GARMIN GPS 24xd

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
jimh
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GARMIN GPS 24xd

Postby jimh » Fri Oct 16, 2020 9:30 am

For some time I have been wondering when marine GNSS receivers were going to catch up the Smartphone GNSS receivers and be able to use multiple GNSS constellations and the L5 civilian signals. GARMIN have pushed marine GNSS receivers into a new tier with their just announced model GPS 24 xd.

The GPS 24xd GNSS receiver is capable of receiving four different global navigation system constellations:
  • US Air Force NAVSTAR GPS
  • European Union GALILEO
  • Russian GLONASS
  • Republic of China BEIDOU

In addition to jumping ahead of the pack with four constellations, the GPS 24xd also supports both L1 and L5 frequencies on GPS (and possibly GLONASS and others when available). The L1 C/A (Coarse Acquisition) signals are the ones that have been in civilian use since the earliest days of GPS. The L5 signals are newly added civilian signals available from GPS. The L5 signals have been available from the GPS constellation beginning with the GPS IIF in May 2010, but until now very few (if any) recreational grade marine GPS could receive them.

The L5 signals should have a stronger signal (by about 1.5 dB). Another advantage of L5 is decreased doppler shift of the chip frequency. (This is a rather complex technical topic. For explanation see a separate article published elsewhere.)

The most significant advantage to being able to receive both L1 and L5 signals from the same satellite is the ability to estimate and compensate for ionospheric delay. Variation in ionosphere delay introduces errors in the position solution. Along with spaced-based augmentation systems (SBAS such as WAAS), compensation of ionospheric delay allows an improvement in the accuracy of the position solution to a level where the CEP (Circular Error Probability) will be not more than 1-meter for 88-percent of the time.

GARMIN did not stop there. They have also included a magnetic heading sensor that is said to have an accuracy of ± 3 degrees.

In addition to the technology step forward, the GPS 24xd is a NMEA-2000 device that can be powered by the network (Load Equivalent Number =3), and a 6-meter drop cable and network T-connector are included. It also outputs NMEA-0183 sentences. A pole-mount adaptor, a surface mount adapter, and an under deck mount adaptor are included. The retail price is $299.99.

The GARMIN website page for the GPS 24xd is located at https://buy.garmin.com/en-US/US/p/684270

The GARMIN Press Release can be found at https://www.businesswire.com/news/home/20201015005340/en/

Jefecinco
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Re: GARMIN GPS 24xd

Postby Jefecinco » Fri Oct 16, 2020 10:28 am

Does this GPS antenna's received signal automatically install as the preferred antenna on MFDs with built-in antennas? As it is NEMA 2000 network compatible I assume it is connectable to all existing GPS chip devices on a network. I also assume software updates will be required for some GPS chip equipped devices. I wonder if it will be easily connectable and operable with Simrad/Lowrance and other manufacturers devices. On the list of compatible devices only Garmin devices are listed. I also wonder if the antenna's electronics will be included with Garmin's next generation devices.

Perhaps this new antenna will drive the other marine electronic device manufacturers to develop and market their own devices. At Garmin's listed price of $299.99 it is inexpensive given the price of a new nine inch MFD.
Butch

jimh
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Re: GARMIN GPS 24xd

Postby jimh » Fri Oct 16, 2020 10:43 am

Jefecinco wrote:Does this GPS antenna's received signal automatically install as the preferred antenna on MFDs with built-in antennas?


The GPS 24xd is not an antenna; it is a complete receiver. How other devices will change their setting automatically is dependent on the other devices and how they are configured. I think it would be very strange behavior for a display device to change configuration of the data it is displaying automatically.

I expect there may be some advantage to using the GPS 24xd with other GARMIN display products, such as perhaps being able to perform some configuration of the GPS 24xd using a GARMIN display. But I also expect the NMEA data sent out for values like position, speed, and heading will all be in a standard format that any NMEA device could use.

Knowing where the boat is to an accuracy of 1-meter may not be necessary for navigation, but it would be fun. Knowing where the boat is to an accuracy of 0.5-meter (also likely with the GPS 24xd) will be really quite amazing.

Jefecinco
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Re: GARMIN GPS 24xd

Postby Jefecinco » Fri Oct 16, 2020 5:31 pm

Thank you. Ben Ellison agrees with your comments.
Butch

jimh
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Re: GARMIN GPS 24xd

Postby jimh » Mon Oct 19, 2020 9:16 am

The irony of having a position sensor with sub-one-meter accuracy on a boat is the use of navigation charts whose land portions were probably not surveyed with anywhere near that accuracy; the seaward points of interest like shoals or hazards were probably surveyed with even less accuracy.

The “big deal” regarding L1 and L5 signal reception is the feature I mentioned regarding being able to compensation for ionospheric delay. By having two signals from the same source, following the same path, but at different frequencies, by some very fancy math the ionospheric delay can be deduced. This removes uncertainty in the position solution, giving more precision.

I did not see any comment from Ben about that, or in the GARMIN press release. I would hope that the GPS 24xd is utilizing that ability.

GARMIN is somewhat like APPLE in that they both tend to hide technical mumbo-jumbo from their users. GARMIN is selling the GPS 24xd as a newer and better GNSS receiver than their old one. They don't like to raise the hood too far and show the insides to the users.