Aerospace and Electronic Systems Magazine August 2017 - 11

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tional observations (e.g., multi-GNSS and inertial navigation) are
needed to reduce the convergence period of PPP.
Attention has been focused on reducing the convergence
period of PPP [24]. Of the techniques considered, the two most
prevalent are (1) leveraging the use of observations from multiple
GNSS constellations [25], [26] and (2) integrating PPP with inertial navigation systems (INSs) [27]-[29]. While the RTGx supports
multiconstellation GNSS data processing following the first consideration, this article follows the second consideration, detailing
the tight integration of INS within RTGx and assessing its benefits
for airborne kinematic applications.
This work is a significant extension of our work in [30] and
[31]. In Watson et al. [30], tight integration of INS within PPP filters was conducted using simulated data to demonstrate when and
how much the incorporation of INS are beneficial for airborne kinematic PPP applications. In the simulation study, it was determined
that INS is most important for accurate positioning when confronted with high-multipath, poor-troposphere models and low-quality
GNSS orbit and clock products. Then, using recorded flight data
provided by the National Geodetic Survey's (NGS's) Kinematic
Processing Challenge, our work in Gross et al. [31] validated the
implementation of tightly coupled INS models in the Jet Propulsion Laboratory's (JPL's) RTGx software by comparing PPP/INS
and PPP filter solutions to postprocessed reference PPP solutions.
In this article, the benefits of incorporating tightly coupled INS
in PPP filters when considering the accuracy (i.e., latency) of the
orbit and clock products and the troposphere modeling approach
are presented to offer insight to those considering the use of PPP
for their application. First, a review of the details of tight INS integration is presented. Next, two global positioning system (GPS)
and INS data sets recorded during long-baseline (i.e., ≥700 km)
flights over Alaska are used to process PPP filters with and without
the incorporation of tight INS. Then, the sensitivity of tight INS
benefits is shown when using the GPS broadcast ephemeris, the
National Aeronautics and Space Administration (NASA) global
differential GPS (GDGPS) system's real-time GNSS orbit and
clock products, and JPL's final postprocessed orbit and clock products. Finally, an assessment of the INS benefit sensitivity to troposphere modeling approaches is presented. A final contribution of
this study is to share with the community that JPL's RTGx software
incorporates a new tight INS capability, so it may be considered for
use in airborne geodetic applications.
AUGUST 2017

The rest of this article is organized as follows. The next section
provides background information on the processing software used
in this study. Then, the integration architecture and INS formulation are covered. Finally, an analysis of flight data sets processed
using varied ephemeris and troposphere products is presented to
demonstrate the benefits of tight INS/PPP integration.

BACKGROUND
This section provides a short overview of the GNSS software tools
and data products developed at JPL that are used in this study.

GIPSY-OASIS
JPL's GNSS-Inferred Positioning System and Orbit Analysis Simulation Software package (GIPSY-OASIS) has been the primary
geodetic and positioning software for several NASA missions: TOPEX/Poseidon [32], Jason [33], and Gravity Recovery and Climate
Experiment [34] low-Earth-orbiting spacecraft. In addition, it is
operationally used to generate JPL's precise GPS orbit and clock
products [35]. GIPSY-OASIS is licensed for free by the California Institute of Technology to academic institutions for research
purposes.

JPL'S GDGPS
The GDGPS is a GNSS monitoring and augmentation system composed of a large network of GNSS receivers and real-time processing software. The real-time processing software is RTGx, which
produces subdecimeter RTK positioning for a large number of
GNSS tracking sites globally (www.gdgps.net).

RTGX SOFTWARE
JPL's new geodetic and navigation GNSS processing software,
RTGx [36], is a redesign of JPL's GIPSY-OASIS and Real-Time
GIPSY software libraries and can be configured for real-time or
postprocessed GNSS orbit and clock determination, low-Earth orbiter (LEO) precise orbit determination, or PPP for both static and
kinematic applications. RTGx supports multiconstellation GNSS
and inherits unique features from the legacy GIPSY software, such
as a single-receiver integer ambiguity resolution [37]. RTGx is

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http://www.gdgps.net

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2017