PLA AEW&C, SIGINT, EW and MPA thread

[RedMercury]

In your example, if the radar sent out a second pulse while waiting for the first pulse, then all is screwed because the radar needs to distinguish which pulse is which. That's why there is a wait period, because the radar knows the pulse it is receiving is the one last it sent out. If a pulse that was sent out earlier, echoed against a much farther target, then received by the radar which is also receiving other echoes simultaneously, it would be disregarded. The radar will only accept the echoes of the last pulse as valid.

I realize this problem you raised, and in my previous post I asserted that this disambiguation can be done using a probabilistic filter. Yes you would get two echoes, but only one would make sense given the prior information you have about the target track, so you can reject the other. The radar's matching filter for the pulses it sent out can be modified to match for any recent pulse, instead of just the previous one. This doesn't seem like magic to me, just some more signal processing work, and it seems pretty easy if this is all done in software after digitization.

 

[crobato]

I realize this problem you raised, and in my previous post I asserted that this disambiguation can be done using a probabilistic filter. Yes you would get two echoes, but only one would make sense given the prior information you have about the target track, so you can reject the other. The radar's matching filter for the pulses it sent out can be modified to match for any recent pulse, instead of just the previous one. This doesn't seem like magic to me, just some more signal processing work, and it seems pretty easy if this is all done in software after digitization.

The problem is what happens if the signal is being received while a new pulse is being sent out? In which case, that signal is not received. In every cycle, you assume at least 50% send, 50% receive time.

I'm just telling you pulse radar basics. That's how radar works.

 

[RedMercury]

Okay that makes sense. So how do continuous wave radars send and receive at the same time?

 

[crobato]

Okay that makes sense. So how do continuous wave radars send and receive at the same time?

CW has an entirely different principle from pulse radars. Pulse radars go Send then Receive, Send, then Receive. All in a single antenna.

In CW, you got two separate antennas, one that is continuously sending, and one that is continuously receiving.

However, pulse radars can send a signal farther, because it can compress much more power into a single pulse, while CW has its power distributed continuously all the time. While CW radars have shorter range as a result, they are also less likely to be detected as well.

With pulse, the radar uses the time when the signal is first sent, against the time when the echo is received to determine range. With CW, however, this is not possible because there are no markers that let you do this. Instead, with CW, you use the Doppler shift (increase or decrease in received frequency) of the echo by measuring the original frequency vs. the echo frequency, to indicate if the target is heading towards you or receding, which means CW measures speed most accurately. Thus pulse radars are best for measuring range, while CW radars are best for measuring speed. That's why pulse radars are used for detection and early stages of tracking, while CW are used for late stage tracking, gun and missile guidance. CW are also used for speed traps and police radar guns to get speeders.

You can get range on CW if you modulate a marker into the CW wave, like a signature spike or an interruption, then try to read this marker on the returning wave. That's called Frequency Modulated CW and Interrupted CW.

 

[A.Man]

● 于泽远
北京

　　中共建国60周年庆典将于10月1日上午10时在北京天安门广场正式开始，备受关注的阅兵式将持续66分钟左右。有关军事学者指出，除海军舰艇外，这次阅兵展示的武器装备基本代表了解放军列装的最新水平，其中最大的亮点首推空警2000预警机。

　　据了解，庆典开始后将首先举行升旗仪式，随后中共总书记胡锦涛将乘国产阅兵车出天安门，过金水桥，接受阅兵总指挥、北京军区司令房峰辉简短汇报，依次检阅排列在长安街北侧的各兵种方队。随后，胡锦涛将返回天安门城楼发表讲话。

　　胡锦涛讲话后，由陆、海、空、二炮、武警和后备力量六大阵容组成的14个徒步方队、30个装备方队、12个空中梯队自东向西依次通过天安门广场。最后是各地彩车和民众游行队伍通过天安门广场。

　　与10年前阅兵不同的是，官方对这次阅兵训练不再保密。官方主流媒体以及一些市场化程度较高的媒体纷纷报道参阅部队的训练情况，从官兵的精神风貌到吃喝拉撒睡都成为采访内容。与阅兵有关的报道显然已成为官方营造国庆看点和喜庆气氛的主打内容。

　　但官方媒体很少透露阅兵式上将亮相的武器装备型号，只是引述军方将领派的一些笼统说法，强调武器装备的国产率和先进性，似乎要为已被大事炒作的阅兵式保留一些神秘色彩。

　　中国空军副司令员赵忠新日前透露，这次阅兵空中梯队领队机，由中国自行研制并正式装备空军的大型预警机担任。这型国产预警机集空中指挥、预警探测、电子对抗等多种功能于一身，是空军信息化建设和装备发展的重要成果，有“空中指挥所”之称，是空军实现攻防兼备战略转型的标志性装备，也是空中作战体系能力的关键要素，代表着近年来空军信息化建设的新成就。

　　有关军事学者指出，赵忠新所说的就是kj-2000（空警2000）大型预警机。这款预警机采用俄罗斯制造的伊尔-76为载机，但固态有源相控阵雷达、软件、微波单片集成电路、高速数据处理电脑、数据总线和接口装置等都为中国设计和生产。预警机采用三部相控阵雷达，呈三角形排列，每部负责扫描120度空间范围，三部可构成360度全方位探测，可同时引导30架歼10、歼11、苏27或苏30等战斗机进行攻击拦截作战，对隐形战机也有较强的探测能力。

　　这名学者说，空警2000预警机是本世纪初中国头号军事重点工程之一。据悉，空警-2000的雷达天线并不像传统预警机那样是旋转的，而是固定不动的，说明空警-2000采用的是先进的固态有源相控阵雷达，只需以电子扫描进行俯仰和方位探测，不需要再采用落后的机械扫描转天线。空警2000预警机的服役填补了中国没有预警机的空白，使中国空军的作战能力和防空能力得到显著增强。

　　赵忠新也表示，空军预警机已形成了战斗力，在日常防空战备以及奥运空中安保中发挥了重要作用。

　　10月1日的阅兵中，空军航空兵、陆军航空兵和海军航空兵共同组成空中梯队，届时将有151架飞机构成12个梯队通过天安门广场上空，飞机规模和梯队数量都将创造中国历次阅兵之最。

　　另据赵忠新透露，今后一个时期，空军将着力提高侦察预警、空中打击、防空反导和战略投送能力，重点发展新型战斗机、大型运输机、防空反导武器、指挥自动化系统等装备，陆续配置一批较先进的信息化装备和空空、空地精确制导弹药，改进现役装备电子信息系统，完善情报保障、作战指挥、武器控制和通信基础网络，不断提高空军武器装备建设水平。

　　阅兵武器装备的另一大亮点是各型导弹，包括由长底盘卡车运载的东风-31改进型洲际导弹。东风-31和巨浪-2实际上是同一类型的导弹，一个部署在陆地上，另一个部署在战略核潜艇上，射程可达8000公里以上。另外，改进型“东风-11”及“东风-15”近程导弹、掠海型“鹰击-83”反舰导弹以及“东海-10”远程巡航导弹等先进武器也将在阅兵中亮相。

　　

由中国自行研制的大型预警机集空中指挥、预警探测、电子对抗等多功能于一身，是空军信息化建设和装备发展的重要成果，有“空中指挥所”之称，是空军实现攻防兼备战略转型的标志性装备，也是空中作战体系能力的关键要素，代表着近年来空军信息化建设的新成就。

 

[maozedong]

this looks like Y-9 going on assembly, but som source say KJ-200 actually use Y-9 as platform, named 8F600, but it's not Y-9 transport plane, the Y-9 transport plane still not in service yet.

 

[williamhou]

I wish people need to learn more about radars.

Power is not the main factor in range. Frequency is. A low power signal can travel hundreds and even thousands of kilometers if the frequency is long enough to reduce atmospheric attenuation. However, what long frequency requires is a big antenna, since each element has to be at least 1/2 the length of the frequency. You can bounce radio signals off the moon with a ham radio.

The second determinant of range is the length of the receiving time. That means the radar has to wait for the receiving signal to come back. If the radar has to broadcast a second signal before the echo of the first signal is received, the echo of the first signal is then lost. So the radar has to wait. The longer these wait times, the longer the range. The disadvantage of this, the longer your wait times are, the less signals you can broadcast over time, which means lower resolution in terms of tracking and discrimination.

Any radar that is looking for a balance between range, tracking and discrimination needs to find a sweet spot in frequency and pulse times, basically PRF. Its just physics. You want longer range, you're going to suck when it comes to tracking and discrimination. You cannot expect a detection range of 1000km to have the discrimination and resolution rate you get with 300km.

The result is that all radars have various operating modes that has to bias the radar either for longer range detection, to shorter range tracking and discrimination.

Do you mean wavelength (rather than frequency) long enough to reduce atmospheric attenuation?

Also as radio wave is EM wave, which travels at the speed of light, I expect the time between sending and receiving a pulse is actually very short (unlike sound) and it should be good enough to measure both the location and the velocity of the object fairly accurately over a range of several hundred km?

 

[crobato]

Do you mean wavelength (rather than frequency) long enough to reduce atmospheric attenuation?

Also as radio wave is EM wave, which travels at the speed of light, I expect the time between sending and receiving a pulse is actually very short (unlike sound) and it should be good enough to measure both the location and the velocity of the object fairly accurately over a range of several hundred km?

Location yes, velocity no. But you don't need very accurate velocity readings anyway, you have to let a higher resolution PRF & target tracking radar for that. You need very precise and ongoing velocity figures for missile engagement which is ultimately done with CW systems rather than pulse.

You have primary radar systems intended for detection, then followed by secondary radar systems used for targeting & tracking. The primary radars cue secondary radars to the detected target.

 

[williamhou]

Location yes, velocity no. But you don't need very accurate velocity readings anyway, you have to let a higher resolution PRF & target tracking radar for that. You need very precise and ongoing velocity figures for missile engagement which is ultimately done with CW systems rather than pulse.

You have primary radar systems intended for detection, then followed by secondary radar systems used for targeting & tracking. The primary radars cue secondary radars to the detected target.

Is it always the case to use one pulse and one CW radar? If the detection range is short enough (maybe not AWACS), would it be possible to search and track using one pulse radar? If it is what range would that be?

Also for surface targets, would pulse radar alone be good enough to do the search / tracking as surface targets move much slower?

 

[crobato]

Is it always the case to use one pulse and one CW radar? If the detection range is short enough (maybe not AWACS), would it be possible to search and track using one pulse radar? If it is what range would that be?

Also for surface targets, would pulse radar alone be good enough to do the search / tracking as surface targets move much slower?

Its nearly always been the case where you have a detection-pulse radar supported by a fire control-CW radar. The examples are consistent from ships to aircraft to SAM batteries.

In active seeking missiles, fire control-CW radar becomes the missile seeker itself. Active seekers are found in AAMs and antiship missiles, and gradually into SAMs.

With active missiles against ships and aircraft, search pulse radar helps brings the missile within the range radius of the missile's seeker, which finishes the remaining journey.