open source, home radar receivers for imaging Comets/meteors

[promacjoe]

While watching a video on the Internet, about Imaging comets and meteors using radar receivers. I know that multiple radars that are interconnected can give a high resolution image. I was wondering how hard it would be to set up a radar system at home.  if this type radar system could be interconnected via the Internet over hundreds of thousands of homes. What kind of resolution could be achieved.

The microwave signal could be transmitted from a central location. That signal could be received by each of the home units. A time correlation would have to be established and verified for each of the units along with calibrating the amplitude of each receiver.

Of course all of this would be well over my head, but it might make an interesting discussion.

Would something like this be feasible. How large would each individual dish need to be. Could you get by with a 24" disk, Or would you need a larger disk. What kind of down converters/amplifiers would need to be used. How would you calibrate it. it might be an interesting open source project, if some University/Universities wanted to work with us on the project. if it is feasible, it might establish the Worlds largest radar array possible.

Just a thought. Joe.

[radar_macgyver]

From Wikipedia data, let's assume that the average comet can be approximated by a sphere 5 km in diameter. Let's also assume that it's a perfect conductor, so its radar cross section is that of a 5 km metallic sphere, or 19e6 m^3. Since you're interested in observing comets in space, let's assume a range of 400 km, or low-earth orbit.

The highest EIRP one can radiate without a license in any ISM band (in the US) is 53 dBm at 5.8 GHz. Let's assume this is split up as 30 dBm (1 watt) transmitted power, and 23 dBi of antenna gain. When limited in transmit power, one must resort to FMCW radar to achieve decent sensitivity. Due to use of FMCW, peak transmit power and average transmit power are equivalent.

Plugging these numbers into the radar equation (and assuming no attenuation due to gaseous attenuation or rain fade), one gets a receive signal strength of 4e-17 watts = -144 dBm. For this to be above the thermal noise floor of a perfect receiver (no added noise), one would need a receiver with a bandwidth of ~1 kHz - which doesn't leave much room for frequency-sweeping an FMCW transmitter.

Turning this around, if we assume a receiver sensitive to -113 dBm (0.5 MHz bandwidth, with a 3 dB noise figure) then the maximum range at which you can pick up the echo from a target with a comet-sized RCS is about 21 km.


One way this can be made to work is to use lots of radars and add their outputs in-phase (interferometry). This would require synchronization between the radar nodes - not impossible, one can use GPS for this, but it would be expensive, and you would need a lot of radars to get the type of sensitivity required to detect a comet at 400 km range.


If you disregard FCC rules and set up a 5.8 GHz radar using a surplus TVRO 14 foot [4.3 m] reflector, the antenna gain goes up to about 43 dBi. The received signal would be higher in power by 20*2 = 40 dB, which is in the realm of what one can receive and process with an SDR.


As always with FMCW, the trick is to prevent the transmit signal from desensitizing the receiver. Separate transmit/receive antennas are one way of doing this. Active carrier cancellation is another, but is tricky to implement.

[promacjoe]

I was thinking more on the lines that a University could maintain and control the transmitter portion. At the beginning, the receiver dish could be located at local schools. Then maybe Amateur astronomers and maybe ham radio operators, would host a receiver.

At this point it's just a thought discussion. Would it be feasible. would it be useful. What kind of resolution could we expect. And could we achieve a reasonable distance. It does no good to detect an object that is going to hit the earth in less than 30 minutes. We want to be able to detect them well out  into space. Preferably as they exit the asteroid belt. But I know this would be a stretch. What I'm hoping for is to augment the capabilities of detecting near Earth objects before they could cause damage. Giving us enough time to do something about them. We just don't have enough detectors yet. For instance when that meteor exploded over Russia a while back, our instruments were looking at another near earth asteroid. And it came from the opposite direction, Which no one expected. even if we could just detect these objects, and have larger instruments do the actual imaging, it would be a big help. And doing It as an open source project, could put these detectors anywhere in the world.

I just thought I would put the idea out there to see what we could come up with.

I appreciate any and all information. As I said this is well over my head.

Joe.

[radar_macgyver]

Sorry if my post came across as too negative. I've tried this once, didn't get much interest. I thought setting up a radar equivalent of CoCoRaHS might help fill in the gaps of the existing weather radar networks. I even came up with a design for a low-cost 24 GHz FMCW. Nobody was interested in funding it though. University-owned radar networks do exist for weather observation, none that I'm aware of for space object tracking (mostly because it's so hard to get the sensitivity required).

Also, what you're talking about requires *tracking* (in addition to volume search), a much harder problem to solve. The easy way to get steerable, high resolution, directional antennas is to mechanically scan a parabolic reflector. Doing this to scan even a small patch of sky requires continuously scanning antennas (aka mechanical wear and tear). The other approach is electronic scan, which is horribly expensive.

You're talking about tracking objects in outer space. Well, here's a radar meant to track deep space objects. Note the size of the antenna. The ESA built this cutie as a test-bed, and it can track low-earth objects the size of earth-observing satellites. It's a precursor to a larger system similar to the FPS-85. Here's a summary of what it takes to do this. The authors have looked at this problem a lot longer than I have, and their conclusion, like mine, is that for this type of application (small, > 5000 km range) you're better off tracking objects optically instead of using radar.

[mark03]

Tracking comets and meteors would be completely different problems.  When a bright comet gets closer than 1 AU (150 million km), amateur astronomers get excited; any comet closer than the moon would be a pretty big deal, even if it were not on a collision course. 

Meteors, by contrast, are obviously less than ~ 100 km altitude.

There is plenty of amateur work with pre-existing high-power transmitters for meteor detection (actually they are seeing the ionized trail rather than the meteor itself, which is usually pebble-sized).  The US used to run a good one, the NAVSPASUR (?) or "space fence" but it was shut down years ago.  I think most recent work has focused on other transmitters not necessarily designed for radar purposes, like TV stations.  There is probably great scope for clever signal processing of these data, and combining data from widely distributed receivers.

[janoc]

I think a 5km sized piece of rock moving at cosmic speeds 400km from the Earth surface would give everyone much different worries (aka end of the world) than the amount of radar power needed to track it!

Even 36000km from the surface (geostationary orbit) is considered extremely close and the rare near misses within that distance with even much smaller asteroids give people cold sweat already.