IEEE Geoscience and Remote Sensing Magazine - December 2014 - 54

Technical committees
James L. Garrison, Purdue University
Estel Cardellach, Institut de Ciencies de l'Espai

The IEEE GNSS and Signals of
Opportunity Working Group

INTRODUCTION
he Instrumentation and Future Technologies (IFT)
Technical Committee (TC) of the IEEE Geoscience
and Remote Sensing Society (GRSS), has the following
vision: "To foster international cooperation in advancing the state-of-the-art in geoscience remote sensing instrumentation and technologies that improve
knowledge for the betterment of society and the global
environment." Supporting this vision, the IFT-TC has
three primary missions: 1) Assess the state-of-the-art in
remote sensing instruments and technology; 2) Identify new instrument concepts and technology trends;
and 3) Recognize enabling technologies for future
instruments.
Presently, the IFT-TC is composed of six working
groups; 1) Active Microwave; 2) Microwave Radiometers;
3) Lidars; 4) Optical Instruments; 5) Remote Sensing
Instruments and Technologies for Small Satellites; and
6) Global Navigation Satellite Systems (GNSS). In the
last 20 years, several different measurement concepts
have developed for using GNSS signals in Earth remote
sensing. Among them; reconstructing ionospheric and
tropospheric structures from GNSS signal delays; vertical sounding of the atmosphere from bending of the
radio-links in radio-occultation (RO) geometry; and
analyzing GNSS signals after their reflection from the
Earth surface, a relatively new field known as GNSS
reflectometry or GNSS-R. GNSS-R continues to generate
interest with increasing numbers of publications appearing in IEEE-GRSS journals and conferences. Research
disclosed in those publications addresses a wide range
of Earth science research problems and applications,
as shown in FURTHER READING. Additionally, several spaceborne GNSS-R missions have been launched
or selected for funding in recent years, demonstrating

Digital Object Identifier 10.1109/MGRS.2014.2367318
Date of publication: 26 January 2015

the technical maturity of reflectometry for certain Earth
observations. Finally, techniques originally developed
for GNSS signals in L-band are now being applied to
digital transmissions from communication satellites in
other bands. These new satellite sources are referred to
as "signals of opportunity," or SoOps. In light of these
recent changes to the GNSS remote sensing landscape,
we have proposed to change the name of the working
group to "GNSS and Signals of Opportunity Working
Group" to broaden its scope so as to include both GNSSspecific measurements (radio-occultation, atmospheric
and ionospheric tomography, and reflectometry) and
the application of similar approaches to other SoOps.
This article will first give a brief history of GNSS
remote sensing. We will then present a few representative examples of current work in GNSS reflectometry
that have reached the highest technical maturity, as
evidenced by recent and upcoming satellite launches.
We will then summarize some recent research in SoOp
reflectometry. Finally, we will announce two upcoming
conference events and suggest some additional sources
of information for the interested reader.
HISTORY OF GNSS REMOTE SENSING
Global Navigation Satellite Systems (GNSS) refers,
generically, to a navigation system using one-way ranging signals broadcast from a constellation of satellites.
These signals are designed to allow a measurement of
travel time (and hence apparent distance) to a receiver.
Observation of these signals from four or more satellites
allows a user to solve for their position and synchronize
their clock to the time standard used by the constellation. NAVSTAR, now commonly known as the Global
Positioning System (GPS), was the original GNSS, with
the first GPS satellite launched by the U.S. Air Force in
1978. This was followed by the first GLONASS launch by
the former Soviet Union in 1982. Galileo is in the process of being deployed by the European Union, with the
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Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - December 2014