# How does an onboard atomic clock help interplanetary navigation?

On 2019 June 22, JPL will launch and test the accuracy of an onboard atomic clock. Its FAQ says that it will improve interplanetary navigation because for the ship to determine its position, instead of measuring how long it takes for a radio signal to go from Earth to ship and back, now a one-way signal suffices. Halving a multi-hour duration is obviously helpful.

But how can just one signal determine the ship's location? Doesn't that just say how far the ship is from Earth? Or are these clocks so accurate that measuring the ship's distances to, say, Arecibo, Australia, and Greenwich is enough to triangulate its position during an orbit insertion burn to Enceladus?

• If the satellite knows its orientation (star trackers) and signal direction (communications pointing systems), then the signal delay gives a distance, which places it in a known position relative to the source.
– user20636
Jun 9, 2019 at 21:37

But how can just one signal determine the ship's location? Doesn't that just say how far the ship is from Earth?

tl;dr: The atomic clock increases the precision of the measurement and could even enable one-way orbit determination, but navigation is still done with some combination of range, range-rate, Delta-DOR and calculation of gravitational effects using numerical integration, solar system ephemerides, and other force models relevant to the spacecraft trajectory.

Navigation of spacecraft in deep space is done with several tools. Three of the main ones are:

Range-Rate is a measurement of the frequency shifts from signals broadcast from one location on Earth and received by the spacecraft (range-rate as in the rate of change of the range, expressed in units of range divided by a time, typically km/s). It is most commonly done in a two-way fashion whereby the spacecraft will return the same signal in a frequency-coherent way and received back on Earth. An atomic clock enables one-way range-rate measurements because the frequency of the signal generated onboard the spacecraft is extremely precise, and therefore, the measurement of Doppler shift is most entirely due to the relative velocity of the spacecraft compared to the ground station, instead of being due to a poor signal generation onboard the spacecraft. Similarly, an excellent onboard clock allows the spacecraft to precisely discriminate the ranging tones sent from the ground station: this means a very precise reading of the time of flight information. This October 2020 paper details how an onboard atomic clock helps for orbit determination.

A range-rate measurement is often combined with a range measurement, which is the time of flight of a signal. The combination of range and range-rate measurement is often called delay-Doppler in radar systems. You get a range from the delay, and a rate-of-change of range (a 1 dimensional velocity) from the Doppler shift.

Delta-DOR: is a one-way measurement of signals broadcast from a spacecraft and received by two widely separated deep-space ground stations on earth and the difference in the times of signal arrival is precisely measured (and used to calculate a bearing). This is corrected using information about the current delays due to Earth’s atmosphere, obtained by simultaneously tracking (from each ground location) radio signals from a quasar (within 10 degrees of the same direction).

Numerical Integration In spacecraft navigation you would use the above two methods plus extremely accurate simulations of the history of the spacecraft's trajectory based on all known forces, gravitational and otherwise, to build a complete picture of the trajectory of the spacecraft. Masses and positions of all large solar system objects already exist as ephemerides and these are inputs to such a calculation.

So

• Range-Rate gives a distance and speed without an accurate direction
• Delta-DOR gives an accurate direction
• Numerical integration augments these two and completes the picture, providing more accuracy than possible with the first two alone

How does an onboard atomic clock help interplanetary navigation?

@DavidHammen's excellent answer explains how having an atomic clock aboard a spacecraft allows the range-rate measurement to be done one-way instead of two way. I recommend you read the full answer, but briefly, the spacecraft broadcasts a signal encoded with time information from the atomic clock. These signals are received on Earth and compared to known time here, and the difference, once corrected for relativistic effects gives the distance. Doppler shift of the time signals or the frequency (if locked to the atomic clock) gives the rate or relative velocity.

• Just jumping in to correct that range-rage (also called a Doppler measurement, I've never seen "delay Doppler") only does a Doppler shift measurement from the ground: the ground station knows what frequency it sent, knows the expected return frequency from the SC, and then measures the frequency shift. This provides information on position and velocity of the spacecraft (via the measurement sensitivity matrix). Oct 1, 2021 at 15:45
• @ChrisR can you help me find exactly what part needs a correction, or is it that there's an omission? To me it seems we've said the same thing. The signal is from Earth but requires two-way signal propagation. If the spacecraft had an atomic clock then a one-way signal propagation would be sufficient for the spacecraft to obtain tracking information. It could then inform the ground of the results by normal telemetry. Delay doppler is an older radar term for the same idea that's called range-rate by space folks.
– uhoh
Oct 1, 2021 at 17:55
• Your question is exactly what I was trying to address, so I'll address it there. Oct 1, 2021 at 20:36
• @ChrisR Thank you for your answer there! Now back to here; I am not sure what needs to be corrected. If you like, please feel free to just edit this to make it right. Or you can help by suggesting which sentence needs fixing. Thanks!
– uhoh
Oct 3, 2021 at 23:34
• I didn't figure out how to suggest an edit, so I had to edit your answer directly. Please feel free to edit it again for clarity and style. Oct 4, 2021 at 5:02