PPS: Why if you forget to use it you can mess up a whole survey?

by Umberto on 20 October, 2020 in Online

Let’s get this straight. Our job can sometimes be really easy.

When?

Well… imagine a positioning job on a dive support vessel where you don’t even have to record videos.

Easy Peasy, isn’t it?

But most of the time it isn’t like that at all. You know it better than me.

How many times you had to run MBES, SSS, SBP altogether, maybe after performing Environmental analysis and 2DHR Seismic and right before starting the Geotechnical part?

A lot, right?

Well, we know that clients want to optimize times and to compact all the operations in the least number of stages, so this is what happens the most. A multi-instrumental and multi-phase job.

And since it is like that more and more we must be sure that the synchronization between all the instruments is perfect.

How?

Thanks to our fellow PPS.

PPS: what is it? And what advantage does it give?

What is PPS?

As Wikipedia would say it:

pulse per second (PPS or 1PPS) is an electrical signal that has a width of less than one second and a sharply rising or abruptly falling edge that accurately repeats once per second. PPS signals are output by radio beacons, frequency standards, other types of precision oscillators, and some GPS receivers. Precision clocks are sometimes manufactured by interfacing a PPS signal generator to processing equipment that aligns the PPS signal to the UTC second and converts it to a useful display.

[…]

PPS signals are used for precise timekeeping and time measurement. One increasingly common use is in computer timekeeping, including the NTP protocol. Because GPS is considered a stratum-0 source, a common use for the PPS signal is to connect it to a PC using a low-latency, low-jitter wire connection and allow a program to synchronize to it. This makes the PC a stratum-1 time source. Note that because the PPS signal does not specify the time, but merely the start of a second, one must combine the PPS functionality with another time source that provides the full date and time to ascertain the time both accurately and precisely.

Source: Wikipedia


As I would say it, and as you probably already know:

The PPS is an electric pulse, sent every second by the GPS that, together with the NMEA ZDA string, lets you synchronize all the survey equipment you have on-board, starting from your Navigation PC.

It’s almost like the DGPS is the orchestra conductor and the PPS is its baton (the wand) used to coordinate all the instruments to get a perfect harmony.

PPS to Serial Adapter - The bridge between the DGPS and the PC

Unfortunately, it is not as poetic as it seems because the Navigation computer doesn’t directly digest the PPS signal. It needs something to decode it, otherwise, it’s just a useless 5v signal.

To do this, one of the things that I have mostly used in my career is the QPS TTL and FIX Adapter. It receives the Pulse on a BNC and transforms it into a serial message understandable by the pc. Besides, it has also had a selector to change from Rising Edge or Falling Edge (depending on your DGPS synchronization) and a LED light that indicates when a PPS signal is received.

If you want more info about it go check it here: QPS PPS Adapter

PPS Adapter

QPS PPS adapter

Which kind of survey can benefit the most out of use of the PPS?

Let’s get this straight. The type of survey that benefits the most of a PPS Synchronization is a Multi-Beam Survey. I would say that the use of PPS is ESSENTIAL in an MBES survey.

But why only MBES?

Because it is not strictly needed in other kinds of operations.

  • Positioning job? It’s an Overkill. A simple ZDA will do the trick.
  • Side Scan Sonar? It doesn’t need it.
  • Sub Bottom Profiler? Same as for the SSS.
  • Magnetometer? Not at all.
  • 2D Seismic? They need a trigger for sure, but it is space-based, rather than time-based (I have never worked on a 3D Seismic vessel, but I don’t think it will change much from 2D).

The only case in which I see the necessity to have a PPS signal sent to the acoustic instruments above listed (apart from MBES) is with a multi-sensor survey with the instrument located too close to each other (probably on the same pole). In this case, I would use the PPS to fire a trigger board set up with a different delay for each instrument.

But BE AWARE, I’m talking about necessity, but it doesn’t mean you cannot set it anyway.

If you have time and can set it all up nicely without compromising all your other tasks, just go for it!

You could do also a job with one DGPS and one pc and still use the PPS. No one will ask you the 0.001s (one millisecond) precision in that case, but you can do it anyway if you prefer.

Which errors are caused by bad timing?

Once we assured that the biggest influence of PPS is notable with the MBES survey, let’s see what happens if we forget to set it (or set it wrong).

For simplicity of the example, we will use an SBES, but the same principles apply to the MBES.

 Let’s say that, for this example, even if we have not set up the PPS, we have anyway fed the ZDA to the navigation PC and the Single Beam Echo Sounder.

Let’s assume we have an average delay of 400 milliseconds (0.4 seconds). These are caused mainly by the DGPS computing the satellite signals into position data and creating the GGA message and then sending it.

Time

GGA String

SBES Value

10:20:30.000

...

10.4

10:20:30.100

...

10.2

10:20:30.200

...

10.6

10:20:30.300

...

10.9

10:20:30.400

$GPGGA,102030.00,…

10.7

10:20:30.500

...

11.1

10:20:30.600

...

11.4

10:20:30.700

...

11.2

10:20:30.800

...

11.6

10:20:30.900

...

11.9

10:20:31.000

...

11.7

10:20:31.100

...

12.1

10:20:31.200

...

12.4

10:20:31.300

...

12.2

10:20:31.400

$GPGGA,102031.00,…

12.6

10:20:31.500

...

12.9

As you can clearly see, confronting the SBES value at the right time versus the one when the navigation Pc receives the GGA, we have huge differences on the values recorded due to the latency:

  • 0.3 in the first case;
  • 0.9 in the second.

This, as you can imagine, can lead to obvious problems of positioning data, especially with MBES.

Why?

Because this same error applies to the MRU data, meaning that the MBES data will not be corrected at all, the DGPS data will not be accurate, so the combination of all will be a total mess.

I am using PPS but I am still having problems…What am I doing WRONG?

So, you went on and set up everything to the state of the art, but…IT DOESN’T WORK. Your navigation system keeps giving you errors about the PPS signal being received too late (or too early) and not being able to match it with any ZDA string, your MBES data are compensated well and you have double checked everything 3 times!!

So this PPS thing is only a useless gimmick!!!

No, my dear friend. There could be something you missed along the way.

You have to understand that the PPS, as every electric signal, has a rising and a falling edge before the pulse itself. You can see it clearly in the image below.

PPS waveform

The PPS seen using an oscilloscope. The rising and falling edge can be clearly detected. 

Unfortunately not all DGPS use the same convention for the PPS, so some use the rising edge as a reference while other the falling one.

So how can you solve all of this? I’ll give you a couple of tips that should sort this out.

List of Tips to sort PPS related problems

Tip 1

You should look for any damage on the cable (both the PPS one that the ZDA one) that could lead to sporadic shorts and data stops.

Tip 2

You must assure that the cable used to transmit the PPS is not too long. A longer cable can cause delays and poor signal (this can only happen if your navigation pc is far from the receiver).

Tip 3

You must assure you are not splitting the PPS signal using only a T-type connector. To split a PPS signal avoiding loss you need a PPS splitter.

PPS Splitter

Example of PPS and ZDA splitter

Tip 4

Check the manual and see how your DGPS works.

Once done that, set the Navigation software or the TTL to serial adapter to match the DGPS setup (rising or falling).

After this four tip you should have solved any issue (PPS related) that could have appeared on your path to a perfect survey.

I was glad to have you on board.

See you around,

Umberto


P.S. Some of you asked me privately why for this article I only referred to MBES surveys and not SBES surveys.

The reason is only one and can be led back to my personal experience.

I have never ever seen a PPS input on any SBES I used.

I have used just a few of them, but none had a PPS input. So, if you have something to say about this, please fill free to shoot a comment below.


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