NAVSPASUR Radar Experiments
April 6, 2006
Volker Grassmann, DF5AI
The US Naval Space Surveillance Mission (NAVSPASUR) detects, identifies and tracks all man-made objects orbiting around the Earth. A multi-static radar system employs fan-shaped radar beams forming a radar fence across the Unites States. These radars operate several thousands of dipol antennas, each dipole is driven by an individual 300 watts transmitter which results in an enormous effective radiated power on 216 MHz. This radar can therefore generate strong moon echoes, of course.
NAVSPASUR moon echoes
In fact, whenever the moon passes through one of the radar beams, intense lunar radio echoes may be observed even with small receiving antennas. This has attracted the interest of radio amateurs, one of those is Rob (PE1ITR) in Eindhoven who experiments with NAVSPASUR moon echoes since early 2004. Rob is using a 220 MHz 8 element Yagi together with a frequency converter which transfers the moon echoes to his 144 MHz transceiver. The above diagram displays an example of the radar's moon echoes measured in April 2004. Interested readers are requested to refer to Rob's excellent web site where he explains the technical and operational details and where he presents his impressing results.
Reflections from satellites illuminated by NAVSPASUR
However, the NAVSPASUR radar system has been designed to detect satellites in space orbits - can radio amateurs use the radar to detect satellites too? Yes, they can. In 1995, Tom (K4GFG) has published his experiments in the detection of radar signals from satellites illuminated by the NAVSPASUR radar (we all know Tom from his landmark paper in 1982 where he has introduced the phenomenon of mid-latitude E-region irregularities to the ham community, see the Aurora and FAI section of this web site). Tom writes: "the fact I was tracking satellites did not escape the attention of the U.S. government" - in October 1994, he was invited to the Pentagon "to give a briefing". The latest measurements were conducted by Rob (PE1ITR) who receives weak satellite reflections within time periods of some 10 seconds to 10 minutes. Located in the Netherlands, Rob must refer to satellites in very high orbits, of course. One of the Ariane 5 rocket boosters therefore became his preferred space object in the experiments, see the details in Rob's article.
Time-Doppler analysis of NAVSPASUR moon echoes
On this web site, we have already discussed the echo depth of the moon, i.e. moon echoes originating from the disk center return to Earth earlier than reflections originating from the lunar limb. In another paper, we have also addressed the Doppler effect resulting from the moon's rotation, i.e. we are facing a positive and a negative Doppler component caused by the left and right side of the lunar disk, respectively (see the articles in the Moonbounce section of this web site). Thus, the total moon echo represents a complicated mixture of signal components with varying time delay and Doppler shift. Markus (DF6NM) has specialized on the measurement of time-Doppler spectrograms derived from NAVSPASUR moon echoes. One of his results is displayed on the right which is obtained by combining the data of four measurements in October 2005 and January 2006. See the details and the results on Markus' web site.