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Down to Earth Discussions of Global Positioning and Wireless Module Issues
This section contains down to earth discussions of GPS receivers and Wireless Modules at a board and chip set interface level. Synergy Systems’ Chief Applications Engineer has written papers which discuss various aspects of technical support problems encountered throughout his many years experience in the field.
In general, this is not advisable. The RTC was only intended to keep APPROXIMATE time in order to decrease search time when the receiver is restarted, it was not designed to be used as an accurate time source unless satellites are being tracked. When the receiver is actively tracking satellites, the time being reported by the receiver is extremely accurate as the RTC is slaved to GPS system time. However, if the receiver suffers a loss of lock due to satellite blockage or other system problems, the RTC will enter a free-running mode. The RTC will continue to run, but errors of up to several seconds a day may develop, depending on component tolerances.
How is this possible? The key point here is that the satellite visibility number is a CALCULATED value, not the result of a real time activity being performed by the receiver. A receiver containing a valid almanac, time, and position can CALCULATE which satellites SHOULD BE visible (above the horizon) and will report these as VISIBLE. If a newly activated receiver has determined it’s position but has yet to download an almanac, it will continue to report zero satellites visible until the almanac has been downloaded (approx. 15 minutes after the first satellite is acquired).
Generally, yes. If you are in a single story wood framed building you will probably be able to track some satellites with the antenna indoors, but performance will be degraded. Buildings with a lot of metal in their frames (or foil-backed insulation in the walls and ceiling) make extremely effective shields. Even so, placing the antenna next to a window will generally prove to be sufficient for basic testing. Because of the orbital paths of the satellites, users in the Northern hemisphere should avoid North facing windows since satellite visibility will be poor. Users in the Southern hemisphere should likewise attempt to avoid South facing windows.
No, you do not have a defective unit. When you first power up a new i-Lotus receiver, it is in a default condition, in Motorola binary mode, with all messages inactive. In order to obtain data from the receiver you must request the messages using your application or test software.
This really depends on your application. Using a back-up battery allows the receiver to store data such as time, position and satellite visibility so the receiver can quickly establish a valid fix and so the same output messages will be active once power is restored.
This is an especially important consideration for users operating a receiver in NMEA mode. Receivers that do not have EEPROM, and without battery back-up the receiver will revert to the native Motorola binary mode when main power is removed. Since many NMEA based software applications do not support the binary protocol, the only way to command the receiver to return to NMEA mode is to use another application to establish communications in binary mode, order a switch to NMEA, and then return to the NMEA application. A battery saves a lot of frustration and re-initialization headaches
No. i-Lotus receivers that are not fitted with the back-up battery at the factory do not have all of the components installed necessary to support the onboard battery option. Back-up power must be supplied through Pin 1 of the main 10 pin header.
Here is a general description of the protocols and where they are used:
- Motorola Binary (All ONCORE™ receivers) This is the native Motorola communications protocol, operating at 9600 baud, 8/N/1. Use of this protocol offers the richest command set and control over most of the receiver operating parameters. Most commonly used for timing applications
- NMEA (National Marine Electronics Association) protocol is an ASCII text based standard operating at 4800 baud, 8/N/1. NMEA is a standard in marine navigation applications, and is used almost exclusively in Automatic Vehicle Location (AVL) and in vehicle tracking applications. It is also quite popular with hobbyists and newcomers to GPS since the data is output as human readable text that can be viewed with generic terminal programs (Procomm,etc.) receivers running in NMEA mode can also supply data to applications such as Microsoft Expedia Streets.