is not possible for surveyors to remove the SCIGN dome which encloses the adaptor
(and geodetic reference point).
Therefore, a final important measurement is from the ERP to the geodetic reference point.
This value is determined by adding the distance from the ERP to the bottom of the antenna adaptor
(typically 0.1016m; listed as the "antenna adaptor divot distance" in the Monument section of
the site log) PLUS the distance from the bottom of the antenna adaptor to the top of the
center support screw (0.02286 m): 0.1016 + 0.02286 = 0.12446 m.
Surveyors leveling to a SCIGN eccentric reference point should keep in mind that this height
is x meters (0.12446 m in the example above) below the published antenna height,
which is measured at the geodetic reference point.
Finally, some software uses the L1 phase center reference point.
Differences in height from this point to the antenna reference point can be found on the
IGS Antenna Dimensions page. For example, the distance from the L1 phase center to the
antenna reference point for an Ashtech Dorne Margolin antenna is 0.110m.
Overall adaptor, antenna, and dome assembly
Main reference height drawing,
for 'fixed height' D3 adaptor
Eccentric reference mark drawing,
for restoring adaptor to its original location should the adaptor be removed