RTK receiver for iPhone: what actually works over Bluetooth

Short answer: yes, an iPhone can give you centimeter-accurate positioning, and it has to be done with an external receiver. The phone contributes the screen, the app, and the data connection; the receiver contributes the antenna, the multi-band tracking, and the corrections. What varies between products — and what people get caught out by — is how the position reaches your apps.

Why the phone cannot do it alone

Three hard limits, none of which a software update reaches:

• The antenna. A few millimetres of metal in the edge of a slab, with no ground plane and your hand over it. It picks up reflections as happily as direct signals.
• The chip. Most phones track a single frequency, so they cannot measure ionospheric delay and have to apply a rough model instead.
• No corrections. Satellite clock, orbit and atmospheric errors need an external correction stream to remove.

That is the whole story of why phone positioning sits at 3–5 metres; the long version is in why your phone’s GPS is only accurate to 3–5 metres.

The part nobody explains: two ways in

This is the distinction that decides whether a receiver fits your workflow, and vendors are often vague about it.

Route 1 — system-wide location (MFi accessories)

Apple’s Made for iPhone programme allows a certified accessory to supply location to iOS Location Services itself. When one is connected, every app on the phone gets the better position without knowing anything has changed. It is invisible and universal, and it is why some long-standing external GPS products work with mapping apps that have never heard of them.

The cost is that MFi certification is a commercial and technical programme with real overhead, which is a large part of why accessories that do this are priced the way they are.

Route 2 — app-level integration (Bluetooth Low Energy)

A BLE receiver publishes position and status over a Bluetooth service, and apps that have integrated with it read from that service directly. It needs no certification programme, which keeps the hardware cheaper and the protocol open — but an app only benefits if it has done the integration.

Neither route is a trick; they are different trades. The question to ask any vendor is simply: does this feed iOS Location Services, or do apps integrate with it? A confident answer either way is a good sign. A vague one is not.

Northing takes the second route deliberately: the Bluetooth protocol is published, and a free Swift SDK is on the way, so any developer can build on the receiver without asking permission or signing anything. Tractor GPS is the first app to use it. That openness is the point — but it does mean an app has to adopt it, and we would rather say that here than let you assume otherwise.

Is Bluetooth fast enough?

Yes, and by a wide margin. The traffic in each direction is small:

• Corrections in: about 1 kbps. A trickle.
• Position and status out: a position several times a second, plus fix type, satellite counts, correction age, and health.

Bluetooth Low Energy handles that without effort. The genuine engineering problems are elsewhere, and they are the ones that separate a receiver that works on a bench from one that works in a cab:

• Packet framing. BLE has a small default payload and negotiates a larger one per connection. Correction messages routinely exceed it, so they must be split and reassembled correctly. Getting the boundary arithmetic wrong truncates messages in a way that shows up as a fix that will not hold — and only under load.
• Background operation. iOS suspends apps that have not declared they need Bluetooth in the background. An app that has not is fine while you look at it and stalls the moment the screen locks.
• Reconnection. Walking out of range and back, or a phone call, or a low-battery moment, all break the link. Silent, automatic recovery is a feature you only notice the absence of.
• Power. The phone is now driving a radio link all day as well as its screen. A receiver that pushes ten times more data than it needs to costs you battery for nothing.

Android, for comparison

Worth knowing if you run both, because the platforms differ in one structural way.

Android supports mock location providers: with developer options enabled, a small helper app can feed an external receiver’s position into the system, and every other app on the device sees it. That gives Android something close to Route 1 without any certification programme, which is why the DIY precision community leans Android.

iOS has no equivalent for third-party apps. On an iPhone the choice really is MFi or app-level integration — there is no side door, and any vendor implying otherwise is describing Android.

Neither platform is better for this; they are differently shaped. Bluetooth Low Energy itself, and everything in the section above about framing, background operation and reconnection, applies identically to both.

Mounting and power, briefly

The receiver goes where it can see sky — roof, bar, or pole — and the point you mount is the point you measure, so a bracket that flexes is a measurement that flexes. Power is 5 V USB from whatever supply is already on the machine; there is no battery inside to age in a hot cab, and no proprietary harness to source.

Your phone, meanwhile, is now running a screen and a Bluetooth link all day. Plan on it being plugged in, the same as you would for any full day of navigation.

What to check before you buy

1. Multi-band, multi-constellation. Multi-band is not a luxury; it is how the ionospheric error gets measured instead of guessed. Anything single-band is a metre-class device however it is marketed.
2. How position reaches your apps — Route 1 or Route 2 above, and if Route 2, which apps have actually integrated today, not on a roadmap.
3. Where the corrections come from, and what they cost. A cheap receiver with an expensive or metered correction subscription is not a cheap system. Ask what year two costs.
4. Whether it works without a subscription. Some receivers become metre-class paperweights the day you stop paying. Ask what happens on cancellation, and get it in writing.
5. Coverage at your location, not at the vendor’s head office. Boundaries are polygons, and near an edge the answer is specific to you.
6. Whether it tells you when it is wrong. This is the one nobody asks and everyone should — see below.
7. How firmware updates work. Over the air, signed, with automatic rollback, or a trip to a dealer?

A note on “works with any app”

The phrase appears on a lot of product pages and means at least three different things: that the receiver feeds iOS Location Services and so genuinely does work with everything; that it emits a common text position format any app could read if it chose to; or simply that the vendor’s own app works and others are welcome to get in touch. Only the first is literally true, and it is the one that requires certification.

Ask which it is. With Northing the answer is the published protocol and the SDK — open to every developer, adopted today by Tractor GPS, and adopted by others as they integrate. That is a real answer rather than a comfortable one, and you should expect a real answer from anyone else too.

The question nobody asks

When a correction stream dies quietly — the phone drops off the network, an app is killed, a link breaks — many GNSS engines keep reporting centimeter confidence while the real error drifts. We measured it happening: quality flags, accuracy estimates and correction-age fields all frozen while true error grew from 7 cm toward 40 cm.

A receiver that misreports its own accuracy is worse than one that is simply less accurate, because you cannot tell afterwards which work to redo. Every Northing receiver therefore runs an independent watchdog and raises “Stale Corrections” to every connected app within 30 seconds, whatever the engine underneath claims. Ask any vendor what their hardware does in that situation. It is the fastest way to find out how much of their product was designed in a field.

What it looks like in use

Mount the receiver where it can see sky. Pair it once in the app. That is the setup — no correction server address, no mount point, no port, no username and password to type at a field gate.

Measured on our logged drive sessions:

• Fixed within seconds of a cold start, no warm-up period.
• ±2.5 cm repeatable accuracy, RTK Fixed, open sky.
• Fixed for 88% of a 40-minute drive including deliberate tree cover, a bridge and buildings, recovering in 1–2 seconds after each.
• 24 seconds back to fixed after a full corrections outage, holding centimeter accuracy for about a minute unaided while it lasted.

And if the subscription ever lapses, the receiver keeps working on Northing Global — 10–20 cm class, anywhere on Earth, free, forever.

The bottom line

An iPhone plus an external multi-band receiver is one of the cheapest credible centimeter-accuracy setups available, because the expensive parts — a bright screen, a fast processor, a data connection, and a device you already replace every few years — are already in your pocket.

What you are buying is the antenna, the correction stream, and the honesty of the software in between.

Next: what is RTK GPS for the underlying technique, or how much RTK actually costs for what the market charges.

Common questions