Aerospace and Electronic Systems - November 2018 - 47
Maheshwarappa, Bowyer, and Bridges
2. Two signals with Doppler offsets varying at 50 Hz/s and centered at different frequencies. Figure 9A shows the Doppler effect at 1k2 and 2k4, where the rate of change of frequency is
prominent as the bandwidth is smaller when compared to Figure
9B, where the drift is less prominent due to larger bandwidth.
In reality, the functionality from an open source satellite tracking and orbital prediction program, such as "predict," can be integrated to acquire the location and direction information of the
satellites and to therefore obtain the Doppler information. The receiver, on the other hand, was able to track the frequency offset
accurately and decode the signals. Algorithm implemented on the
receiver for multiple signal reception includes:
1. Software scripts, which configure the front-end such as gain,
filters, and bandwidth.
2. Software FFT, which scan the spectrum and find the signal(s).
3. Each signal is fed into a different DDC block depending on the
configuration.
4. Each DDC block has a register, which can be configured using
a Python script.
5. The decimated signal stored at different offset addresses using
DMA.
6. The memory is accessed by different demodulator threads,
which extract the signal around the chosen centre frequency.
The demodulator could be different for different satellite signals, but in this case, the same FUNcube-1 modulation/coding
technique is used, in order to reduce the time in implementing
various standards. However, parameters such as center frequency, symbol rate were changed to test the receiver's adaptability
along with frequency offsets due to Doppler effect. The received
signals were decoded on a single board using two different configurations.
Configuration 1: 2 x DDC > 2 x Rx
Configuration 1 is shown in Figure 10A where the signals from
both the transmitters (1st signal has an offset, while 2nd signal
has 20 kHz offset) are connected to a combiner, then the signal is
transmitted via a coaxial cable to a splitter.
The output from the splitter is connected to two Rx ports of the
same board. These ports are connected to individual DDC blocks
within the firmware, where the signal is decimated for further processing. The drawback here is as the number of transmitters increase, the Rx ports required would have to increase; that requires
additional boards. In order to overcome this issue, configuration 2
was implemented.
Figure 9.
Two signals at different data rates and frequencies (moving at 50 Hz/s). (A) 1k2 and 2k4. (B) 1k2 and 19k2. (Date: 20/09/2015, Time 15:00:00, X-Axis:
10 kHz, Y-Axis: 10 dB).
NOVEMBER 2018
IEEE A&E SYSTEMS MAGAZINE
47
Aerospace and Electronic Systems - November 2018
Table of Contents for the Digital Edition of Aerospace and Electronic Systems - November 2018
Contents
Aerospace and Electronic Systems - November 2018 - Cover1
Aerospace and Electronic Systems - November 2018 - Cover2
Aerospace and Electronic Systems - November 2018 - Contents
Aerospace and Electronic Systems - November 2018 - 2
Aerospace and Electronic Systems - November 2018 - 3
Aerospace and Electronic Systems - November 2018 - 4
Aerospace and Electronic Systems - November 2018 - 5
Aerospace and Electronic Systems - November 2018 - 6
Aerospace and Electronic Systems - November 2018 - 7
Aerospace and Electronic Systems - November 2018 - 8
Aerospace and Electronic Systems - November 2018 - 9
Aerospace and Electronic Systems - November 2018 - 10
Aerospace and Electronic Systems - November 2018 - 11
Aerospace and Electronic Systems - November 2018 - 12
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Aerospace and Electronic Systems - November 2018 - 14
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Aerospace and Electronic Systems - November 2018 - 18
Aerospace and Electronic Systems - November 2018 - 19
Aerospace and Electronic Systems - November 2018 - 20
Aerospace and Electronic Systems - November 2018 - 21
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Aerospace and Electronic Systems - November 2018 - 25
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Aerospace and Electronic Systems - November 2018 - 68
Aerospace and Electronic Systems - November 2018 - Cover3
Aerospace and Electronic Systems - November 2018 - Cover4
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