Secrets of APRS Radio Explained: How To Use APRS For Emergency Communications
What is APRS?
APRS stand for Automatic Packet Reporting System. It is a system that automatically shares a position, bearing, speed, and altitude, weather data, and text across the VHF radio bands. APRS runs in simplex on a single frequency nationwide, 144.390 MHz. By monitoring a single frequency you can find position of other hams, the frequencies they are monitoring, near real-time weather data, and local repeater data out upto 100+ miles away.
With the dawn of the cellular broadband era, smartphone APRS applications are now available to monitor this positional information and share it through the internet. We will discuss this more later in the article.
How Does APRS via Radio Work?
APRS operation on 144.390MHz starts with packet operation traditionally using a 1200 baud signal. Your radio will include or will be connected to a TNC and a GPS to get your positional information. This information is then transmitted over the air. This process is repeated every few minutes, each time providing the updated positional information.
The same holds true for receiving. Your radio receives the ARPS signal and the TNC decodes the data. The decoded information is then shown on the radio display or the connected computer screen an shown on a map.
DigiPeaters – Extending the Range of VHF Radio
While APRS signals via radio can be heard by local stations around you, that is only heard over a few miles. Digipeaters are what really make APRS shine for emergency communications. They are essentially a simplex data repeater with an antenna at a high location (in many cases). The digipeater hears your data on 144.390MHz, reads the path information on the packets of data, and then retransmits the information again over 144.390 MHz, but now from a location with a much higher altitude and clear of obstructions. Since it is higher and unobstructed, your signal travels farther.
Then, another digipeater in the area hears the retransmitted information, reads the data and resends it again, giving you additional coverage, and again at an elevated unobstructed location. This process repeats for a total of 2-3 hops until the path information in the data tells the next digi to no longer send the information.
This digipeating of data and multiple hops from digi to digi are how the information can be heard at such great distances using a simplex VHF radio.
Great Coverage, No Single Point of Failure
The APRS coverage available across the country is extremely impressive. I have been on multiple trips over 1000 + miles and had position coverage for the entire trip, even in remote areas, with only a few dead spots of 30-50 miles. In addition, I was receiving RF beacons with positional information from other APRS users 190+ miles away in some cases, all made possible by digipeaters.
This coverage is made available by no single ham operator, but many operators and clubs across the country. Each installing and maintaining their own equipment to support the network. With this type of system, there is no single point of failure. There are multiple paths at any given time to get information across the airwaves.
VHF Propagation with APRS
When environmental conditions are right, APRS data can be heard from 200+ miles away via propagation. Now APRS propagation really isn’t “dependable” from a preparedness perspective but it is interesting to see. Here is a current map.
iGates – Integrating APRS into the Internet Era
Earlier in the article we mentioned that APRS data is integrated into the internet. iGates help to make this happen. When your APRS radio packet hits an iGate, it sends the information to the APRS-IS (APRS Internet Service) where it will then show on websites such as APRS.fi.
iGates essentially integrate the traditional APRS via radio (APRS-RF) with the APRS internet sources. In many cases, they are integrated with digipeater sites so you get 2 benefits from the same site. A retransmission of your position via radio, and a post of your position online.
This allows family and friends (or others) to follow your location and historical track when you are on trips, family outings, or even evacuating from a hurricane or wildfire. Turn it on or off as much as you would like, as you probably don’t want to be tracked all the time.
APRS Features and Preparedness Benefits
APRS packets include a number of other pieces of data within its transmission, let’s take a look at these features and how they can be of benefit for emergency preparedness.
Position is included in every transmission, it can also include speed, bearing, and altitude, however these options can usually be turned off at the radio or application level. This information can be used for tracking group members locations and direction of movement. This can be useful in rural or backcountry locations where there is no cell service and typical tracking tools such as Google Maps, Find My Friends, or other location sharing tools are unavailable.
Allows up to 42 alphanumeric characters which are sent with your position packets. Think of it as a message tag you can send along with your location. This is a very powerful tool you can use in your family communications plan.
Status text is “preset” by you in your APRS device and sent each time with the position data. It can be changed at any time. This can be seen by other radio users around you and if it makes it through an iGate, it will also get posted on APRS-IS sites so it can be seen as well.
Any message you can fit in this 42-character limit can then be seen by other team members locally via radio, or abroad that have an internet or cellular connection. It is also sent automatically each time my radio sends a position report. Lets examine a couple use cases.
Use Case 1 – Daily Use
Friends and I are taking a cross country trip into territory with limited cellular coverage. I want my family to know we’re ok on the trip. When I hit the road, I can change the status to:
“On The Road – All Systems Go”.
As I get closer, I can change my status to:
“On The Road – ETA to Destination 15 mi”.
When I hit my destination with no cell coverage (but iGates or Digipears still available), I will set my status text to:
“At Destination – Joe Smith State Park”.
At any given time, they can look us up on their phone or computer to see what our status was and if we made it to our destination safely. Let’s look at one more example and examine how the automated sending can be beneficial.
Use Case 2 – Emergency Use
I am called into work for a recent disaster and have to leave my family to assist with rescues and medical support. Cell coverage is intermittent or unavailable, power is out in a large portion of town, and internet is limited due to power.
I have an APRS capable radio with my family and one with myself. I can leave my radio on showing my current status “Status: Ok” and update it as needed while I am out. I can set it and leave it temporarily to go back to work.
Back at home, my family can look in the list of received data on their radio (usually known as the APRS history) and check my status. All via radio.
I basically have a “black box” logger to keep my family posted of my whereabouts and status. But how do I find out about their status? Do we have power at home? Is there internet? Do we still have a roof on our house?
They can share this information in the status text on their radio transmitting APRS. When I have a break and can go check my radio, I can see their status in my APRS history.
Use Case 2 Continued: How Can We Make It More Secure?
Maybe you are concerned with your Operational Security (OPSEC) and don’t want to risk this information plastered across the internet or known by others who monitor APRS. No problem! Status text can still be a viable solution. As part of your family or team communications plan, you set up a list of “signal codes” to be used as status text. These can be pre-programmed into your radios. For example:
Status: Ok = All Ok, No Problems
Status: Signal 1 = Family Secure, No Power
Status: Signal 2 = Family Secure, No Power, House Damaged
Status: Signal 3 = House Unlivable, At Neighbors, Contact Immediately
Status: Signal 4 = Evacuating to Site A
You get the idea. Now we can use these status codes and only those part of my communications plan know what they mean. If you are REALLY concerned about OPSEC you can use status codes with cyphers as part of your communications plan and make it much more secure.
Final Thoughts on Status Text
Bottom line is, the automated transmission capability allows you to “set it, and (temporarily) forget it” unless your status changes. Finally, there is a higher probability that the information will be received by your family members if going through digipeaters when compared to an APRS message (as not all digipeaters accepts APRS messages).
Position comments are fixed and not editable. Depending on your device, the following position comments are available:
- Off Duty
- In Service
- EMERGENCY! (only for emergency use at it will physically alarm other ARPS stations)
This information can be a useful tool to define the current “priority” of your situation. Typically, the most common seen is Off Duty, Enroute, or In Service. From a “general use” perspective anything other than “emergency” is not considered high priority. EMERGENCY! However, will physically trigger alarms on other ARPS stations that hear the message unless they have turned off the settings in their APRS unit. This comment does gain attention and should only be used in emergency situations.
Messaging allows you to contact a single user or group directly via the APRS-RF system. Think of it as a text message through APRS to another user. By entering their callsign or selecting them from my APRS history, I can send a message via radio. Each message will send up to 6 times (depends on the settings in the radio or software you are using), each time waiting for an acknowledgement from the receiving station. If it receives an acknowledgement, it will stop sending.
Messaging is great for more private messages or announcements amongst a group, when you receive a message in your inbox, there is a special alert notification on your radio to let you know and usually an icon notification on the display as well.
More Private But Limited Range
If within 50-75 miles or less of your party, it can be a useful messaging tool when there is no cellular infrastructure available (otherwise a simple group text would be easier). Beyond this distance, they may become unreliable as some digipeaters do not accept APRS messages and they may not be received by the intended party if there are multiple digipeater hops.
Who Can See the Message?
While these are sent over the air and can technically be seen by all monitoring the raw packet data, APRS radios and software do not post them in the radio message inbox unless you are the intended recipient. They are not secured or encrypted and do pass over the airwaves, so keep that in mind.
Ping Position to Get Messages
You should always be beaconing your position if you want to get your APRS messages. While these are two separate “features” of APRS, digipeaters look to see who is on them. This information is then used to route APRS messages.
If you are just waiting for a message and not sending a position, you are not using the digipeater. Therefore, if someone sent you an APRS message and that digipeater hears it, it won’t send it to you because it doesn’t think you are there.
If you want the best chance to get your APRS messages, it is best to be beaconing your location. Even if not moving.
More on Security
Keep in mind also that APRS doesn’t just have to be ran on 144.390MHz. It could be ran on another amateur VHF/UHF frequency for a more “private” APRS set up for your group. The APRS data is still readily available for anyone tuned to that band with APRS mode engaged, but they would have to know where to listen. However, without setting up your own, coverage and range is limited because there are no Digipeaters or iGates listening.
APRS Packets Don’t Always Follow the Same Path
When APRS packets are going through digipeaters, they don’t always take the same path when multiple digipeaters are in the area. Remember digipeaters “repeat” the packet but have a limited number of hops. The route your packet takes is completely dependent on what digipeater it hits first, who hears it second, etc.
Why Is This Important?
- When sending APRS messages: As they may not always get to the intended recipient because of the path the packet takes across the digipeater hops. Because of this, you may need to retry your message multiple times.
- Positional Packets and Status Text: On the brighter side, with positional packets, the multiple route concept can be beneficial as your first packet transmission may not hit an iGate, the second or third packet transmission taking a different path does, thus posting your information online. The automatic repetition of positional packets allows for multiple “attempts” and different routes to get your message out further via radio and higher potential to get it on APRS-IS.
APRS vs a Satellite Communicator
Satellite text communicators and satellite phones are an extremely effective communications solution and allow you to get a message across at nearly any location across the globe. I have used satellite devices in areas where the only two things that got out were HF radio or satellite. I highly recommend them for families who don’t have ham licenses or need the easiest method to get a message across. You know, the KISS method (Keep It Simple Stupid).
The downside to these solutions? They come with a price. They require an initial hardware cost of anywhere from $300-2000 dollars plus monthly fees for service at rates of up to $5 per minute. And in most cases, if you don’t use the time, you lose it.
Looking for a cost-effective satellite solution? We recommend and use the Garmin InReach which operates on the Iridium Satellite network. It provides, two-way text messaging via satellite for a great monthly price. At this time of this writing, the cheapest plan available is around $12 a month for a estimated total of $144 a month per unit. After you buy the hardware of course.
In our area we have a robust APRS network. APRS requires just the initial cost of the equipment and is much more cost effective over the long term.
Once APRS is set up, it is easy to use. With a little practice, anyone, even school-age children can be taught how to initiate an APRS beacon or send an APRS message.
What do we use? We use both. We use APRS for keeping our ham group, family ham operators, and radio base stations up to date with our current status during trips and emergencies. We invested in Garmin InReach in order to have non-terrestrial communications as a Plan B in our preparedness toolbox. However the investment was also made because the InReach can be used during hiking trips in the wilderness and while away on business travel, situations where carrying a mobile APRS units was not feasible.
Both work well, the APRS solution is much more cost effective over the long run comparatively speaking, but remember, these are two different communication technologies each with their pros and cons.
Take a few minutes and review your emergency communications plan. If cellular infrastructure goes down, do you have a way to continue to communicate and share vital information with family and friends? Consider APRS and how it can fit into your plan.
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If this article was useful for you, checkout our related articles on the 4 T’s of Emergency Communications Planning and Ham Radio for Beginners: Simplex and Duplex Communications for Preparedness.