The satellite clocks made by Meinberg have been designed to provide extremly
precise time to their users. The clocks have been developed for applications where
conventional radio clocks can´t meet the growing requirements in precision. High
precision available 24 hours a day around the whole world is the main feature of the
new system which receives its information from the satellites of the Global Positioning System.
The Global Positioning System (GPS) is a satellite-based radio-positioning, navigation, and time-transfer system. It was installed by the United States Departement of
Defense and provides two levels of accuracy: The Standard Positioning Service (SPS)
and the Precise Positioning Service (PPS). While PPS is encrypted and only available
for authorized (military) users, SPS has been made available to the general public.
GPS is based on accurately measuring the propagation time of signals transmitted
from satellites to the user´s receiver. A nominal constellation of 24 satellites together
with some active spares in six orbital planes 20000 km over ground provides a
minimum of four satellites to be in view 24 hours a day at every point of the globe.
Four satellites need to be received simultaneously if both receiver position (x, y, z)
and receiver clock offset from GPS system time must be computed. All the satellites
are monitored by control stations which determine the exact orbit parameters as well
as the clock offset of the satellites´ on-board atomic clocks. These parameters are
uploaded to the satellites and become part of a navigation message which is retransmitted by the satellites in order to pass that information to the user´s receiver.
The high precision orbit parameters of a satellite are called ephemeris parameters
whereas a reduced precision subset of the ephemeris parameters is called a satellite´s
almanac. While ephemeris parameters must be evaluated to compute the receiver´s
position and clock offset, almanac parameters are used to check which satellites are in
view from a given receiver position at a given time. Each satellite transmits its own set
of ephemeris parameters and almanac parameters of all existing satellites.
5
GPS167PC Features
The hardware of GPS167PC is a plug-in board designed for computers with standard
ISA bus architecture. The rear slot cover integrates the antenna connector, two status
LEDs, and a 9 pin sub-D female connector.
The antenna/converter unit is connected to the receiver by a 50 ohm coaxial cable
with length up to 200m. Power is supplied to the unit DC insulated across the antenna
cable. Optionally, an overvoltage protection and an antenna distributor are available.
The antenna distributor can be used to operate up to 4 Meinberg GPS receivers using a
single antenna/converter unit.
The navigation message coming in from the satellites is decoded by satellite clock´s
microprocessor in order to track the GPS system time with an accuracy of better than
250nsec. Compensation of the RF signal´s propagation delay is done by automatical
determination of the receiver´s position on the globe. A correction value computed
from the satellites´ navigation messages increases the accuracy of the board´s temperature compensated master oscillator (TCXO) to ±5•10
tes the TCXO´s aging. The last recent value is restored from the non-volatile memory
at power-up. Optionally, the clock is also available with a higher precision time base.
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and automatically compensa-
A monitoring software shipped with the board can be used to check the clock's
status and configure some operational parameters.
Time Zone and Daylight Saving
GPS system time differs from the universal time scale (UTC) by the number of leap
seconds which have been inserted into the UTC time scale after GPS has been initiated
in 1980. The current number of leap seconds is part of the navigation message
supplied by the satellites, so the satellite clock's internal real time is based on UTC.
Conversion to local time including handling of daylight saving year by year can be
done by the receiver´s microprocessor. For Germany, the local time zone is UTC +
3600 sec for standard time and UTC + 7200 sec if daylight saving is in effect.
The clock's microprocessor determines the times for start and end of daylight saving
time by a simple algorithm e. g. for Germany:
Start of DST is on the first Sunday after March, 25th, at 2 o'clock standard time.
End of DST is on the first Sunday after October, 25th, at 3 o'clock daylight time.
The monitoring software shipped with the board can be used to configure the time
zone and daylight savings parameters easily. Switching to daylight saving time is
inhibited if for both start and end of daylight saving the parameters are exactly the
same.
6
Asynchronous Serial Ports
Two asynchronous serial interfaces (RS232) called COM0 and COM1 are available to
the user. Only COM0 is available at the rear panel slot cover, COM1 must use another
submin-D connector which can optionally be connected to the 5 pin jumper block on
the board. The monitoring program can be used to configure the outputs. In the default
mode of operation, the serial outputs are disabled until the receiver has synchronized
after power-up. However, they can be configured to be enabled immediately after
power-up. Transmission speed, framing and mode of operation can be configured
individually for each port. Both of the ports can be configured to transmit either the
Meinberg standard time string (once per second, once per minute, or on request with
ASCII ´?´ only), or to transmit capture strings (automatically when available, or on
request). The format of the output strings is ASCII, see the technical specifications at
the end of this document for details.
Time Capture Inputs
The board provides two time capture inputs called User Capture 0 and 1 (CAP0 and
CAP1) which can be mapped to pins at the 9 pin connector at the rear panel. These
inputs can be used to measure asynchronous time events. A falling TTL slope at one of
these inputs lets the microprocessor save the current real time in its capture buffer.
From the buffer, an ASCII string per capture event can be transmitted via COM1 or
displayed using the monitoring program. The capture buffer can hold more than 500
events, so either a burst of events with intervals down to less than 1.5 msec can be
recorded or a continuous stream of events at a lower rate depending on the transmission speed of COM1 can be measured. The format of the output string is described in the
technical specifications at the end of this document. If the capture buffer is full a
message "** capture buffer full" is transmitted, if the interval between two captures is
too short the warning "** capture overrun" is being sent via COM1.
Pulse and Frequency Outputs
The pulse generator of GPS167PC generates TTL level pulses once per second
(P_SEC) and once per minute (P_MIN). A DIL switch on the board can be set up to
map one or both of the pulses to pins at the 9-pin connector at the rear slot cover.
A TTL level master frequency of 10 MHz is derived from the TCXO. By default,
this frequency is available only at the 5 pin jumper block on the board.
In the default mode of operation, the pulse outputs are disabled until the receiver has
synchronized after power-up. However, the monitoring program can be used to enable
these outputs immediately after power-up.
7
DCF77 Emulation
The GPS167PC satellite controlled clock generates TTL level time marks (active
HIGH) which are compatible with the time marks spread by the German long wave
transmitter DCF77. This long wave transmitter installed in Mainflingen near Frankfurt/Germany transmits the reference time of the Federal Republic of Germany: time
of day, date of month and day of week in BCD coded second pulses. Once every
minute the complete time information is transmitted. However, the clock generates
time marks representing its local time as configured by the user, including announcement of changes in daylight saving and announcement of leap seconds. The coding
sheme is given below:
MStart of Minute (0)
RRF Transmission via secondary antenna (0)
A1Announcement of a change in daylight saving
Z1, Z2Time zone identification
Z1,Z2 = 0,1:Daylight saving disabled
Z1,Z2 = 1,0:Daylight saving enabled
A2Announcement of a leap second
SStart of time code information (1)
P1, P2, P3Even parity bits
Time marks start at the beginning of a new second. If a binary "0" is to be transmitted,
the length of the corresponding time mark is 100 msec, if a binary "1" is transmitted,
the time mark has a length of 200 msec. The information on the current date and time
as well as some parity and status bits can be decoded from the time marks of the 15th
up to the 58th second every minute. The absence of any time mark at the 59th second
of a minute signals that a new minute will begin with the next time mark. The DCF
emulation output is enabled immediately after power-up.
8
Connectors and LEDs in the Rear Slot Cover
The coaxial antenna connector, two status LEDs and a 9 pin sub
D connector can be found in the rear slot cover. (see figure).
The upper, green LED (LOCK) is turned on when after powerup the receiver has acquired at least four satellites and has
computed its position. In normal operation the receiver position
is updated continuously as long as at least four satellites can be
received.
The lower, red LED (FAIL) is turned on after power-up until
the receiver has synchronized or if a severe error occurs during
operation.
The 9 pin sub D connector is wired to the GPS167PC's serial
port COM0. Pin assignment can be seen from the figure beside.
This port can not be used as serial port for the computer.
Instead, it can be uses to send out Meinberg's standard time
string to an external device. A DIL switch on the board can be
used to wire some TTL inputs or outputs (0..5V) to some
connector pins. In this case, absolute care must be taken if
another device is connected to the port, because voltage
levels of -12V through +12V (as commonly used with RS232 ports) at TTL inputs or outputs may damage the radio
clock.
Behind the little hole in the slot cover there is a push button
which is needed if the clock's firmware shall be updated. See the
chapture about firmware updates for details.
Installing the Radio Clock
Before the clock can be installed in the computer, some configuration may be required
if the desired settings differ from the default settings.
Setting The Port Base Address
Programs can read data from or write data to the board using 2 I/O ports from a block
of four addresses. The base I/O address can be set up in a wide range using the DIL
switch on the board. When being shipped, the board´s address is set to 300h corresponding to the default address used by the utility software. If one of the levers 8, 9, or
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