TIME TAGGINGTIME TAGGING

HYPACKHYPACK®® Timing Basics Timing Basics

The internal clock on your computer is a lousy The internal clock on your computer is a lousy clock.clock.

HYPACK SURVEY and DREDGEPACKHYPACK SURVEY and DREDGEPACK®® use a use a proprietary clock model called proprietary clock model called VERITIME.VERITIME.

When you start SURVEY or DREDGEPACKWhen you start SURVEY or DREDGEPACK®®::VERITIME is initially set to the Windows clock time.VERITIME is initially set to the Windows clock time.VERITIME and Windows clock times slowly drift apart. Don’t VERITIME and Windows clock times slowly drift apart. Don’t

worry about it!worry about it!

In ‘Normal’ mode, all devices receive their time In ‘Normal’ mode, all devices receive their time tags from the VERITIME clock model.tags from the VERITIME clock model.All devices are time tagged off the same time standard.All devices are time tagged off the same time standard.

Time TaggingTime Tagging

TIMETIME

Device drivers monitor a Device drivers monitor a serial/network port. They have serial/network port. They have the highest priority!the highest priority!

When the 1When the 1stst character of a character of a message arrives, the program message arrives, the program gets a time tag from the gets a time tag from the VERITIME routine.VERITIME routine.

To adjust for the delay in To adjust for the delay in transmission from the time of transmission from the time of measurement to the time of measurement to the time of transmission, a ‘Latency’ transmission, a ‘Latency’ correction is entered in the correction is entered in the Offsets in HYPACK HARDWARE Offsets in HYPACK HARDWARE and is subtracted from the and is subtracted from the VERITIME time tag.VERITIME time tag.

LATENCY BETWEEN GPS LATENCY BETWEEN GPS AND ECHOSOUNDERAND ECHOSOUNDER

TheoryTheory

There is a delay in time There is a delay in time from when your GPS from when your GPS computes a position computes a position until it is transmitted until it is transmitted out the serial port.out the serial port.

If you don’t correct for If you don’t correct for it, your depths will not it, your depths will not be in the correct be in the correct position.position. In most cases, they In most cases, they

will be backwards will be backwards along the track line.along the track line.

A 0.5 sec latency error at 8 knots = 6.75’ or 2+ metersA 0.5 sec latency error at 8 knots = 6.75’ or 2+ meters

Test Survey LinesTest Survey Lines

Run reciprocal survey lines Run reciprocal survey lines up/down a slope or over a up/down a slope or over a known feature.known feature.It’s best if your planned lines are It’s best if your planned lines are

perpendicular to the bottom perpendicular to the bottom slope!slope!

If the latency correction is If the latency correction is perfect, the profiles will overlay perfect, the profiles will overlay each other.each other.If there is an error, the profiles will If there is an error, the profiles will

be early or late.be early or late.

The The SINGLE BEAM LATENCY SINGLE BEAM LATENCY program varies the time tags to program varies the time tags to determine the best ‘fit’ latency determine the best ‘fit’ latency time.time.

The Keys to a Good TestThe Keys to a Good Test

Run perpendicular to the bottom slope.Run perpendicular to the bottom slope. Running non-perpendicular to the slope will generate Running non-perpendicular to the slope will generate

errors unless the reciprocal track-lines are ‘dead on’.errors unless the reciprocal track-lines are ‘dead on’. Run the test at slack tide.Run the test at slack tide.

The test does not factor tide, draft or heave The test does not factor tide, draft or heave corrections.corrections.

Keep as close to the planned line as you can.Keep as close to the planned line as you can. Watch the GPS constellation.Watch the GPS constellation.

No big changes in HDOP during the test.No big changes in HDOP during the test. You might consider setting your GPS Elevation mask You might consider setting your GPS Elevation mask

to +20to +20°, so when a new satellite arrives, it won’t jump °, so when a new satellite arrives, it won’t jump your position too much. (Set it back after the test!)your position too much. (Set it back after the test!)

Jumps in position due to poor GPS positioning can Jumps in position due to poor GPS positioning can result in false latency results.result in false latency results. Run the LATENCY TEST three times, each time with a Run the LATENCY TEST three times, each time with a

separate pair of lines and use the average latency time.separate pair of lines and use the average latency time. Results are more easily repeated with RTK GPS than Results are more easily repeated with RTK GPS than

with DGPS.with DGPS.

LATENCY TEST ProgramLATENCY TEST Program

Do 3 separate tests. Take the average result.Do 3 separate tests. Take the average result.

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Before and After ProfilesBefore and After Profiles

BeforeBefore

AfterAfter

Setting the Latency in HARDWARESetting the Latency in HARDWARE LATENCY TEST LATENCY TEST computes a combined latency between the GPS computes a combined latency between the GPS

and the Echosounder.and the Echosounder. In the HARDWARE program for your GPS device:In the HARDWARE program for your GPS device:

Take the current latency time for the GPS (1.000 in example).Take the current latency time for the GPS (1.000 in example).Subtract the result from the average of 3 LATENCY TESTS. (1.00 Subtract the result from the average of 3 LATENCY TESTS. (1.00

– 0.75 = 0.25 )– 0.75 = 0.25 ) (for this example).(for this example).Set the Latency time of your GPS to Set the Latency time of your GPS to 0.25 0.25 (for this example).(for this example).Make sure the latency time of your echosounder is set to 0.00.Make sure the latency time of your echosounder is set to 0.00.

Now, go back to SURVEY, collect 3 more sets of test lines and Now, go back to SURVEY, collect 3 more sets of test lines and check the results.check the results.

Correcting RAW Data Files in the SINGLE BEAM EDITORCorrecting RAW Data Files in the SINGLE BEAM EDITOR

Correcting Existing Correcting Existing RAW Data Files: RAW Data Files: You can update the You can update the

Latency value used Latency value used during data collection during data collection by changing the value by changing the value in the SINGLE BEAM in the SINGLE BEAM EDITOR.EDITOR.

Enter the correct Enter the correct Latency value as Latency value as determined in the determined in the LATENCY TEST LATENCY TEST program.program.

NOTENOTE: You cannot : You cannot correct data with an correct data with an updated latency that is updated latency that is already in the Edited already in the Edited ALL Format.ALL Format.