GPS Time System

Satellite-controlled time base with DCF time code generator

Features Operates worldwide. Built-in or separate antenna. Automatic setting of UTC time. Programmable time zone offset. Programmable daylight saving time changes. DCF-compatible time code output for master-clocks, digital clocks, recorders, etc. Display of position. Low power operation by intermittent receiver control. Minute pulse output (by menu, instead of DCF code). Fast code output, or time of last sync. (slow code). Menu-driven initialization.

GPS signal

The GPS Global Positioning System of the US Department of Defense, consists of 24 satellites revolving around the earth at an altitude of approximately 20'000 km. GPS receivers use the satellite signals to determine their exact location and altitude on earth. GPS satellites also transmit accurate time and date information, which is being used by the GPS Time System.

General description

The GPS Global Positioning System of the US Department of Defense, consists of 24 satellites revolving around the earth at an elevation of approximately 20'000 km. The satellites make about 1 revolution every 12 hours, thus achieving global coverage. They are being controlled and adjusted by several earth stations.

Using the GPS satellite signals, the GPS receiver determines its precise geographical position and altitude by triangulation. As a by-product, it also receives accurate time and date information (UTC/GMT).

In order to derive its position and altitude, the GPS receiver needs to detect at least 4 satellites at the same time. On the other hand, one satellite is enough to get the time.

The spread spectrum transmission frequency is 1'575 MHz. Every satellite uses its own spread spectrum modulation code, which makes it possible to identify individual satellites.

The major application areas of the system are military, transportation (locating ships and trucks, directing cars according to stored maps) and recreational (private boats, hikers).

The GPS Time System uses the GPS signals for timing purposes. It is designed for delivering accurate time and date information in various commonly-used data formats. A menu-driven interface allows easy programming of output formats, time zones and daylight saving time parameters. As a by-product, it also displays the position (latitude and longitude).


Technical Data

GPS Receiver
Separate Receiver in Weatherproof box (IP65), 5 x 5 x 4 cm, with Corner Plate for Installation.
Connection by 4-Wire Cable, Length 5 Meter

Power input :
9 - 35 VDC.
About 150 mA at 12V or 100 mA at 24V, when receiver operating.

Display :
LCD display indicating UTC time, local time, GPS reception status and operating mode. Menu display for entering parameters. Display of latitude and longitude possible by menu.

Keypad :
For entering parameters.

Accuracy :
Accuracy of DCF output and time marks output when GPS-synchronization is good ("navigation"-status) : better than 10 ms.
If the device is newly installed, the error may be several seconds as long as the GPS Information has not been fully acquired.
Quartz time base (between GPS synchronizations) : 11 MHz, 10 ppm from -10 to +60 degrees centigrade.
Accuracy of Position Data: Better than 100 Meters.

Operating parameters :
Parameters are entered through keypad and saved in non-volatile memory.

• Define output 1 : select DCF code or minute pulse (time marks) output.
• Define outputs 2 and 4 : fast code or RS-232/RS-485 messages, GPS time readings.
• Define time zone : enter time offset between local time and UTC (if desired).
• Define DST time changes (if any) : select preprogrammed european rule, or enter changeover dates and time.
• Inhibit DCF output when GPS not synchronized (Yes/No).
• Geo-version : if desired, activate low power mode (intermittent receiver operation), and define the following parameters :
• Time interval (receiver turned on every 1/2, 1, 2 or 4 hours).
• Automatic GPS synchronization of output 1 (Yes/No)

Operating temperatures :
-25 to +75 degrees centigrade.

Casing :
ABS, IP65, dimensions 200 x 150 x 75. Cable gland clamps PG7.

Weight :
About 1 kg.

Output Ports

Output 1, "DCF", Open Collector, Max. 35 VDC, 500 mA, programmable by Menu, 3 signals at choice :

• DCF time code (slow code, 1 bit/sec.)
• Minute pulses (time marks)
• Hour pulses (optional time marks, 1 second duration).

The time information corresponds to local time (UTC + programmed time shift).

DCF time code :
The DCF time code (as transmitted by the German DCF77 transmitter on 77.5 kHz) is a slow serial time code consisting of 60 bits, transmitted at the rate of 1 bit/s. It takes 1 minute to deliver the full code. This time code is handy for many types of applications and it is becoming widely-used in Europe. The coding is as follows :

Bit rate : 1 bit/sec. Negative pulses, beginning on the second (open collector starts conducting).

• Pulse duration 0.1 second = value "0"
• Pulse duration 0.2 second = value "1"
• No pulse at second 59 of every minute (---> minute identifier !)

The time information is coded as follows :
Bits 0 - 14 : always=0;
15 : quality of radio-synchronization : good =1, bad =0;
16 : upcoming time change : normally=0, before time change=1;
17 : winter=0, summer=1;
18 : winter=1, summer=0;
19 : always=0;
20 : always=1;
21-27 : minute (of next minute);
28 : combines with bits 21-27 for even parity;
29-34 : hour;
35 : combines with bits 29-34 for even parity;
36-41 : day of the month;
42-44 : day of the week (Monday=1);
45-49 : month;
50-57 : year;
58 : combines with bits 36-57 for even parity.
59 : the pulse is missing (minute identifier).

By menu, the GPS unit may be programmed to stop delivering the DCF code when GPS synchronization is bad.

Minute Pulses, Time Marks :

Minute pulses duration gives following indication :

Length		Indication
1 second		Minute mark when GPS synchronization OK
2 seconds		Minute mark when GPS synchronization NOT OK
4 seconds		Minute mark at the full hour (local time)
10 Seconds		Minute mark at 24.00 o'clock (local time)

Geo-version : output 1 (DCF code or minute pulses) may be programmed to operate on the internal quartz time base only, without automatic corrections by GPS. GPS information is then obtained from output 2, and used for compensating the quartz oscillator drift.

Output 2, "F. Code", open collector, max 35 VDC, 500 mA, with the following Ssgnals :

• In normal operating mode (permanent receiver operation) : "Fast Code" time code format (specifications on request). This fast code is not available when an RS-232 or RS-485 format is used (menu selection).
• Output 2 in low power mode (geo-version) : slow code, bits are synchronous with output 1, giving time comparison between internal time and GPS time. This code is useful together with the DCF code operating on the internal quartz time base : it allows to correct for the drift of the quartz time base. The coding is as follows :
  
  Slow code "Time comparison between internal time and GPS time, based on latest GPS Reading" (geo-version) :
  
  Bits 1 to 12 : milliseconds of GPS time at time T (BCD-coded : 4 bits for every digit).
  Bits 13 to 19 : seconds of GPS time at time T.
  Bits 21 to 41 : time T = time of the last GPS reading = bits 21 to 41 of output 1 when the last GPS reading as made (at the full minute).
  Bits 42 to 48 : minute of GPS time (local time) at time T.
  Bits 49 to 54 : hour of GPS time (local time) at time T.
  Bit 56=1 for normal high precision mode; =0 for power saving low precision mode.
  Bit 57=1 : DCF code synchronized when GPS available; =0 : DCF code on quartz time base.
  
  
• OMEGAFACE code (option) : fast code compatible with OMEGAFACE code (repetition rate 1 code/minute, bit length 0.02 second, 1600 Hz modulation).

Output 3, current-loop (optional) : 20 mA current-loop output (passive). Voltage to be applied : 12 - 40 VDC. Minimal voltage at 20 mA : 9 V. 1=20mA, 0=4mA.). Same signals as output 1.

Output 4, RS-232/RS-485 : output of time information in RS-232 ASCII format, 1200 Baud. 5 formats selectable by menu.

Options :		- 5 meter cable with RS-232 sub-D 9-pin connector.
		- RS-485 instead of RS-232.

Pin allocation :

• GND
• RS-232 output
• RS-485 output (option)
• RS-485 output (option)
• Vdd (+5V)

Time message formats :

Legend : HH=hours; MM=minutes; SS=seconds; MIS=milliseconds; DD=day of month; d=day of week (Monday=1); DOY=day of year; mm=month; YY=year; s=status byte.

1. <STX>HH:MM DD.mm.YY0<ETX><BCC>
   Byte format : 1 start-bit, 7 data-bits, 1 parity-bit (even), 1 stop-bit.
   Repetition rate : 1 message every minute, beginning exactly on the minute.
   BCC : XOR over all bytes from H to ETX (even parity).
   
   
2. HH:MM:SS.MIS DD/mm/YY DOY d s<CRLF>
   Byte format : 1 start-bit, 8 data-bits, 1 stop-bit, no parity.
   Repetition rate : 1 message every minute, beginning exactly on the minutes.
   Status byte : normally=30H; leap year=31H; bad GPS synchronization=34H; leap year and bad GPS synchronisation=35H.
   
   
3. <STX>SSMMHHDDmmYYs<ETX>
   Attention : the order of the tens and units is inverted. The units come first, then the tens. Byte format : 8 data-bits, 1 stop-bit, no parity. Repetition rate : 1 message every minute, beginning on the minute.
   Status byte : bad GPS synchronization and winter time=31H; bad GPS synchronisation and summer time=33H; Good GPS synchronisation and winter time=35H; Good GPS synchronisation and summer time=37H.
   
   
4. <STX>/K/Y/Y/m/m/D/D/d/D/O/Y/H/H/M/M/S/S/s/<LRC><ETX>
   Byte format : 8 data-bits, 2 stop-bits, no parity. 0.5 ms time slot between the bytes.
   Repetition rate : 1 message every minute, ending exactly on the minute.
   K=N for standard time (winter); =S for daylight saving time (summer).
   Status byte=S when good GPS synchronisation; =' ' when bad GPS synchronisation.
   LRC : parity byte, XOR over all bytes from "K" to "s".
   
   
5. YYmmDDHHMMSSd<CRLF>
   Byte format : 1 start-bit, 7 data-bits, 1 extra-bit, 1 stop-bit, no parity. Repetition rate : 1 message every minute, beginning on the minute.

Output 5 (RS-232) : GPS-monitoring connection to a PC (using optional C-monitor program).