During our operations, very often, we use a Waterlinked Underwater GPS. This helps us to keep track in 3D of our movements and link video recording’s timing and telemetry to specific places. These experiments were done together with Andalú Sea. Some examples of these 3D tracks are:

Waterlinked GPS – ROV Navigation inside the harbour

Waterlinked GPS – ROV Navigation Open Water

Thanks to this technology, we can position the ROV when it is under the water, but…

How does it work?

Using the Master Waterlinked GPS System (3), the boat knows the ROV’s position under the water thanks to some pulses that the ROV is sending using a Locator (1) installed. This Locator (1) sends pulses every n-seconds, and those pulses are listened to by the Antenna or Receivers (2), which transmits those pulses to the Master (3).

In general, the Master (3) can compute a lot of information about positioning:

Acoustic data (1+2) will be processed to triangulate the position of the ROV Under the water regarding the Antennas or Receivers (2) position.
Master (3) reads the Global position from a GPS Chip installed in Master (3). This way, Master (3) can find its position on the Earth.
Rov’s Global Position is inferred from Acoustic Position + Global Position.

How is the acoustic position accurately inferred?

To determine the relationship between Acoustic and Global positioning, the Waterlinked GPS requires pairing the Locator (1) with the Master (3) before getting to the water.

The Locator (1), which has a GPS Chip installed, can synchronize its timing with the available GPS satellites. Once the Locator (1) is synchronized, it sends pulses.

The Master (3) is also synchronized with the available GPS satellites and will start reading those pulses.

When Antenna (2) is set, the system can accurately find out the position of the ROV (distance from the antenna) using synchronized timing. The Master (3) knows when precisely the Locator is sending its pulses. Now it can infer the delay and the distance from each Receiver in the Antenna (2) to the Locator (1).

The rest is just maths.

How to set up a Waterlinked GPS?

Our Waterlinked GPS is set up with an Antenna (2).

The first thing to do is to attach it properly to the boat, so it will not move. Both the Antenna (2) and the Master (3) have a printed arrow on them, so it is mandatory to point both in the same direction when the Antenna (2) and the Master (3) is attached to the boat.

Close correctly the Locator (1), link it with Master (3), attach it to your ROV, and put it in the water (so it will start sending its position):

Start the pairing process as pointed by the instructions, and once ready, your ROV is prepared for duty.

You can get the full User Manual for Waterlinked GPS here.

How to read data with Python?

I did a few basic programs to read data from a Waterlinked GPS and save it in files. Then others can plot the information on your screen using Matplotlib or convert it to GPX or KML format used by Google Maps.

You can find those visiting Waterlinked GPS on GitHub.

Let’s get more technical

The system is composed by:

1.- Master (3), a connection box outside the water that will communicate with the computer using a wifi connection (the box works in AP mode with ESSID “UnderwaterGPS” and Password “waterlinked”, the connection box will not deliver a DHCP connection so you must set your own one using 192.168.7.x IP address):

40-pin Molex Interface

Signal	Pin	Pin	Signal	Function
GPIO 2*	1	2	GPIO 3
D1 TX+	3	4	D1 TX-	Locator-D1
D1 RX+	5	6	D1 RX-	\|
GND	7	8	12 V	Power out
R1 TX+	9	10	R1 TX-	Receiver-D1
R1 RX+	11	12	R1 RX-	\|
GND	13	14	12 V	Power out
R2 TX+	15	16	R2 TX-	Receiver-D1
R2 RX+	17	18	R2 RX-	\|
GND	19	20	12 V	Power out
R3 TX+	21	22	R3 TX-	Receiver-D1
R3 RX+	23	24	R3 RX-	\|
GND	25	26	12 V	Power out
R4 TX+	27	28	R4 TX-	Receiver-D1
R4 RX+	29	30	R4 RX-	\|
ANT RX 1+	31	32	ANT RX 1-	Antenna
ANT RX 2+	33	34	ANT RX 2-	\|
ANT RX 3+	35	36	ANT RX 3-	\|
ANT RX 4+	37	38	ANT RX 4-	\|
A1 TX+	39	40	A1 TX-	Locator-A1

* Used to control relays for switching between Receivers and Antenna.

6-pin NMEA 0183 GPS + PPS (not currently implemented)

Signal	Pin	Pin	Signal
GND	1	2	12 V
NMEA GPS TX	3	4	NMEA GPS RX
NMEA GPS PPS	5	6	GND

6-pin NMEA 0183 Compass (not currently implemented)

Signal	Pin	Pin	Signal
GND	1	2	12 V
NMEAS COMPASS TX	3	4	NMEA COMPASS RX
NC	5	6	GND

6-pin NMEA 0183 Out (not currently implemented)

Signal	Pin	Pin	Signal
GND	1	2	12 V
NMEA OUT	3	4	NC
NC	5	6	GND

16-pin TTL GPIO

Signal	Pin	Pin	Signal
GND	1	2	12 V
DNC	3	4	DNC
DNC	5	6	DNC
DNC	7	8	DNC
GND	9	10	PPS OUT
GND	11	12	PPS IN
GPIO 0	13	14	GPIO 1
GND	15	16	3.3 V

Note: TTL voltage 3.3 V (5.0 V tolerant)

2-pin MicroFit 3.0 Power

Signal	Pin	Pin	Signal
GND	1	2	VIN*

* 10 – 30 VDC, 0.7 A (12 V nominal).

2.- Antenna (2), several sensors that will be held underwater near the water line:

3.- API Access:

There is an available API available at: http://192.168.7.1
It can be accessed using the URL: http://192.168.7.1/api/v1/
You can find those visiting Waterlinked GPS on GitHub.

4.- Locator (1): this is one of the most important parts, and we will describe it in detail.

Locator (1) in detail

The external USB-C connector and switch ON button will be pushed when the cap is attached and closed. There is also a potentiometer it may be used for channel configuration: