# Measuring velocity and position in space **URL:** \ **Category:** Factual Questions\ **Created:** [May 21, 2012, 1:35am UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572 "2012-05-21T01:35:19Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [May 21, 2012, 1:35am UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/1 "2012-05-21T01:35:19Z")
I’m used to navigating terrestrial vehicles (cars), which have two fairly easy reference points for determining speed and position - tire rotation and in some cars, GPS. But watching Stargate Universe tonight got me wondering about how to determine these things in other contexts. 1. How did aircraft measure velocity and position pre-GPS? 2. How did orbital spacecraft pre-satellite, and how do they post-satellite? 3. I’m guessing this could potentially get recursive when we send up new satellites! How do satellites (or the first one up) whose purpose is telling other devices where they are determine where they themselves are? 4. How did/do spacecraft/probes navigate that travel past the range of terrestrial orbit? ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 21, 2012, 1:59am UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/2 "2012-05-21T01:59:33Z")
> [@jackdavinci](#): > > How did aircraft measure velocity and position pre-GPS? They used dead reckoning, various radio-based techniques (VOR, Tacan, etc.) and sometimes celestial and inertial navigation. > [@](#): > > How did orbital spacecraft pre-satellite… I’m guessing most orbital spacecraft were post-satellite. > [@](#): > > How do satellites (or the first one up) whose purpose is telling other devices where they are determine where they themselves are? In many cases (e.g. GPS) they are tracked by ground stations which measure and tell them where they are. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 21, 2012, 7:23pm UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/3 "2012-05-21T19:23:01Z")
> [@jackdavinci](#): > > 1. How did aircraft measure velocity and position pre-GPS? Aircraft used a combination of ground-based radio navigation (such as LORAN and previous systems), inertial navigation, and dead reckoning, combined with aircraft surveillance when over the continental United States and Europe. While flying standard routes with locator beacons (RNAV) this allowed for a good degree of precision, when flying over less regulated routes this could result in substantial navigation errors, the most infamous of which is Korean Air Lines flight 007, which was shot down over Soviet-controlled Sea of Japan when it went of course. “Strapdown” inertial navigation systems are still used with integrated GPS receivers on many commercial and most (US) military aircraft. > [@jackdavinci](#): > > 1. How did orbital spacecraft pre-satellite, and how do they post-satellite? Orbital spacecraft used ground-based tracking such as the Spacecraft Tracking and Data Acquisition Network (STADAN) plus inertial navigation, supplemented for some systems by celestial measurements. The NASA-operated Tracking and Data Relay Satellite System (TRDSS) is used today for most government launch systems and satellites. GPS and commercial SATCOM are generally used for commercial launch vehicles and spacecraft, again with supplementary integrated inertial navigation. Once the six orbital elements are determined to the desired precision, an orbit can be determined, and perturbances are due to small variances in the gravitational field are determined by Earth Gravitational Model 1996 (EGM96) and (for spacecraft in Low Earth Orbit) estimates of drag in the upper thermosphere. > [@jackdavinci](#): > > 1. I’m guessing this could potentially get recursive when we send up new satellites! How do satellites (or the first one up) whose purpose is telling other devices where they are determine where they themselves are? Ground tracking gives the orbital elements (eccentricity, semimajor axis, inclination, longitude of the ascending node, mean anomaly, argument of periapsis). The GPS satellites actually refine their positions based upon doppler measurements from each other was well as known ground tracking beacons. > [@jackdavinci](#): > > 1. How did/do spacecraft/probes navigate that travel past the range of terrestrial orbit? Historically, interplanetary spacecraft have primarily used ground tracking via the Deep Space Network (ground based) combined with inertial “dead reckoning” and sometimes celestial navigation. Because these facilities can only support a limited number of missions (and only function in line-of-sight of Earth), future efforts at higher frequency and at different trajectories may require a deployed satellite system analogous to GPS. I’ve seen several proposals for a Solar Positioning System-type concept (typically having spacecraft located in the Sol-Earth stable libration points) but there is currently no plan to deploy such a system. These are not trivial questions. What appears to be a simple trajectory plot on a viewgraph or animation is the result of hundreds of thousands of engineering man-hours of effort to develop and refine the systems that are used to track and determine position to a degree of precision that dwarfs any terrestrial navigation or positioning systems. ---
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [May 21, 2012, 7:38pm UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/4 "2012-05-21T19:38:55Z")
Great answers, thanks! What’s the vertical range of the ground based systems? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 21, 2012, 8:38pm UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/5 "2012-05-21T20:38:24Z")
> [@jackdavinci](#): > > Great answers, thanks! What’s the vertical range of the ground based systems? I’m not sure what you mean by “vertical range”. With enhancements, the DSN has maintained contact with the Voyager probes which are out at ~120 AU. As the probes are well beyond local gravitational perturbances their trajectories are close to being purely ballistic about the Sun, so measurement based upon “inertial” data from past known positions is pretty accurate. Stranger ---
**Author:** ![davidm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/davidm/32/225_2.png) [@davidm](https://boards.straightdope.com/u/davidm)\ **Post date:** [May 21, 2012, 8:54pm UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/6 "2012-05-21T20:54:12Z")
A related question. For spacecraft traveling beyond Earth orbit, what frame of reference is used to denote their position? Is it done with three coordinates? A vector? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 22, 2012, 4:25am UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/7 "2012-05-22T04:25:37Z")
The frame of referenced used to specify or simulate an interplanetary spacecraft position is generally a celestial reference frame with the origin at the solar system barycenter (center of rotation of the mass of the system) aligned to the ephemeris of the planets. However, measurements taken from the Earth are generally in an Earth-centered system and translated into a celestial system via coordinate transform. For fly-by maneuvers, the barycenter of the sphere of influence (SOI) may be used with perturbative corrections, e.g. for the Cassini–Huygens mission once the spacecraft entered the Saturnian SOI it would used the barycenter of that system as teh reference frame for local navigation. The type of coordinate frame being used depends on the function being served. For general trajectory prediction the orbital elements (which are the Euler angles and associated reference arguments) are generally used, but for local guidance and navigation, these are generally converted into state vectors that are more readily linearized for iterative numerical simulation and optimization. [This monograph on deep space navigation](http://descanso.jpl.nasa.gov/Monograph/series1/Descanso1_all.pdf) will give more detail on the techniques used for determining and calculating the position and trajectory of interplanetary spacecraft, though it only touches the surface. In general, JPL’s [Deep Space Communications and Navigation Systems](http://descanso.jpl.nasa.gov/index.cfm?force_external=0) is probably the best on-line source of information about deep space tracking and communication. Stranger ---
**Author:** ![Memoe](https://avatars.discourse-cdn.com/v4/letter/m/6bbea6/32.png) [@Memoe](https://boards.straightdope.com/u/Memoe)\ **Post date:** [May 22, 2012, 4:43am UTC](https://boards.straightdope.com/t/measuring-velocity-and-position-in-space/622572/8 "2012-05-22T04:43:40Z")
> [@davidm](#): > > A related question. For spacecraft traveling beyond Earth orbit, what frame of reference is used to denote their position? Is it done with three coordinates? A vector? cartesian coordinates