What Is an MMSI Number, and How Do You Get One?
Updated for 2026
Everything you need to know about MMSI registration and set-up
On July 14, 2026, a 49-foot cabin cruiser carrying 20 people capsized near Alcatraz Island. Sixteen people were rescued, one person died, and three remained missing when the Coast Guard suspended its active search. The cause of the sinking is still under investigation, and we do not yet know what communications equipment was aboard or how the emergency was reported.
What we do know is that emergencies on any body of water can unfold with astonishing speed.
The Bay is a beautiful and and exciting place to boat, but we do contend with cold water, strong currents, commercial traffic, fog, sometimes wind-driven chop and conditions that can change quickly. When something goes wrong, the ability to call for help clearly and transmit your exact location matters.
Many boat owners already have one of the best emergency communication systems available. They just don’t know it isn’t fully set up. It’s the small red DISTRESS button on a modern marine VHF radio.
But the button must be properly set up before it can do its full job.
That means the radio needs Digital Selective Calling, commonly called DSC. It needs the correct MMSI number programmed into it. It also needs current GPS position information.
What is an MMSI number?
MMSI stands for Maritime Mobile Service Identity. It is a unique nine-digit number used by marine radio equipment to identify a vessel.
The simplest way to think of it is as your boat’s electronic identity.
An MMSI is programmed into equipment such as a DSC-capable VHF radio and an AIS transceiver. The registration attached to that number can include information about the vessel, its owner and emergency contacts. The Coast Guard can use that information when responding to a distress alert.
The MMSI belongs with the vessel, not with the person at the helm.
If you own several boats, each boat should have its own MMSI. If one boat has two permanently installed DSC radios and an AIS transceiver, those units should ordinarily use the same vessel MMSI. The Coast Guard makes exceptions for equipment such as AIS search-and-rescue transmitters and handheld VHFs that are not dedicated to one vessel.
The useful rule is: One boat. One vessel MMSI.
What is DSC, and what does the red button do?
DSC stands for Digital Selective Calling. It is built into most modern fixed-mount marine VHF radios.
The easiest way to recognize a DSC radio is to look for a red button marked DISTRESS, usually protected by a small cover so it cannot be pressed accidentally.
A traditional VHF emergency call is made by voice on Channel 16. You say “Mayday” three times, identify the vessel, give your position, explain the emergency and state how many people are aboard.
That remains an essential skill.
DSC adds a digital alert before the conversation begins.
When the system is correctly configured, pressing and holding the distress button can transmit:
- the boat’s MMSI;
- its GPS position;
- the time of that position;
- and, when selected, the nature of the emergency.
This gives rescuers critical information quickly, even when the person operating the radio is frightened, injured or struggling to explain the boat’s location.
After transmitting the DSC alert, make a voice Mayday call on Channel 16 whenever you are able. The red button gets the alarm out. The voice call tells rescuers what is happening, how many people are aboard and what help is needed.
Without an MMSI, your VHF is not useless. It still works for ordinary voice communication, including calling the Coast Guard on Channel 16. What is missing is the vessel identity that allows the DSC system to perform as intended.
Why the GPS connection matters
The MMSI tells the Coast Guard who you are. The GPS tells them where you are. Both are needed for your DSC distress system to provide the most useful information in an emergency.
Some newer VHF radios have their own internal GPS receiver. Others receive position information from the chartplotter or another GPS through an NMEA 0183 or NMEA 2000 connection.
Do not assume this connection is working merely because both screens light up.
Look at the VHF display. It should show current latitude and longitude or indicate that it has a valid GPS position. Depending on the radio, a problem may appear as blank coordinates, a flashing position, an old timestamp or a message such as “No GPS” or “No Position.”
Your chartplotter may know exactly where the boat is while your VHF remains completely uninformed.
A DSC radio with the correct MMSI but no current GPS input may identify the vessel without automatically transmitting an accurate current position. That can leave rescuers with the name of the boat but not the answer to the most urgent question: where are you?
If your VHF has built-in GPS
Many modern VHF radios include their own GPS receiver. Once the radio has a clear view of the sky, it can determine its own position without being connected to your chartplotter.
Turn the radio on and look for current latitude and longitude on the display. If you see your position, your radio is already receiving GPS data.
If your VHF does not have built-in GPS
Most boats simply obtain their GPS position from the chartplotter.
On newer boats, this is usually done through an NMEA 2000 network—a single data cable that allows the chartplotter, VHF, AIS and other onboard electronics to share information. In many cases, the connection is already there, and only needs to be enabled or configured.
Older boats often use NMEA 0183, which connects the chartplotter and radio with individual wires. If your VHF is not displaying a position, a marine electronics technician can usually verify the wiring and have the system working quickly.
The easiest way to know if everything is working
Simply turn on your chartplotter and your VHF radio. Within a few minutes, your VHF should display your current latitude and longitude. If it does, the radio is receiving GPS information.
If the screen says “No GPS,” “No Position,” or doesn’t display coordinates at all, your DSC system is not yet receiving your location and should be checked before relying on it in an emergency.
Once your radio has:
- a properly registered MMSI,
- a current GPS position, and
- a working antenna,
You’re ready to make a DSC test call using the Coast Guard’s automated test service to confirm the system is working correctly.
Does the Used Boat I Just Bought Already Have an MMSI Number?
Maybe—and it’s one of the first things you should check.
Many boats are sold with the original VHF radio still installed. If that radio has Digital Selective Calling (DSC) capability, there’s a good chance the previous owner already programmed an MMSI into it.
Before you apply for a new MMSI, find out if your radio already has one.
This is important because many marine VHF radios only allow an MMSI to be entered once. If you accidentally program a second number into a radio that already has one, you may need to send it back to the manufacturer or an authorized dealer to have it reset.
Fortunately, checking is easy.
Turn on the VHF radio and look for the DSC or MMSI menu. Most radios will display the programmed nine-digit MMSI if one has already been entered. If you’re not sure where to find it, search online for your radio’s make and model followed by “display MMSI.” The owner’s manual will usually show you exactly where to look.
If there isn’t an MMSI programmed
Great. You’re starting with a clean slate.
You can obtain a new MMSI, program it into your radio, and complete the rest of the setup described in this article.
If there is already an MMSI programmed
Write the number down before you do anything else.
Next, ask the previous owner if they have the original MMSI registration paperwork. If they do, the process is usually straightforward.
If they don’t, don’t panic.
Contact the organization that issued the MMSI—such as BoatUS, America’s Boating Club, Shine Micro, or the FCC—and explain that you’ve purchased the boat and would like to transfer or update the registration. They can tell you whether the MMSI is registered through their system and what information they need from you.
In many cases, you won’t need a new MMSI at all. You’ll simply update the existing registration with your name and current emergency contact information.
Once that’s complete, turn your attention back to the radio. Make sure it is receiving a current GPS position, then perform a DSC test call to confirm the entire system is working properly.
Deb’s Tip
Whenever you buy a used boat, take a few minutes to photograph the VHF model number, the MMSI screen, and any registration paperwork before you leave the dock. Those photos can save a surprising amount of time if you later need help from BoatUS, the FCC, or a marine electronics technician.
Where do you get an MMSI number?
The correct source depends on where the boat will operate.
A recreational vessel that remains in United States waters and is not otherwise required to carry a marine radio can obtain a domestic MMSI through a Coast Guard-recognized private registration organization. The Coast Guard currently lists:
A vessel that will visit foreign ports or is otherwise required to have an FCC ship station license should obtain its MMSI through the FCC as part of that licensing process.
Being Coast Guard documented does not, by itself, mean that your recreational boat must obtain an FCC MMSI. The deciding questions are whether the vessel is required to have a ship station license and whether it will operate internationally.
A recreational yacht staying on San Francisco Bay and elsewhere within the United States will generally use a domestic MMSI registration service.
A yacht heading to Mexico, Canada or another foreign destination should use the FCC process from the beginning.
That choice matters because changing an MMSI later may require the existing number to be removed from the radio or AIS equipment with assistance from the manufacturer.
How do you test DSC safely?
Never test the system by pressing the red DISTRESS button.
The Coast Guard’s Rescue 21 network provides an automated DSC test-call service. On a compatible radio, use the DSC menu to make a Test Call to:
003669999
Follow the instructions in your radio manual because the menu sequence varies by manufacturer.
A successful acknowledgment indicates that the radio sent and received the DSC test through its antenna system. You still need to verify separately that the VHF shows a current GPS position and that the MMSI registration information is accurate.
Do not make routine radio checks on Channel 16, and never transmit a distress alert unless there is a genuine emergency.
Download our handy MMSI and DSC Setup Checklist
We created a practical checklist you can take aboard and complete beside your own equipment. That little red button is useful only when the systems behind it know who you are, where you are and how to send the call.
It walks you through how to:
- identify your VHF and GPS equipment
- check for an existing MMSI;
- obtain, transfer or update the correct registration;
- verify that the VHF is receiving current GPS coordinates;
- make a proper DSC test call;
Download the Boat Owner’s MMSI Setup Checklist and complete it before your next trip.
Frequently Asked Questions About MMSI, DSC and AIS
What is an MMSI number?
An MMSI is a unique nine-digit identity programmed into marine DSC radios, AIS transceivers and certain other marine communication equipment.
Is an MMSI number legally required?
Not for every recreational boat. However, a properly assigned MMSI is necessary to use the identity functions of DSC and transmitting AIS equipment. International operation and certain regulated vessels may trigger FCC licensing requirements.
How do I get an MMSI number?
Recreational vessels remaining in U.S. waters and not required to have an FCC ship station license may use BoatUS, United States Power Squadrons or Shine Micro. Boats visiting foreign ports or otherwise requiring a ship station license should apply through the FCC.
Does every boat need a separate MMSI?
Yes. Each vessel should have its own vessel MMSI. Permanently installed DSC and AIS equipment aboard that vessel should ordinarily use the same number.
Is an MMSI the same as a Coast Guard documentation number?
No. A Coast Guard official number identifies a documented vessel for documentation purposes. An MMSI identifies the vessel through marine radio systems.
How do I know whether my VHF has DSC?
Look for a protected red DISTRESS button. You can also check the menu for DSC, MMSI, position request or test-call functions and verify the make and model in the owner’s manual.
Can I change an MMSI already programmed into a radio?
Possibly, but many radios restrict user changes. You may need assistance from the manufacturer or an authorized marine electronics dealer. International standards intentionally make MMSIs difficult to alter to prevent incorrect vessel identities.
Does my VHF need GPS?
A voice VHF will operate without GPS. For a DSC distress alert to include your position automatically, the radio must have a current GPS position through an internal receiver or a connection to the boat’s navigation network.
What happens to the MMSI when I sell the boat?
The registration should be transferred or updated for the new owner. Any DSC or AIS equipment removed from the boat must have the old vessel MMSI deleted before being used elsewhere.
How do I test my DSC radio?
Use the radio’s DSC Test Call function with the Coast Guard test identity 003669999. Follow the manufacturer’s instructions. Never test the system by pressing the red distress button.
Can the Coast Guard continuously track my boat through its MMSI?
No. An MMSI is an identity, not a continuous satellite-tracking service. A DSC transmission is sent when a call is made. An operating AIS transceiver periodically broadcasts navigational information, but the MMSI itself does not continuously track the vessel.
Want to have more boating choices? Book a Luxury Yacht Charter on San Francisco Bay, join our Boating Club for private boating without the hassle of ownership, or take Private Lessons and learn faster and with greater confidence.
Looking to transform your stress and burnout on the water? Join one of our Day Retreats where we combine sailing with yoga, meditation, creativity, and stress reduction. About Us.