# How does an iphone directional compass work? **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2023, 2:13pm UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940 "2023-07-12T14:13:35Z")\ **Posts on this page:** 6\ **Page:** 3
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [August 3, 2023, 7:29pm UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/41 "2023-08-03T19:29:09Z")
> [@Robot\_Arm](#): > > Two receivers in close proximity, picking up signals from the same satellites, at the same time, will have the same positional error. That’s true to a large extent. The biggest source of error is the ionosphere, and the fact that “weather” up there can change the path length of a radio wave. Some of this can be corrected for, but not all. On the other hand, it varies fairly slowly, so you can use a nearby fixed reference point as a correction. The ionospheric error also averages out over a very long period (days, months), so you can also get a good result by sampling over an extended time. Geologists use this to track continental plate movement to millimeter precision. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 3, 2023, 8:42pm UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/42 "2023-08-03T20:42:11Z")
> [@Robot\_Arm](#): > > I read something once (a couple decades ago) about how very busy harbors had a system that improved the accuracy of GPS. They would find a high position, like the top of a building, and survey the location very accurately. Then they put a GPS receiver there. By comparing its known location with where the GPS receiver thought it was, they could determine how much of an error the GPS receiver was experiencing. Then they would broadcast that error to the ships in the harbor, which were calculating their GPS position from the same satellite signals as the reference location. By applying that correction, they were accurate within centimeters, instead of meters. Yeah, that’s the “differential assistance” that @bump referred to. I think nowadays they do it at cell phone towers, too. But you’re right that that systematic error would cancel out on a GPS compass, so you could make one much smaller than the size of the error circle (though still probably not as small as a phone). ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [August 3, 2023, 10:20pm UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/43 "2023-08-03T22:20:11Z")
> [@Dr.Strangelove](#): > > That’s true to a large extent. > [@Chronos](#): > > Yeah, that’s the “differential assistance” that @bump referred to. Thanks. I did check a couple of pages online, found some info on the WAAS system used by the FAA, but they didn’t answer that specific question. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 4, 2023, 10:03am UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/44 "2023-08-04T10:03:00Z")
> [@Dr.Strangelove](#): > > That’s true to a large extent. The biggest source of error is the ionosphere, and the fact that “weather” up there can change the path length of a radio wave. Atmospheric humidity also plays a role: > **[The effect of absolute humidity on GPS-positioning accuracy | Academic...](https://science.lpnu.ua/istcgcap/all-volumes-and-issues/volume-87-2018/effect-absolute-humidity-gps-positioning-accuracy)** > > Objective. Investigate the effect of absolute humidity on the GPS accuracy for different durations of observation. Methods. The GPS observations with different durations over spring-autumn period at 17 permanent stations in France and 8 stations in... In fact, weather/climate scientists use GPS signal error as a way to measure atmospheric humidity: > **[Evaluating tropospheric humidity from GPS radio occultation, radiosonde, and...](https://amt.copernicus.org/articles/11/3091/2018/)** > > Abstract. While water vapor is the most important tropospheric greenhouse gas, it is > also highly variable in both space and time, and water vapor concentrations > range over 3 orders of magnitude in the troposphere. These properties > challenge all... > **[Radio occultation](https://en.wikipedia.org/wiki/Radio_occultation)** > > Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system. Other satellite carriers onboard GNSS-Radio occultation include CHAMP (satellite), GRACE and GRACE-FO, MetOp and the recently launched COSMIC-2. > Atmospheric radio occultation relies on the detection of a change in a radio signal as it passes through a planet's atmosphere, i.e. as it is occulted by the atmosphere. When electromagnetic radiation passes through t... ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 4, 2023, 10:23am UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/45 "2023-08-04T10:23:08Z")
No mention of Hall effect devices? You can get extra sensitivity by using more than one, e.g. mount two of them in opposite directions and amplify the difference. GPS is not a magnetic compass, obviously. It does usefully tell you where you are at. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [August 4, 2023, 12:19pm UTC](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940/46 "2023-08-04T12:19:52Z")
It seems the solid state compasses used in phones are usually Hall effect sensors. Which was a surprise to me. I didn’t think the needed sensitivity or signal to noise would have been viable. But it seems it is. I had always assumed they used a flux-gate magnetometer. You get some silly capable technology now. In a previous life I used to worry about geological surveys done in magnetics and gravity. Modern surveys are done with tensor output, and use a Squid - superconducting quantum detector for ridiculous resolution. (What is even more brain melting are the flown gravimetric surveys. I still can’'t believe that they can work, but they do.) [Previous page](https://boards.straightdope.com/t/how-does-an-iphone-directional-compass-work/986940.md?page=2)