2 Passive Microwave Remote Sensing of the Ocean
29
observation bandwidths of PMW instruments are typically wider than the protected
bands allocated for PMW remote sensing. Thus, PMW instruments can receive RFI
from legal activity using nearby frequency bands allocated for communication and
other commercial uses. AMSR-E and WindSat are the two PMW instruments most
affected by RFI, while SSM/I and TMI both appear to be relatively unaffected. This
is likely because the lower frequency channels of AMSR-E and WindSat, particularly the 10.7 and 18.7 GHz measurement channels, are sensitive to frequencies
used extensively for media broadcasting. WindSat has more significant RFI than
AMSR-E due to wider observation bandwidth. Observing more bandwidth tends to
yield less noise, but also leads to more interference from frequencies further from
the channel’s center observation frequency. For example, at 18.7 GHz, WindSat
receives interference from DirecTV nationwide broadcast beams. AMSR-E, with
narrower bandwidth at 18.7 GHz, does not appear to be significantly affected by
nationwide broadcast frequencies, but does receive RFI from DirecTV spot beams,
which broadcast at frequencies closer to the center observation frequency of the
18.7 GHz channel.
Power and direction are also important factors affecting RFI. Satellite media
broadcasts appear to direct most signal power very carefully to specific markets.
Powerful signals can result in large RFI errors within certain regions. To serve
smaller but growing geographically dispersed markets, media satellites also broadcast wide, low power beams to cover much larger, less populated regions. These
lower power beams induce more subtle RFI effects that can be difficult to detect and
remove. Assuming the Earth observation point is within the footprint of a geostationary broadcast, the magnitude of RFI is highly dependent on the glint angle, or
how close the observation reflection vector comes to pointing at the RFI source.
RFI induced errors in AMSR-E ocean products were investigated over the entire
7 year mission data set. The effects of the different sources of RFI are listed in
Table 2.4, including which PMW passes are affected and the time period of interference. Because most geostationary broadcast power is directed toward the northern
hemisphere, many broadcast beams only reflect into the descending pass AMSR-E
field-of-view.
From the start of the AMSR-E mission in 2002, HotBird, which is positioned
over 13.0 ◦ East longitude, and Astra, located at 19.2 ◦ East, have steadily increased
RFI in European waters over time. DirecTV-10 at 102.8 ◦ West and DirecTV-11 at
99.2 ◦ West have produced RFI in American waters sinc, and Atlantic Bird 4A at
7.2 ◦ West has been contributing to Mediterranean Sea RFI since 2009. Also from
beginning of mission in 2002, SkyBrazil has directed power toward the southern
hemisphere, therefore reflecting into ascending passes of ASMR-E and producing
RFI off the coasts of southern Brazil and Argentina.
Ground-based RFI sources are also growing stronger and more numerous over
time. Unlike the Geostationary RFI, the ground-based RFI affects both ascending
and descending swaths, though to different extents. This is likely due to differing levels of RFI activity at the AMSR-E local observation times of 1:30 AM or
1:30 PM. Although errors caused by these ground-based sources cover fairly small
regions, the size and intensity of these RFI effects have been increasing over the
29
observation bandwidths of PMW instruments are typically wider than the protected
bands allocated for PMW remote sensing. Thus, PMW instruments can receive RFI
from legal activity using nearby frequency bands allocated for communication and
other commercial uses. AMSR-E and WindSat are the two PMW instruments most
affected by RFI, while SSM/I and TMI both appear to be relatively unaffected. This
is likely because the lower frequency channels of AMSR-E and WindSat, particularly the 10.7 and 18.7 GHz measurement channels, are sensitive to frequencies
used extensively for media broadcasting. WindSat has more significant RFI than
AMSR-E due to wider observation bandwidth. Observing more bandwidth tends to
yield less noise, but also leads to more interference from frequencies further from
the channel’s center observation frequency. For example, at 18.7 GHz, WindSat
receives interference from DirecTV nationwide broadcast beams. AMSR-E, with
narrower bandwidth at 18.7 GHz, does not appear to be significantly affected by
nationwide broadcast frequencies, but does receive RFI from DirecTV spot beams,
which broadcast at frequencies closer to the center observation frequency of the
18.7 GHz channel.
Power and direction are also important factors affecting RFI. Satellite media
broadcasts appear to direct most signal power very carefully to specific markets.
Powerful signals can result in large RFI errors within certain regions. To serve
smaller but growing geographically dispersed markets, media satellites also broadcast wide, low power beams to cover much larger, less populated regions. These
lower power beams induce more subtle RFI effects that can be difficult to detect and
remove. Assuming the Earth observation point is within the footprint of a geostationary broadcast, the magnitude of RFI is highly dependent on the glint angle, or
how close the observation reflection vector comes to pointing at the RFI source.
RFI induced errors in AMSR-E ocean products were investigated over the entire
7 year mission data set. The effects of the different sources of RFI are listed in
Table 2.4, including which PMW passes are affected and the time period of interference. Because most geostationary broadcast power is directed toward the northern
hemisphere, many broadcast beams only reflect into the descending pass AMSR-E
field-of-view.
From the start of the AMSR-E mission in 2002, HotBird, which is positioned
over 13.0 ◦ East longitude, and Astra, located at 19.2 ◦ East, have steadily increased
RFI in European waters over time. DirecTV-10 at 102.8 ◦ West and DirecTV-11 at
99.2 ◦ West have produced RFI in American waters sinc, and Atlantic Bird 4A at
7.2 ◦ West has been contributing to Mediterranean Sea RFI since 2009. Also from
beginning of mission in 2002, SkyBrazil has directed power toward the southern
hemisphere, therefore reflecting into ascending passes of ASMR-E and producing
RFI off the coasts of southern Brazil and Argentina.
Ground-based RFI sources are also growing stronger and more numerous over
time. Unlike the Geostationary RFI, the ground-based RFI affects both ascending
and descending swaths, though to different extents. This is likely due to differing levels of RFI activity at the AMSR-E local observation times of 1:30 AM or
1:30 PM. Although errors caused by these ground-based sources cover fairly small
regions, the size and intensity of these RFI effects have been increasing over the