488BGPS satellite time synchronisation for master clocks and clock systems |
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The Wharton 488BGPS GPS time synchronisation system is designed to
provide an extremely accurate, yet simple to install and maintenance
free, time reference for the 4850 system master clock and 4860 NTP master clock.
The 488BGPS package comprises an active antenna mounted on a stainless
steel post mounting bracket, a receiver/decoder module housed
in a rugged, IP65 rated, wall mounting aluminium enclosure, a
5m coax cable between the active antenna and the receiver/decoder
module and a 25m, four core cable between the receiver/decoder
module and the master clock.
| what is GPS? |
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| GPS is a system of satellites in six orbits,
each orbital plane equally spaced about the equator
and inclined at 55 degrees. The satellites transmit
highly accurate, real time, worldwide navigation
information at a frequency of 1575.42 MHz that
anyone with a suitable GPS receiver can use to
identify their position, together with a precise
local time. The GPS system was developed by the
US government initially for military use and is
now available for unlimited commercial access.
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| how GPS time
synchronisation works |
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| GPS satellites travel in nearly circular
orbits every 12 hours at an altitude of 20,200km
(10,900 nautical miles). The exact position of
each satellite can be determined at all times.
Each satellite continuously transmits a unique
code sequence, derived from atomic clocks, which
is tracked by the GPS receiver. |
| The transmitted information includes current
orbital data for each satellite. The GPS receiver
is able to identify each satellite by its code
and is able to calculate the satellite's exact
position in space. The GPS receiver uses the calculated
information to determine its distance from several
satellites. The receiver then uses a process called
triangulation to compute its own position together
with a precise utc time. |
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| active antenna |
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| The active antenna module has a rugged, weatherproof
housing rated at IP65 and is mounted on a stainless
steel post mounting bracket. GPS signals are received
by the antenna, amplified within the antenna assembly
and then relayed to the GPS receiver module for
processing. The gain pattern of the antenna is
designed for full upper hemispherical coverage
with the gain diminishing at low elevations. This
cross-section is consistent through 360 degrees
and so the 3-dimensional gain pattern is a symmetrical
spheroid surface. |
| The antenna should be horizontally mounted
with a clear view of 75% of the sky. If the sky
view is reduced, the interval between 'switch-on'
and system time synchronisation will be considerably
increased. The antenna module transmits the received
GPS signals and receives power (5Vdc @ 25mA) from
the GPS receiver/decoder module via a single 5
metre long RG58 coaxial cable. The post mounting
clamp supplied enables the antenna to be fixed
to a suitable horizontal or vertical post of up
to 20mm diameter. The antenna may also be mounted
on the roof of a building or under a suitable
skylight. |
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| receiver/decoder
module |
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| The GPS receiver/decoder module contains an advanced
Motorola® 12 Channel parallel receiver and a microprocessor
based communications interface. The receiver/decoder
module is housed in a robust cast aluminium case
fitted with mounting points for wall mounting
and is protected to IP65. The module should be
mounted in a protected location within 5 metres
of the antenna. |
| The 488B GPS system is designed
to self initialise without the necessity of operator
data input. On power-up the receiver will automatically
begin to search the sky for all available satellites.
After three satellites have been acquired a precise
date and time will be calculated from the satellite
data transmissions. When accurate satellite time
information is available synchronising time signals
are transmitted from the receiver/decoder module
to the 4850/4860 master clock. |
| The connection between the receiver/decoder
module and the 4850/4860 master clock is by a
four core, screened twisted pair cable. A 25m
length of cable is supplied with the 488B system.
The cable length may be extended to 50m using
7/0.2 screened twisted pairs cable or 200m using
16/0.2 cable. This cable also provides power to
the receiver/decoder module. When the 4850/4860
master clock is synchronised with the GPS receiver/decoder
module all transmitted signals are accurate to
within +/- 50uS. |
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| environment |
|
| Power supply |
The 488BGPS system is powered via the master clock at 12v
150mA dc. |
| Operating temperature |
- Active antenna: -40°C to +100°C
- Receiver/decoder module: -20°C to +80°C
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| Relative humidity |
- Active antenna: 0% to 100%
- receiver/decoder module: 0% to 95% non-condensing
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| MTBF |
›50,000 hours |
| Accuracy |
- Antenna/receiver +/- 1uS
- Master clock outputs +/- 50uS (100uS for 4850)
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| EMC |
The 488BGPS time synchronisation system, when installed and
used in accordance with our recommendations, complies
with the European Community Electromagnetic Compatibility
Directive 89/336/EEC and Low Voltage Directive 73/23/EEC
and conforms to the following standards:
- EN 50081-1 55022 class B
- EN 50082-1 IEC 801-2 Level 3, IEC 801-3 level
3 & IEC 801-4 level 3
- EN 60950
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| special
designs |
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| If you have a special GPS, master clock,
ethernet NTP time synchronisation or time reference requirement that is not
covered by our standard range of products please
do not hesitate to contact our technical department. |
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