The iPhone's own GPS is good to a few metres. An external receiver does better, and
a receiver fed with correction data over NTRIP does far better — centimetres rather than metres.
Both connect here.
which receivers work
MFi receivers — nothing to do
Bad Elf, Garmin GLO, Dual and other Made-for-iPhone receivers work automatically through iOS. Pair them in the iOS Settings app and every app on the phone, this one included, simply uses the better fix. They do not appear on this screen and do not need to.
BLE NMEA receivers — pair here
Receivers that stream NMEA over Bluetooth Low Energy, including RTK rovers such as Emlid Reach and SparkFun RTK, are what this screen is for. iOS does not adopt them system-wide, so the app talks to them directly.
Power up the receiver and make sure it is advertising over Bluetooth.
Open Settings → GPS Source → External GNSS Receiver.
Tap Scan under Devices. The list reads Searching for receivers… while it looks, and Tap Scan to search for nearby receivers. when idle.
Tap your receiver to connect.
Switch on Use External Receiver so the app takes its position from the receiver instead of the phone.
The status panel shows the connected device name and the satellite count. While the
link is up but no sentences have arrived yet it reads Waiting for NMEA data….
Auto-Connect reconnects to a known receiver when it comes back in range, so
switching the rover on is the whole procedure next time.
The GPS Source row in Settings shows the connected receiver's name, or Off,
so you can confirm at a glance which instrument the app is actually using.
antenna height
Antenna Height (m) is subtracted from every fix the receiver reports.
A pole-mounted receiver measures its own antenna, not the ground. Enter the pole
height and the app reports the ground point — which is what a control point, a spot height
or a survey mark actually means.
Leave it at zero for a handheld receiver. Set it to your pole length — 1.8 m
or 2 m are common — the moment you put the rover on a pole.
the satellite sky view
The Satellites section draws a polar sky plot of every satellite the receiver can see:
azimuth around the edge as a true bearing, elevation from 0° on the outer ring to 90° at
the centre, with signal-strength bars underneath.
GPS
Green
GLONASS
Red
Galileo
Blue
BeiDou
Amber
QZSS
Cyan
Other
Grey
Satellites actually used in the fix are marked as such, so you can tell a
receiver that can see twenty from one that is using six.
The plot is the fastest way to diagnose a bad fix. Signals clustered on one side
mean terrain or a building is masking the sky, and moving twenty metres often fixes it.
Before any observations arrive the panel reads No observations yet.
ntrip corrections PRO
NTRIP streams correction data from a reference station over the internet. The phone fetches it
and forwards it to the receiver over Bluetooth, which is what turns a metre-class rover into an
RTK float or fixed solution.
Connect your receiver first — corrections have nowhere to go without one.
Fill in Caster (the host), Port, and Username and Password if the service needs them. Switch on Use TLS if the caster supports it.
Tap Choose next to Mountpoint. The app downloads the caster's source table and lists the mountpoints nearest to you first.
Pick one and tap Connect.
Watch Corrections age. A few seconds is healthy.
Your password is stored in the device Keychain, not in ordinary settings.
For a network (VRS) service the app uploads your position every ten seconds so the
caster can generate corrections for where you actually are — using the receiver's own sentence
when there is one.
Auto-Connect re-establishes the correction stream with a backoff after a
drop, so a tunnel or a dead spot does not end the session.
Some receivers cannot accept corrections. If yours has no writable input
the app says so plainly: this receiver doesn't accept data from the phone, so corrections
can't be forwarded to it. NTRIP needs a rover that takes an RTCM input.
With corrections flowing, the fix type in the status panel shows RTK float
or RTK fixed. Fixed is the centimetre-class solution.
Combine RTK with
Position Averaging to set a control point: the
averaging tool uses whichever source is live, so it averages the corrected fixes.
raw nmea logging
Log Raw NMEA writes every sentence from the receiver to a session file you can share.
It is off by default.
Switch it on when a receiver is behaving oddly — the log shows exactly what it
sent, which is the difference between guessing and knowing.
It is also a record of the raw observations for post-processing elsewhere.
altitude source
Settings → GPS Source → Altitude Source chooses the instrument that reports your height:
GPS or Barometer. Under the picker is a live reading with its accuracy and a label
naming the instrument producing it.
The barometer is usually the better instrument — GPS vertical error typically runs
two to three times its horizontal error. Selecting it is also what triggers the motion-sensor
permission prompt, and the row only appears on devices that have a barometer.
Inside a sealed vehicle or an aircraft cabin the barometer confidently reports
cabin altitude, which is not your altitude. That is why there is no "Automatic" option: the
app will not choose for you when the right answer depends on where you are sitting.
A barometric altitude also benefits from a lower Elevation Noise Filter in
the Tracker, since it is far less jittery than a GPS height. See
Tracker.
