Operating North of 72N

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Lou
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Re: Operating North of 72N

Post by Lou »

This from TC AIM:

"All GPS orbits cross the equator at a 55° angle, so it is not
possible to see a GPS satellite directly overhead when north of
55° N or south of 55° S latitude. This does not affect service in
polar areas adversely; in fact, on average, more GPS satellites
are visible at high latitudes since receivers can track satellites
on the other side of the pole."
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Rich
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Re: Operating North of 72N

Post by Rich »

Lou wrote:"... on average, more GPS satellites
are visible at high latitudes since receivers can track satellites
on the other side of the pole."
I think this is a myth/urban legend. At high latitudes (starting around 50 degrees) you will see GPS satellites on opposing longitudes at the moment when they're the farthest North but as you go North you also lose satellites that are more Southern on your "own side" of the globe.

I did some digging and found a couple of examples of using GPS right at the North Pole, which show 3-4 suitable satellites available. I haven't been able to find any other cases showing more. Right now sitting inside my home (roughly 44 deg N latitude) I show 9 usable satellites. It seems to vary between 8 and 10.

But I think WAAS functionality in North America gets compromised as you move North. There are couple of reasons. For one, once you get beyond 80-ish degrees N. (depends somewhat on altitude and relative longitude) you lose line-of-sight to the geosynchronous satellites, The WAAS correction satellites are in this orbital path. And the ground stations (known as Wide-area Reference Stations - WRS) that send the critical correction information to the WAAS are very sparse. There are none in Yukon or NW territories. The 2 most Northern are at Point Barrow and Uqaluit in the far eastern portion of Nunavut. Barrow is the most Northern, around 70 deg N.
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Re: Operating North of 72N

Post by hkavasch »

we are planning a flight from Spitzbergen to Alaska.. over the northpole..
have been warned about the issue... One guy told me, that GPS gets mad if you circle the northpole (e.g. to take pictures..)

but what is the resolution? Handheld GPS? Which brand will work 100% sure? Compass is useless... only Sun could be used... What are the recommendations and experiences?
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Lou
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Re: Operating North of 72N

Post by Lou »

The map showing current WAAS LPV availability is here: http://www.nstb.tc.faa.gov/RT_VerticalP ... nLevel.htm
There is a slight reduction in service in the Eastern Arctic, likely because it is further away from Alaska? After all, it is a US system and Eastern Arctic service is not a priority.

Here's another data point. I just checked my hand held GPS: at 53 31'N I can see between 10 and 11 satellites. Seems to give some credence to the TC AIM paragraph quoted.

Hans, I think the commercial operators use inertial systems. There is a lonely VOR at Alert that I would guess is used to set them.
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Re: Operating North of 72N

Post by Rich »

It might be worth a clarification of my previous post. I was quoting the number of satellites that the Stratus had signal lock on. Right now that number is 11. The total number of satellites visible at this moment is 14.
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Re: Operating North of 72N

Post by rwtucker »

In the links I posted on the previous page, there was some discussion that the observed limitations might have been built into some GPS software rather than being a theoretical limit based on satellites. If so, one then wonders why that would be. The Earth is flattened at the poles and bulges at the equator so this might represent a minor challenge for polar GPS but nothing that was not overcome analogously at the equator.
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Re: Operating North of 72N

Post by Rich »

Just because I contribute little to society these days, here's a bit of moderate thread drift.

Over the last couple of hours I've monitored the GPS Status page in Foreflight with Stratus sitting at my home here in central Oregon.

Number of visible satellites has varied from 13 to 16, 16 being rare. 14-15 is most common.

Number of satellites "locked in" (whatever that means, but they're green, so it must be good) varied from 7-12, 10 being the most common. This number is constantly changing, several times per minute.

At no time have a seen it show any satellites in the Northwest quadrant of the map. I don't know for sure what that means, as there are no meaningful obstruction over that piece of sky. But I was also monitoring this site:

http://www.nstb.tc.faa.gov/RT_WaasSatelliteStatus.htm

This did show a scarcity of satellites in this area during this time, though not zero.
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Re: Operating North of 72N

Post by ThomasD »

hkavasch wrote:we are planning a flight from Spitzbergen to Alaska.. over the northpole..
have been warned about the issue... One guy told me, that GPS gets mad if you circle the northpole (e.g. to take pictures..)

but what is the resolution? Handheld GPS? Which brand will work 100% sure? Compass is useless... only Sun could be used... What are the recommendations and experiences?
For the flight that I filmed that landed at the North Pole they used a simple hand-held GPS; I think it was a Garmin eTrex that only displayed lat/long [no moving map] but it seemed to work well.
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