Power-aware improvement in signal detection. Page: 3 of 15
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(TEC) of the ionosphere along the direction of the signal travel,
the given frequency, and the signal time-of-arrival if ionospheric
dispersion did not exist .
Figure 1. Time vs. Amplitude Profile of Chirp
The total electron content (TEC), represents the number of
electrons in a unit-area cross-section of an ionospheric column
along the signal path. This atmospheric property is related to
the propagation of radio signals through the ionosphere which
can distort or bend the signals over the horizon. TEC is also
related to the surface temperature of the Earth, and thus, could
be viewed as an indicator for storm severity.
2.2 FORTE RF Hardware
FORTE receives RF signals either from two orthogonal
monopoles mounted at the satellite's base or by passive
moderate-gain antennas mounted on a 35-foot nadir-directed
boom . There are two types of receivers tunable in 30-300
MHz bands which consist of a mixer, bandpass filter, and a
second mixer stage. The first mixer up-converts the antenna
signal to a higher frequency then passes the signal through the
bandpass filter. The second mixer then converts the band-
limited signal to baseband. Depending on the type of receiver,
either a 12-bit high-speed digitizer or an 8-bit digitizer is used.
The digitizers are in constant operation. An analog trigger box
processes the output from the second-stage mixer and
determines whether or not the digitized data is to be recorded in
payload memory. The recorded data can then be down linked.
Data analysis is carried out as part of the ground operation at
LANL (Los Alamos National Laboratory). Figure 2 shows a
conceptual block diagram of the RF hardware.
Analog Trigger times &
Figure 2. Detection and Data Acquisition Hardware
A signal is declared detected by the analog trigger box. The
analog signal is passed into frequency-separated channels
through a set of bandpass filters in the trigger box. Each of the
eight channels has an analog trigger associated with it. Once the
signal present in the channel causes the threshold to be broken,
detection in the channel is declared. A full signal detect is not
declared until a second threshold number, M (1 5 M 8), of
the channels declared in-channel detect. This double threshold
detection scheme produces better detection characteristics than
that of a single channel alone. Harrington's method was use to
determine the optimal in-channel thresholds for a given signal-
to-noise ratio . Figure 3 shows the receiver operating
characteristic (ROC) curves for the simulated hardware of this
paper. Both curves, the continuous and discrete, of the figure
were derived from closed form analytical solutions.
Receiver Operating Characteristic: 3 dB SNR
*. 98 + double t hr eshol d (theory)---------
---- si ngl e channel ( t heor y)
0. 95 - - - - - - - - -
0.80 ---------- ------ - - - ------- -
0.70 ---------- --- - 2/8- -
0.60 - - - - - - - - - - -- ---- --- ----
0.50 - - - - - - - - - - ----- - - - - -4-- - ---
-- - - - ---
0.430 -318--- - ----
0.20-------------------- -- ----
0.10 , - - - _
.02 - --------- - -- ------
10P t 10
Probability of False Alarm
Figure 3. Detection Performance of Analog Trigger Box
Full-signal detect, yields two sets of data used by the parameter
estimation algorithms. First the time of each channel's detection
is logged. The center frequency of each bandpass filter can be
paired with these time values. This set of data yields a time
history of when different frequencies of the chirp signal arrived
at the satellite. For the chirp signal, the highest frequency
should arrive first then the next highest frequency should arrive
and so on. The function associated with these arrival times
should be nonlinear in nature . The second set of data
yielded by full-signal detect is the digitized time-domain
waveform. These two sets of data can be processed to estimate
the value of TEC and further more the estimate of TEC can be
used to reduce false alarms.
Unfortunately LANL failed to field any capability for on-orbit
processing of the data sets. An instrument known as the FORTE
Event Classifier was development and installed aboard the
FORTE satellite. The mission of the Event Classifier instrument
was to implement on-orbit digital signal processing algorithms
which could be obtained from either an onboard library or
uplink from the ground. However, the Event Classifier was
never fully operational at launch and was turned on for testing
only one time after launch . Thus the Event Classifier
instrument failed to provide any means for testing of new
algorithms or concepts on orbit. Therefore the work of this
paper does not involve space-base implementations.
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Briles, S. D. (Scott D.); Shriver, P. M. (Patrick M.); Gokhale, M. (Maya) & Harikumar, J. (Jayashree). Power-aware improvement in signal detection., article, January 1, 2003; United States. (https://digital.library.unt.edu/ark:/67531/metadc934100/m1/3/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.