smos_key-visual_FINAL_L.jpgLast week IEEE Spectrum published an article about how interference is a problem for NASA and ESA efforts to measure soil moisture levels to predict droughts.  With over half of North America experiencing a drought, this is a timely issue.  The cause of the interference is “rogue transmitters”.  The ESA has contacted the authorities in the areas of the offending interference to ask them to go after the sources.  My first thought was might I somehow be on their list?


The radiometry works by monitoring a 27-MHz-wide band centered on 1.410GHz.  Satellites orbiting at an altitude of 700km monitor faint signals radiated from a region of ground with a high-gain antenna focused on a region of 50 kms in diameter.  The receiver sensitivity is in the -100dBm range.  That’s impressive because that’s -100dBm over a bandwidth of 27MHz.


How hard would it be to interfere with such a system by accident?


Let’s look at that cost between my ham radio and the satellites.

FSPL = 20*log(d[km]) + 20*log(f[GHz]) + 92.45 = 20 * log(700) + 20*log(1.41) + 92.45 = 152dB.


I can’t find any info on the gain of the antenna on the satellite, but let's suppose it’s 30dB. 


For the satellite to detect a transmission its effective radiated power would need to be FSPL - RX Sensitivity - RX Antenna Gain = 150dB - 100dBm - 30dB = 20dBm = 100mW.  So if someone had a 1W handheld transceiver (HT) designed for the 23cm band (1.24GHz - 1.30GHz) that was operating on at 1.4GHz for some reason, he could conceivably interfere with the satellite if here were transmitting as the satellite went overhead.  The receiver integrates over time, so unless he were transmitting continuously every time the satellite took a measurement his interference would fall into the noise. Accidental emissions in the lab would be in the noise even I left them on 24/7. 


RFIs_2011-2012_L.jpgThe real sources of interference are military radar.  Governments have worked to reduce this interference drastically.  NASA is planning to launch a satellite that monitors shorter periods of time, so that time-variant interference sources like radar can be identified and those samples can be discarded. 


When I first experimented with radio as a hobbyist, I was fastidious about not transmitting outside a band I was licensed for.  In the lab, now, I don’t sweat it if I accidentally cause equipment to transmit briefly out of band.  My understanding is that such truly accidental transmissions in the course of development are not illegal.  To avoid interfering with important efforts like this, though, if I have any reason to think a transmitter might go out of band, I try to have it connected to a resistive “dummy” load and not an antenna outside. 


For Further Reading

ELBARA II, an L-Band Radiometer System for Soil Moisture Research - Technical details of the satellites’ receivers, amplifiers, and antennas. - Some technical details of radiometry receivers - Shows how governments have reduced intereference in response to requests from ESA.