ON IMPROVED GRAVITY MODELING SUPPORTING DIRECT GEOREFERENCING 
OF MULTISENSOR SYSTEMS 
Dorota A. Grejner-Brzezinska“”, Yudan Yi®, Charles Toth”, Robert Anderson“, James Davenport“, Damian Kopcha® and 
Richard Salman“ 
"Department of Civil and Environmental Engineering and Geodetic Science, 470 Hitchcock Hall, Columbus, Ohio 43210, USA 
dbrzezinska(@osu.edu 
"Center for Mapping, The Ohio State University, USA 
*National Geospatial-Intelligence Agency, Geospatial Sciences Division, St. Louis, Missouri, USA 
SS 3: Mobile Multi-Sensor Mapping Systems 
KEY WORDS: GPS/INS, navigation, calibration, direct georeferencing. 
ABSTRACT: 
Typically, inertial navigation systems assume the gravity field to be normal (ellipsoidal), meaning that the deflections of the vertical 
(DOV) are ignored in the gravity compensation procedure. This is one of the primary error sources in inertial navigation, especially 
detrimental in the stand-alone mode. Errors due to gravity field and system noise grow rather fast in the vertical channel, which 
normally requires some external aid (such as GPS or barometric altimeter), while the horizontal error growth is much slower and 
bound within the Schuler period. In this paper we present some preliminary results of testing GPS/INS navigation, supported by 
accurate external DOV information. The principal objective of our investigation is to determine to what extent accurate gravity data 
can improve georeferencing of airborne and land platforms, and how this accuracy propagates to a digital imaging sensor error - 
model. The two primary interests addressed in this paper are: (1) the effect of accurate gravity information on the inertial sensor 
error estimation, and (2) the accuracy of stand-alone inertial navigation during a GPS outage with the DOV compensation. The high 
accuracy navigation grade LN 100 INS was tested in stand-alone mode and tightly integrated with dual frequency GPS data. The 
DOV compensation was performed using the unclassified 3D 2'x2' NGA (National Geospatial-Intelligence Agency) DOV grid, and 
tests comparing the navigation and calibration results with and without accurate gravity compensation under varying navigation 
conditions were analyzed. Due to the limited scope of this paper, 
focus on the land-based test results. 
1. INTRODUCTION 
Despite fundamental operational differences, GPS and 
inertial navigation systems (INS) are considered 
complementary positioning systems. GPS is, essentially, a 
geometry-based system, with the advantage of long-term 
position accuracy. Differential GPS, where systematic errors 
can be eliminated, can provide highly accurate cm-level 
position determination. Unlike GPS, an INS system is based 
on the laws of Newtonian physics and the initialization errors 
propagate throughout the trajectory. Although the long-term 
accuracy of a stand-alone INS cannot compare to that of 
GPS, its navigation solution is still necessary during the 
times of GPS signal loss. The GPS/INS systems based on 
high-quality inertial systems and supported by differential 
carrier phase GPS data can reach accuracies of a few 
centimeters per coordinate at the sensor’s altitude (see, for 
example, Abdullah, 1997; Grejner-Brzezinska and Wang, 
1998; El-Sheimy and Schwarz, 1999). 
Typically, navigation algorithms consider the gravity field to 
be normal (ellipsoidal), meaning that the deflections of the 
vertical (DOV), defined as the difference between the actual 
gravity and the gravity model used (see Figure 1), are 
ignored in the gravity compensation procedure. This 
normally results in the inertial navigation error growth with 
time, especially detrimental for stand-alone INS operations. 
DOV is generally on the order of several arcseconds, except 
for in rugged terrain, and the global max/min values of 86/- 
113 arcsec (& north DOV) and 108/-93 arcsec (7, ea.t DOV) 
occur in the Himalayan region. In the US, particularly large 
DOVs occur in the Rocky Mountains and around trench 
regions (e.g., Hawaii). These are also the areas where the 
only a sample of the airborne test results is presented, with a main 
DOVs change most rapidly. To limit the navigation error 
increase, some military systems incorporate active gravity 
field compensation, such as real-time DOV estimation from 
models. The horizontal error growth due to gravity field and 
system noise is much slower than in the vertical channel, and 
is bound within the 84.4-minute Schuler period. A typical 
horizontal error growth reaches 0.5-1.0 nm/hr for navigation- 
grade commercial systems. The vertical channel normally 
needs an external aid, such as GPS, to control its error 
growth. 
1.1 DOV effects on inertial navigation 
In this paper, the impact of the DOV compensation on sensor 
errors, position and attitude solutions is analyzed. A 3D 
2'by2' grid of NGA (National Geospatial-Intelligence 
Agency) DOV data was used in this study together with the 
WGSS84 gravity model; DOVs were provided at eight 
altitudes: 0K, 10K, 15K, 20K, 30K, 50K, 70K, and 90K ft, 
and were interpolated for the sensor’s altitude. The two main 
questions we attempt to answer are: (1) To what extent can 
accurate gravity information improve the accuracy of stand- 
alone inertial navigation during a GPS outage? (2) Can 
gravity compensation, combined with the INS static 
calibration technique (ZUPT, zero update point) provide 
better navigation accuracy during a GPS outage? We 
analyzed land-based and airborne test data representing 
different mission environments and dynamics. The results 
clearly indicate a positive effect of DOV compensation 
primarily on pitch and roll, but also on the horizontal 
coordinates. The details are discussed in section 2. 
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