Frequently Asked GPS/DPGS/LORAN Questions - SailNet Community
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Frequently Asked GPS/DPGS/LORAN Questions

What is a GPS satellite receiver?
A device that receives signals from the US Department of Defense (DOD) Navigation Satellite Timing and Ranging (NAVSTAR) Global Positioning System (GPS). A constellation of 24 satellites transmits precise positioning data by means of spread spectrum digital technology. This free service operates 24 hours a day, 365 days a year. GPS accuracy is largely unaffected by weather and electrical interference.

What type of GPS satellite receiver do you recommend?
I recommend versatile hand-held units because they can be bracket mounted and are portable. These units can run your computer-based navigation system or become independent navigation devices. Hand-held units can operate from internal batteries or ship's power and can utilize external antennas for below deck use.

Which GPS satellite receiver do you recommend?
I recommend the Garmin 48 hand-held unit as the best combination of features, quality, and price. The GPS 48 features 500 waypoint memory and a database of navaids. If price is no object and you don't need portability, the best is the Northstar. PC-Card GPS units are available, however I don't recommend them as they require a computer to operate and can't be used as a backup in an emergency.

Can I use a LORAN receiver with my computer?
Yes, however Long Range Aid to Navigation (LORAN) uses an Amplitude Modulation (AM) signal which is subject to atmospheric and electrical interference that affects accuracy. Despite the name, LORAN signals are limited in range and are accurate for coastal use only. LORAN has excellent repeatability, or ability to return to the same position, which makes it popular with fishermen. LORAN also makes an excellent backup to your GPS.

What is DGPS?
The Differential Global Positioning System (DGPS) is a system operated by the US Coast Guard (USCG) to correct the DOD Selective Availability (SA) error introduced into standard GPS signals. DGPS is an AM signal similar to LORAN with a range limited to less than 200 nautical miles (nm) with 16-33 ft (5-10 m) accuracy.

Which DGPS receiver do you recommend?
I recommend the Garmin GBR 21 DGPS receiver because it is a compact, extremely accurate unit that will easily connect to the Garmin 48 or most other differential ready GPS units on the market.

I'm Having trouble getting GPS information into my IBM type notebook computer. Can you explain the computer com port /GPS connection in more detail?
Every Windows compatible computer ever made has at least one connector for receiving and sending information in a serial mode (one character after another) to external devices, such as a modem, or even a GPS. This serial port connector is usually called COM1 (short for Communications Port No. 1). You must have an unused serial port, and the cable from the GPS must be plugged into that port before you can use GPS data in a navigation and electronic charting program. If you have an IBM Thinkpad Notebook computer, be advised that they come from the factory with the COM1 connector port disabled - IBM uses that port for infrared devices. The procedure for disabling the infrared serial device and enabling the 9-pin COM1 port seems to vary from one Thinkpad model to another. So read your manual, or call IBM. If you have an internal Fax-Modem card installed, it can also interfere with the COM1 connector. The Fax-Modem may have been set up to use COM1. Your choices are to set it up for COM2, or re-configure your computer so that the serial connector on the back is COM2. In some cases, you may need to remove the PC Card modem from your computer when you want to receive GPS data.

I know that I have made all the connections properly but I can't see my boat on the chart. Is there a simple way to determine if the GPS data is getting to my computer?
Yes,. you can use Window's Hyperterminal feature to see the GPS data as it is sent to the computer. For Windows 95 or 98, click on the the Start button, select Programs, then Accessories, and finally Hyper Terminal. Double click on the Hypertrm.exe icon to get started. 1. Enter a name such as GPS Test. 2. When the Phone Number window opens, select Direct to COM 1 in the "Connect Using:" box and then click on OK. Note: if you are using a different COM port, then select Direct to COM X (where x is the Com port you are using, i.e. 2, 3 or 4.) 3. In the Port Settings window, select 4800 Bits per Second and click on OK. Note: The majority of GPS units use 4800 but there are some newer GPS units that can also use 9600. The other default settings are correct and there is no need to change them. 4. If everything is set correctly, your GPS is locked on, and your selected COM port is OK, then you will see GPS data being displayed in the Hyper Terminal window. If the GPS data is there then the problem of not seening your boat displayed is a matter of selecting the correct settings in the software and you should check out the Help feature, read the Manual or call that company for instructions. If there is no data there, check to ensure that the GPS is working and recheck all the wires. If everything is set correctly then your COM port may be out.

Jim Sexton is offline  
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