h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hey

I'm really enjoying these entries. I have a quick question. under the freq modulation paragraph, towards the end, you point out that an uncompressed 20 micro-s pulse has a range of 3000 m, but the compressed pulse has a range of 50 m. Based on what you said earlier, I'm thinking that the compressed pulse has a higher intensity, and that it doesn't spread out in the air. If it has higher intensity, shouldn't the range be longer? but I don't know. Either way, can you explain why the range is affected by the pulse compression? Thanks,

dave

Hi Captain!

When being transmitted and travelling through the air, the FMOP pulse is just as long as any other pulse. The only difference is that its frequency isn't constant (so it would sound something like 'ooooeeeeiiii' rather than 'oooooooooooo' which represents a 'simple' pulse). This signal is 12 digits long and if each letter has a power of 1 Watt then we have transmitted 12Watts over the time of, say, 12 µs. *Both* the ''ooooeeeeiiii' and the 'oooooooooooo' signal stretch out for, say, 3000m in the air.

The compression takes place within the radar receiver. The pulse compressing device (SAW) is something where 'i's travel faster than 'e's and 'e's travel faster than 'o's. Therefore, the 'o's, 'e's and 'i's come out of it at the same time, to make something like 'AAAA'.

This 'AAAA' is three times as strong (3 Watts, because each 'A' is obtained by adding an 'o', an 'e' and an 'i') as the original signal. We still have 12 Watts in total, but they are compressed into a shorter time (4 µs) which represents 3000/3 = 1000m. We are now able to get *three individual* target blips in the same display area that would be occupied by one big 'splat' if an unmodulated+uncompressed pulse were used. Therefore, range resolution (not range) has been improved threefold.


Bossel