The Best GNSS Receiver

Electrical and electronic topics for small boats
jimh
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The Best GNSS Receiver

Postby jimh » Sat Oct 22, 2016 1:48 pm

Is the SIMRAD HS60 the best GNSS receiver for your boat? Perhaps--not because it is the best GNSS receiver you can get, but because it is also a compass or heading sensor. The HS60 is an interesting combination of technologies. In one housing it has:

--a NMEA-2000 interface to connect to other navigation electronics

--two GNSS antennas and a GNSSreceiver to determine both position and heading

--a tilt sensor, and

--a gyro compass.

The NMEA-2000 interface is as you'd expect on a modern device. In addition to providing a path for the data, it also powers the unit, which only consumes 2-Watts of electrical energy. This greatly simplifies installation.

The GNSS receiver is able to receive the Air Force GPS constellation L1 coarse acquisition signal, the normal civilian signal, and augment with correction data from various spaced-based augmentation systems (SBAS) like WAAS (in North America) and EGNOS (in Europe). The receiver is a modern implementation which retains SBAS correction data for as long as 40-minutes. This allows the receiver to continue to deliver an enhanced precision position solution even if the SBAS satellite is out of view for short durations. SIMRAD rates the receiver when SBAS is available for 1-meter-RMS accuracy 95-percent of the time. Without SBAS the receiver is rated at 3-meters-RMS 95-percent of the time.

Most significant is the inclusion of two antennas in the housing. This permits the GNSS receiver to calculate a compass heading solution with an accuracy of 2-degrees. Since this method does not use a magnetic sensor, it is free from interference from the usual magnetic influences found on boats from ferrous metals or strong electrical currents.

The HS60 also has tilt sensors that deliver data on heave, roll, and pitch. The inclusion of heave data is going to be attractive to SONAR enthusiasts. Some of the higher-end Simrad SONAR devices can accept heave data. What is heave? It is the up-down vertical motion of your boat, usually due to waves or swell. By having heave data available, a sophisticated SONAR can correct its depth measurements to reduce the influence of waves on the boat. You've all seen what a SONAR display looks like when you are sounding in waves--the bottom contour looks like a sine-wave curve as the depth to the bottom changes as your boat rides up and down in the waves. Heave compensation can eliminate that. Of course, you need a sophisticated sounder to be able to do that.

The gyro compass also augments the GPS compass by providing faster updates of heading changes and also taking over (for as long as three minutes) if the GPS compass cannot get a heading solution. Note that this is a gyroscope compass, not a magnetic compass. Again, no problems with magnetic interferences from the boat.

All of this comes with a price. The HS60 is not in the same market as a POINT-1. This device has an MSRP of $1,000. If that seems a bit expensive, compare the HS60 to its bigger cousin, the HS70 which sells for $2,250. See more on the HS60 at

http://store.navico.com/Products/000-12308-001

simradHS60.png
SIMRAD HS60 GPS Compass
simradHS60.png (23.53 KiB) Viewed 6407 times

jimh
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Re: The Best GNSS Receiver

Postby jimh » Sat Oct 22, 2016 2:09 pm

There is also some suspicion that the Simrad HS60 is actually a Hemisphere VECTOR V104 GPS COMPASS. Compare at

https://hemispheregnss.com/Products/Products/Position-Heading/vector-v104e284a2-gps-compass-763

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Don McIntyre - MI
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Re: The Best GNSS Receiver

Postby Don McIntyre - MI » Sat Oct 22, 2016 2:19 pm

Same? Looks mighty suspicious to me. Wonder what the price is.

Regards - Don

jimh
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Re: The Best GNSS Receiver

Postby jimh » Sat Oct 22, 2016 3:48 pm

There is a Si-Tex version. The MSRP is the same. Discounted to about $720. See

http://www.hodgesmarine.com/Si-Tex-Vector-Compact-N-Worlds-Smallest-Gps-Co-p/sitvectorcompactn.htm

Here is the Si-tex webpage; they don't try to hide the source:

http://si-tex.com/item/vectorcompactpgscompass

Acseatsri
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Re: The Best GNSS Receiver

Postby Acseatsri » Sat Oct 22, 2016 4:35 pm

After the problems with the Point1 on my autopilot, I question the sanity of putting a compass/heading sensor in the same housing as the GPS receiver, as they really need totally different mounting locations to provide optimal performance. The heading sensor should be mounted as close to the waterline and center of the keel as possible to negate the effects of rocking, while the receiver needs an unobstructed view of the sky.

jimh
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Re: The Best GNSS Receiver

Postby jimh » Sat Oct 22, 2016 4:58 pm

The heading sensor IS the GPS receiver in the HS60--it cannot be mounted below deck. These devices are not a GPS with a magnetic compass. They are GPS compasses.

A good magnetic compass heading sensor would be the PRECISION-9, but it costs about $650. You can mount it where you like, but it is still subject to magnetic interferences.

Also, a GPS compass would always show TRUE heading, not MAG heading, because it is not using a magnetic sensor. A magnetic compass will always show a MAG heading, to which you would apply a correction (DEVIATION) to get back to TRUE.

conch
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Re: The Best GNSS Receiver

Postby conch » Sat Oct 22, 2016 6:13 pm

You can see the sights for aligning the antenna molded into the top cover in the picture.

With high end transducers for sounders costing much more being sold and installed all the time, I do not think the price is out of line. This will be a welcome addition to those installing or improving their autopilots.

jimh
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Re: The Best GNSS Receiver

Postby jimh » Sat Oct 22, 2016 9:03 pm

From the instruction manual, here is more about how the GPS receiver works:

Vector II technology supports multiple RF front ends--enabling tighter coupling of measurements from separate antennas for use in heading-based products. Users will achieve excellent accuracy and stability due to its more accurate code phase measurements, improved multi-path mitigation, and fewer components. The HS60’s GPS antennas are separated by 13.5 cm between their phase centers, resulting in better than 2-degree rms heading performance. The HS60 provides heading and position updates of up to 10 Hz and delivers position accuracy of better than 1.0 m 95% of the time when using differential GPS corrections from Space Based Augmentation Systems (SBAS)....

The HS60 provides accurate and reliable heading and position information at high update rates. To accomplish this task, the HS60 uses a high performance GPS receiver and two antennas for GPS signal processing. One antenna is designated as the primary GPS antenna and the other is the secondary GPS antenna. Positions computed by the HS60 are referenced to the phase center of the primary GPS antenna. Heading data references the vector formed from the primary GPS antenna phase center to the secondary GPS antenna phase center. The heading arrow located on the bottom of the HS60 enclosure defines system orientation. The arrow points in the direction the heading measurement is computed (when the antenna is installed parallel to the fore-aft line of the vessel). The secondary antenna is directly above the arrow.


Source: http://www.navico-commercial.com/Root/SimradProSeries_docs/HS60_OM_EN_988-10836-001_w.pdf
There is a lot of interesting information in that publication. It gives a nice overview of how the device works.

The wavelength of a signal at 1575.42-MHz is 19.04-cm. The 13.5-cm separation between the antennas is thus 0.7-wavelengths. By comparing the carrier phase of the L1 signal between the two antennas, the receiver can deduce the orientation of the antennas. It has to know the true bearing to the satellite, but since the receiver knows its location and the satellite's location, it already knows the true heading to the satellite.