How can Voyager send a signal strong enough for us to receive, in spite of its enormous distance from us? And how can it have the power to do so more than 20 years after its launch?
It’s better to reverse the question and ask not how Voyager can send a signal strong enough, but how Earth can receive a signal so weak. The signal that leaves Voyager is only about 6 times as powerful as the signal from the average cell phone. It is much, much weaker when it gets to Earth.
NASA maintains what is called the Deep Space Network (DSN). It is composed of antenna sites in California, Spain, and Australia (each 120 degrees of longitude apart so that the the full Earth is covered). At these sites are giant dish antennae, ranging from 85 feet to 230 feet across. The antennae are parabolic (bowl shaped) – meaning they are designed so that every signal that hits them is reflected off of the curve towards the center where they are combined, making the signal stronger.
NASA维护着所谓的深空网络（ Deep Space Network ，DSN）。它由位于加利福尼亚，西班牙和澳大利亚的天线站点组成（彼此相隔120度经度，以便覆盖整个地球）。在这些站点上是巨大的天线，范围从85英尺到230英尺。天线是抛物线形的（碗形）-意味着它们的设计使得击中它们的每个信号都从弯曲处反射到组合它们的中心，从而使信号更强。
These giant antennae manage to pick up the weakest of signals. In fact, it is believed that the antennae could continue to receive Voyager signals even if it traveled for another century (although we expect Voyager’s power supplies to be too weak by about 2025). Voyager is powered by heat produced from radioactive decay. When it left Earth, that system had an output of about 450 watts. Today it is down to less than 250 watts.
When Voyager transmits a signal, from its high gain antenna that is pointed at Earth, the signal power is at about 19 watts, but it becomes weaker and weaker as it travels about 16 hours at the speed of light, to reach Earth. By the time it hits one of the DSN antenna it is at about 10E-16 watts. Fortunately Voyager transmits at a very high frequency (x-band 8.4 GHz) which is not commonly used for terrestrial purposes, so the signal doesn’t get lost in the clutter from things like cell phones, TV, and GPS.
当旅行者号从指向地球的高增益天线发射信号时，信号功率约为19瓦，但是当它以光速传播约16小时到达地球时，信号功率变得越来越弱。当它碰到DSN天线之一时，它的功率约为10E-16瓦。幸运的是， 旅行者以很高的频率（x波段8.4 GHz）进行传输，该频率通常不用于地球上其他的用途，因此信号不会因手机，电视和GPS等杂物的干扰而丢失。
The DSN is unfortunately underfunded and much of the equipment is very old, so outages sometimes occur and we miss data. Also rain bouncing on the dish is enough to cause us to lose the signal.
Better to show a picture of a 70 meter antenna instead of a 34 meter, since the 70′s are what are used exclusively to communicate with Voyager nowadays. This is good one, showing what it would look like in the Rose Bowl:
Is it true that we only need to worry about terrestrial interference at that frequency band? Or does that band frequently contain random signals from outer space that we need to carefully sift through?
There isn’t much interference of any kind at 8 GHz
How do they choose 8Ghz only?is there any way using which they find that the best for the purpose?
That’s the frequency allotted to them. Authorities on the ground, completely separate from NASA, regulate the communications spectrum, allotting frequencies to television, radio, cell phones, aircraft, and spacecraft.
But more the frequency, less it’s convenient for longer distance, right??
In space, there’s nothing to actually absorb your signal; it simply spreads over a larger area, requiring a bigger antenna to intercept the same fraction of transmitted power. This is the famous “inverse square law”. It comes from the fact that the area of a sphere increases with the square of its radius.
在太空中，没有什么东西可以真正吸收您的信号。 信号只是散布在更大的区域，需要更大的天线来截取相同比例的发射功率。 这就是著名的“平方反比定律”。 这是由于一个球体的面积随其半径的平方而增加。
If the spacecraft transmits uniformly in all directions (an “isotropic” radiator) and is received by a dish antenna of a given size, the fraction of transmitted power intercepted by the dish does not change with frequency. But if the spacecraft uses a dish (and they almost always do), it can more easily focus its signal on the earth at higher frequencies, wasting less on deep space. So things actually get *better* as you go up in frequency. Also, the higher frequency bands are usually wider. These two properties combine to allow faster communications with the same power and antennas.
如果航天器在各个方向（“全向”发射器）均匀地发射，并被给定尺寸的碟形天线接收，则碟形天线截取的发射功率不会随频率变化。 但是，如果航天器使用碟形天线（并且它们几乎总是这样做），它就可以更轻松地将信号以较高的频率聚焦在地球上，从而减少了在深空的浪费。 因此，随着频率的增加，事情实际上会变得“更好”。 而且，较高的频带通常较宽。 这两个属性相结合，可以在相同的功率和天线下实现更快的通信。
But there is a fly in the ointment. The signals have to travel through the earth’s atmosphere, which contains two things relevant to microwave propagation: water vapor and oxygen gas. Water does not actually resonate in the microwave spectrum (this is a common misconception about microwave ovens) but as a polar molecule it does absorb radio signals more effectively as you go up in frequency. This can cause higher frequency signals (e.g, on the 8.4 and especially the 32 GHz bands) to fade out in the rain before those on the lower “S-band” at 2.2 GHz. That’s why the deep space network sites are generally in dry deserts.
但是总会有美中不足的。 信号必须通过地球大气传播，其中包含与微波传播有关的两件事：水蒸气和氧气。 水实际上不会在微波频谱中产生共振（这是对微波炉的常见误解），但作为极性分子，当您提高频率时，它确实可以更有效地吸收无线电信号。 这可能会导致较高频率的信号（例如，在8.4频段，尤其是在32 GHz频段）在雨中逐渐淡出，而在2.2 GHz的较低“ S频段”上则逐渐消失。 这就是为什么深空网络站点通常在干旱的沙漠中。
Also, oxygen is an extremely strong absorber at 60 GHz and 120 GHz making those frequencies useless for space/earth communications. But they’re very useful for links between spacecraft that never have to pass through the atmosphere.
而且，氧气在60 GHz和120 GHz时是一种极强的吸收剂，因此这些频率对空间/地球通信毫无用处。 但是，它们对于永不穿过大气层的航天器之间的通信非常有用。
Deep space communications is allocated dedicated frequency bands by the Federal Communications Commission (FCC) in the US, which works with the International Telecommunications Union (ITU) to coordinate their use internationally. It’s a “radio service” just like FM or TV broadcasting, ham radio or police radios. Some of the bands are allocated for near earth satellites and others (like Voyager’s signals) are in bands reserved for deep space. There are also separate frequency bands used for command uplinks to these same spacecraft. Other uses are not allowed to avoid interference.
NASA coordinates with the space agencies in other countries to pick specific frequencies for each spacecraft. Because the ground antennas have sharp beams, interference between two spacecraft on the same frequency is very unlikely unless they happen to be orbiting the same planet (e.g., Mars).
The 1–10 GHz range is ideal for space communications. The atmosphere is largely transparent (though heavy rain can cause problems) and the background noise from space is relatively low.
The higher you go, the more easily you can focus your radio beams on the receiver and the more radio bandwidth is available. That means higher data rates, so the long-term trend has been to move up in frequency when the technology permits. Voyager was one of the first spacecraft to use the 8.4 GHz “X” band operationally. Before then, the 2.2 GHz “S” band was the main workhorse. S-band is still very heavily used, mainly by spacecraft closer to earth. The Apollo spacecraft used a “Unified S-band” (USB) system for all communications with the ground beyond earth orbit.
频率越高，就越容易将无线电聚焦在接收器上，并且可用的无线电带宽越多。这意味着更高的数据速率，因此长期趋势是在技术允许的情况下提高频率。旅行者号是最早使用8.4 GHz“ X”频段的航天器之一。在此之前，2.2 GHz“ S”频段是主要力量。 S波段仍被大量使用，主要是更靠近地球的航天器使用。阿波罗号飞船使用“统一S波段”（ Unified S-band ，USB）系统与地球轨道以外的地面进行所有通信。
The trend continues today with more interplanetary spacecraft using “Ka” band at around 32 GHz. Rain absorption gets worse as you go up in frequency, but the advantages can still outweigh the disadvantages, especially when sending bulk prerecorded data that can be resent if not received the first time.
今天，这种趋势继续发展，越来越多的行星际航天器使用32 GHz附近的“ Ka”频段。随着频率的增加，雨水吸收的影响会越来越大，但是高频段的优势仍然大于劣势，尤其是在发送大量已经预先记录下的数据时，如果第一次接收不完则可以重新发送。