Teledyne PDS Clock Synchronization Considerations Version 1.2 TELEDYNE RESON B.V. Stuttgartstraat 42-44 3047 AS Rotterdam The Netherlands Tel.: +31 (0)10 245 15 00 www.teledyne-reson.com Dated: 01-05-2015 Copyright 2015 TELEDYNE RESON B.V.
Amendment Record Sheet Rev. Date Reason for Modifications 1.2 01/05/2015 Text modified. PDS2000 changed into Teledyne PDS 1.1 12/12/2014 Version after review 1.0 11/12/2014 First version of this Technical Bulletin.
1 Introduction For a correct multibeam survey it is essential clock synchronization is applied correctly. Different methods for clock synchronization are in use. For accurate clock synchronization a Pulse Per Second (PPS) and a time message are needed. This technical note describes the clock synchronization considerations. 2 Why clock synchronization? Different sensor types as heading, multibeam, positioning etc. measure and output asynchronous data to the computer. A delay will be noticeable between the time of the measurement and the time of reception by Teledyne PDS. In Teledyne PDS we call this dynamic delay. Time correlation of the different data streams is needed as they have to be combined. In other words: for each individual depth measurement the related position and attitude for compensation for the related position and attitude needs to be determined. Time tagging of all the messages are therefore needed using the same time basis. Clock synchronization is required for this purpose. 3 Clock synchronization signals For clock synchronization a time message is needed from a time source. Teledyne PDS supports the following time sources: Different brands of GPS receivers. (Trimble, Leica, Topcon, Ashtech, Novatel, Aquarius) NMEA GGA and NMEA ZDA messages (including PPS) Simrad PPS+GGA message TSS POSMV PPS The most common used time message is the NMEA ZDA message. The ZDA message contains the UTC time from the moment of the start of the ZDA message transmission. Using this time message alone to time synchronize the computer clock is not accurate enough. Commonly present symptoms to be aware of are: The GPS receiver can have a delay in output of the message. Message queuing can occur resulting in unknown latency. Or in the ZDA message only the integer seconds are transmitted. In order to have accurate time stamping PPS is used. PPS means Pulse Per Second. It is a digital pulse output by most GPS receivers every second, very accurately without any delays. This accurate (in time) PPS is used to clock synchronize the clock with the time message (e.g. ZDA). Teledyne PDS does not require a special interface for the PPS. The PPS signal must be connected to the Data Carrier Detect (DCD) line of the computer s serial port and the ground to the port s ground line. The schematic diagram for the connection is illustrated in Figure 3-1. Clock synchronization Page 1
Computer D9 serial COM port Computer D25 Serial COM port 1 2 5 3 7 8 PPS Pulse Time message (e.g. ZDA) Ground (PPS and message) Time message (e.g. ZDA) Ground (PPS and message) PPS Pulse Figure 3-1 Clock synchronization signal (message and PPS) at COM port computer The time message (e.g. ZDA) can instead of connected to the serial port also input by Ethernet to the computer In Teledyne PDS the proper device driver must be selected in the clock synchronization dialog. (See section 4.1 on page 3) There are also devices which need clock synchronization signals in order to time tag the output messages. These devices have in common a coax input socket for the PPS signal and a serial port for the time messages. An example is the RESON T20-P. In these cases it is not necessary to synchronize the computer clock. Figure 3-2 RESON T20-P with serial time port and coax PPS port Although the signals are now synchronized in the device as the T20-P it is still recommended to synchronize the computer as discussed in section 4.2 on page 7. The same applies for other device on which the clock synchronization signals are connected as the Odom RTA. Clock synchronization Page 2
4 Clock synchronization methods Several clock synchronization methods can be used: Clock synchronizing the computer clock; Clock synchronizing the interface/processor. 4.1 Clock synchronizing the computer clock This method is used when devices are connected to the computer. The clock synchronization signals (message and PPS) are connected to a computer serial port. Schematically this setup can be illustrated as in below figure. Time source (e.g. GPS) Time message (E.g. ZDA) PPS Time signals connected to the device (multibeam) or time messages received from TeledynePDS COM1 Teledyne PDS computer Multibeam sensor Time message Data COM2 Ethernet COM3 Sensor x (E.g. heading, attitude etc.) Data Figure 4-1 Devices connected to computer. Computer is clock synchronized. Different sensors are interfaced to the computer. The time source as for example a GPS receiver output the time message and a PPS signal to a serial com port of the Teledyne PDS computer. Clock synchronization Page 3
The Teledyne PDS computer clock needs to be synchronized with this signal. See Figure 4-2. a. Open the Teledyne PDS clock synchronization dialog. b. Enable the clock synchronization. c. Select the correct device driver. d. Select the correct port. e. Test if the messages are received correctly. Note Figure 4-2 Teledyne PDS clock synchronization It is not possible to start the Teledyne PDS Realtime acquisition when the clock synchronization is enabled but the clock signals are not or incorrect received. Clock synchronization Page 4
In Figure 4-1 it is seen that the clock synchronization of the multibeam device is accomplished with time signals (message and PPS) added to the multibeam device from the time source, or alternatively Teledyne PDS provides a time message to the multibeam device. If Teledyne PDS provides a time message to the multibeam device: Use the Output option in the time synchronization dialog to output a time message. (See below figure). This method was used for older multibeam systems as the RESON 8K series. Now, in common, clock synchronization is added straight to the multibeam device. Figure 4-3 Time output Next step is to define the correct timestamp modes for the devices. Timestamp modes are set in the device properties. Clock synchronization Page 5
Figure 4-4 Timestamp mode In the above example the timestamp mode for the GPS and multibeam are set to Time in message as these data streams contains synchronized time. For most multibeams timestamp mode cannot be set as it is fixed Time in Message. For other sensors as heading and attitude the time stamp mode Computer clock should be used as these messages in common do not contain time. (Cannot be set for most devices as already fixed to computer clock) The message will be time stamped on the first character received by Teledyne PDS. This is correct as the Teledyne PDS clock is synchronized. Clock synchronization Page 6
4.2 Clock synchronizing the interface/processor In this case the devices are connected to an interface box or a processor. Examples are an Odom MB2 RTA box or RESON T20-P. All devices are connected to this interface/processor including the time source with the time synchronization message and PPS. From this interface/processor an Ethernet cable is connected to the Teledyne PDS computer. Schematically a setup can be illustrated as in Figure 4-5. Time message (e.g. ZDA) Time source (E.g. GPS) PPS Multibeam sensor Sensor data Interface/ processor (E.g. Odom RTA, RESON T20-P) Data stream (ethernet) Teledyne PDS computer Sensor x (e.g. heading, attititude etc.) Sensor data Figure 4-5 Clock synchronization of device All messages as received by Teledyne PDS are time synchronized by the interface/processor. This means in Teledyne PDS timestamp mode Time in message is selected. Be aware that the devices (Odom RTA, RESON 7K, etc.) use different protocols for the data streams. Clock synchronization Page 7
Figure 4-6 Timestamp mode NOTE Although it is not necessary to clock synchronize the computer clock it is strongly recommended to do! In common the survey law states Proper survey is done with proper clock synchronization. Advantages: In Teledyne PDS the dynamic delay is indicated. The dynamic delay is the time difference between the time in the message and the computer clock time at which it is received. When the computer is not synchronized properly this indication cannot be used. The time of the log files matches with sensor time when the computer is clock synchronized. When something is wrong with the time source, error messages will be displayed in Teledyne PDS and it is not possible to start Realtime. Clock synchronization Page 8
5 Notebooks and clock synchronization (No serial port) Flexibility in installation, mobility and use are great advantages of notebooks compared to desktop computers. However current notebooks are in common not provided with serial com ports. When devices are synchronized by the interface or multibeam processor as described in section 4.2 this is not a problem; the messages are time synchronized correctly and the notebook does not need clock synchronization. However it is still strongly recommended to clock synchronize the notebook by an Ethernet message for the in section 4.2 on page 7stated reasons. For RESON systems the Teledyne PDS clock synchronization dialog has features to synchronize correctly. (See section 5.1 on page 9) For Odom Multibeam system the RTA (MB1) or RRIO (MB2) port can be used to synchronize. (Refer to the Teledyne PDS Odom manual) For other multibeam devices time synchronization can be acquired by Ethernet. The time source (e.g. POSMV) should able to output time messages by Ethernet. (See section 5.2 on page 10) Be aware this clock synchronization is not accurate. For accurate clock synchronization (with PPS) a serial port is needed! 5.1 RESON Multibeam systems For RESON multibeam systems the RESON I/O module is used for clock synchronization. Refer to the RESON documentation how to setup the I/O module. For RESON multibeams in the Teledyne PDS Clock synchronization dialog two options are available. Select Use RESON 7k I/O module for clock synchronization when the 7K I/O module runs on the same computer as Teledyne PDS. Figure 5-1 Clock synchronization by 7k I/O module Select Use PDS Clocksync module with remote RESON 7K I/O when the IO module runs on an external computer. Select the address of the connected external computer from the Remote 7k I/O module address drop down list. Figure 5-2 Clock synchronization by Remote 7k I/O module Clock synchronization Page 9
5.2 Using Ethernet time messages Perform the following procedure for clock synchronization of messages available by Ethernet. The time source device (e.g. GPS receiver or POSMV) should be able to output time messages by Ethernet. Step Action 1 See the figure below: a. Enable clock synchronization b. Use PDS clocksync module c. Select the correct device d. Click Select e. Click Select to select the correct port. Proceed to step 2 when the port is not listed. Clock synchronization Page 10
Step Action 2 When the Ethernet port is not listed it can be created. Click Add. Select Socket. Give the port a name. Set the local port number. (127.0.0.1 as local IP address) Clock synchronization Page 11
Step Action The port is now added. Clock synchronization Page 12
5.3 Adding a Com port For accurate clock synchronization of a notebook a serial com port is needed. (See section 3 on page 1) 5.3.1 USB serial com ports Serial com ports by a USB adapter can NOT be used for clock synchronization purposes. USB has a low priority for the operating system and therefore has time delays in signal treatment. Figure 5-3 USB-serial adapter 5.3.2 Com port by Express Cards Alternatively it is possible to use an Express card RS232 Serial Adapter card. The notebook should be provided with an Express card slot for this purpose. Figure 5-4 Com port Express card 6 Conclusion The computer must be clock synchronized with a time message and a PPS pulse when devices are connected to the computer ports. When the devices are connected to an interface box or multibeam processor and clock synchronized accurate (with PPS) it is not necessary to accurate clock synchronize the computer clock. It is however still strongly recommended to clock synchronize the computer. In this case the computer can be synchronized with a time message only as high accuracy is not needed. Clock synchronization Page 13