There
are three types of GPS receivers, which are available
in today's marketplace. Each of the three types offers
different levels of accuracy, and has different requirements
to obtain those accuracies.
C/A
Code receivers
C/A Code receivers typically
provide 1-5 meter GPS position accuracy with differential
correction. C/A Code GPS receivers provide a sufficient
degree of accuracy to make them useful in most GIS applications.
C/A Code receivers can provide
1-5 meter GPS position accuracy with an occupation time
of 1 second. Longer occupation times (up to 3 minutes)
will provide GPS position accuracies consistently within
1-3 meters. Recent advances in GPS receiver design will
now allow a C/A Code receiver to provide sub-meter accuracy,
down to 30 cm.
Carrier
Phase receivers
Carrier Phase receivers
typically provide 10-30 cm GPS position accuracy with
differential correction. Carrier Phase receivers provide
the higher level of accuracy demanded by certain GIS applications.
Carrier Phase receivers measure
the distance from the receiver to the satellites by counting
the number of waves that carry the C/A Code signal. This
method of determining position is much more accurate;
however, it does require a substantially higher occupation
time to attain 10-30 cm accuracy. Initialising a Carrier
Phase GPS job on a known point requires an occupation
time of about 5 minutes. Initialising a Carrier Phase
GPS job on an unknown point requires an occupation time
of about 30-40 minutes.
Additional requirements, such
as maintaining the same satellite constellation throughout
the job, performance under canopy and the need to be very
close to a base station, limit the applicability of Carrier
Phase GPS receivers to many GIS applications.
Dual-Frequency
receivers
Dual-Frequency receivers
are capable of providing sub-centimetre GPS position accuracy
with differential correction. Dual-Frequency receivers
provide "survey grade" accuracies not often
required for GIS applications.
Dual-Frequency receivers receive
signals from the satellites on two frequencies simultaneously.
Receiving GPS signals on two frequencies simultaneously
allows the receiver to determine very precise positions.
|
What is NMEA and SiRF II?
|
In
order to relay computed GPS variables such as position,
velocity, course etc. to a peripheral (e.g. computer,
screen, transceiver), GPS modules have a serial interface
(TTL or RS-232 level). The most important elements of
receiver information are broadcast via this interface
in a special data format. This format is standardised
by the National Marine Electronics Association (NMEA)
to ensure that data exchange takes place without any problems.
Nowadays, data is relayed according to the NMEA-0183 specification.
SirfII is an alternate to NMEA,
providing for a new communications protocol. SirfII protocols
provide for more complete control of the GPS receiver
as well as higher speed communications between the GPS
receiver and the mapping/display application
|
| What is WAAS and EGNOS? |
The
North-American WAAS system (Wide Area Augmentation System)
is a network of approx. 25 ground reference stations (WRS,
Wide Area Ground Reference Station) that receive GPS signals.
They have been surveyed exactly in terms of their position.
Each reference station determines actual and target pseudo-range
deviation. The error signals are relayed to a master station
WMS (Wide Area Master Station). The WMS's calculate the
differential signals and monitor the integrity of the
GPS system. The precisely processed DGPS correction values
are transmitted to two geo-stationary satellites (Inmarsat)
and beamed back to Earth on the GPS L1 frequency (1575.42MHz).
The WAAS signals are received by GPS receivers equipped
for this task and further processed.
|
| |
 |
EGNOS
(European Geo-stationary Navigation Overlay System) is
a satellite-based augmentation system for existing GPS
and Glonass satellite navigation systems. A European network
of GPS/Glonass receivers has been built up to receive
the corresponding satellite signals and relay these to
central data processing stations. The signals received
at these data processing stations are evaluated taking
into account the exact known position of the receiving
stations. In this way, correction data can be determined
that is ultimately broadcast to users via geo-stationary
communications satellites. With the help of these corrections
positional accuracy of around 7 m can initially be achieved.
In addition, a level of data integrity is attained that
enables instrument approaches to be made in aviation.
|
 |
|
|
|
|
