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Titelmaster
On the Nomenclature of Reference Points
and Local Ties
Axel Nothnagel
Institute of Geodesy and Geoinformation
University of Bonn
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Definition of Reference Points
A reference point is a point which is used to describe a location in
space and time, representative for a certain purpose.
Similar to the system and frame issue:
 a physical point with coordinates
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Ideally reference points have a physical meaning
If more than one or no physical point exists, it has to be defined by
convention.
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GNSS Reference Point
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The GNSS antenna reference point (ARP) is defined as the
center of the bottom of pre-amplifier
An alternative would be the bottom center of the ground plane
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ARP is a truely conventional reference point
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GNSS Phase Center
Phase center variations are modelled as a fixed vector plus azimuth and
elevation dependent variations
PCV =
dϕ * λ /360°
z
ideal phase front
r0
ax

dr (α , ε ) = a ⋅ r0 + λ ⋅ dϕ (α , ε )
ε
E
a
ARP
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real phase front
az
x
Görres 2010
− The choice of E is not unique, dφ(α,ε) depends on the choice of E
− PCVs can only be applied to the phase observations, never to the
coordinates
 GNSS coordinates are always referred to the ARP
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DORIS Reference Point
The DORIS (STAREC) antenna reference point is defined as the
center of the red painted ring (= 400 MHz phase center)
An alternative would be the 2 GHz phase center
(Offset by –Δh)
Center of „red ring“ is a
truely conventional reference point
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SLR Reference Point
The SLR antenna reference point is physically defined as
the intersection of the two axes (azimuth and elevation)
This is a physical/mathematical reference point
No alternative from physics or mathematics
Any technique-specific corrections are applied
to the observations (various dependencies)
− No need for a convention
− No need to call it „conventional reference point“
 The SLR reference point
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VLBI Reference Point
The VLBI antenna reference point is physically defined as
− the intersection of the two axes
− projection point of the secondary axis on the primary axis
This is a physical/mathematical reference point
No alternative from physics or mathematics
Any technique-specific corrections have to be
applied to the observations (elevationdependent)
− No need for a convention
− No need to call it „conventional reference
point“
 The VLBI reference point
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Conclusions (1/2)
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Use the expression „reference point“ in a correct manner
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DORIS and GNSS have Conventional Reference Points
− There is nothing like an electrical reference point
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SLR and VLBI have Reference Points
− No need for further adjectives
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Local tie measurements/local surveys or local ties in general can
only link these reference points
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Conclusions (2/2)
If global coordinate differences between techniques do not fit
separate (at least conceptually) between
− Local tie/survey discrepancies
− Technique-specific errors
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Avoid saying „the local ties don‘t fit“
Proposal for a Paris 2013 Declaration of all participants on the correct
usage of terms
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