Here's the result of some receive tests to determine the minimum signal detection capablity of the FFTDSP program relative to it's potential use to detect the Mars Global Surveyor. Information on what Mars Global Surveyor is all about is available at: Mars Global Surveyor Home Page
Signal Source: -101 dBm in a 290 deg K load
(Sealed with only a single N-connector. Measured on HP equipment)
Attenuator: 0 - 82 dBm (professional unit. Measured for leakage)
Receiver: ICOM 781 with Microwave Module transverter
Preamp: 0.5 dB NF (35 deg K)
Detection Program: FFTDSP41h running on Pentium 133 Mhz Computer (Audio from receiver connected directly to sound card)
All equipment was warmed up for 24 hours. The signal source and attenuator were placed in a thermally insulated box.
FFTDSP program was set at the recommended 16x Integration mode with a 1300 Hz search window for the -171 and -173 dBm level tests. A very long integration at 100x was also tested with a -179 dBm level. These signal levels are calculated for a 100 deg K system. The actual tests were run in a 325 deg system at 5 dB higher than the caculated level.
Receiver was set for WB CW mode
The FFTDSP images showed excellent detection at the -171 and -173 dBm. This level corresponds to a just two 10' Helix antennas for -171 and two 5' Helix antennas for -173. Even the -179 dBm signal was quite detectable with 100x integration but is not recommended because of the requirement for doppler correction at that level. Here are the images:
Spectrogram of -171 dBm signal in a 100 deg K system - expected performance of a two 10
foot helix antenna system
(-166 dBm signal in the 325 deg K test system): Expand to Full Size (27 Kbytes)
Spectrogram of -173 dBm signal in a 100 deg K system - expected performance of a two 5
foot helix antenna system
(-168 dBm signal in the 325 deg K test system): Expand to Full Size (27 Kbytes)
My station showed a non linear drift rate of aprox 0.02 Hz/sec during the test. I doubt that many ham stations will be more stable. At the 2 Hz BW for FFTDSP, the signal needs to remain in the bin for aprox 50 seconds. 50 seconds is the period of integration for the 100x setting. This setting would only be useful when the doppler rate was at a minimum.
Here's a plot near the limit of detectability of FFTDSP:
Spectrogram of -179 dBm signal in a 100 deg K system:
(-174 dBm signal in a 325 deg K test system): Expand to Full Size (27 Kbytes)
-179 dBm will be the minimum detectable signal for a ham station using a good preamp (0.5 dB NF) when running the FFTDSP program. If doppler is considered, -173 dBm is easily seen when pointing at cold sky with a low temperature array.
(For those of you familiar with the UNKN422 weak signal challenge, as a comparison, the UNKN422 signal was aprox -156 dBm)
Here's my calculation for the MR Beacon level: (Someone please correct me if I am wrong.)
MR 437.1 Beacon Tx level = 1.3 watt = +31 dBm
Distance, Earth to Beacon = 5.6E6 Km
= 92.4 + 20 log (km) + 20 log(fghz) = 92.4 + 20 log(5.6 e6) + 20 log (.437) = 92.4 + 135 - 7.2 = 220 dB
Signal level at Antenna =-189 dBm
Assumed Antenna Gain = 18 dBci (include coax loss)
Signal level at Preamp =-171 dBm
I believe this test confirms the possibility of detecting the probe with a 18 dBci antenna (including coax) and a 0.5 dB NF preamp.
Assuming 0.5 dB coax/combiner loss, two 10ft Helix antennas would provide 18 dBic. With the new receive levels, I think this system would provide an excellent chance of detection. In fact, just two 5 ft Helix should be enough for detection. Also, I would recommend mounting these antennas with non conductive masts and combining the coaxes behind the reflectors. You can easily loose 1.5 dB if conductive masts are used.
With the 16x integration mode, Doppler Chirp correction will not be necessary. At that
integration mode, the display trace will have a slope showing the Doppler Chirp which
should help confirm that it is from the Mars Global Surveyor. A 30 minute period will fill
the screen and the trace will show a linear slope of aprox 100 Hz.
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