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Globalstar Testing

While on a Memorial Day trip to visit family I decide to test the Globalstar satellite telephone in typical situations where a sportsman might be using it. The test was conducted in a heavily wooded area on the border between Pennsylvania and New York.

The Globalstar satellite system has 48-plus satellites in orbit, and any place service is provided there are at least two satellites in view. As a result, the satellites can hand-off a call when the signal strength starts to diminish, thus preventing the call from being dropped. This assumes the caller has a clear view of the sky from about 10 degrees above the horizon. However, in tests around my home, with obstructions much higher than ten degrees (including on my back porch, under a roof), the unit worked very well.

The satellite telephone used was the Qualcomm GSP-1600 in which only the satellite side of the phone was active. The phone was not designed to work in the situations presented here; however, since customers ask me about these situations all the time I wanted to report on what a Globalstar� user could expect.

Test 1 - The Car

In the first trial I was in a van without an external roof antenna. I wanted to check my voice mail while some else was driving the van along the interstate. Seated in the front passenger seat, I pressed the Globalstar� phone against the window, receive a good satellite signal and placed the call. It worked, but I had to keep the phone pressed against the window. There was also a strong chance of the call being dropped because the phone could see less than half of the sky and the other Globalstar satellite(s) would not be in view for the call hand-off. The sky was blocked on one side to 90 degrees and the other side to about 20 degrees.

In spite of these potential difficulties I was able to check my voice mail. I had two calls. The entire voice mail recording was heard well and the call lasted about three minutes. I decided to return one of the voice mail calls and was able to contact the caller and carry on a conversation with him. The call was getting "choppy" and sounded like it would fail soon, so I cut the conversation short.

By the way, if your call is dropped, you simply press the "send" key again and the last number called will be re-dialled.

Test 2 - Light Woods

This test was made walking along a horse trail in the woods. The trees were less than 20 feet tall, less than 6 inches in diameter and spaced about 20 feet apart. The branches provide light shade against the sun, but the sky was still easily seen. Other trees and hills obscured the horizon.

The satellite phone held a good signal level (4 bars) most of the time, but fades to zero were occasionally observed as we walked. Signal strength was quickly recovered, however.

A three-minute test call was placed. The call was successful and voice quality was good.

Test 3 - Heavy woods

In this test the trees were large, 12 to 24 inches in diameter, up to 100 feet tall and spaced 20 to 30 feet apart.

When walking in the edge of the woods, typically along an open field, the signal levels were strong (4 bars) with occasional fades, much the same as the light woods tests. A call of two minutes was placed with no problems.

We then ventured into the deep woods. Trees blocked the horizon, and the sky peeked through the branches only above 45 degrees. A call was attempted and the first try failed. The voice was robotic; that is, the fades were cutting out portions of the call so the voice was broken. The call did not drop, but seemed to come close many times during the three-minute duration. We were able to talk to each other, but I felt it necessary to frequently repeat things in order to ensure I was being understood.

Test 4 - Heavy woods and ravine

I conducted the final test in heavy woods in a ravine. The idea was to simulate someone falling down in a ravine while hunting or hiking. I sat on the ground while placing the call. The sky was visible at about 45 degrees on one side and 60 degrees on the other.

Patience was needed to wait for a satellite signal; a good signal appeared only every 2 to 3 minutes. The satellite phone would quickly register and a one-minute call could be placed before the call faded out. The call could be re-dialled a couple of minutes later. The voice was "choppy" but intelligible. It would be necessary to talk slowly and precisely to get any emergency information out, but it would be possible.

Why did it work this way?

The Globalstar satellite system works the opposite of the way we are used to seeing a ground- based cellular system work. In a ground-based system, the cell sites are fixed and the caller moves through them. An area that does not have cell coverage will always be blocked.

However, with the Globalstar satellite system the caller basically stands still and the cell sites pass overhead. Typically, with a perfect horizon-to-horizon view, there will be two or three satellites within sight of the caller. The caller is only linked to one satellite at a time, but the other satellites are there in case the caller's signal level begins to drop. The satellites communicate with each other about the signal strength of the caller's unit, and when the signal level begins to fade the connection is handed to another satellite that is receiving a stronger signal.

In the tests we conducted we never had horizon-to-horizon coverage. In fact, we often had only a single satellite in view, and thus there was no other satellite to which our call could be passed. As a result we saw the unit loose satellite contact until another satellite came into view. The more sky that was visible, the quicker a new satellite would be acquired.

In our most extreme case the unit in a wooded ravine it took several minutes before a satellite passed nearly directly overhead. Even then, we only had a good signal for a minute or so before the satellite sank behind the hills and trees.

*** Here is some technical info direct from Globalstar that explains a bit more about the operation of the satellite phone:

The handset can get the signals from two satellites. Both are being "tracked" at the same time. The signal from each satellite is combined together to give 3 dB gain minus some small combining losses. The Third Satellite's power is being tracked to serve as a possible hand-off when either of the other two Satellites' power levels decrease to a point. This is the user getting the signal from the Satellites.

The technical term is called Diversity with Maximal Ratio Combining. It is the optimal design for a Maximum Likelihood Ratio Receiver.

The other direction is even better. Up to 6 signals from the user terminal can be collected at a gateway via the Satellites, either from multiple beams on a Satellite, or from different Satellites. These are combined using maximal ratio combining to really get all the power advantage possible, without the combining detracting too much from the process. So in principal the links are made stronger in both directions by MULTIPLE LINKS actually being in place. The handoff is also more smooth since there are only two of three links actually transitioning. So Fading or cutting out does not mean that the only link is about to die, but that the two out of three links are about to switch and the new one will gain strength in a minute.

It is even better for the return link where up to six signals can be followed at a time. But of course it is more important here too since the User Terminal is power limited.

Bottom line

The point I am making is that with the Globalstar satellite system you will eventually get a good signal from the satellite if you can see the sky at all. In an emergency, this will be the "window" during which you can make a quick distress call. You can improve the duration of the call if it is possible to move to a position where your view of the sky is better.