# What's stopping space probe communication from being jammed?

Space probes use radio waves to communicate back to Earth - I would imagine a probe sent many decades ago, using very little energy and from very far away would be sending an incredibly weak signal back to Earth.

Just wondering, wouldn't it be easy for anyone on Earth to broadcast a signal in the same frequency and jam the probe signal (whether unintentionally or with intent).

• Mostly location and the frequencies that probes use don't bounce off ionosphere (for obvious reasons) for the skywave approach to jamming to work. Then there's also legal reasons and such methods would basically be an act of war / terrorism / sabotage / pick any scary opsec word. But it's of course technically achievable (to jam, it wouldn't be so easy to do a man in the middle attack). Just re-transmit the same message half a turn (180°) out of phase and it will cancel the transmitted message out exactly. Apr 6 '15 at 11:46
– mins
Apr 6 '15 at 11:49
• – user
Apr 7 '15 at 11:11

A broadcast from a random location won't work. The radio antennae used at the Deep Space Network sites to send data to and receive data from deep space probes are highly directional. To jam the signal, you would have to transmit from a location that is between the receiving Deep Space Network antenna and the deep space probe.

That would mean using a drone, and it would need to be a rather sophisticated drone. The airspace around those three DSN sites are highly restricted. Fly a drone into that restricted airspace and you will be in a boatload of trouble.

• I'm wondering if sending a signal, with the appropriate Doppler shift, reflected by the ionosphere or the Moon, or even by an aircraft at a reasonable altitude would be successful. The signal from deep space is so weak that it is easy to compete with it.
– mins
Apr 6 '15 at 12:29
• @mins - Bouncing off the ionosphere won't work. The ionosphere has little effect on the frequencies used for deep space communications. Bouncing off aircraft won't work; aircraft avoid restricted airspaces (and the operators send NOTAMs (NOtices To Air Men) to avoid the areas in which the antennas will be tracking). Bouncing off the Moon won't work with the possible exception of probes sent to the Moon. Even then, the Moon's very low albedo would require sending a very powerful signal. There are only a few such antennae in the world. Amateur antennae wouldn't cut it. Apr 6 '15 at 12:45
• @mins - Yes, X band bounces off the troposphere at low angles of incidence. Reception doesn't begin until the target spacecraft is well above the horizon -- and then there's no using troposcatter to interfere with the received signal. Moon bounce is also irrelevant unless the spacecraft happens to be very close to the Moon in the sky, so close that the Moon is within one of the antenna's lobes. Finally, receiving and interfering with reception are very different concepts. Apr 6 '15 at 15:11
• No, not buying it. The signal level being received from some spacecraft is so incredibly low (e.g. the Voyagers), that I'm sure it could be jammed with a strong, entirely off-axis signal at the appropriate frequency aimed at the antenna from the side. Every parabolic dish antenna pattern has noticeable side lobes. They are several orders of magnitude down, but since the signal being received is so weak, you need very little power into the antenna to swamp that signal. Apr 6 '15 at 21:21
• Good! See @RossMillikan's answer below about the Voyager signal strength. That and your rough calculations validate my comment, showing that it would be quite easy to swamp the Voyager signal with only -80 db sidelobes. This answer is wrong. Ross Millikan's is right. Apr 6 '15 at 22:14

So, first of all, what does it take to send the signal in the first place? Each of the Deep Space Network sites has a 34m antenna that is typically used for inner solar system communications. The power is approximately 200- 400W, in a band where that much power is difficult to achieve. In order to jam the signal, you need to increase the noise significantly. Overall, it would be rather difficult to jam the signal going to the spacecraft. It could be done, but building dishes of the required size is a tricky matter, although I don't doubt that a dedicated source could make this happen, given the appropriate desire.

Okay, so what about the return signal? Well, that one is a bit easier, as the signal is rather weak on the ground. I don't have the full rf budget of a mission, but it is safe to assume it is weak. However, this is going to have some serious difficulties. First of all, the antenna is very directional, very little RF will get in to it except in the direct path of the antenna beam. In fact, from this paper, the feed horn, which is essentially the receiver, at 22 degrees off of the beam there is a 38 dB, and that is the best sidelobe! If the horn is properly shielded, as is likely being in the dish, then the side lobs drop down dramatically. Even if it is not properly shielded, while it would be possible to jam this signal if you can find the correct location to do it from, it would have to get pretty close to the direction the antenna is pointed to take advantage of that. In addition, the bands of interest are essentially line of site only. You would have to get close to the antenna to have any success, and the signal would lead directly to the device that was transmitting. It could theoretically be done, but it would be difficult to achieve.

Bottom line is, the most likely scenario for jamming would leave the guilty party quickly found. All spacecraft I'm aware of have a method to re-transmit lost data. It would be painful for NASA, but of limited damage, and the person would end up in jail rather quickly, no doubt.

• Sidelobe jamming is possible. Apr 6 '15 at 16:59
• It most certainly is, just difficult. I suspect the sidelobes for DSN are quite low, although I don't have any information for sure on that one. It would still have to be line of sight, which would make finding the jammer easy. Apr 6 '15 at 17:19
• I was able to find a NASA study for a 64m DSN antenna. Eyeballing the antenna pattern on page 11, I'd estimate the 1st sidelobes are ~38db at 22 degrees. (Assuming the spots around 12 and 16 degrees where the curve partially levels off aren't sidelobes that are suppressed out of existence anyway.) Apr 6 '15 at 18:00
• @DanNeely: Nice find. That is actually only for the feedhorn, however, and thus isn't likely to affect the entire system. I'll have to think about how that might affect the system as a whole. Hmmm... Apr 6 '15 at 18:12
• Whooops. Missed that wasn't the full picture. It's also uncertain how much the mid 80's upgrade of the 64m dishes to 70m was driven by a study from a decade ago. (No date on the pdf itself; but it's newest reference was published in 73). I had that in mind primarily as a representative system; even if not what NASA is actually operating. Apr 6 '15 at 18:41

Jamming the downlink signal is quite easy. It is extremely low in power, so if you just set up a signal source in the vicinity of the receive antenna, there will probably be enough off-axis gain that yours will be the stronger signal. This page gives the received signal level of Voyager 1 as $-150.7$ dBm. Even if you generate a few Watts at the proper frequency you will swamp this if the receive antenna has gain in your direction more than $-150$ dB or so. But would you want to?

• Jamming the uplink seems practically difficult due to the high ERP from DSN stations, but stations are not pointed 24x7 to a given spacecraft, so it seems quite possible to send something between DSN activity periods. That is rather frightening. But on the other hand, as software updates are possible after launch, I assume some security measures have been included, if not present since the beginning.
– mins
Apr 6 '15 at 15:14
• @mins: It would be very difficult to get enough EIRP to send a command-that is why the DSN has such large antennas. Not something an amateur can do. That is why I focused on the downlink. Apr 6 '15 at 15:25

Long time ago I studied in aerospace university of the former USSR and we briefly covered subject of space communications. The main reason was that those radios used a new technology of that time (~1985) called something like "broad spectrum communications below noise level". This was combination of encryption and radio transmission, so what you receive would be a broad spectrum white noise of very low level that is indistinguishable from regular noise. However, if you know the modulating sequence then you could combine the sequence with the received white noise and extract the signal. Mind you that this technology wasn't digital and was based on analog computing. The main advantages were that it was impossible to detect, impossible to jam and encryption was given. As far as I know the similar technology was used by cell phone network operators for very same reasons.

• Yes, spread spectrum modulation is much more difficult to jam, but I am not aware of any deep space spacecraft that uses it. Apr 7 '15 at 21:14

It is theoretically possible but would be tricky as NASA does not publish information such as the frequency in the public domain.

In addition, it is not a major concern as there is really no motive for anyone to want to jam the signal.

• Not in the public domain? How to receive Voyager 1 and 2, and many other.
– mins
Apr 6 '15 at 14:33
• Frequency is public, click e.g. on "more details" link on DSN Now after selecting any currently active dish. There also isn't any point in hiding the frequency, it's too easy to scan the lot nowadays. Plus, it has to be registered and approved. Apr 6 '15 at 15:43