1 INTRODUCTION
1.1 The Global Positioning System (GPS) is a space-based positioning,
velocity and time system that has three major segments: space, control and user. The GPS
space segment will normally be composed of 24 satellites in six orbits. The satellites
operate in circular 20,200 km orbits at an inclination angle of 55° with a 12 h period.
The spacing of satellites in orbit will be arranged so that a minimum of four satellites
will be in view to users world-wide, with a position dilution of precision (PDOP) of ≤
6. Each satellite transmits on two ‘‘L’’ band frequencies, L1 (1575,42 MHz) and L2
(1227,6 MHz). L1 carries a precise (P) code and coarse/acquisition (C/A) code. L2
carries the P code. A navigation data message is superimposed on these codes. The same
navigation data message is carried on both frequencies.
1.2 Receiver equipment for the GPS intended for navigational purposes on ships
with maximum speeds not exceeding 50 knots should, in addition to the general
requirements contained in resolution A.694(17), comply with the following minimum
performance requirements.
1.3 These standards cover the basic requirements of position-fixing for
navigation purposes only and do not cover other computational facilities which may be in
the equipment.
2 GPS RECEIVER EQUIPMENT
2.1 The words ‘‘GPS receiver equipment’’ as used in these performance standards
include all the components and units necessary for the system properly to perform its
intended functions. The equipment should include the following minimum facilities:
-
.1 antenna capable of receiving GPS signals;
-
.2 GPS receiver and processor;
-
.3 means of accessing the computed latitude/longitude position;
-
.4 data control and interface; and
-
.5 position display and, if required, other forms of output.
2.2 The antenna design should be suitable for fitting at a position on the ship
which ensures a clear view of the satellite constellation.
3 PERFORMANCE STANDARDS FOR GPS RECEIVER EQUIPMENT
The GPS receiver equipment should:
-
.1 be capable of receiving and processing the Standard Positioning Service
(SPS) signals as modified by selective availability (SA) and provide position
information in latitude and longitude World Geodetic System (WGS) 84 co-ordinates
in degrees, minutes and thousandths of minutes and time of solution referenced to
UTC. Means may be provided for transforming the computed position based upon WGS
84 into data compatible with the datum of the navigational chart in use. Where
this facility exists, the display should indicate that co-ordinate conversion is
being performed, and should identify the co-ordinate system in which the position
is expressed;
-
.2 operate on the L1 signal and C/A code;
-
.3 be provided with at least one output from which position information can
be supplied to other equipment. The output of position information based upon WGS
84 should be in accordance with IEC Publication 1162;
-
.4 have static accuracy such that the position of the antenna
is determined to within 100 m (95%) with horizontal dilution of precision HDOP ≤ 4
(or PDOP ≤ 6);
-
.5 have dynamic accuracy such that the position of the ship is
determined to within 100 m (95%) with HDOP ≤ 4 (or PDOP ≤ 6) under the conditions
of sea states and ship’s motion likely to be experienced in ships;footnote
-
.6 be capable of selecting automatically the appropriate
satellite-transmitted signals for determining the ship’s position with the
required accuracy and update rate;
-
.7 be capable of acquiring satellite signals with input signals having
carrier levels in the range of –130 dBm to –120 dBm. Once the satellite signals
have been acquired, the equipment should continue to operate satisfactorily with
satellite signals having carrier levels down to –133 dBm;
-
.8 be capable of acquiring position to the required accuracy within 30 min
when there is no valid almanac data;
-
.9 be capable of acquiring position to the required accuracy within 5 min
when there is valid almanac data;
-
.10 be capable of re-acquiring position to the required accuracy within 5
min when the GPS signals are interrupted for a period of at least 24 h but there
is no loss of power;
-
.11 be capable of re-acquiring position to the required accuracy within 2
min when subjected to a power interruption of 60 s;
-
.12 generate and output a new position solution at least once every 2
s;
-
.13 the minimum resolution of position, i.e. latitude and longitude, should
be 0.001 minutes; and
-
.14 have the facilities to process differential GPS (DGPS) data fed to it
in accordance with the standards of Recommendation ITU-R M.823 and the appropriate
RTCM standard. When a GPS receiver is equipped with a differential receiver,
performance standards for static and dynamic accuracies (3.4 and 3.5 above) should
be 10 m (95%).
4 PROTECTION
Precautions should be taken to ensure that no permanent damage can result from an
accidental short circuit or grounding of the antenna or any of its input or output
connections or any of the GPS receiver equipment inputs or outputs for a duration of 5
min.
5 FAILURE WARNINGS AND STATUS INDICATIONS
5.1 The equipment should provide an indication of whether the position calculated
is likely to be outside the requirements of these performance standards.
5.2 The GPS receiver equipment should provide as a minimum:
-
.1 an indication within 5 s if either:
-
.1.1 the specified HDOP has been exceeded; or
-
.1.2 a new position has not been calculated for more than 2 s.
Under such conditions the last known position and the time of the last valid fix, with
explicit indication of this state, so that no ambiguity can exist, should be output
until normal operation is resumed;
-
.2 a warning of loss of position; and
-
.3 differential GPS status indication of:
-
.3.1 the receipt of DGPS signals; and
-
.3.2 whether DGPS corrections are being applied to the indicated ship’s
position.