# How does a spacecraft know that it is in orbit?

After a 5 year long journey in space, Juno finally reached and started orbiting Jupiter. How does the probe actually know that it is in orbit, so that it can send confirmation message like 'Welcome to Jupiter!' ?

For the purposes of this question, please treat "know" in the usual colloquial sense that we use for computers. That is, the question could be phrased: "What triggers the Juno probe to send the confirmation message?" Consciousness is not required.

• Sadly, at this time (2016-07-05 19:30Z) there 4 contradictory answers from 4 high-rep users. How am I, an interested reader, to know whether the accepted answer is the correct one? Or if any of them are? Especially since none of the answers links to a reference specific to Juno... – davidbak Jul 5 '16 at 19:32
• @davidbak -- The answer may well be unknowable.I generally know how to poke, but the JPL and Lockheed Martin web sites appear to be in a poke-free zone. Specifically, an export controlled and proprietary poke-free zone. See page 4 of this presentation: wiki.sei.cmu.edu/aadl/images/a/a5/Juno_project_112009.pdf. We need someone who knows the details of how the spacecraft's GNC and mission management software work, but that someone may well not be allowed to answer the question because of the export-controlled / proprietary nature of the answer. – David Hammen Jul 6 '16 at 0:29
• Given that there is a communication delay of 48 minutes each way, it would be impossible for the probe not to have some autonomy. If mission control on Earth needed to send a signal to say "you're in orbit now, stop engines", the probe would react an hour and a half too late. Colloquially, a system which autonomously performs actions based on sensed input is often referred to as "deciding" to act based on "what it knows", without actually implying sentience. – IMSoP Jul 6 '16 at 15:30
• I have cleaned up all comments related to whether or not the probe has knowledge. Please discontinue arguing over the definition of knowledge. I have added a note to the question for clarity. We do not need to belittle the OP or anyone else over this complicated matter. – called2voyage Jul 6 '16 at 20:28
• @davidbak The answers aren't contradictory. Each contains part of the story: the detection done on Earth, the sensors used for attitude and acceleration determination, and the software on Juno to interpret the data. All that is missing is whether Juno had a program able to calculate whether it was in orbit and switch modes in response, or all commands come from Earth, which is part of the information not released to the public. – kim holder Jul 7 '16 at 14:27

The Juno spacecraft has no means to directly measure and compute that it is in orbit. It did not send any such confirmation message. All it sent was an FSK tone indicating that it had completed the activities it was commanded to do. After the spacecraft turned back to Earth, it transmitted all of the recorded engineering data from the event, providing much more information on how it had completed the activities it was commanded to do.

The spacecraft can determine its attitude, but that gives no information whatsoever about its trajectory. The only way that, in theory, Juno could determine it was in orbit on its own would be to use the outreach camera to observe Jupiter periodically after JOI and see its position against reference stars, comparing that to a prediction of what it would see if it were in orbit vs. not. However the difference between those is initially small, so it might take a few hours or days to make the determination. That capability has been developed at JPL, called AutoNav, but it is not being used by Juno.

Juno could infer onboard that it is in orbit by integrating the accelerometer readings. But that is not a direct orbit determination.

The most immediate way that we knew Juno was in orbit was the two-way Doppler signature. We knew the trajectory of Juno approaching the planet, and from that what the change in the Doppler shift of the X-band signal along the line of sight to Earth would look like during a successful orbit insertion burn. We could then look for that signature in real time. Lo and behold, there it was.

The Juno trajectory was designed so that the spacecraft would be in view of Earth through the entire burn. (It's quite common for the spacecraft to go behind the planet as seen from Earth for part of an orbit insertion burn.)

Two-way Doppler works by sending a very precise frequency derived from an atomic clock on Earth to the spacecraft, and having the spacecraft turn that frequency around with coherent phase, where that frequency is multiplied by an exact rational number (usually 880/749) for the downlink. The signal received on Earth is converted appropriately and beat against the same atomic clock to get the Doppler shift. This can measure the velocity component of the spacecraft relative to Earth along the line of sight to Earth to within a few millimeters per second. Two-way Doppler can be done with just the carrier, so no data needs to be carried on the link. This allows for Doppler tracking support with relatively low signal strength from the spacecraft.

• What's the significance of 880/749? Is it just a convention, or is there some reason to prefer that over other multipliers? – Kevin Jul 5 '16 at 21:28
• It's a convention to allocate the X-band spectrum. See deepspace.jpl.nasa.gov/dsndocs/810-005/201/201B.pdf – Mark Adler Jul 5 '16 at 21:34
• Well, the difference in "knowing" here is: NASA sending the craft "You are at x1 with v1, we want you to perform optimal burn to land on x2, with v2" and the craft developing the burn, vs "perform a burn of t seconds, at attitude a." - if the current orbital elements vector is in RAM of the probe, it can be said to "know" it. – SF. Jul 5 '16 at 22:11
• All the spacecraft can say is that it completed the activities that it was commanded to do, and there were no unexpected events. There is no message that claims "I am now in orbit", just "I did what you said to do." However as I outlined in the answer, the navigators on the Earth can determine that it is in orbit from the Doppler shift of the radio signal. – Mark Adler Jul 6 '16 at 21:59
• Is it possible to plot a trajectory through incremental reports of the altitude for instance taking 10 snapshots of the altitude over an hour? Can a probe calculate how far it is away from the orbit's perapsis using methods like that? Or is all of that programmed as part of the program logic beforehand by physicists? Also, did you typo altitude as attitude? Or is attitude different from altitude? – Magic Octopus Urn Jun 29 '18 at 20:10

Using attitude determination devices, (including doppler shift of radio signal from Earth), it can determine* its location and velocity relative to Jupiter, and from that data, and knowing Jupiter mass, trajectory can be calculated. If the trajectory forms a loop around Jupiter - it's an orbit!

* the actual determination is performed on Earth, Juno just bounces a signal from Earth back, and sends telemetry from the rest of own instruments.

• @JoeBlow: Mark explains, that while very precise, this method only gives the velocity component of the spacecraft relative to Earth along the line of sight to Earth, that is only one of three velocity vector coordinates. The rest must be derived using other instruments; position too (at this distance, while we can very precisely determine the distance we have no chance of determining two remaining components of position within any sensible accuracy, without Juno locally determining them). So, the radio gives us only 2 out of 6 orbital elements. The rest comes from other instruments. – SF. Jul 5 '16 at 19:55
• This answer has a big problem: No one has said which navigation sensors Juno used. It certainly had accelerometers and a star tracker, and it may have an optical navigation sensor. What else it had at its disposal is hidden in the non-technical nature of JPL's public websites. But I did not down vote. – David Hammen Jul 6 '16 at 23:38

First a clarification. If one insists that "knowing" requires self awareness and intelligence, then the Juno spacecraft of course doesn't "know" anything. Rather than getting hung up on the silliness of what "knowing" means, it's better to look for an alternative way of answering the question. That alternative: How sophisticated is Juno's onboard computer software? Did the spacecraft software know, in some limited way, that it had indeed achieved orbit?

I'm making this community wiki because this is not an answer. The correct answer is very hard to find. (As far as I can tell, none of the proferred answers are correct.) Both JPL and Lockheed Martin release very little, if any, technical details on their vehicles' inner workings. The vehicle's guidance, navigation, and control systems and mission managers are apparently stamped as ITAR restricted and as proprietary. I'll look at two cases, one in which the vehicle is not aware that it is in orbit about Jupiter and the other in which it is.

One way that the vehicle could have performed the orbit insertion burn autonomously would have been to orient the vehicle in a predetermined orientation, start the rocket, and stop when delta V to go (the difference between a predetermined desired delta V and accumulated sensed delta V) reached zero. Suppose both the predetermined orientation and predetermined desired delta V had been a part of a command sequence sent to the spacecraft from Earth. If this is the case, the vehicle did not know it was in orbit. All it knew was that the burn was complete. The software to do this is very simple. If simple software is good enough to do the job, simple is best.

A good deal more sophistication might be needed if that simple approach would not suffice. For example, the spacecraft might need more sophisticated guidance, navigation, and control software, and more complex mission management software. (It apparently has rather extensive failure detection, isolation, and recovery software). In this more sophisticated version, the mission manager may well have a on-orbit mode, the transition to which is triggered by the GNC software assessing that the desired orbit has been achieved. While this is not self-aware software, it is software that is aware that the vehicle is indeed in orbit about Jupiter.

Note that Juno is controlled by a 200 megahertz RAD750 computer with 128 megabytes of memory and 256 megabytes of flash storage. That's the equivalent of a low cost personal computer from 1999. There's not much room in that rather limited system for very much sophistication.

• Your last paragraph inspires a question at movies.stackexchange.com/questions/56416/… – Criggie Jul 6 '16 at 3:55
• "There's not much room in that rather limited system for very much sophistication." Beg to differ. The PC from 1999 would have been running an OS with a GUI and doing loads and loads of things for the user, with software of very bad quality. The spaceship program surely is optimized as good as humanly possible and very, very single-minded / focused (compared to a general use PC). A single-minded program can do a lot with that amount of computing power and memory. – AnoE Jul 6 '16 at 10:24
• You are correct. We might not be able to ascertain the answer due to export restrictions. – called2voyage Jul 6 '16 at 20:21
• @called2voyage - Where can I find the export restrictions for stuff you're exporting to the outer solar system? They're probably interesting reading. – davidbak Jul 6 '16 at 22:59
• @davidbak -- If some other nation state sent a vehicle to capture Juno and reverse engineer its flight software, that would essentially be an act of war. For example, see Sea Hunt, Inc. v. Unidentified, Shipwrecked Vessel or Vessels. To avoid having to declare war with Spain, the treasure found by Sea Hunt, Inc. had to be relinquished to Spain. Note also that one of the vessels found by Sea Hunt was the Juno. – David Hammen Jul 7 '16 at 0:05

The duration of the orbital insertion burn was controlled by an accelerometer on the spacecraft. Mission control knew the spacecraft's speed at entry and the desired speed needed for the planned orbit. While the engine is running, the spacecraft measures its deceleration rate over time, and when the total change in speed reaches the necessary figure, it shuts off the engine.

The confirmation message in this case was sent when the probe finished the insertion burn, and mission control is able to confirm the speed via Doppler shift as indicated in other answers here.

• integrating the acceleration is not sufficient to measure change in speed. You need at least a gyro scope on top of that. And even then, any imprecision in the accelerometer is integrated, and compounded. – njzk2 Jul 6 '16 at 14:19

The probe doesn't know, really, unless it is told so by Earth. The way it can be tested is by finding its location from Earth, tracking its path, and determine if it is in fact in orbit. This is explained in great detail in the Basics of Space Flight—Navigation. The short of it is, they use two techniques: ranging, where a pulse is sent and responded to immediately, giving the distance; and doppler shift, showing the relative speed between the spacecraft and the Earth. These two, with enough measurements, will allow one to determine the orbit of an object.

It should be noted what the spacecraft was able to do by itself. What it can do is tell the change in velocity, assuming the instruments are correctly calibrated to do so on board. That is what the spacecraft was looking for, or a time of the burn, either of which would allow it to know that it had hit the correct burn. However, if the aim was wrong, the instrumentation wrong, or something similar, the burn might not have been successful. Only measurements from Earth could really let the system know that it had been successful in its capture burn.

• This cannot be correct. Juno had to perform its JOI burn last night autonomously: the burn duration was shorter than the light-distance between here and Jupiter, so it was over before Earth even heard it had begun. Juno's computer determined when it was in the proper orbit (1 sec. difference from the predicted burn time) and shut off at the correct moment, without any input from Earth in the meantime. – Jacob Krall Jul 5 '16 at 18:30
• Hi Jacob - the journey to Jupiter took forever. Sure, it was able to perform the particular operation you speak of autonomously. But there's no sense in which it "flew the whole flight itself, and 'knew when it go to Jupiter'". You see? – Fattie Jul 5 '16 at 19:31
• @JacobKrall I added a few details that discuss what the spacecraft was actually capable of doing. It knew the burn time or change in velocity, but while it could infer everything was correct, only measurements from Earth could confirm that. – PearsonArtPhoto Jul 5 '16 at 20:36
• @PearsonArtPhoto -- A well-designed Kalman filter can detect errors such as navigation sensor biases and misalignments. I haven't the foggiest how sophisticated Juno's flight software is. We're all just guessing. – David Hammen Jul 5 '16 at 21:43
• @JoeBlow Neither the question, nor Jacob's comment, assert that Juno flew autonomously the whole way to Jupiter. Both discuss the fact that the probe performed an autonomous insertion into orbit, and must therefore have had some defined end to that maneuvre. This might have been as simple as "after X seconds, stop engines", which would translate colloquially to "assuming it was in orbit"; or it might be "when condition X is met, stop engines", colloquially "knowing it was in orbit". – IMSoP Jul 6 '16 at 15:35

The Juno spacecraft uses a combination of techniques to determine its location and trajectory. Onboard systems, known as "star trackers" help the spacecraft to determine its orientation in space. Combined with positioning data determined from radio signal analysis conducted on Earth, the vehicle can "triangulate" its position along its trajectory, and calculate deviations. Once the spacecraft knows its actual current trajectory, it can perform calculations to determine what thrust vectors to apply (using its various engines) to achieve the correct course. In this case, that would be orbital insertion. Please see this article for more information.

• Juno doesn't do those calculations, they are done on Earth. – kim holder Jul 7 '16 at 14:46
• Some calculations are done on Earth, but the final integration of acceleration, vector, and thrust data must occur in real time, and is performed by the spacecraft itself. – Byron Jones Jul 7 '16 at 15:40

It's all about gyroscope. If you don't have your physics sharp, you might have to put your imagination to work a little :-)
It is quite a simple process, and it is all done autonomously (has to be: it takes 48 minutes for a signal to reach Juno from Earth, and, not surprisingly, the same amount of time from Earth to Juno. So 48+48 = DISASTER!!!).

1 - You have Jupiter's gravity component. It is pulling Juno towards it. Let's call this the down-force;
2 - To escape that down-force, Juno has to accelerate forward so fast that it will eventually do escape, if he keeps on accelerating;
3 - The gyroscope actually acuses (reads) that acceleration downwards and sets the propulsion rockets to the forward direction until the downward acceleration is equal to zero. This is called free fall.

These forward rockets have to be set on from time to time (gyroscope sets that - autonomously) because, despite what you might have heard, there is friction (little, but there is) that will slow down the forward speed of Juno when in orbit, because of Jupiter's atmosphere.

Don't confuse acceleration with speed.
Imagine you in your car. When you step on the gas, you accelerate. You feel the pressure of your back to the car's seat. When you get to the desired speed, you stop accelerating (no more back pressure), and just maintain the gas pedal pressed enough to keep that speed (to overcome friction and other - but no more acceleration).

If your imagination is good enough, think that, for you to walk you have to lean your body forward to start falling. To prevent smashing your face on the floor, you stretch your leg forward and put forward speed, until your acceleration is zero and you are enjoying that walk in the park.

Nice site about Juno, with lots of videos and nerd facts: http://spaceflight101.com/juno/juno-mission-trajectory-design/

• Gyros does not measure acceleration, it measures relative angles. What measure acceleration is a accelerometer. Also having a latency between juno an earth doesnt make it need to be autonomous. Remember that the distances are very big and usually takes time to something wrong happen. You can perfectly estimate the stuff and take decisions based on 48min time lag. Also it is very hard to the probe calculate itself velocity to know if it is in orbit, thats why they usually uses a Dopler Radar from earth to probe to measure their speed. – Lucas Teske Jul 6 '16 at 18:35
• Ok.... so if you measure how fast your angle changes in a period of time, could you derive your acceleration? If you don't know how to answer that, do your research before posting trash. – user3293101 Jul 7 '16 at 18:15
• And I am still laughing here about the Dopler Radar from earth. I can hardly imagine those sound waves travelling in space... – user3293101 Jul 7 '16 at 18:16
• Quanta asneira numa resposta só. – user3293101 Jul 7 '16 at 18:19
• user3293101 you can derive your angular acceleration (actually, electronic gyros measure angular velocity, that you can integrate and find the angles). So you can only find how fast the spaceship is turning around its axis. You cannot find acceleration just by having the gyros info. – Lucas Teske Jul 7 '16 at 20:31