EM-406 A

The datasheet can be found here.

Summary

The EM-406A is a 20-channel GPS receiver with built in patch antenna available at Sparkfun.com for $60. It supports the NMEA 0183 protocol for GPS data over an RS-232 serial connection, making it easy for a microcontroller to communicate to it. The datasheet also contains examples of each supported command, which is helpful when dealing with an unfamiliar protocol. It also is WAAS-enabled, which allows for a higher level of accuracy than standard GPS alone. (I’ll explain WAAS further later.) Considering its high level of accuracy, small form factor, and easy setup, it seems like an excellent choice for GPS applications.

Relevant Features and Specs:

• SiRF star! high performance GPS Chip Set*
• Very high sensitivity (Tracking Sensitivity: -159 dBm)
• Extremely fast TTFF (Time To First Fix) at low signal level
• Support NMEA 0183 data protocol
• Built-in SuperCap to reserve system data for rapid satellite acquisition (This isn’t explained anywhere else in the datasheet.)
• Built-in patch antenna
• LED indicator for GPS fix or not fix

General

Chipset

SiRF Star!

Channels

20 channel all-in-view tracking. This allows the unit to acquire data from up to 20 satellites, which should be enough redundancy for city and indoor use (unverified).

Sensitivity

-159 dBm

Accuracy

Position

10 meters, 2D RMS, 5 meters, 2D RMS, WAAS enabled

Velocity

0.1 m/s

Time

1us synchronized to GPS time

Datum

Default

WGS-84

Acquisition Time

Reacquisition

0.1 sec., average

Hot start

1 sec., average

Warm start

38 sec., average

Cold start

42 sec., average

Power

Main power input

4.5V ~ 6.5V DC input

Power consumption

44mA

Protocol

Electrical level

TTL level, Output voltage level: 0V ~ 2.85V RS-232 level

Baud rate

4,800 bps

Output message

NMEA 0183 GGA, GSA, GSV, RMC, VTG, GLL

Physical Characteristics

Dimension

30mm*30mm*10.5mm "0.2mm (pretty freakin' small)

Theory of usage

From Wikipedia: Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time.

It is not necessary to understand the exact nature of these signals in order to use a GPS receiver. Like nearly all GPS receiver modules, the EM-406A decodes these signals, determines its position, and provides the developer with a means of acquiring this data.

Protocol

The EM-406A supports the NMEA 0183 protocol for GPS data over an RS-232 serial connection, making it easy to interface with a microcontroller. A typical command looks like this: $GPGGA,161229.487,3723.2475,N,12158.3416,W,1,07,1.0,9.0,M,,,,0000*18 This is an output command from the EM-406A. All commands start with a $, then the command itself, followed by any parameter data. All of the supported commands and the data fields are described in the data sheet. This command translates to:

Name	Example	Units	Description
Message ID	$GPGGA	GGA	protocol header
UTC Time	161229.487		hhmmss.sss
Latitude	3723.2475		ddmm.mmmm
N/S Indicator	N		N=north or S=south
Longitude	12158.3416		dddmm.mmmm
E/W Indicator	W		E=east or W=west
Position Fix Indicator	1		See Table B-3
Satellites Used	07		Range 0 to 12
HDOP	1.0		Horizontal Dilution of Precision
MSL Altitude1	9.0 meters
Units	M		meters
Geoid Separation1			meters
Units	M		meters
Age of Diff. Corr.		second	Null fields when DGPS is not used
Diff. Ref. Station ID	0000
Checksum	*18
<CR><LF>			End of message termination

The EM-406A also supports WAAS, which is a secondary network of satellites and ground stations used to provide higher resolution positioning data. This network was created by the U.S. FAA and DOT.