Glenn Baddeley - GPS - Garmin GPS38 Information

Contents Now Obsolete Specifications NMEA sentences The End-Of-Week-Rollover problem GPS38 Links [Glenn's GPS Contents Page]

The GPS38 became quite a popular receiver following its release in February 1996. It was seen as a lower cost replacement for the GPS40, with improved software. The '38 was retired from the market by Garmin in 2000. As a single channel receiver it could no longer compete with the many multi-channel receivers which became available at reasonable prices.

This also means that the GPS38 is readily availabe on the used GPS receiver market for a very cheap price. It makes an ideal entry-level or experimental unit.

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Specifications

Straight from the sales blurbs...

The GARMIN GPS 38 is the one navigational tool for the great outdoors that offers big features in a small, lightweight package - all at a truly affordable price. Mark your favorite fishing spot, tree stand or camp site or retrace your step back to the safety of your truck using our all new TracBack feature. The GPS 38 shows you exactly where you are, where you've been and where you're going.

The GPS 38 features easy, one-thumb operation and weighs only 9 ounces. There's a resettable trip odometer, graphic compas and highway steering guidance. And it provides up to 20 hours of use on a set of 4 AA batteries. The GARMIN GPS 38. The affordable way to bring you back.

Basic Features

• MultiTrac8 operating system continuously tracks and uses up to 8 satellites for fast, accurate positioning.
• User-friendly rocker/keypad system allows room for a large, backlit LCD display with easy-to-read characters and full-featured graphic plotting.
• Innovative TracBack feature lets you quickly navigate your track log back home without manually storing waypoints.
• Resettable trip odometer keeps track of your distance travelled.
• User-selectable navigation screens for compas or graphic highway steering guide.
• Ultrasonically welded, dry nitrogen filled case with internal antenna.
• EZinit feature allows fast "point and shoot" initialization.

Performance

Receiver: Differential-ready MultiTrac8^TM continuously tracks and uses up to eight satellites to compute and update position

Acquisition Times:

• Warm: approx. 20 seconds
• Cold: approx. 2 minutes
• Autolocate^TM: approx. 7.5 minutes

Update Rate: 1/second, continuous

Accuracy:

• Position: 15 meters (49 feet) RMS*
  5-10 meters (16-33 feet) RMS with
  GARMIN GBR 21 DGPS receiver (optional)
• Velocity**: 0.1 knot RMS steady state

Dynamics: 3g's

Interfaces: NMEA 180, 182, 183 and RTCM 104 DGPS corrections

Antenna: Internal

Temp. Range: 5°F to 158°F (-15°C to 70°C)

* Subject to accuracy degradation to 100m 2DRMS under the United States Department of Defense-imposed Selective Availability Program.

Physical Specifications

Size: 6.15 x 2 x 1.23 inches (15.6 x 5.1 x 3.1 cm)

Weight: 9 ounces (255g) w/ batteries

Display: 2.2 x 1.5 inches (5.6 x 3.8 cm) high-contrast LCD w/ electro luminescent backlighting.

Case: Waterproof tested, ultrasonically welded, dry nitrogen-filled

Power

Memory Backup: Internal lithium battery

Power Source: 5-8 VDC (4 AA batteries)

Usage: 0.75 watts

Battery Life:

• Up to 12 hours (normal mode)
• Up to 20 hours (battery saver)

Navigation Features

Waypoints:

• Total: 250
• Nearest: 9 (automatic)

Routes: 20 reversible routes with up to 30 waypoints each, plus MOB and TracBack^TM modes.

Track Log: 768 points

Map Datums: Over 100

Coordinates: Lat/Lon, UTM/UPS, plus 7 grids, including Maidenhead

** Speed Limitation GARMIN products designed for non-aviation use will display a maximum speed of approximately 90 knots or 103 MPH. This unit will not display speed or position information if this limit is exceeded.

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NMEA sentences

Here is a list of the group of NMEA sentences transmitted by the GPS38 in its two NMEA 0183 output modes The sentences are transmitted in this order every 2 seconds approximately. A link is provided to a description of the format of each sentence.

NMEA 0183 1.5	NMEA 0183 2.0	Code	Contents
YES	YES	$GPRMC	Recommended Minimum Specific GPS/TRANSIT Data
YES	YES	$GPRMB	Recommended Minimum Navigation Information
-	YES	$GPGGA	Global Positioning System Fix Data
-	YES	$GPGSA	GPS DOP and Active Satellites
-	YES	$GPGSV	GPS Satellites in View (transmitted twice in each group)
-	YES	$PGRME	Estimated Position Error
YES	YES	$GPGLL	Geographic Position, Latitude/Longitude
YES	YES	$PGRMZ	Altitude Information
YES	YES	$PGRMM	Map Datum
YES	-	$GPXTE	Cross-Track Error, Measured
YES	YES	$GPBOD	Bearing, Origin to Destination
-	YES	$GPRTE	Routes (not transmitted in every group)
YES	-	$GPBWC	Bearing & Distance to Waypoint, Great Circle
YES	-	$GPVTG	Track Made Good and Ground Speed
YES	YES	$GPWPL	Waypoint location

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The End-Of-Week-Rollover problem

After 22nd August 1999 the internal week counter in some Garmin GPS receivers (including the GPS38) rolls back to zero because it has insufficient precision to count beyond a certain value. If the GPS receiver firmware is not updated to a new version the unit will experience slow acquisition of satellites and take a long time to get a fix on its position. This is because the GPS receiver thinks it is in a different week to the real week and it will attempt to use the wrong ephemeris data to locate satellites.

There is a Windows PC GUI program which will update the GPS38 firmware via a serial port cable connected to the GPS receiver:

• Garmin Readme file (3 KB)
   
• 'GpsEow.exe' program version 1.20 in ZIP archive file (188 KB)

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GPS38 Links

• GPS38 Owner's Manual at the Garmin web site. Scroll down to the Discontinued Receivers. PDF file, 476KB, Revision B, June 1997, Software version 3.0, Free download. It is also available here.
   
• Specifications at FPS Corporation
   

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