RE: CODAR bandwidth -- from looking at the CODAR SDR shot on Wikipedia (taken by a ham -- it looks like it was a CODAR intrusion into the 20 meter ham band) it appears that CODAR sweeps are about 20 khz wide? Is this pretty standard for most CODARS?
Sorry I missed this before.
There is no “standard” swept bandwidth for CODAR. There are some general guidelines as to what you might expect to see. In general the lower the frequency of operation the narrower the swept bandwidth. I have seen CODAR use anything from 12 kHz to over 200 kHz and I have no idea what their real limitations are.
Around the 5 MHz region I typically see about 26 kHz, up around 13 MHz I typically see about 50 MHz., and up around 25 MHz I often see 100 kHz. On the other hand, I have also seen 24 kHz in the 25 MHz area, so as I said the “narrower at lower freqs” is just a rough guideline.
Sweep rates are generally between 0.5 and 4 sweeps per second. Chirp rate (pulse compression) will be sweep rate times bandwidth.
Many CODAR run 24 hours a day, others cycle on and off at different times.
Some pictures of various CODAR:
This is two interleaved CODAR centered on 4550 kHz. They are both the same width and they stay in sync (probably GPS timing based), this way they do not interfere with each other despite occupying the same frequency range. The width measures out at about 26 kHz.
The above 2 CODAR can be seen operating here:
http://www.youtube.com/watch?v=8OF-yhrtqKoThis is a CODAR centered on 12190 kHz. It is about 52 kHz wide. Another can be seen to the left of the one in question, it is also roughly 52 kHz wide.
And last is a CODAR centered on 13500 kHz, with a width of about 150 kHz.
CODAR, ionospheric sounders, and “real” radar (radars looking for man made and moving targets, such as ships and aircraft, or rapidly changing natural features, such as weather) are all radar and often use identical techniques. But I typically break them out into CODAR, sounders, and radar, and speak about them as if they are not all “radar” to reduce confusion.
CODAR sweep relatively wide (compared to “real” radars) and relatively slowly because their target set is slow moving and such a technique lowers equipment cost. Sounders typically either sweep or sample (two different things) even more slowly than CODAR because the sounders target set changes even more slowly than a CODARs target set.
On one hand the sweep rate determines maximum unambiguous range for an unencoded FMCW waveform when applied to a radar or CODAR. The slower the sweep rate, for a given swept width, of an FMCW radar the greater the possible unambiguous range. However, a sweep rate of about 7 times a second yields a possible unambiguous range that could stretch all the way around the World. So the typical CODAR sweep rate of 1 sweep per second would yield a range of over 7 times the distance around the World, but CODAR typically looks at data at 300 km or closer. In other words the sweep rate of the CODAR, unlike “real” radars, has nothing to do with maximum unencoded and unambiguous range, and more to do with target set velocity and cost of the technology.
“Real” radars sweep faster because they are designed for a realistic maximum range, based on detection thresholds (how small a target RCS, Radar Cross Section, that can be tracked at a given range, a complex relationship, search “radar range equation” for an introduction), technology, and probable propagation in the frequency range of the hardware. If the desired maximum range is 6000 km than a sweep rate of 25 Hz would be used (and 50 Hz yields a maximum of 3000 km). Of course, a lower sweep rate (say 10 Hz) would also yield at least that range (10 Hz yielding a max possible range of 15000 km), but at the cost of illuminating the target less often, lowering the probability of detection.
Several other factors define the width swept and chirp rate (pulse compression), based on the performance of the desired target set. But, this post is long enough already ;-)
T!
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