Inmarsat At High Latitudes?

[ka4wja]

For simplicity, I did not provide details earlier....but I got a question about this....so...

Just a clarification, when I wrote earlier that Iridium coverage is the SAME everywhere on the Earth, although it might seem counter-intuitive, it IS the truth...

Originally Posted by Delancey

One quick thought, is Iridium coverage actually better at the poles than at the equator, for having the polar orbit? As in, because the pole-orbiting satellites cross the poles while the earth spins, do they actually end up spending more time at the poles than they do at any other point over the surface of the earth?

While the satellites are closer together at the poles, the Iridium system's inter-satellite-linking does not occur with high relative deltaV (which happens with satellites going in opposite directions, near the poles)....and most importantly to this particular point, Iridium satellites actually shut-down some of the communications cells on-board as they approach the poles...Here is a quote from a 15 yr old paper.

The vehicle footprints overlap greatly near the poles. In order to reduce interference between so many spacecraft in one area, the constellation begins to shut down specific cells on individual spacecraft in these areas.

As, you can see from this one example, in addition to still being singularly unique, the Iridium system is quite elegant....
(oh, and it works good, too!)



I hope this helps explain why/how Iridium coverage IS the SAME everywhere...

Fair winds..

John
s/v Annie Laurie

[svlamorocha]

Originally Posted by ksanders

I use satellite at high latitudes all the time because that is the area that I am home ported out of.

We are right now at 60 degrees north and get 2 mbps download speeds.

We use a KVH minivsat service, which is not tied to any one satellite constellation. KVH rents transponder space from a variety of companies to provide global coverage.

I disagree unless the caveats are spelt out. There are three versions of the KVH Mini VSAT service. If you see the maps in the link below you will find that:

- the V3 (most common) and V7 services are limited to Ku band and have very little coverage in the oceans of the Southern hemsphere.

- the V11 service also has C-band that gives decent coverage in the Southern hemisphere, but it does not have global coverage. These are geostationary satellites with the limitations discussed upthread.

miniVSAT Broadband Airtime Service

I experienced all this when crossing to the Marquesas and the Mini VSAT V3 would not work past the Galápagos. Back to good old SSB and Pactor!

KVH´s response to those who complain about this shortcoming ios to make them get an Iridium Pilot unit that has global service.. See KVH TracPhone V3-IP Goes Fully Global with Integrated Iridium OpenPort Broadband Service

All that said, I suspect we have not said that Iridium´s communication requires more than two hops, hence at times you may not get through even if you have good signal to the satellite that you are connected to.

Charlie

[Dauntlessny]

Originally Posted by Delancey

I have a friend who worked in the media business as a producer for war-correspondents, and later a bureau chief for a major network.

In his opinion Iridium if fine if you want to chit-chat with grandma and grandpa but in his professional experience, if the call you are making is a call that counts, the last thing you want is to drop it, and because of this he is very firm in his endorsement of Inmarsat.

He is a very savvy guy and I tend to place a lot of stock in his opinion on something I have no experience with. However, I know that most of his experience tended to be in what you might call "hotter" climates than where we are going, fifty north or higher.

From my reading of an Inmarsat see-me map I found somewhere it looks like the altitude of the geostationary satellite where we are headed is <20 degrees, which seems pretty low to me. Any thoughts?

Second question, is there a basic way to work out the declination of the azimuth of the satellite? Or is it far enough away that if you aim in a southerly direction and more importantly have the right altitude, you can get a link?

If you are looking for simple comm, check out
InReach

They have text plans to match with GPS that can be pinged.

Also, you should be able to find satellite azimuth by doing a google for it.

[ka4wja]

Just a heads up guys...

Delancey mentioned he is looking for an inexpensive handheld sat phone, for a trans-Atlantic delivery, in order to make a few phone calls (not for on-board weather, etc.)....

Those of you suggesting a $75,000 KVH V11 Vsat system (or even a smaller $15,000 - $25,000 Vsat system), are a bit over-the-top....don't 'ya think??

Just saying he's gotten his answers....and it ain't gonna' be a 240lb, 4 foot dia, 5' tall, $75k dome!!


Fair winds..

John
s/v Annie Laurie

[svlamorocha]

Originally Posted by ka4wja

Just saying he's gotten his answers....and it ain't gonna' be a 240lb, 4 foot dia, 5' tall, $75k dome!!

Sorry; I thought it still there was no consensus that a plain Iridium phone is what OP needs...

Cheers C

[ksanders]

Originally Posted by svlamorocha

I disagree unless the caveats are spelt out. There are three versions of the KVH Mini VSAT service. If you see the maps in the link below you will find that:

- the V3 (most common) and V7 services are limited to Ku band and have very little coverage in the oceans of the Southern hemsphere.

- the V11 service also has C-band that gives decent coverage in the Southern hemisphere, but it does not have global coverage. These are geostationary satellites with the limitations discussed upthread.

miniVSAT Broadband Airtime Service


I experienced all this when crossing to the Marquesas and the Mini VSAT V3 would not work past the Galápagos. Back to good old SSB and Pactor!

KVH´s response to those who complain about this shortcoming ios to make them get an Iridium Pilot unit that has global service.. See KVH TracPhone V3-IP Goes Fully Global with Integrated Iridium OpenPort Broadband Service

All that said, I suspect we have not said that Iridium´s communication requires more than two hops, hence at times you may not get through even if you have good signal to the satellite that you are connected to.

Charlie

Thanks for pointing out the limitations!

For my V3 unit I am glad that they have coverage along the coastlines of pretty much everywhere on the planet. For ocean crossings I agree you would need another unit.

The good thing is for my situation, where I need coverage in the North Pacific and eventually along the coastlines of north and Central America they have coverage.

[N3VR L8]

Originally Posted by Delancey

Preaching to the choir.

In this situation, the owner has neither the finances or the need for it once the boat gets to the other side, so for us it's a case of opting for lower cost and push-a-button functionality of a sat phone over the higher cost and learning curve of the radio.

I am at work (yuck) so will watch the vid later, but I will, and thanks again. Cheers!

Another thing to keep in mind about "the call that counts" is I assume the phone will be off until that call needs to be placed. Both the Isatphone 1 and Isatphone 2 have registration times that are substantially longer than an Iridium phone and do require you to point the antenna. An Iridium phone will register as soon as it is powered up.

I would not recommend the Isatphone 1 at all, it simply isn't on the same build level as the Iridium handsets. The Isatphone 2 is much nicer, durable, but very large.

If it were me and my life were potentially on the life, it would be Iridium.

Let me know if you have further questions about either, I'd be more than happy to answer.

[Delancey]

Just to follow-up, our plan is to rent an Iridium, with the low altitude of the Inmarsat satellites when viewed at high latitudes being the determinative factor, which is pretty much the answer to the OP.

I did chat a bit with my friend again and he mentioned experience using both types of phones. When I explained that it was sounding to me like we would be far enough on the fringe of Inmarsat coverage, he conceded that if that were the case it would be beyond his knowledge. I think his preference has to do with dropped calls.

Yes, Iridium has a vast armada of satellites circling the globe but I think as they zip overhead they have to trade off your call and sometimes it gets dropped. Whereas with Inmarsat, once you get a link you can talk as long as you want. Specifically he mentioned calls of an hour or more without concern.

Haven't quit nailed down my vendor for the rental, any recommendation would be appreciated.

Thanks

[N3VR L8]

Originally Posted by Delancey

Just to follow-up, our plan is to rent an Iridium, with the low altitude of the Inmarsat satellites when viewed at high latitudes being the determinative factor, which is pretty much the answer to the OP.

I did chat a bit with my friend again and he mentioned experience using both types of phones. When I explained that it was sounding to me like we would be far enough on the fringe of Inmarsat coverage, he conceded that if that were the case it would be beyond his knowledge. I think his preference has to do with dropped calls.

Yes, Iridium has a vast armada of satellites circling the globe but I think as they zip overhead they have to trade off your call and sometimes it gets dropped. Whereas with Inmarsat, once you get a link you can talk as long as you want. Specifically he mentioned calls of an hour or more without concern.

Haven't quit nailed down my vendor for the rental, any recommendation would be appreciated.

Thanks

Inmarsat satellites are geostationary, meaning 20,000+ miles up, so certainly you only need to talk to one bird.

Your concern with Iridium satellites zipping by and having to hand off calls is a bit misunderstood IMO. It takes 9-10 minutes for one bird to go from horizon to horizon... yes the Iridium birds hand off calls, all the time. The Iridium earth terminals only talk with one satellite at a time, so unless you are in Arizona (or very near) you can bet your calls are being passed from bird to bird to bird (and so on) until they reach the one bird passing over Arizona at the moment.

Let me know if you have any questions

[chala]

Originally Posted by ka4wja

3)…
There is NO pointing required for using an Iridium handheld phone, nor any Iridium device....as their satellites orbit at only 485 miles altitude, vs. 22,300 miles....


5)…
Regarding "pointing"....as I wrote above, Iridium requires NO pointing at all...

In the case of Iridium would the mounting of an external aerial on the top of the mast improve the communication or would the greater movement of the aerial at the top of the mast deteriorate the communication.

[ka4wja]

chala,
Typically you want the satcom antenna to be mounted low, and in a stable environment as possible....
So, "top-of-the-mast" mounting is a NO!!!
Also with Iridium, using a "passive" external antenna, you want to use a little cable as possible between the phone and antenna, thereby reducing signal loss....(you do NOT want 75' - 100' of cable...)

Originally Posted by chala

In the case of Iridium would the mounting of an external aerial on the top of the mast improve the communication or would the greater movement of the aerial at the top of the mast deteriorate the communication.

But FYI, you do need to mount it in a location that has as good of view of the sky as possible....
Think of it similarly to a GPS antenna, just one that needs to be outdoors....


I hope this helps...

Fair winds...

John
s/v Annie Laurie

[chala]

Thanks John.

If you add the risk of lightning strikes and extra weight up the mast it does not make the mounting of the satcomm antenna at the top of the mast very attractive. Furthermore I sail in the tropics and random motion (pitch, roll) can make good communication hard to achieve.

[N3VR L8]

Originally Posted by chala

Thanks John.

If you add the risk of lightning strikes and extra weight up the mast it does not make the mounting of the satcomm antenna at the top of the mast very attractive. Furthermore I sail in the tropics and random motion (pitch, roll) can make good communication hard to achieve.

Chala,

An Iridium passive antenna weighs about a pound.. I would definitely recommend having it high enough so that your mast and or rigging does not obstruct the line of sight between the antenna and the bird.

As far as cable runs, dB loss is important, so the longer the run the thicker diameter cable.

Do you have a specific cable run length in mind?

[chala]

Originally Posted by N3VR L8

Do you have a specific cable run length in mind?

The length would be at least 19 meters (62.33 ft). I am not yet convinced that the improvement given by installing an antenna at the top of the mast justifies such an installation. Also one pound at the top of the mast would require an extra 17.5 pounds to balance the load of the antenna.

[estarzinger]

Originally Posted by Delancey

From my reading of an Inmarsat see-me map I found somewhere it looks like the altitude of the geostationary satellite where we are headed is <20 degrees, which seems pretty low to me. Any thoughts?

Second question, is there a basic way to work out the declination of the azimuth of the satellite? Or is it far enough away that if you aim in a southerly direction and more importantly have the right altitude, you can get a link?

Don't know if the OP is still interested . . . but, just fyi you can in fact maintain an inmarsat connection all the way thru the NW Passage. The coverage chart makes it look iffy, but I have worked with two boats that have done it. It has been the gear of choice for people who have to maintain work VPP type connections, while iridium has been the primary choice for the "gribs and e-mail" crowd. Iridium pilot might be able to give you the 'work bandwidth' but I at least don't have much feedback from people actually using it.

Second, you can in fact get exact locations for the sat's (its public info), so can work out their exact azimuth if you want to. But the inmarsat equipment I am familiar with on boats have all done this tracking automatically.