Customer Reviews

Garmin 18x LVC GPS Navigator Unit

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The Garmin 18x LVC is a bare lead OEM GPS unit with a measurement pulse output aligned to the Universal Coordinated Time (UTC) second, within a maximum error of 1.0 microsecond (1.0 millionth of a second) Only the LVC variety outputs the measurement pulse (Pulse Per Second) signal. As a retail item this unit is primarily used for very accurate timekeeping. One could theoretically use it for navigation, but with bare leads you need to solder a connector on the unit, and there are other 18x models, not the LVC, that have connectors from the factory.

But the 18x models with connectors DO NOT output the precision measurement pulse, so they are useless as reference clocks. You may wonder why anyone would need a clock accurate to within 1.0 microsecond. The clocks inside computers are notoriously inaccurate, sometimes up to an hour off per day. So, the Internet is peppered with reference clocks. People interested in time keeping will place a reference clock server online, and thousands of Desktop PCs also connected to the Internet can poll these servers to accurately set their clocks. The reference clocks need to be accurate, because of dispersion in distribution (i.e it takes a small bit of time for the client PCs to receive the time from the time server. Having microsecond accurate time servers insures no client will ever drift past 1/2 second from UTC.

It's all automated. The user doesn't do anything. It just happens. So that is what this particular unit is for, from a retail perspective. There is a web page, [...] , explaining in very simple terms how to make a time server. Soldering skills are required for individuals to make use this unit. The 18x USB, not the 18x LVC, is handy for navigation via a laptop.

This unit is fine for experimenters who can manage a little soldering and a little software, too. You can provide it with 5 volt power and simply use the 1pps output, or you can also connect it to a serial I/O port (default 4800 bps) or a USB-to-serial converter.

I use it with my Ubuntu 10.04 Linux system for ntp timekeeping and for GPS position measurement using xgps and other software. See [...] and [...] for more information.

The 1 usec timing precision is nice, but with common PCs and operating systems, your software timing is subject to millisecond or greater fluctuations. The GPS clock does assure good long-term stability, however, independent of Internet connections.

This unit performs well, no trouble getting a signal indoors.
It quit working after 13 months.
UPDATE: failure found due to broken wire, not GPS unit. Unit is still functional.

We have the unit up and installed, it was shipped and delivered on time, good fast response. Wei will be planning to purchase more of these units as our customers ask for them.

I purchased the GPS 18x LVC for NTP timekeeping purposes due to its highly-accurate 1-pulse-per-second output. This output is not available on the other GPS 18x models. For those interested only in navigation and not precision timekeeping, the GPS 18x USB model is probably the best option.

As the LVC model is a "barewire" model meant for OEM integration, it requires a little bit of soldering to get setup the way one wants. I opted to connect the GPS to a small circuit board that supplies power to the GPS, has a fuse, and has indicator LEDs to show the state of the GPS: one LED shows that power is applied, with another LED shows when the PPS signal is transmitted. The circuit board has a DB-9 serial connection to the computer. Other people I know have connected the PPS and data wires directly to a DB-9 serial port and the power/ground wires to a male USB-A plug without any LEDs. Either way works fine -- the GPS draws about 60mA by itself (mine draws 75mA with the LEDs) and so can be powered easily off of a USB port (which can supply 100mA without the device needing to ask for more power).

It's difficult to rate the GPS unit itself as it's so simple and there's not a whole lot of features: supply it with power, let it see the sky, and it starts outputting data. It managed to find an initial lock and output PPS signals within about 3 minutes of when I turned it on for the first time. Since then, it's able to reestablish a lock within about 5 seconds after being powered down and turned back on again. For testing purposes, I have it inside my apartment on top of a box placed next to my northeast-facing window and it is able to see between 5 and 9 satellites depending on the time of day. While no GPS receiver is meant to operate indoors due to the very weak strength of the GPS signals, the 18x is quite sensitive and gets quite reasonable signal indoors.

The receiver is WAAS/EGNOS-enabled for improved precision (~3m vs. 10-15m). As the window in my computer room is on the northeast side of my apartment, the receiver cannot see the WAAS/EGNOS satellites in the southern sky.

For those interested in setting up this GPS receiver for either navigation or timekeeping, the GPS 18x technical specification document, available from Garmin at [...] is of invaluable help, as well as the Garmin configuration software SNSRXCFG (Windows only, but supposedly works under WINE for Linux) which is available at the "Software" link at[...] . The out-of-the-box configuration for the GPS 18x was quite good, and I only changed a few settings with SNSRXCFG to reflect my specific needs (e.g. increasing the PPS duration from 100ms to 200ms, turning off the PPS signal if GPS lock was lost so that I don't inadvertently use bad timing information, etc.).

Once I constructed the circuit board and soldered all the necessary components (about an hour or so), I simply followed the directions at [...] (specifically setting the "low_latency" flag with setserial, installing and configuring gpsd, and configuring NTP), and I had everything working in about 15 minutes. The GPSd software makes interfacing the GPS 18x with the computer trivial. After connecting the GPS to the computer and letting NTP run and stabilize for a few hours, ntpq -p reported that the clock had an offset of 0.000 +- 0.003ms. That is, the clock was syncronized to the correct time with an uncertainty of three microseconds. That's excellent, and about a thousand times better than using only internet timeservers.

In short: this GPS receiver is well-suited for precision timekeeping, and is quite inexpensive. A person with basic soldering skills can fabricate the necessary connector with little trouble. The configuration software from Garmin makes it very easy to change any settings on the device itself, and the device is compatible with GPSd and NTP. If you're interested in GPS timekeeping, this is almost certainly the best option.

This little GPS is great. It plugs straight into my existing NMEA systems. It is extremely accurate, gets a fix within seconds of turning on, and has a great update rate.

I upgraded to this from a 2 year old garmin hand-held. The tracks on the hand-held only had points once every 20 seconds (verses this units once per second), and often had times with large errors. This unit makes very smooth, accurate tracks. It gets great signal, even being mounted in the lazarette under a layer of fiberglass.