GPS Tracking for Open Water Swimming

What follows is a minimally-technical, practical overview of current GPS tracking technology, with a focus on open-water swimming. In providing tracking services to numerous solo swimmers and races through my web app, I’ve noticed a few common questions and misunderstandings, so this is my attempt to compile some pertinent information in one place.

GPS tracking data is an essential component of a well-documented marathon swim, providing valuable information about your swimming speed, navigational efficiency, and surface currents. GPS data do not “prove” a swim claim, but any “funny business” will often show up first in the tracking data.

GPS technology is ubiquitous, inexpensive, and starting this year (2017) tracking data are a hard requirement for MSF Documented Swims. Remember, though: A GPS track does not determine the official distance of a swim. In open water swimming, distances are measured with route lines.

The Basics

What is GPS?

The Global Positioning System (GPS) is a network of satellites, each of which continuously broadcasts two pieces of information: (1) the time, and (2) its own location in space. This information is then received on Earth by any GPS receivers in the satellite’s direct line-of-sight. GPS receivers are typically connected to a device (e.g., your iPhone or Garmin watch), which uses the information received from the satellite to calculate its own geolocation.

In a nutshell, that’s how all our GPS enabled devices “know” where they are - by continuously trilaterating their position with respect to GPS-networked satellites.

GPS Trackers

A “GPS tracker” is a device that combines a GPS receiver and some method of either (a) logging the geolocation data to memory or (b) transmitting the data to an external server for storage.

GPS trackers commonly used for tracking marathon swims include:

Details and recommendations on specific GPS trackers are provided below. But first…

Conceptual Issues

Logging vs. Transmitting

Any GPS tracker can log tracking data during a swim. These data are what you need for swim documentation - they allow someone analyzing your swim later to plot your progress on a map.

Satellite messengers (such as SPOT), Marine AIS, and smartphones have an additional capability: they can transmit data “back to civilization” via cellular or satellite networks. While this is not strictly necessary for purposes of swim documentation, it’s a nice feature that allows your friends, family, and fans to monitor your progress in real-time.

Transmitting data via cellular or satellite networks costs money (subscription fees), while mere logging is free beyond the cost of the device itself.

Bottom line: You do not need an expensive satellite messenger to merely log GPS tracking data for your swim. Any old iPhone or Garmin watch will do that, even without cellular reception.

Raw Data vs. Visualization

Any GPS device should be able to either (1) store raw tracking data and provide some method of exporting it in a standardized format (such as GPX), or (2) transmit the data somewhere else for storage.

But raw tracking data are difficult for humans to interpret. We must transform the raw data into something we can understand - like a map, or a plot. In other words, GPS data are fairly useless unless we can visualize them. visualization of Sarah Thomas’ 80-mile swim in Lake Powell.

It is important to understand that logging and visualizing are separate functions. Some devices do one or the other, and some devices do both. Notably, smartphones and tablets (combined with certain third-party apps) can provide powerful real-time visualization of GPS tracking data. In contrast, fitness watches and satellite messengers usually provide limited or no built-in visualization.

This also helps explain what does: It provides sophisticated, customized visualization — but not the raw data itself. Thus, users must provide their own device (such as a smartphone or SPOT).

Device-Specific Recommendations

Smartphones and Tablets

A smartphone or tablet installed with a suitable third-party app can be a highly powerful GPS tracker. Even without cellular service (and even in airplane mode) it can easily log, save, visualize, and export tracking data. If cellular service is available, smartphones and tablets can also transmit tracking data externally.

You’ll need to obtain a specialized GPS tracking app through the Apple App Store or Google Play Store. The app should, at minimum, log tracking data and provide a method of exporting the data in a standardized format (such as GPX). Some apps provide additional features such as navigation, real-time statistics, and transmission to external servers or webpages.

Apple App Store (for iPhones and iPads)

For basic tracking needs, I recommend Track ($4.99 on the App Store). Track is slickly-designed and dead-simple to use. A very nice bonus: Track also provides optional, integrated public sharing of your track via the domain.

More advanced / technical tracking features are available in Navionics Boating (free, with in-app chart purchases) and MotionX GPS ($1.99): nautical charts, route planning and navigation, custom waypoints, speed metrics, and more. Note: If you’re using an iPad, choose the “HD” versions of these apps.

Navionics Boating apps for iOS and Android devices

Google Play Store (for Android)

I have yet to find a basic Android tracking app as slick as Track for iOS. Literally dozens of apps on the Play Store claim GPS logging functionality, but few distinguish themselves. For a “no frills,” free, and ad-free tracker on Android check out GPS Logger by Mendhak. Press the big “Start Logging” button, and you’re good to go! Easily accessible option to export GPX file later.

For more advanced/technical tracking features, try GPS Essentials (free, with a donation to remove ads) or Navionics Boating (for phones) and Boating HD (for tablets), which like their iOS counterparts are free but with add-on purchases for NOAA nautical charts.

Fitness Watches

Any GPS-enabled fitness watch from Garmin, Suunto, or similar brand can log a GPS track and save it for later analysis. Higher-end models such as the Garmin Fenix have navigation features and advanced metrics that can be useful for observing or crewing a swim.

But these super-watches can be expensive, and, since you’re not allowed to wear them on official marathon swims anyway, in my opinion they offer little advantage over the smartphone you probably already own.

The Apple Watch is an intriguing new entry in this category, and can transmit your GPS track via cellular networks for real-time public tracking.

Satellite Messengers

The key feature of satellite messengers such as SPOT is the ability to transmit tracking data “back to civilization” from anywhere on the surface of the Earth via satellite. So, even if your swim is way offshore, with “zero bars” of cell service, people can still follow your swim progress in real-time.

Examples of satellite messengers include the SPOT Gen3 and the Garmin (formerly DeLorme) InReach. The SPOT device is cheaper upfront, while the pricier InReach has more bells and whistles (such as visualization and text messaging).

Both devices require subscriptions to their respective satellite networks: SPOT uses Globalstar, while InReach uses Iridium. The Iridium network has superior coverage, but in most near-shore cases you will not notice a difference. However, there are some well-known gaps in SPOT’s coverage - including the Kaiwi/Molokai Channel!

SPOT and InReach both provide web-based real-time mapping for friends back home to monitor your progress. But it’s fairly primitive, and this was the original raison d’être for - to provide better mapping and visualization than the default “FindMeSPOT” page.

Marine AIS

The Automatic Identification System (AIS) is an industrial strength, extremely reliable but extremely expensive tracking option, and typically not practical for individual swimmers. However, if your boat (e.g., the English Channel fleet) is already AIS-equipped, it’s a nice option. Data export is more complicated than the above options, and may cost money.

In contrast to consumer GPS trackers, intended to log a single individual’s movements (and perhaps send that data to a server somewhere), AIS is a system for monitoring the movements of thousands of vessels, all over the world, in real-time. Per Wikipedia:

AIS is intended to assist a vessel’s watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Vessels fitted with AIS transceivers can be tracked by AIS base stations located along coast lines or, when out of range of terrestrial networks, through a growing number of satellites that are fitted with special AIS receivers which are capable of deconflicting a large number of signatures.

Courtesy of Orgcomm.

AIS devices provide data logging and transmission only - visualization is available through third-party services such as MarineTraffic and

Terrestrial-AIS vs. Satellite-AIS

AIS-networked receivers communicate via VHF radio, and come in three flavors:

  1. On-shore receiver stations.
  2. Receivers on AIS-equipped vessels.
  3. Receivers mounted on orbiting satellites.

Tracking a given vessel depends on the vessel’s AIS transponder being within range of another AIS-networked receiver (either on-shore, on board another vessel, or on a satellite). In heavily trafficked, near-shore areas such as the English Channel, this is never an issue. But out in the middle of the ocean, there’s a good chance the nearest terrestrial (on-shore or vessel-based) receiver is out of range.

Satellite AIS covers these situations. But there’s a catch: only Class A transponders are powerful enough to reliably transmit to satellites. If you have a cheaper Class B transponder out in the middle of the ocean (like Ben Hooper in the Atlantic earlier this year), chances are you’ll be off the grid and untrackable for long stretches.


Which GPS tracking option should you choose for a given swim? Well, it depends!

How to choose among these three?

Learn more about - advanced tracking & analytics for endurance sports.

Posted 24 April 2017 in: knowledge base , product reviews Tags: GPS , documentation