Various GPS Receivers Compared

[porthole]

I have multiple GPS devices. Getting the idea from another thread, I took all but one of my GPS devices and connected them to power and set the antennas on the 21 Outrage's T-top. Each of the antennas are within a circle of about 16"-20"
The Lowrance HDS Gen3 does not have an external antenna for this demo. The GPS units were on for about an hour before I started taking pictures.

See: http://continuouswave.com/forum/viewtopic.php?f=9&t=191#p1244


Units tested:
Humminbird 997c
Lowrance HDS 7 Gen3 (in dash)
Northstar 951 D (differential) (differential not connected as I had no differential coupler antenna whip)
Northstar 952 DW (differential and WAAS)
Northstar 952 W (WAAS) (in dash)
Northstar 958 W (WAAS)



Humminbird 997c



Lowrance HDS Gen3



Northstar 951 D





Northstar 952 DW





Northstar 952 W




Northstar 958

[porthole]

Difference of the main chart display. Note that the Humminbird has an odd name for my area, "Shark River"

Humminbird


Lowrance HDS


Northstar 951D


Northstar 952 DW


Northstar 952 W


Northstar 958

[Jefecinco]

Conclusions?

[fno]

It seems that the Lowrance and Humminbird don't really know where they are. There are two different lat-lons displayed on the chart page. [Moderator's note: one position is the cursor position on the chart, the other position is the receiver position.--jimh]

I am assuming you were standing still during the photo session. Notwithstanding that observation, the Northstars are better units and wrote the book on what defines most "good" nautical navigation displays. Duane, you have too many Northstars and the slight differences of position will only confuse you. Send one to me to eliminate the confusion!!!

[jimh]

Let me present some of the data in tabular form:

Number of Satellites In view (PRNs), HDOP, Lat/Long

Humminbird 997c = 13 (8, 13, 14, 15, 18, 20, 21, 22, 24, 27, 46*, 48* ,51*), 0.89, 40-11.278N / 074-05.048W

Lowrance HDS Gen3 = 12 (13, 14, 15 18 20, 21, 22, 24 27, 70** 73**, 86**), 0.800, 40-11.272N / 074-05.055W

Northstar 951 D = 7 (10, 14, 22, 15, 18, 21, 24), 1.5, 40-11.275N / 074-05.049W

Northstar 952 DW = 7 (14, 15, 18, 20, 21, 22, 24), 1.2, 40-11.277N / 074-05.046W

Northstar 952 W = 8 (10, 14, 15, 20, 21, 22, 24, 27), 1.2, 40-11.278N / 074-05.046W

Northstar 958 = 8 (14, 15, 18, 20, 21, 22, 24, 27), 1.3, 40-11.278N / 074-05.049W

* = WAAS
** = GLONASS

NOTES:

TIME: It appears that some of the screen images were taken at different times so the position of the satellites in view is different. This affects the HDOP outcome.

HDOP: The most accurate position solution should be the one with lowest HDOP, which is the Lowrance HDS Gen3.

PRN 32 to 64: NMEA numbers in the range 32 to 64 are used for space based augmentation system satellites. To correspond these NMEA numbers to PRN numbers, add 87 to the NMEA number to get the real PRN. So we see that:

46 + 87 = PRN 133

48 + 87 = PRN 135

51 + 87 = RPN 138

These PRNs are for the United States FAA space-based augmentation system called WAAS. Lowrance's satellites in view page often does not show the space-based augmentation satellites, so we see it has omitted any NMEA numbers in the range 33 to 64 in its display, even though it is receiving at least one of the three WAAS satellites, indicated by the notation of "WAAS" in the description of the position fix. Hummingbird's satellites in view page shows all three WAAS satellites (as NMEA numbers 46, 48, and 51). Of the three, only PRN 135 and 138 transmit pseudo-ranging signals which can be used for the position solution. The augmentation information transmitted is the same on all three satellites.

PRN 65 to 96: The Lowrance receiver shows reception of NMEA numbers greater than 64, NMEA numbers 64 to 96 are used for GLONASS satellites; the SLOT ID of the GLONASS satellite is found by subtracting 64 from the NMEA number. So we see that

70 - 64 = SLOT ID 6

73 - 64 = SLOT ID 9

86 - 64 = SLOT ID 22

The Lowrance HDS Gen3 is receiving three GLONASS satellites in addition to the GPS satellites. This helps it get the lowest HDOP.

The FAA space-based augmentation system WAAS was described in an earlier article. That article also gives more details about WAAS satellites, their locations, and the look angle to them from North America.

The method for converting NMEA numbers to PRN numbers was described in an earlier article.

An excellent source of information about GNSS satellites in operation is found at GPSWORLD.COM's The Almanac page.

SNR: All the receivers show the received signal strength metric as "SNR" or signal-to-noise ratio. This is incorrect, although extremely common to see in receiver reports. The figures shown are the carrier-to-noise density. For a brief explanation see "SATELLITES IN VIEW--What the Data Means."

[jimh]

In some GNSS receivers, the satellites used in the position solution may be selected on the basis of their apparent elevation at the receiver's location. Signal measurements with satellites with elevation angles of less than 5-degrees or sometimes less than 10-degrees are often discarded because:

--low elevation signals travel farther through the atmosphere and are thus subject to more atmospheric effects on their propagation;

--low elevation signals tend to have more multi-path effects; and

--low elevation signals tend to have lower signal strength due to longer path length and the receiver antenna gain possibly being lower at low elevation angles.

Some GNSS receivers impose a low elevation mask in selecting signals to be used in the position solution. In the above comparison, some receivers indicate they are tracking satellites with very low elevation angles, such as the Humminbird which is tracking PRN 8 and 13 just above the horizon or the Lowrance Gen 3 which is tracking PRN 13 just above the horizon, while others appear to either not be able to receive those satellites or ignore them in their tracking, such as the Northstart 952 DW, which seems to only be tracking satellites that are well above the horizon.

[porthole]

Some notes; I will check the time stamps of the pics when I can, but all the pictures should have been taken within the 18 minute window the times show. The two 3 o'clock hour pictures are not DST adjusted.

The four Northstars are all set for a minimum SAT elevation of 10 degrees. I'll have to double check the HDS and Humminbird.
The Northstar 951 and 958 use the "ball" shaped antennas, although they are not the same. The 951 uses a simple COAX cable where the 958 uses a multi conductor cable (7 conductors maybe?)
The two 952 series Northstars use the disc style antenna.

Update:

I cannot find any page on either the Humminbird or the Lowrance GPS units or manuals as to the limits with the degrees of elevation for satellites in view.
I did find that the Humminbird can and will use up to 16 satellites if they are in view.

Your present position is determined by receiving signals from up to 16 satellites and measuring the distance from the satellites.


Also this from the Humminbird manual:
A dark grey bar indicates that the satellite is being used to determine your current position. A light gray bar indicates that the satellite is being monitored, but is not yet being used

So in the picture I used of the Humminbird, the #8 satellite is not being used for position calculation.

[porthole]

I am assuming you were standing still during the photo session. Notwithstanding that observation, the Northstars are better units and wrote the book on what defines most "good" nautical navigation displays. Duane, you have too many Northstars and the slight differences of position will only confuse you. Send one to me to eliminate the confusion!!!

Standing still, yes parked in the driveway.

The Northstar has always been my favorite since the 9XX series. Really disliked the 800 series.
I ran the 9XX series Northstars when I was doing SCUBA charters, there simply was nothing better at the time. The reason for multiples is - when you are running charters, customers don't want to hear my "stuff" is not working.

I have several because I would buy them on eBay whenever they came up and went for a really good price (think middle of the night ending auctions). Even non working units for the parts.
My 951 was new and no discounts when I got it, $2600. I doubt I spent more then $1000 for all the other units I have purchased through eBay (those shown here plus two others that went with the boats).

Purpose of this demo - none really. Other then I could and it was mentioned casually in another thread.

I did have to do a function test for the Northstars prior to putting them up on eBay. I just got three of the Northstars back from service for a battery change, software update and tide table replacement. The 958 apparently can not be updated for tides.

The HDS was purchased for my 17 Outrage, as an all in one, only one GPS device.
The 21 Outrage came with the Humminbird, but I had this brand new HDS and 3 updated Northstars that I really liked.

So the Humminbird came out and the HDS and a Northstar 952 got flushed mounted.
After some research on the Humminbird, I am thinking it may be a viable replacement for the Northstar 952. So, all the GPS devices were plugged in and this demo is what you get.

Seriously considering getting another piece of black Starboard and flush mounting the Humminbird in next to the HDS.

[jimh]

To compare the Humminbird and Lowrance, the Humminbird shows 13 satellite and the Lowrance shows 12. That is not a true indication because three of the satellites the Humminbird is counting are the WAAS signals and a fourth is a very low elevation GPS satellite. That brings the Humminbird tally down to nine satellites. The Lowrance is also receiving those nine, and it adds three more GLONASS satellites. And it also is getting one or more of the WAAS signals, which is often does not show in the polar plot or satellites in view page. On that basis I think the Lowrance has the best receiver, with 12 satellites in the position solution compared to perhaps nine on the Humminbird.

[porthole]

Well most of your comments are above my pay grade :-)

How accurate do you think the device displayed error is?
Humminbird = 5'
Lowrance = 13.8'

Using your math the Humminbird would be looking at 9 sats, but only using 8 for a solution, no?

Given the age of all the devices, I would hope that the Lowrance HDS Gen3 does indeed have the best receiver, although it is showing a higher position error.

Now if this was 15 years ago, I'd be placing all my bets on the Northstar's. Back in their heyday I found nothing that could beat them for both navigation and repeatable position accuracy. The claim to fame during the SA concern was resolved with the implementation of differential corrections, Northstar was bragging of 6-9 feet accuracy solutions. And in using them I believe the 6-9 was accurate.

[porthole]

Question posted to Lowrance tech support:
On a HDS7 Gen3, I do not see a way to limit the degrees off the horizon for the satellite view. Is there a user selectable setting? If not, what is this unit's satellite view limit in degrees?

Answer:
There is not a user setting for this and there is no spec for degree of range for the GPS receiver