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-hour 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;*
.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.
* Refer to res. A.694(17), Publications IEC 721-3-6, IEC 945 and IEC
1108-1.