GPS Antenna Installation Tips

Many marine electronics – including AIS transponders, chart plotters, some DSC radios and some LTE routers – have an embedded Global Navigation Satellite Systems (aka GNSS) module or what we often refer to as GPS receivers. GPS refers to the US Global Positioning System but other GNSS such as the European Galileo and Russian GLONASS systems are often supported by these devices as well. I use the term “GPS” in this document since that is the term that most people use but in reality, this guidance applies to all GNSS antennas.

I get a lot of questions on how best to install GPS antennas. As electronics become more functional, we see more and more devices with embedded antennas and GPS receivers. For example, the first generation of AIS transponders used external VHF and GPS antennas typically connected via coax to antennas mounted outside the cabin. Now we have several models with embedded GPS receivers and antennas.

This article provides some tips for getting the most out of your devices with embedded GPS antennas as well as how and when to use external GPS antennas to improve performance.

For tips on VHF/AIS antenna installations, see my guide here: AIS/VHF Antenna Installation and Troubleshooting

Top 10 GPS antenna installation tips

Here are my top ten tips for getting good GPS performance with your marine electronics:

1. Imagine the path: Unlike other antennas that are looking for other antennas on the horizon, GPS antennas must have a clear path to the sky to receive signals from GPS satellites. Keep that in mind as you install devices with embedded GPS antennas inside a cabin. If there is a lot of metal clutter directly above the location of the GPS antenna, then it is unlikely you will get a good GPS signal.
2. Location: It is very tempting to place a new piece of marine electronics in an electronics cabinet, especially if there is no need to interact with the device as is the case with an AIS transponder. Usually, these are wood cabinets and unfortunately wood – especially varnished wood – can be a blocker of radio signals. There is an interesting paper put together by University of Virginia and Microsoft Research that shows how wood and especially wood in combination with water (think wet teak decks) can be more of a blocker than glass or concrete – but not as bad as steel – for GPS signals. In my own testing, I have found GPS, Wi-Fi and cellular signals can be significantly degraded if the equipment and antennas are located in a wood cabinet. In addition, if you have a wood deck or hull, or a steel boat, you will likely need to hunt for a good location where signals are not blocked by these materials or consider an external GPS antenna. Fiberglass and glass on the other hand do not typically block GPS and other signals.
3. Wiring and other metal objects: Related to the above point, be sure to not have lots of wiring running above the GPS antenna. I have seen some installations where all the wiring and other metal objects make the enclosure look like a virtual Faraday cage. Again, the GPS antenna – regardless of type – needs a path to the sky.
4. Antenna separation: NMEA recommends three feet of separation between GPS antennas and most other antennas including VHF, cellular and Wi-Fi antennas. In my experience, this is typically not the case and there are many combo antennas on the market that combine GPS with other antennas in a single enclosure. But if you have GPS reception issues, try testing a few locations before choosing the final location to be sure the GPS is not impacted by other antennas. That said, if you have radar on your boat do not install your GPS antenna in the horizontal path of the radar. Place the GPS antenna well above or below the radar beam.
5. Use diagnostic tools: Almost all marine electronics with a GPS receiver will have some type of diagnostic screen – either on the device or via a computer or mobile device diagnostic application. This can be used to look at GPS satellite reception strength. Typically you need a connection to at least three satellites to get a decent fix but more is better. The vertical bars will tell you the relative strength as well as whether the connection is being used to determine your position. Some devices use multiple GNSS systems such as the US GPS system, GLONASS and Galileo. Be sure to use these tools to verify that you are getting a strong signal before you do your final installation.
   
6. Right side up: Be sure your GPS antenna is installed right side up. Consult the manual for devices with embedded antennas to get recommendations on how the device can be installed. If you use a portable “puck” GPS antenna, be sure the rounded side is facing towards the sky.
7. Low on deck: Unlike other antennas, you want to keep your GPS antenna closer to deck level. If it is installed high up on a mast or arch and you are in rough seas, the GPS will give inaccurate readings for course over ground and speed over ground and cause jitter in your navigation program.
8. Choose the correct GPS antenna: Most modern marine devices with GPS capability have the GPS module in the device case and then either use an embedded GPS antenna also in the case or rely on an external GPS antenna connected to a TNC or SMA antenna port. These GPS antennas are active antennas with a low noise amplifier (LNA) built into the body of the antenna. The LNA is powered by 3-5 volts sent via the antenna cable to the antenna body and the signal comes back down to your device with the GPS module. Most AIS systems and LTE routers use this combination. However, to be sure consult the manual for your device to be sure. In some cases with older equipment, a passive antenna is used or in some cases the GPS module is in the antenna dome and the wiring sends NMEA data. USB GPS modules on the other hand have the GPS module, antenna and interface electronics all in the “puck” at the end of the USB cable. So, double check the manual and be sure to use the correct antenna type if you decide to add an after-market GPS antenna.
9. AIS transponders use their own GPS system: I get a lot of questions from AIS transponder customers asking if they can use GPS data from another GPS source via a NMEA 0183 or NMEA 2000 connection. While this seems like it should work, the AIS standards require Class B transponders to use their own GPS system and therefore will ignore GPS data coming in from another source. The exception to this is the AMEC CAMINO-108 AIS transponder family which supports using a NMEA source as a backup to the built-in GPS system. This is also the case with Class A transponders. Regardless, you should always use the GPS antenna (embedded or external) that came with your transponder or install a compatible GPS antenna recommended by the manufacturer or dealer.
10. Consider an external GPS antenna – either inside or outside the cabin: In some installation scenarios for devices with embedded GPS antennas, it is just not possible to install the device in a place where you get good GPS reception. It may be time to consider using an external GPS antenna. All AIS transponders include a port for an external GPS antenna – even models that have an embedded GPS antenna already in the case. Generally speaking there are two types of external GPS antennas. A small portable “puck” antenna comes with 16 feet of cable and is intended to be used inside the cabin or in the dash area of a driving station.These offer the benefit of allowing you to install your device where you want and then place the GPS antenna where you will get decent reception. These work well as long as they are not located directly below large metal objects such as deck winches and will usually shoot through fiberglass just fine. Use the device diagnostic software to check reception and find a location that provides a good signal.
    If you have a boat with a metal deck or even a thick wood deck, you may find getting a decent GPS signal is just not possible inside the cabin. Your final option is to install a marine-grade “mushroom” GPS antenna outside the cabin on a cabin top, rail or other appropriate location. These antennas typically come with 30 feet of cable and are built to withstand the elements and have a 1 inch 30 thread receiving hole for mounting on a standard ratchet or other style antenna mount. These will almost always work really well and provide the best signal. Some customers choose to use the large “mushroom” style inside the cabin which given its size will usually work better than a portable “puck” antenna. Again, use diagnostic software to check reception in your chosen location before doing the final install.

The two biggest takeaways from all of this…

• Be sure there is a clear path to the sky from your GPS antenna and ideally there is only fiberglass or glass between the antenna and the sky.
• Use the diagnostic software or tools that are included with your device as you evaluate and select GPS antenna locations.

Feel free to contact us if you have specific questions.

Safe boating! 

Doug Miller