Radar thread - specs & discussions

Locked
Arun_S
BRF Oldie
Posts: 2800
Joined: 14 Jun 2000 11:31
Location: KhyberDurra

Post by Arun_S »

Dileep wrote:and what about cooling the phase shifters themselves. The loss in them goes out as heat.
My info is dated but the greater challange was space to physically fit the phase shifter controller (in the interstitial space) AND power distribution (including capacitor size).
maitya
BR Mainsite Crew
Posts: 622
Joined: 02 Feb 2001 12:31

Post by maitya »

Arun_S wrote:maitya: I am sure you realize that Radar peak power requirement is different from average power requirement, mostly determined by duty cycle. Ultracap or other storage devices easily take care of few microseconds of Tx power (as most AESA use long tx period and use pulse compression signal processing)
Arun, the above calc was for peak transmitted power. Obviously, for a avg transmitted power value, the power requirement would be lower ... not sure if there's any linear relationship with the duty cycle though!! I mean for, say a Bars-class AESA radar, the peak transmitted power is 4-5Kw implying a avg power o/p of 1.2Kw (assuming a duty cycle of 4, say) - not sure does that mean a corresponding 4-fold reduction in i/p power requirement!!

Another question is regarding the pulse compression capability of a AESA (vis-s-vis, say a PESA) - I mean, yes the radar sensitivity (roughly range resolution) for a given transmitted power can be increased by increasing it's bandwidth using pulse compression techniques.
My question is, does solid-state transistor based T/R modules have any inherint advantages for doing the same? If so what's that? Of course, like everything, I guess there's limit to that also - IIRC, the range sidelobes tends to increase thereby making it more difficult for the receiver to process them!!
Dileep
BRF Oldie
Posts: 5882
Joined: 04 Apr 2005 08:17
Location: Dera Mahab Ali धरा महाबलिस्याः درا مهاب الي

Post by Dileep »

not sure if there's any linear relationship with the duty cycle though!!
Yes, the power ratio is exactly the same as duty cycle ideally.
Another question is regarding the pulse compression capability of a AESA (vis-s-vis, say a PESA) - I mean, yes the radar sensitivity (roughly range resolution) for a given transmitted power can be increased by increasing it's bandwidth using pulse compression techniques.
Pulse compression is for range RESOLUTION, not range SENSITIVITY. Range resolution is needed to count the fan blades of the engine, and to do terrain mapping etc.

1st Gen AESA and PESA have the same capabilities and challenges for pulse compression IMO.
Arun_S
BRF Oldie
Posts: 2800
Joined: 14 Jun 2000 11:31
Location: KhyberDurra

Post by Arun_S »

maitya: Typical Tx duty cycle would be in range of 1000 - 50,000.

Pulse compression becomes a requirement for Solid state devices based radar (or even for TWA designs) that are peak power constrained rather than average power constrains. Unlike Magnatron, these devices have linear capabilities thus can exploit more complex modulation also to get processing gain to compensate for Tx power. Of course longer pulse width allows it to transmit same energy in the Tx pulse as shorter pulse from Magnatron style RF generators.

Linear power devices generally also have wider bandwidth capability and wide band spread spectrum techniques can be better applied.

Just my 2 cents.
Dileep
BRF Oldie
Posts: 5882
Joined: 04 Apr 2005 08:17
Location: Dera Mahab Ali धरा महाबलिस्याः درا مهاب الي

Post by Dileep »

maitya: Typical Tx duty cycle would be in range of 1000 - 50,000
You mean pulse repetitive frequency right?

We need to distinguish between range sensitivity and range resolution. The sensitivity is basically a function of the carrier amplitude alone. But you need a minimum pulse width for the receiver to detect the signal. IOW, you can not have just a few cycles of the carrier transmitted. Now, once we achieve the minimum pulse width necessary for reliable detection, more width is useless. The pulse width needed depends upon the bandwidth of the AM detector and FM discriminator(for Doppler shift), and also the DSP system. There is no inverse relation between the amplitude and pulse width for a given range. IOW, you can't compensate for lower amplitude with wider pulse.

The range resolution, ie distinguishing two objects that are at same line of sight to the radar, but positioned at different distances, also inversely depend upon the pulse width. The pulse compression technique is a compromise. Using wider pulses for easy detection and compression for easy range resolution.

The pulse repetition frequency depends upon the target. for sttionary and slow oving targets, you need only slow repetition. For fast moving targets like aircraft, you need faster repetition. The duty cycle is the ratio of the pulse width to the repetitive frequency, and it can be very small for scan function, and higher for tracking.

Also, the longer the range, slower the repetitive rate. A 300km range scan will take 2 milliseconds round trip travel, so you can send only 500 pulses per second max. Assuming you are looking for 1km resolution, you have a duty cycle of 1/600. Now, tracking a target at say 50km with 10 metre resolution will give a duty cycle of 1/10000. That is pretty small.
Arun_S
BRF Oldie
Posts: 2800
Joined: 14 Jun 2000 11:31
Location: KhyberDurra

Post by Arun_S »

Dileep wrote:
maitya: Typical Tx duty cycle would be in range of 1000 - 50,000
You mean pulse repetitive frequency right?.
No sir. Duty cycle being defined as the ratio of the time Tx is On to the time it is Off.

The following is not for the Guru but his students. So for E.g. the radar may transmit a RF pulse of say 0.5 microSec and depending on range will have to wait for ~600uSec (100Km range) before transmitting again (yes the reciprocal of max PRF) . Thus the dutycycle of the Transmitter's Power Amplifier is ~ 1 : 1,200 (The heatsink cooling and power rating of most devices largely go by average power, yet the max pulse power is dependent on many factors, one of them is generally the peak current rating of transistor, apart from safety margin for max VSWR/reflection due to load impedence mismatch).
Dileep
BRF Oldie
Posts: 5882
Joined: 04 Apr 2005 08:17
Location: Dera Mahab Ali धरा महाबलिस्याः درا مهاب الي

Post by Dileep »

Arun, the duty cycle is always a fraction. So, do you mean 1/1000 to 1/50000?

And duty cycle is the ratio of ON time to TOTAL time, not OFF time.

And thanx for the "Bhaashyam" ;-) I was interrupted multiple times by the SHQ while I typed it and had to edit it after posting to fix numerical errors.
Sumeet
BRFite
Posts: 1616
Joined: 22 May 2002 11:31

Post by Sumeet »

just a question for the Gurus here does an AWACS radar also have a LPI feature ?
Arun_S
BRF Oldie
Posts: 2800
Joined: 14 Jun 2000 11:31
Location: KhyberDurra

Post by Arun_S »

Dileep: Thanks for making it clear and correction. I did mention the 1: 1000 somewhere in the post. You are right its a fraction, but for radar its such a obscene number that I find the 1: X more useful. Of course when X is very large the X+1 makes little difference to the ratio ;) Hum to Baal Ki Khhaal Nikal Rahain Hain, its a Friday after all. How about lunch next week?
Locked