Most running and multisport GPS watches tell you when they are locked by the GPS satellites above their heads. Unfortunately, the Apple Watch doesn’t, at least not without using a third-party app like Runkeeper or Intervals Pro. But there are a few workarounds.
Most of the time, your Apple Watch’s GPS should work invisibly. In a minute or two of starting an outdoor workout or other activity tracked by GPS, it will just have a good lockdown. And, if there are any gaps or short sections that it is losing signal for, it will use your pace history, stride length, accelerometer, and other personalized information to estimate and complete things.

However, even if you cannot confirm that your watch has a GPS mid-workout lock, you can verify that it is using “Location services”. These are basically GPS and Assisted GPS (aGPS), Cellular Positioning, Wi-Fi Network Positioning, and any other way for your watch to know where you are.
To check, slide your finger up on the main watch face to access the Control Center. If your watch uses location services, there will be a solid purple arrow in the upper right corner. (If the purple arrow is just a hollow outline, it means that an app could potentially receive location data, but currently does not.)
Now it just tells you that your watch is using location services. That doesn’t mean it gets accurate location data (or none at all). Assuming all is well with your watch, it will receive a GPS signal ASAP. If you are indoors, under trees, or if something else is interfering, it may not have GPS at the moment.
If you’re not sure if your GPS is working and you need to troubleshoot it, or if you just want to make sure you have a lock before an important timed event, there is a way to force it: use the Maps application.
On your watch, open the Maps app. To find your location, it has to use Location Services (you can check in the Control Center, if you like.) More importantly, it will show the results right in front of you.

The size of the location circle will give you an idea of the quality of the GPS signal you have (or if you have one). GPS is accurate to a few feet, so if the circle is larger than that, your watch does not yet have a solid satellite fix. If you stand out in open ground and still can’t get a lock, there’s probably something wrong with your GPS and you should contact Apple Support. (Or, I guess, nuclear war is about to break out; in that case, there’s no need to contact Apple.)
Point: Your watch also overlays your iPhone’s GPS signal. If you’re trying to troubleshoot your Apple Watch’s GPS signal, turn off your phone, and then open Maps on your watch.
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