|Jan21-13, 01:28 PM||#1|
Phased Array Radar
Let's say I want to use a phased array detector on a satellite to detect other objects in space. The radar would need to be able to detect objects .5cm or larger, and would have to get an accurate read on their position (within 10-15 m). From there, the goal would be to maximize the volume of space scanned per second. How much volume of space could I expect to scan this accurately per second? Obviously, the more the better. I am open to any variant of phased array (or any other method of radar if that would work better). It would also be a plus to be able to detect the velocity components and size of the object. Would there also need to be some kind of wave emitter to "paint" objects for the radar to detect?
Also, I am trying to understand how phased array works when detecting objects. I understand that the beam can be steered. Do most phased array radars work by scanning, say, a cone with a 2 degree angle, then just determining if an objects lies within that cone? Or can they determine where (in multiple dimensions) an object lies within that cone?
Kinda confused here. Thanks!
|Jan21-13, 06:18 PM||#2|
guss, I tried to extract specific questions you seem to be asking but I stopped this effort after writing out five questions because it became apparent that all the information you are asking for is contained in these two Wiki pages:
If, after studying these two sources, you have some doubts or further questions, please do post them here on Physics Forums.
|Jan21-13, 09:14 PM||#3|
"Objects in space" sounds like "big distance"... People use to search for orbital debris to avoid collisions, and at a few 1000km range, 5cm tends to be the absolute lower limit.
Did you already check if you can detect one single object within a single pass? At 800km altitude, objects appear at 3,000km distance, stay visible for 10+min, then disappear at 3,000km. Serious challenge!
Just for comparison, a stealth combat aeroplane has like 1000cm2 radar cross-section and is detected at <100km range. But you could try to find figures from the Norad, if any is public.
|Jan22-13, 12:47 PM||#4|
Phased Array Radar
So, the radar beam would be steered, and when an object is detected, the beam could be focused to get more accurate readings. From my understanding, this is possible.
I still have some questions, though. It seems to me a very small setup (say, the size of a couple soda cans) could have a range of about 15km. This would probably compromise the close-range (less than 1km) capability, though, since you can't have close and long range on the same radar. What would be the smallest sized objects you could detect with a setup like this?
Additionally, I have learned a little bit about LIDAR. It seems like that could be used well here. I have had a lot of trouble finding information about optical phased array and LIDAR, though. Is this technology available? As in, with LIDAR, is it possible to steer the sensor and the beam electronically?
Thanks a lot!
|Jan23-13, 04:24 AM||#5|
A “very small setup (say, the size of a couple soda cans)” does not determine the smallest sized objects you could detect. This depends more on the power in the transmitted pulse, the frequency, and the target characteristics. A metal object, for example, will scatter (reflect) more incident energy than the same size refractory tile. The term for this is the “radar cross-section”. The aspect (orientation) of the object also determines how much energy is reflected. A jet fighter aircraft seen “head on” reflects far less energy than its “side aspect” would.
The physical size of a radar set does not determine what maximum and minimum ranges it can detect. See the below tutorials for detailed explanations of these radar set performance characteristics. Variables include transmitted pulse power, antenna gain, frequency, minimum discernible signal, transmission losses, and target characteristics.
Yes, the same radar set could be used to detect targets at both long ranges and also at short ranges. In nature, bats using acoustic pulses do exactly this by adjusting the pulse repetition frequency (prf). For long range detection of prey they transmit pulses with long time periods between them (a low prf). As they get closer to their targets, called the “homing phase” of the collision course they fly, they increase the prf so as to get more information rapidly to use for more detailed and fine course corrections. They end up with dinner in their mouth. Some modern radar guided air-to-air missiles do exactly this as they close on the target.
LIDAR systems are used to detect and track space “junk”. LIDAR systems are basically optical telescopes. As far as I know electromagnetic radiation in this region (optical) of the spectrum cannot be controlled by phase-controlled modulation as are radar beams. Therefore, no, it is not possible to steer the LIDAR sensor or beam electronically. There are systems in telescopes, however, that use piezoelectric actuators to deform the reflecting surfaces, so this may be construed as “beam steering”, but it is clearly a mechanical change in the reflecting surface.
Here are two radar tutorials that I’ve selected. They are clearly written and will help you get a good basic understanding of radar operation and performance.
As for your main questions about detecting and tracking space debris, please see this Wiki page:
Saved the best for last: On this Wiki page if you look down at the references, please see these three sources for excellent descriptions of the equipments used:
113. A powerful radar facility for the detection and tracking of space debris and the imaging of space objects is available in the form of the 34 m dish radar at the Research Establishment for Applied Science (FGAN) at Wachtberg near Bonn, in Germany. This offers lots of the technical specifications of the radar.
114. Laser Tracking of Space Debris, Ben Greene, Electro Optic Systems Pty Limited, 55A Monaro Street Queanbeyan NSW 2620 Australia
The laser ranging equations are well documented in the literature. These clearly show that ranging to space debris is possible if the following parameters are scaled upwards sufficiently:
115. Orbital debris: Optical Measurements", NASA Orbital Debris Program Office. Optical telescopes.
|Jan23-13, 12:54 PM||#6|
Thanks! I will read the tutorials. It seems to me like radar is the way to go here, since phased array comes with redundant reliability and no need to mechanically steer the parts. I believe that I should be able to detect small objects at longer ranges without too much of a hassle, as well. I'll keep LIDAR in mind, though.
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