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Author Topic:   Lowrance HDS GPS Data Display
jimh posted 11-05-2011 09:11 AM ET (US)   Profile for jimh   Send Email to jimh  
Earlier this year I posted a series of article about changes to the FAA'S satellite-based augmentation system (SBAS) which is generally called the Wide Area Augmentation System or WAAS. The article described changes in the psuedo-random noise (PRN) codes used to modulate the GPS signals, particularly the PRN codes associated with the WAAS satellites. Of particular concern was the manner in which my Lowrance HDS GPS receiver presented information about the GPS signals it was receiving, and particularly about the WAAS PRN codes it was receiving.

My HDS-8 unit provides a display that shows a great deal of information about the satellites it is receiving. Below is a screen capture of a typical display. We explore in more detail what information the various presentations on this screen intend to convey to the viewer:

Graphic: Screen capture of HDS-8 satellite information display
HDS Satellite Status Display Screen

On the left side of the display there is an azumith-elevation (Az-El) chart on which the current positions of the GPS satellites which are in view are plotted; this is called the sky view. The outer ring of the sky view represents the horizon, and the inner ring represents an elevation of 45-degrees above the horizon. The center of the sky view is a point directly overhead at the GPS receiver's current position. The satellites are identified by their PRN code number. In the example above, the chart shows nine satellites are in view, PRN's 3,6,7,13,16,19,23,31, and 133.

Below the Az-El chart are a series of vertical bar graphs which represent the relative signal strength of the various satellite signals being received, with a taller bar representing a stronger signal. (Signal strength is also given in the tabular data on the right.) If a WAAS signal is being received, its signal strength is indicated by the right-most bar, which is marked with an asterisk below the bar to identify it as the WAAS signal.

If the GPS receiver is utilizing a satellite in the position fix solution, the color of the position indicator and signal strength indicator is changed to blue, otherwise it remains gray. In the example above the satellite signals used in the position fix are those with PRN 3,6,13,16,23, and 31. Satellites with PRN 7 and 19 are received but not used in the position fix solution.

If the GPS receiver has received a correction signal from the WAAS, the satellite signals which are being corrected with the WAAS correction data are indicated by placing as asterisk above their signal strength bar.

On the right side of the display is a tabular listing of data regarding the GPS receiver status. There are three subsections: status, position, and time. The data in the position and time subsections is quite straightforward and easily understood. The data in the status section deserves closer inquiry:

The parameter FIX refers to the nature and precision of the position fix. A fix can be 2D (i.e., only latitude and longitude) or 3D (i.e., including elevation) and can be of enhanced precision if a WAAS correction has been applied.

The parameter EPE is an acronym for estimated position error, and it expresses the estimated accuracy of the horizontal position fix in distance (usually feet). It is based on the dilution of precision and satellite signal quality, that is, the signal quality being transmitted by the GPS system, not particularly the receiver signal quality. There does not seem to be a strict standard for this calculation, and the EPE reported by one receiver may not be comparable to the EPE reported by another. Each manufacturer seems to implement the EPE calculation in their own manner. However, the EPE is a relative indicator of a particular receiver's estimate of its accuracy. Lower numbers are better.

The parameter HDOP is an acronym for horizontal dilution of precision. This expresses the potential for accuracy of the position fix based on the particular alignment of the lines of position which are available from the current satellite position geometry. The best HDOP would occur if all satellites were on the horizon and nicely spread around in azimuth at every 45-degrees. Of course, at any moment the geometry of the satellites in view is variable according to the particular alignment of their orbits at a particular time and place on Earth. A lower number for HDOP indicates a better geometry for position solution, with "1" being ideal. Values of HDOP in the range of 2 to 5 are considered good. Values below 2 are considered excellent. (See for more information.)

The parameter "SNR best 4" indicates the Carrier-to-noise Density average of the four best signals being received. (The acronym SNR is not actuall a signal-to-noise ratio but is rather the somewhat more complicated Carrier-to-Noise density. For more information see a companion article.) The parameter "SNR avg all" is the Carrier-to-Noise density average for all signals being received. In general a higher Carrier-to-Noise density means better reception and better performance from the receiver.

To get back to the corollary topic of this article, how the Lowrance HDS GPS receiver displays information about WAAS, the above screen shot has the PRN 133 satellite in its sky view diagram. The natural inference is to assume that this means the WAAS correction is being taken from that satellite. However, according to Lowrance, this is not necessarily true. Lowrance says

We do not display the number of the WAAS satellite being used on the unit. The unit automatically searches satellites 120 to 138 one by one and uses the one with the best signal strength.

Thus with a Lowrance HDS receiver it is apparently not possible to determine precisely which satellite or PRN code source is being used for the WAAS correction. It appears that it is possible for WAAS correction to be used, and NO WAAS satellite could appear in the sky view chart. Below is another screen shot that shows just that presentation.

Graphic: Screen shot of HDS display of satellite status showing WAAS but no WAAS PRN in sky view
HDS Satellite Status screen shots shows WAAS fix but has no WAAS satellites in its sky view.

My inference from the above is that perhaps there is a limit of eleven satellites that will be shown in the sky view diagram. If there are eleven satellites in view then the higher number PRN satellites, that is, the WAAS satellites, are bumped out of the sky view presentation.

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