|
DIFFERENTIAL GPS
(...continued)
Cable for
DGPS
Most folks fabricate a custom cable to work
with the beacon receiver. Here is a diagram for a fairly complicated
version, but you may not need a setup that is this complicated
depending on what else you may be doing.
For simple DGPS connections you can just wire a beacon receiver
output signal along with its ground to the data input terminals
of the GPS. If you need to be able to control the beacon receiver
from the GPS receiver then you will also need to send the
output for the GPS receiver to the beacon receiver. A standard
computer interface cable can usually be used for this connection.
If you also need to talk to a pc at the same time things start
to get a little more complicated. To talk to a pc in NMEA
mode you can simply send the output of the GPS to both units.
Wire the output signal to the input on the computer and the
input on the beacon receiver. There is sufficient power in
the signal from the Garmin to drive both units and even a
third item. Note if the beacon receiver doesn't need to receive
commands from the GPS then there is no reason to send the
signal both places but the ground wire is still needed. Finally
if the GPS needs to talk to the pc in Garmin mode or other
proprietary mode and also to the beacon receiver you will
need a switch to permit the beacon receiver to transmit difference
signals or the pc to interact with the GPS. You won't be able
to do both at the same time. This should not present any real
problems since the bi-directional proprietary mode is used
to upload and download waypoint, route, and track data which
does not need the beacon receiver to be operational.
WAAS (WADGPS)
WAAS, Wide Area Augmentation System,
is the latest method of providing better accuracy from the
GPS constellation. It is similar in principle the the DGPS
capability that is built into all Garmin and many other units
except that a second receiver is not required. Instead of
a beacon receiver the correction data is sent via a geo-stationary
satellite and is decoded by one of the regular channels already
present in the GPS receiver. Thus one of the 12 channels can
be designated to decode regular GPS signals or can be used
to decode the WAAS data. Actually, as currently implemented,
when WAAS is enabled two channels will be dedicated to WAAS.
While WAAS is the name of the implementation of this technology
in the US the system is intended for worldwide use. The generic
name for WAAS is SBAS (Space Based Augmentation System) or
WADGPS (Wide Area Differential GPS). Since most receivers
call the correction signal WAAS as a generic term this article
will use WAAS in this way.
The way this works is that a set of
ground stations all over the US (as shown below) collect correction
data relative to the area of the country they are located
in. The entire data is then packaged together, analyzed, converted
to a set of correction data by a master station and then uploaded
to the geo-stationary satellite, which in turn transmits the
data down to the local GPS receiver. The GPS receiver then
figures out which data is applicable to its current location
and then applies the appropriate corrections to the receiver.
Similar systems are being set up in other areas of the world
but they are not yet operational.
In addition to correction information
the ground stations can also identify a GPS satellite that
is not working within specification thereby improving the
integrity of the system for aviation use.
EGNOS and MTSAT As
stated, the intent is to have worldwide coverage of WADGPS
corrections, however the name of the correction system varies.
In Europe it is called EGNOS while in Asia
the Japanese system is called MTSAT, but whatever it is called the
system is designed to be compatible worldwide through a cooperative
effort of member countries. The European ground station network
is shown below.
This system is still in test mode
until 2004 but is displaying correction data since April 2003.
The picture shows a test in 2003 with 33 and 44 representing
the two EGNOS satellites as seen on a Garmin GPS-V from a
location in Europe. When EGNOS is finally released it will
use ARTEMIS as the main satellite. A figure
showing the USA WAAS satellites on a Garmin Etrex Vista is
included below.
Viewing the satellites The
Garmin unit identifies these geostationary satellites on the
satellite status screen by using numbers greater than 32.
The system is still new and will be improved with more satellites
in the future (possibly 19 of them world wide), however since
they are all geo-stationary you will need a clear view of
the southern sky to use them from the northern hemisphere.
This means they are very useful for an airplane or perhaps
a boat, but less useful to someone on the ground particularly
in areas of tree cover or high northern latitudes. While a
GPS receiver can possibly receive satellite data from outside
the ground coverage area there will be very little correction
capability without the correct ground data.
| |
|
Satellite |
# |
Garmin |
| The NMEA standard WAAS labels are used
by Garmin receivers. This system maps the PRN number
to the NMEA label by subtracting 87 from the PRN number.
(source: EGNOS help desk) The mapping from the Garmin
numbers to the standard names and PRN numbers are: |
|
AOR-E |
120 |
33 |
| AOR-W |
122 |
35 |
| ARTEMIS |
124 |
37 |
| IOR-W/F5 |
126 |
39 |
| MTSAT1R |
129 |
42 |
| IOR |
131 |
44 |
| POR |
134 |
47 |
| MTSAT2 |
137 |
50 |
Magellan shows the geostationary satellites
on the status screen as well but does not assign them a number.
Other WAAS capable receivers may or may not show any indication
of satellite reception other than indicating a differential
fix. Some units such as Garmin and Magellan can receive up
to 2 WAAS satellites while other systems may be designed to
receive only one. Once satellite contains all of the corrections
for an area while a second can be used to support redundancy
or provide reception when the other one is blocked.
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