What is NMEA?
The acronym stands for "National Marine Electronics Association" and is an industry association that sets data transmission standards. In this case, the real name is NMEA 180. It is a set of standard messages defining the possible outputs of a GPS receiver. There are several "sentences" available; each of which has some unique data associated with them. They are in ASCII format, in the form of comma delimited strings. String lengths vary from 30 to 100 characters and are output at the interval chosen, usually once per second. The most common string (or sentence) is called the "GGA" string. It contains the Time of the Fix, Latitude, Longitude, Height, Number of Satellites used in the fix, DOP, Differential Status, and the Age of the Correction (RTCM). Others have Speed, Track, Date, etc.
NMEA is available in virtually all GPS receivers and is the most commonly used data output format. It is also the format used in most software packages that interface with a GPS receiver.
How often will I get a fix from my GPS receiver?
This question is often preceded by "How often does OmniSTAR transmit corrections"? The perception is that a user will only get a position fix when a new correction is received from OmniSTAR. That is not the case. The user's GPS receiver will output at the selected rate (usually at 1-second intervals, but for auto-steer applications it may be > 10 times per second), regardless of the corrections. The ability to continue solving for corrected positions between differential updates is based on the fact that the errors in GPS - with Selective Availability (SA) turned off - change very slowly and a correction will be valid for several minutes. When SA was "On", the errors changed very rapidly and corrections had to be updated every few seconds. This is no longer the case.
What is RTCM?
The correct name is RTCM-104, Version 2 or 2.1. The acronym stands for Radio Technical Committee, Marine and is the name of a committee that governs standards for passing data between different equipment used in the Marine Electronics Industry. In the early days of Differential GPS, they set up a standard format for sending differential correction data to a GPS receiver. The actual format is complex and lengthy, but it contains three main elements: (1) The time of the measurement at the Base Station, (2) measured range errors (Corrections) for every satellite in view at that Base Station, and (3) the "range error rate" for every satellite in view.
As explained above, the Range Rate term is currently set to a zero value. If SA were to be turned back ON, the rate term would be measured and sent in every message.
Is OmniSTAR susceptible to interference?
The older C-band OmniSTAR units (Model 7000) were susceptible to a certain type of interference generated by the Telephone Co. Microwave towers. If they were close enough to our frequency or if the user was very near to them, they could totally block the OmniSTAR Receiver. Newer units, which use L-band frequencies, are not susceptible to that particular source of interference. However, normal precautions should be used when using radio receivers near anything that generates noise. There have been reports that certain cell phones and certain laptop computers can cause problems when they are in close proximity (a few feet).
How does OmniSTAR compare to the Beacon Service?
OmniSTAR VBS is superior to Beacons in reliability, accuracy, and coverage. Beacon users usually have only one beacon within reliable range and if it goes down, there are no alternatives. Beacons have a possible accuracy problem in that there is a built-in bias error of about 1-meter for every 100 miles from the beacon location. If a user switches from one beacon to another, the position fix difference may be twice that value. Lastly, OmniSTAR covers the entire continent, versus limited beacon coverage. OmniSTAR is a commercial service that is responsive to its customers needs, as opposed to a free government-run service. (Who do you call if a beacon goes down?) OmniSTAR XP and HP are an order of magnitude more accurate.