IEEE Geoscience and Remote Sensing Magazine - March 2016 - 41

Forest
Monitoring,
Ice Sounding ...
0.435 GHz

Wave Structure,
Geology, Soil
Moisture
...
1.2, 3.2 GHz

HiRes SAR (Dual-Use),
Topography, Land Use, Ice, Wind, Ocean
Surface Motion,
Altimetry,
Altimetry (Land and
Snow/Ice,
Rain Radars,
Ice at High Resolution)
...
...
78 GHz
8.5, 9.3-10 GHz
36 GHz

30 cm
1 GHz

Geology, Oceans,
Sea Ice, Land Use,
Topography, Waves,
Swells, Solid Earth, ...
5 GHz
P Band

L Band

Ocean,
Wind, Ice, ...
13.5 GHz

S Band

C Band

3 cm

3 mm

0.3 mm

10 GHz

100 GHz

1,000 GHz

Vegetation, Snow,
Rain, Wind
17 GHz

X Band

Ku/K/Ka Band

Precipitation
Radars, ...
24 GHz

Millimeter

Cloud Profiling
94, 134, 238 GHz

Submillimeter

FIGURE 1. The choice of frequencies for satellite active sensing is dictated by the physics of the relevant scattering mechanism. Representa-

tive, but certainly not exhaustive, examples of the types of measurements used at each frequency are shown. [Figure used with permission
from "A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum," courtesy of the European Space Agency (ESA).]

are best suited for applications that require good penetration through ice or vegetation. In contrast, high frequencies (i.e., short wavelengths) are needed for the detection
of small microscopic cloud particles (Figure 1).
FREQUENCY ALLOCATIONS
The radio spectrum is used by many types of services, from
radio and television broadcasting to wireless phone communication; weather, military, and remote sensing radars;
and radio and radar astronomy, among many others. Radio regulations and frequency allocations are developed
at both national and international levels. At the international level, regulations are formulated by the Radiocommunication Sector of the International Telecommunications Union (ITU-R). Spectrum allocations for specific
uses are established at the World Radiocommunication
Conference, which is held every three to four years. Spacebased radar remote sensing operates under the Earth Exploration-Satellite Service (EESS/active), and the associated spectrum allocations are shown in Table 1. Within the
United States, spectrum oversight of governmental users
[such as NASA, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Defense
(DoD)] is the responsibility of the National Telecommunications and Information Administration (NTIA), whereas
oversight of private sector users is provided by the U.S.
Federal Communications Commission (FCC). It is important to note that active sensors typically share allocations
with other services, such as communication systems and
march 2016

ieee Geoscience and remote sensing magazine

radiolocation radars. For active systems, it is also important to differentiate between a spectrum allocation, which
is basically the divvying up of the spectrum for different
uses, and a spectrum assignment, which is the actual
permission to radiate at a specific transmit power in a given band over a particular region
of the earth. For active sensors,
having a spectrum allocation
RFI REFERS TO THE
may not entitle a sensor to raUNINTENDED RECEPTION
diate if that sensor is thought
OF A SIGNAL TRANSMITto create harmful interference
TED BY AN UNRELATED
to other primary users of that
SOURCE.
spectral band.
The following two constraints
that active sensors often encounter are associated with the spectrum allocation and assignment process that governs active sensors:
1) The shared nature of the allocations can produce RFI
that can degrade the performance of science sensors.
2) Active science sensors may be denied an assignment or,
otherwise, restricted in their ability to transmit as desired.
RADIO FREQUENCY INTERFERENCE
RFI refers to the unintended reception of a signal transmitted by an unrelated source. When an active sensor receives
such a signal, the intended science measurement may be
corrupted. An active sensor may also act as the source of interference to a communication system, a passive sensing system, or another radar system, which is discussed in the next
41

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2016