Satellite Data Connections Explained

Here at OCENS, we often receive calls from customers having issues using data over their handheld Iridium, Inmarsat, or Globalstar handset. Of these calls, 99.99999998% of the time they end up being an issue with poor signal strength. So, today I thought we should help explain what’s occurring when attempting to use a data service over a handheld satellite phone and what kinds of things can affect having a successful connection.

First, let’s explain the differences between the satellite systems, or “constellations,” as the type of constellation has a lot to do with how your satellite phone interacts with it.

 

Satellite constellations for sat-phone services come in two flavors, LEO or GEO:

LEO stands for Low Earth Orbit and is what Iridium and Globalstar use. In a LEO constellation the satellites are orbiting the planet. Iridium’s constellation, for example, has 66 satellites that polar orbit (on a north-south-north orbit) at a speed of 16,832 mph. At that speed, it takes roughly 100 minutes for any given Iridium satellite to do a complete orbit of the Earth. To help bring this closer to home; at those speeds it means the satellites take roughly 9 minutes to move from horizon to horizon. Suffice it to say, those satellites are moving FAST.

The advantage of a LEO constellation is that you can have truly global coverage. No matter where you are located on the Earth if you have a clear view of the sky you will have a satellite available. The disadvantage, however, is that because the location of the satellites are constantly changing so is the quality of the signal. Also, various elements of the area around you can have a great impact on your signal as the location of the satellites change. We will discuss both of these issues in depth later in this post.

 

GEO stands for Geostationary. An example of a GEO constellation would be Inmarsat’s satellites. Inmarsat’s I4 satellites (providing service to their iSatPhone and FleetBroadband/BGAN systems) are a 3-satellite, High Orbit Geostationary constellation sitting at 22,240 miles above the Earth at the equator.

The advantage of a GEO constellation is that as long as you have “Line of Sight” to one of the satellites, your signal from the satellite is assured. The disadvantages of a GEO, however, are that if you lose “Line of Sight” to the satellite, you will never regain signal until you move to a position where “Line of Sight” is restored. Also, as you move to higher latitudes north or south the angle of your “Line of Sight” drops closer to the horizon and the distance through which your satellite phone must operate to reach the satellite increases.

 

Signal Strength

When talking about signal strength, it’s important to remember that it’s a two-way transaction between the satellite and your satellite phone. The “signal bars” that display on your phone show how well your phone can “hear” the satellite; but they do not, however, show how well the satellite can hear the phone.

When trying to explain the communication relationship between satellites and satellite phones, I like to use a reference involving two people having a conversation. One of those individuals is speaking in a normal tone of voice.  The other has a megaphone. It’s also important to remember that this communication is “Line of Sight” meaning that both parties have to be looking each other in the eye while having this conversation (kind of a funny conversation, isn’t it??)

When signal issues occur, it literally means one of two problems:

  1. Phone can not “hear” the satellite
  2. Satellite can not “hear” the phone

Of those two problems, the MAJORITY of the time it’s because the satellite cannot “hear” the phone because the return signal from the phone is always going to be weaker. Because of this it is much more susceptible to interference.

A popular area where this occurs is in places like a marina where there are an abundance of additional signals occurring within that space: radar signals, RF signals, WiFi signals, and even other satellite devices… all these things can affect the communication between your satellite phone and its constellation. If we go back to our example, imagine a room full of people all talking at the same time and you trying to have a conversation with the megaphone person located on the other side of the room. While you may be able to hear the megaphone clearly, it will be almost impossible to have an enjoyable two-way conversation given the difficulty megaphone man will have in hearing you.

Antenna placement can also affect your signal quality. If you place your satellite antenna too close to something like a radar antenna it’s like trying to have a conversation with a person standing right next to someone else that’s a chatterbox. Now apply the concepts of a LEO, where the satellite (person you are talking to) is constantly moving around the room.  You may be able to barely hear each other while the person is on one side of the room, but as you (or they) move in the direction of the chatterbox, a robust conversation becomes impossible.

 

The Differences between Voice & Data Calls

If you’ve ever called in and talked to one of our technical support people, you’ve probably heard us say that you need “at least 4 bars of signal” when attempting a data call. The reason for this is as follows:

In a voice call, information is sent as it’s created. As you talk, your voice is converted to a signal that’s “streamed” through the system to your recipient on the other end.  Signal strength and quality come into play in how much of your conversation is heard between you and the person you are calling. If you hear words being “chopped off” or “dropped,” or the call itself drops it’s because there is interference. HOWEVER, the over all assessment of the success of your conversation lie with you and your party on the other end. You decide if you understand what’s being said and when to end the call. (The caveat being a complete signal loss, where the call is then terminated.)

In a “Data Call,” however, a whole new set of rules apply. Data is transmitted in containers called “Packets” The majority of data transfers use a protocol called TCP, which stands for Transmit Control Protocol, to send these packets. It’s not really important within the scope of this article to explain all of how TCP works; but what IS important to understand is that this is a two-way rule. When a packet is transmitted, a confirmation is sent from the recipient back to the sender before the next packet is sent. If the sender never gets the confirmation, the connection is considered “Lost” and the data transfer fails. Doing this ensures that all the information requested is received correctly. Applying our example again, imagine having to follow these rules when having a conversation, while also having the “noisy room” conditions explained in the previous section. Kind of a difficult and daunting task, isn’t it?

Because of this TCP rule, signal strength and quality play a MUCH more important role in the ability to transmit data than they do in a voice call. If at anytime one party to a data call does not receive the confirmation it’s waiting for then the TCP rules state that the data is lost.  The data conversation terminates and you have to attempt the data call again, use more of your money, more of your plan’s minutes, and more frustration and headaches. Ensuring that you have the best signal possible when attempting a data call ensures that you have the best chance of making sure both parties in the conversation are able to successfully follow the TCP protocols rules.

 

Hopefully this post helps to better explain what’s occurring when you attempt a data call. If you have any questions, or if you would like to know how to improve your satellite phone’s ability to send data, please give OCENS a call.

Leave a Reply