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ICESat-2 Algorithm Theoretical Document for Precise Orbit Determination, Orbit Design &
Geolocation Parameter Calibration
Release 002
Release Date 28 October 2019
2.0 POD OVERVIEW AND CORE SOFTWARE
The geolocation of the laser altimeter surface return, or bounce point, relies on the precise
knowledge of several components. These include the position of the spacecraft center of mass,
the tracking point offsets of the instrument related to the spacecraft center of mass, the pointing
of the instrument, and the altimeter range observations.
The parameters defining the temporal history of the spacecraft center of mass position and
velocity, or the orbit, can be refined through the minimization of the spacecraft tracking data
residuals utilizing precise tracking data measurement models. This precision orbit determination
(POD) process is capable of estimating both dynamic parameters contributing to the force
modeling of the spacecraft, and measurement modeling parameters which can include a host of
geophysical parameters such as Earth orientation parameters, and tracking system specific
parameters such as antenna phase center offsets. The accuracy of the precision orbit solution
relies on the fidelity of the force and measurement models, and the quality of the tracking data.
The software program to be used in performing the ICESat-2 precise orbit determination is the
GEODYN-II program, which evolved from the original GEODYN program that was developed
at NASA GSFC in the 1960’s. This current version of the software has been operational since
1985, and has been extensively used for satellite orbit determination, altimeter geolocation,
geodetic parameter estimation, tracking instrument calibration, and orbit prediction, and is
capable of handling essentially all types of satellite tracking data.
GEODYN-II has been utilized on every NASA geodetic Earth and planetary altimeter mission
including TOPEX/Poseidon, Jason-1, Jason-2, Mars Global Surveyor, Shuttle Laser Altimeter I
& II, ICESat, GRACE, Lunar Reconnaissance Orbiter, MESSENGER, and GRAIL. Of note is
the use of the GEODYN-II program to achieve and confirm 1-centimeter radial orbit accuracy
for Jason-1 [Luthcke et al., 2003].
Solution strategy can also contribute to the final orbit and geolocation accuracy. For example,
the simultaneous processing of GPS and SLR tracking can provide a more accurate orbit solution
than is possible with either tracking data type alone [Rowlands, 1997]. Similarly, the
simultaneous estimation of altimeter instrument parameters, along with orbit and geophysical
parameters, is a robust solution strategy for improved geolocation accuracy [add reference].
Therefore, the implementation of the estimation process must include the ability to perform
simultaneous solutions for instrument, orbit and geophysical parameters through the reduction of
a combination of instrument range and spacecraft tracking data observations.
The GEODYN-II precision orbit determination and geophysical parameter estimation program is
equipped with a full set of Multi-Beam Laser Altimeter (MBLA) range data processing
capabilities. The direct altimetry and dynamic crossover measurement model algorithms
(Geolocation ATBD, Section ?), along with the classic geolocation algorithms (ATL03g ICEsat-
2 Receive Photon Geolocation ATBD, Section 3.1), are fully implemented into the GEODYN-II
software. GEODYN-II supports the simultaneous estimation of orbit and instrument
measurement modeling parameters through the reduction of a combination of spacecraft tracking