# How does NASA determine the position of their probes?

I'm currently following New Horizons visit to Pluto. I'm just wondering how does the Probe/NASA determine the Position of the Probe? I would assume using earth's GPS wouldn't have the necessary angular resolution if you're billions of km far away.

They flew billions of km and missed by a mere 70km.

What is the accuracy of the positioning system?

• We have very detailed and accurate models of objects in our Solar system. The boys over at NASA also do all calculations for trajectory, delta-v, and flight paths. They do this with a painful amount of precision. Then they calculate exactly how much the spacecraft weighs after each maneuver and how it is pointing. Then they say "go, spacecraft. Do your thing" and make estimates based on how everything performs. The spacecraft also keeps a rough idea of where it is and how it is oriented using various instruments and data....
– Jim
Jul 14 '15 at 16:16
• they then get updates from the s/c and make estimates how far off they are from the flight path in terms of distance, angle, and speed. These can be adjusted en route. Once at the destination, all data collected is sent back and every little thing from the size of Pluto to how long it takes to receive the message indicates where it is. This, plus the path time, length, and various maneuvers gives them a good estimate of where it is.
– Jim
Jul 14 '15 at 16:18
• It's not rocket science.
– Jim
Jul 14 '15 at 16:18
• "All you need is a space ship and a star to steer her by". Well, sort of... www2.jpl.nasa.gov/basics/bsf13-1.php Jul 14 '15 at 16:24
• Would Space Exploration be a better home for this question? Jul 14 '15 at 17:08

I'm just wondering how does the Probe/NASA determine the Position of the Probe?

There are several things that can be directly measured from Earth, as discussed in detail at https://solarsystem.nasa.gov/basics/bsf13-1.php. I describe these briefly below.

1. radial velocity toward/away from Earth: using the Doppler-shift of a well known downlink carrier signal
2. distance from Earth: using a ranging pulse encoded into the uplink signal and a known turnaround time within the spacecraft electronics plus other known delays, one can determine the $\Delta t$ (i.e., time delay) of the travel to/from the spacecraft
3. angular position in sky: using Doppler effects from the Deep Space Network (DSN) stations for low-accuracy values and Very-long-baseline interferometry for more accurate measurements

Using multiple ground stations, we can improve the distance/range, position, and velocity estimates. The uncertainties are generally driven by financial limitations and/or mission necessity (e.g., some spacecraft science objectives do not require that we know their position to better than ~100 km). Some spacecraft can take advantage of the Global Positioning System (GPS), in the same way one would triangulate their position on Earth, if the spacecraft is below the GPS orbit (i.e., ~20,200 km altitude). GPS trackers on spacecraft can be expensive and it can sometimes be easier/cheaper to determine attitude information from the Near Earth Network using similar techniques to those described above using the DSN.

What is the accuracy of the positioning system?

This depends upon the mission and goals/requirements for attitude control. Onboard star and sun sensors provide the spacecraft orientation, generally to less than a degree (sometimes to much smaller tolerances if necessary, e.g., space telescopes often require very high accuracy). The velocities can be determined down to mm/s (or less if necessary) and the position can be known down to less than one kilometer. For instance, Cassini's orbital position must be very well known ($\leq1 \ km$) due to the large amount of orbital debris around Saturn.

Other spacecraft in less hazardous environments require less accurate attitude information. For instance, the Wind spacecraft is currently orbiting the $L_{1}$ point and its position need not be known to better than 10s of km.

They flew billions of km and missed by a mere 70km.

I am not sure from where the distance 70 km arose because New Horizons was roughly ~7800 km from Pluto and ~28,800 km from Charon at closest approach.