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I was reading on one of the GPSr maker's sites about the satellites in the GPS network. It stated that the transmitters only put out about 50 watts. Now I understand that the sats are solar powered so that is a real limitation but even at that 50 watts seems pretty small. If I remember correctly, that's even less then the amount of power that novice ham radio operators can use. It would seem to me that having transmitters even a bit more powerful, say doubling the output, would help a lot with penetrating tree cover and the like. So what am I missing here? <_<

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If I remember correctly, that's even less then the amount of power that novice ham radio operators can use.  It would seem to me that having transmitters even a bit more powerful, say doubling the output, would help a lot with penetrating tree cover and the like.  So what am I missing here?  <_<

It's also 10-20 times the amount of power that many hams use to talk around the world every day. Without getting too complicated, doubling your transmit power only results in a few db signal increase on the other end. So, increasing the power to 100w would make very little difference on our end, but would triple the power requirements of satellite (RF power amps are not all that efficient).

Edited by Mopar
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Adding some more info to the mix, power makes heat and heat is hard to get rid of in space. Circuit cards etc can't be cooled with a fan in space. You can mount heat-sinks (extra-weight) or allow heat to radiate out, but you don't want to generate any more heat than necessary.


When the space shuttle is launched and in orbit they always roll it over to the most optimal angle to take care of their thermal management.


The problem of tree cover is more one of our antenna than their signal. Those of us carrying etrex units wish there was an external antenna port to allow us to plug in something a little more sensitive while out in the trees.

Edited by LETaylor
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I've never heard that fifty watt number. I though it was more like 500 to 750 watts, but I don't have a source to back it up. All the satellites are never of the same vintage so the actual specifications aren’t necessarily the same for any two satellites.


Newer ones have more accurate clocks and probably more powerful transmitters. The next generation in work will have further improvements, but there is a limit to how much power is desirable. If the signal gets strong enough, then the receiver will start picking up multi-path signals more often, rendering the offending satellite worse than useless.

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I've read 50 watts or less, but that's transmit power, what you'll get on the receiving end will be considerably less. As with any transmission at that end of the radio spectrum, line of sight is far more important than wattage. Flying at 37,000' we have no problem talking to ground stations many miles away with only 4 watts.


The same is true for FRS radios. Climb to the roof of a five story building and you'll easily double the range of your radio.

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It's 50 watts transmitter output, with a +13 dB downlink antenna. Thus the effective radiated power is about 1000 watts, focused in a narrow beam toward the Earth. If the effective power was only 50 watts, from 12000 miles altitude, you could bet that the piece of garbage antenna inside the Etrex would never receive it. The path loss for such a long range is dozens if not hundreds of dB.


It is very difficult to get a high power transmitter up to space. Many low earth orbiting sats downlink on 5 watts, some less than a watt. To get 20-50 watts out of a satellite, continuously, is quite a feat. And upping it to 100 watts will only give you a 3 dB increase, which will not help nearly as much as the installation of a proper antenna on the receive end. The transmit antennas are already pretty much maxed out, at +13 dB. Make them higher gain and you will decrease the beamwidth even further, which may cause them to not cover all the ground that they should. Now that I've mentioned that I need to do the math to figure that out, to satisfy my geeky curiosity.


A good antenna could give you 10-20 dB of gain. To accomplish the same on the transmit end you would need to increase power to between 200 and 1000 watts, which is completely out of the question.

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Under powered? wouldn't call them underpowered (as such), they do what they are supposed to do but with most things there is a compromise and trade-off. Simply blasting out heaps of power would create all sorts of other problems.


The GPsIIR-M's in fact will have some increase in transmit power but this has required new design heat dissipation systems. With the IIR-M's and probably anything following the power is now re-allocatable between the different codes.


Cheers, Kerry.

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