Boatist
Rear Admiral
- Joined
- Apr 22, 2002
- Messages
- 4,552
Re: GPS Internal or External antenna???
The basic question was internal or external antenna.
The difference is the external antenna is less likely to be blocked by something on the boat. Things like a depth finder, Radio, window frames, person near the unit can block the signal.
GPS accurracy is not affected much by the signal going thru the atmospheer. On Loran C the signal is affected by the signal going thru the atmospheer because to travels a long way thru the atmospheer.
GPS accurracy is affected more by sattelite wobble in orbit and orbits that are not perfect.
To correct for this the WAAS error correction was put in place. What it amounts to is a serries of ground stations that know their location and when the 11000 mile high GPS satellites come by and tell the station it 30 feet off from it actual position the ground station tell the WAAS Satellites that are in Geostationary orbit 34800 miles high over the equator of the error and a signal it sent back down to GPS units that have WAAS capibilty. It basic tell your unit that in the sector you are in this satellite is off by so much time or distance. Your WAAS GPS unit then corrects for the signal it got from the GPS Satellites and give you a more accurate position.
Now since the WAAS Satellites are in Geostationary orbit they must be over the equator. This makes very little difference if you are in Africia. As you go farther North or South the WAAS Satellite get lower and lower on the horizion. The lower on the horizon it is the more likely it is to be blocked by something on your boat. With the external antenna it is usually mounted up higher on your boat than the where you have the unit mounted and less likely to be blocked.
Also most GPS units will not lock on to weak signals. Most of the time there is no need since units will usually see 8 or more Satellites, but if someone is standing up driving they can block at least 1/4 of the sky from an internal intenna. Not likely to block near as much of the sky from an external antenna. The unit will use at leas 4 birds all the time and many times more but if you body blocks the best sattelites for your position then your location will be less accurrate.
Southern states it is not a big deal but Snorthern states it can be a big deal.
Since they added WASS satellites in 2006 and moved some around it has been very easy to pick up WAAS satellites even with a handheld as long as you do not block the GPS signal fom the south.
Some area of the States like California can now see 4 WAAS birds.
Current locations of WAAS birds are. 0 degrees latitude and
POR Pacific Ocean Reagion #47 at 178 degrees East Longitude.
AOR-W Atlantic Ocean Reagion moved to the Pacific # 35 at 142 degrees West.
ANIK #51 at 107 degrees West.
INMARSE # 34 at 53 degrees West.
AOR-E Atlantic Ocean Region East #33 AT 15.5 degrees West
IOR-W Indian Ocean Reagion West # 39 AT 64 degrees East.
.
The basic question was internal or external antenna.
The difference is the external antenna is less likely to be blocked by something on the boat. Things like a depth finder, Radio, window frames, person near the unit can block the signal.
GPS accurracy is not affected much by the signal going thru the atmospheer. On Loran C the signal is affected by the signal going thru the atmospheer because to travels a long way thru the atmospheer.
GPS accurracy is affected more by sattelite wobble in orbit and orbits that are not perfect.
To correct for this the WAAS error correction was put in place. What it amounts to is a serries of ground stations that know their location and when the 11000 mile high GPS satellites come by and tell the station it 30 feet off from it actual position the ground station tell the WAAS Satellites that are in Geostationary orbit 34800 miles high over the equator of the error and a signal it sent back down to GPS units that have WAAS capibilty. It basic tell your unit that in the sector you are in this satellite is off by so much time or distance. Your WAAS GPS unit then corrects for the signal it got from the GPS Satellites and give you a more accurate position.
Now since the WAAS Satellites are in Geostationary orbit they must be over the equator. This makes very little difference if you are in Africia. As you go farther North or South the WAAS Satellite get lower and lower on the horizion. The lower on the horizon it is the more likely it is to be blocked by something on your boat. With the external antenna it is usually mounted up higher on your boat than the where you have the unit mounted and less likely to be blocked.
Also most GPS units will not lock on to weak signals. Most of the time there is no need since units will usually see 8 or more Satellites, but if someone is standing up driving they can block at least 1/4 of the sky from an internal intenna. Not likely to block near as much of the sky from an external antenna. The unit will use at leas 4 birds all the time and many times more but if you body blocks the best sattelites for your position then your location will be less accurrate.
Southern states it is not a big deal but Snorthern states it can be a big deal.
Since they added WASS satellites in 2006 and moved some around it has been very easy to pick up WAAS satellites even with a handheld as long as you do not block the GPS signal fom the south.
Some area of the States like California can now see 4 WAAS birds.
Current locations of WAAS birds are. 0 degrees latitude and
POR Pacific Ocean Reagion #47 at 178 degrees East Longitude.
AOR-W Atlantic Ocean Reagion moved to the Pacific # 35 at 142 degrees West.
ANIK #51 at 107 degrees West.
INMARSE # 34 at 53 degrees West.
AOR-E Atlantic Ocean Region East #33 AT 15.5 degrees West
IOR-W Indian Ocean Reagion West # 39 AT 64 degrees East.
.
