I am seeking to improve the accuracy of iOS device GPS data, for use in a GPS mapping app. To do this, RTK (Realtime Kinematic) correction data must be applied to the raw GPS unit's output.
I don’t see iOS supporting the direct application of RTK (NTRIP) data to the GPS module's output, before passing this data to apps on the device.
Is there a configuration setting in iOS itself to enable this feature or a third-party app that could run incoming GPS data through corrections (similar to how audio processing apps can transform audio data and route it on to another app) before it gets passed on to the requesting app?
Best Answer
The built-in GPS
The internal GPS implementation in iPhones and iPads do not support apps getting the raw output that is necessary for them to apply RTK corrections themselves, nor does it support inputting RTK corrections into the internal GPS implementation in order for that to apply the corrections.
External Hardware Requirement
In order to get RTK corrected GPS on the iPhone/iPad, you'll need external hardware. The external hardware can either be an RTK-enabled GPS receiver, or it can be a GPS-receiver that supports raw output, which an app on the iPhone/iPad then applies the RTK corrections to.
The former option is readily available on the market in many shapes and forms. The latter currently requires you to develop your own app, and is thus out of reach of many.
iOS device to GPS connectivity
For the former option, there's several different options on how to implement it, depending on your requirements:
If you want the RTK-corrected GPS signal to work in any app (including Apple's built-in apps, such as the Maps app) - the easiest is to buy a RTK-enabled GPS receiver that supports Bluetooth position output. They usually come with some disadvantages, such as for example only supporting a lower rate of position updates. Typically such devices come with their own battery that needs to charged, and so on.
It is also possible to deliver the GPS signal via the Lightning connector to any app, including the built-in apps. That can be seen with for example the (now discontinued) Bad Elf GPS for Lightning. That product is powered through the Lightning port, so you have no battery to charge and no wireless communication signals to consider (besides GPS itself of course).
If you just want the RTK-correct GPS signal to work in a specific app, perhaps even one you have made yourself, there are many more options. A huge range of apps support GPS input via a relatively standardised network option (TCP/IP), which means you can use GPS receivers with WiFi, cabled Ethernet, etc. It is also possible to use RS-232 connected receivers with the right hardware. On iPads (not iPhones) it is also possible to use RTK-enabled GPS-receivers with proprietary USB protocols.
Getting the RTK correction signal to the GPS
For the RTK-correction signal, there's generally 3 options in use in the market place today. The simplest is to use a GPS receiver that supports a correction service delivered by satellite (and obtain a subscription for said service). A bit more involved is to get an RTK-enabled GPS receiver that comes complete with an RTK input system of some sort - that is usually either a cellular modem, RF radio or a variation of LoRA. And the third option is to incorporate the correction signal input handling directly in your own (or third party) app, that just relays the data to the GPS.
Range of choices for GPS receiver
Note that there's a huge range of RTK-enabled GPS receivers to choose from. There are very cheap modules of the "DIY"-sort, going over low performance cheap professional modules and all the way to very expensive systems.
What to choose depends very much on what you need to use the positions for. This can be a bit counterintuitive, as many seem to think of "surveying grade" as being "the best" - but it is actually possible to obtain "surveying grade" accuracy with very cheap equipment available today. However, if you have a moving system (i.e. a vehicle, drone, or similar) that require very frequent position updates, you're in an area with lots of obstructions (i.e. for example multi-pathing becomes a serious issue) and you're dealing with interference, scintilations or even jamming, you're generally looking for higher quality equipment.