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APRS/GPS:

Differential GPS Reference Station Interface Board

This product is no longer available.


Note: The DGPS kit requires the use of the Oncore VP GPS engine. Motorola discontinued the Oncore VP in August of 1999. TAPR is no longer able to supply Oncore VP units for this kit. We suggest you contact Synergy Syetsm (Gina) for any Oncore VP units they might have available. While new units are no longer available, surplus and refurbished units can be found.

DGPS RefSta Interface Board
Differential GPS Reference Station Interface on left
Example of Oncore attached to DGPS Interface on right

Description
The Differential GPS (DGPS) Reference Station Interface Board connects to a Motorola Oncore VP OEM GPS receiver to create a low cost 8-channel DGPS Reference Station.; The Reference Station provides Pseudo-range Differential GPS corrections that conform to the RTCM SC-104 Type 1 message format. Corrections can be transmitted via data link to remote users.; Possible data links are radio modems, terminal node controllers (TNC), or telephone modems.; Remote users receive the corrections and apply them to their DGPS ready receiver to calculate a DGPS solution.

For example, a Radio Amateur installation would use a 2-meter transmitter and TNC.; Using the 2-meter frequency of 144.39 MHz, the new APRS frequency, APRS trackers can take advantage of DGPS accuracies (see performance below).; No special equipment is needed by remote users.; Corrections are received using a 2-meter (mobile or handheld) receiver and TNC.; The output of the TNC is connected to a DGPS "ready" receiver.

Commercial users can connect the DGPS Interface board to commercial data radios.; The responsibility of using the correct frequencies is up to the user.

Possible uses include:

  • Search and Rescue
  • APRS Trackers
  • Parades
  • Marathons
  • Balloon Tracking
  • Research and Development
  • Vehicle Location in a metropolitan areas
  • Anything that requires increased GPS accuracy

Setup of the DGPS Reference Station is easy.; The output baud rate and the Station ID is selectable via jumpers.; The latitude, longitude, and altitude of the reference location and the rate at which corrections will be transmitted are programmed via software commands.; The Reference Station then operates without any further intervention.


Download DGPS Assembly Manual (pdf format, 498K)


Features
  • Operates with the Motorola Oncore VP 8-Channel GPS Receiver
  • Reference Station is Compatible with DGPS "ready" receivers that accept RTCM SC-104 Type 1 Messages
  • Easy setup and operation, suitable for portable operations
  • Programming via RS-232 serial port
  • Outputs RTCM SC-104 Type 1 Message with a selectable RS-232 output baud rate: 1200, 2400, 4800, and 9600 baud
  • Selectable Station ID: 0 - 1023
  • Power Requirements: 7 - 30 VDC
  • Power Consumption: 235 milliamps typical with Oncore VP with Active antenna
  • Lithium; Battery Backup Option for Oncore VP’s without battery backup


Performance
Using Stan Huntting's, KW7KW, SA Watch program, you can see the difference DGPS has over civil GPS.; The following GPS receivers were held stationary and received differential corrections from the TAPR DGPS Reference Station. Differential corrections were transmitted on 145.79 MHz (the previous APRS frequency) and two plots were made for each receiver.; The first plot without differential corrections and the second with.


Zero Baseline Tests
The Zero Baseline test connects the TAPR DGPS Reference Station to a remote GPS receiver under test using the same antenna.; Both receivers connect to the same antenna via a 2-way DC isolated L-band power splitter.


DGPS References

DGPS Links
NOTE: These have all gone dead. If you know or find out where they've moved, please let us know.


Performance of the TAPR Differential GPS Reference Station using a Garmin GPS-20

Garmin GPS-20 Without Differential Corrections
The display below shows the Garmin GPS-20 collected non-differential GPS positions for approximatelly 2 hours.  SA Watch calculated the 95th percentile as 59 meters. The scale of the outer ring is 150 meters.

Garmin GPS-20 without differential corrections

Garmin GPS-20 With Differential Corrections
The display below shows the Garmin GPS-20 collected differential GPS positions for approximatelly 2 hours.  The differential corrections were transmitted every 15 seconds.  SA Watch calculated the 95th percentile as 15 meters.  The scale of the outer ring is 150 meters.

Garmin GPS-20 with differential corrections


Performance of the TAPR Differential GPS Reference Station using a Garmin GPS-45

Garmin GPS-45 Without Differential Corrections
The display below shows the Garmin GPS-45 collected non-differential GPS positions for approximatelly 2.3 hours.  SA Watch calculated the 95th percentile as 64 meters. The scale of the outer ring is 150 meters.

 Garmin 45 without differential corrections

Garmin GPS-45 With Differential Corrections
The display below shows the Garmin GPS-45 collected differential GPS positions for 5 hours.  The differential corrections were transmitted every 10 seconds.  SA Watch calculated the 95th percentile as 16 meters.  The scale of the outer ring is 150 meters.

 Garmin 45 with differential corrections


Performance of the TAPR Differential GPS Reference Station using a Motorola Oncore VP

Motorola Oncore VP Without Differential Corrections
The display below shows the Oncore VP collected non-differential GPS positions for approximatelly 9.5 hours.  SA Watch calculated the 95th percentile as 58 meters.

Oncore VP without differential corrections

Motorola Oncore VP With Differential Corrections
The display below shows the Oncore VP collected differential GPS positions for approximatelly 5 hours.  The differential corrections were transmitted every 10 seconds.  SA Watch calculated the 95th percentile as 11 meters.

Oncore VP with differential corrections

The following plots the scale was changed.  The outer ring is 15 meters

Motorola Oncore VP Without Differential Corrections
The display below shows the Oncore VP collected non-differential GPS positions for approximatelly 2 hours.  SA Watch calculated the 95th percentile as 78 meters.

Oncore VP without differential corrections

Motorola Oncore VP With Differential Corrections
The display below shows the Oncore VP collected differential GPS positions for approximatelly 2 hours.  The differential corrections were transmitted every 15 seconds.  SA Watch calculated the 95th percentile as 11 meters.

Oncore VP with differential corrections


Zero Baseline Test

The Zero Baseline test connects the TAPR DGPS Reference Station to a remote GPS receiver under test using the same antenna.  Both receivers connect to the same antenna via a 2-way DC isolated L-band power splitter.  The plots below were made using a Oncore VP as the remote DGPS receiver.  The reference coordinates were 30:46:58.965N 81:40:30.830W.  The RTCM output of the Reference Station was connected directly to the Oncore VP (i.e., no radio link).

Zero Baseline test were also done with the following GPS receivers as remote receivers:

The below plot positions were collected for 10 hours 17 minutes.  The RTCM output of the Reference Station was at a rate of one message every 1 second.  SAWatch calculated the 95th percentile at 3 meters.  The scale of the outer ring is 15 meters.

 


The below plot positions were collected for 11  hours.  The RTCM output of the Reference Station was at a rate of one message every 5 seconds.  SAWatch calculated the 95th percentile at 4 meters.  The scale of the outer ring is 15 meters.

SA Watch Statistics (edited)

First sample: 01:55:31  10.Jan.1998 — Last sample: 12:26:21  10.Jan.1998 — UTC
Elapsed time: 0:10:30:50 — Logging time: 0:10:30:50 for 18,927 records
Logging interval:  100%@2sec
Sample interval is all samples yielding 18,926 samples
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:58.7460N  30:46:59.1540N  30:46:58.9604N  0:00:00.04935
Longitude       81:40:31.1280W  81:40:30.5880W  81:40:30.8312W  0:00:00.05895
Elevation       -8.500m         27.800m         12.781m         3.6097m
UTM Easting     17 0435384.242  17 0435398.671  17 0435392.171  1.5660m
UTM Northing    R 3405746.518   R 3405758.991   R 3405753.070   1.5261m
2D Position                                                     2.1866m
3D Position                                                     4.2203m
HDOP            0.9             4.5             1.54
SVs Tracking    5               8               6.84
 

The below plot positions were collected for 11  hours 19 minutes.  The RTCM output of the Reference Station was at a rate of one message every 10 seconds.  SAWatch calculated the 95th percentile at 5 meters.  The scale of the outer ring is 15 meters.

SA Watch Statistics (edited)

First sample: 12:41:01  10.Jan.1998 — Last sample: 23:59:57  10.Jan.1998 — UTC
Elapsed time: 0:11:18:56 — Logging time: 0:11:18:56 for 20,370 records
Logging interval:  100%@2sec
Sample interval is all samples yielding 20,369 samples
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:58.632N   30:46:59.292N   30:46:58.951N   0:00:00.0656
Longitude       81:40:31.320W   81:40:30.540W   81:40:30.829W   0:00:00.0642
Elevation       -22.20m         32.30m          11.49m          4.631m
UTM Easting     17 0435379.12   17 0435399.97   17 0435392.23   1.705m
UTM Northing    R 3405743.04    R 3405763.23    R 3405752.77    2.029m
2D Position                                                     2.650m
3D Position                                                     5.335m
HDOP            1.0             3.4             1.40
SVs Tracking    4               8               6.77
 

The below plot positions were collected for 10  hours 35 minutes.  The RTCM output of the Reference Station was at a rate of one message every 15 seconds.  SAWatch calculated the 95th percentile at 8 meters.  The scale of the outer ring is 15 meters.

SA Watch Statistics (edited)

First sample: 04:06:33  11.Jan.1998 — Last sample: 14:41:23  11.Jan.1998 — UTC
Elapsed time: 0:10:34:50 — Logging time: 0:10:34:50 for 19,047 records
Logging interval:  100%@2sec
Sample interval is all samples yielding 19,046 samples
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:58.566N   30:46:59.520N   30:46:58.967N   0:00:00.1037
Longitude       81:40:31.410W   81:40:30.420W   81:40:30.827W   0:00:00.1008
Elevation       -41.70m         52.60m          12.25m          7.427m
UTM Easting     17 0435376.71   17 0435403.20   17 0435392.29   2.679m
UTM Northing    R 3405741.02    R 3405770.23    R 3405753.27    3.208m
2D Position                                                     4.179m
3D Position                                                     8.523m
HDOP            1.0             6.4             1.60
SVs Tracking    5               8               6.86


The below plot positions were collected for 10  hours .  The RTCM output of the Reference Station was at a rate of one message every 20 seconds.  SAWatch calculated the 95th percentile at 8 meters.  The scale of the outer ring is 15 meters.

SA Watch Statistics (editied)
 
First sample: 13:59:09  1.Feb.1998 - Last sample: 23:58:49  1.Feb.1998 - UTC
Elapsed time: 0:09:59:40 - Logging time: 0:09:59:40 for 17,982 records
Logging interval:  100%@2sec
Sample interval is all samples yielding 17,981 samples
 
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:58.032N   30:46:59.706N   30:46:58.956N   0:00:00.1079
Longitude       81:40:31.404W   81:40:30.072W   81:40:30.830W   0:00:00.1025
Elevation       -61.80m         59.80m          3.33m           8.158m
UTM Easting     17 0435376.77   17 0435412.49   17 0435392.20   2.722m
UTM Northing    R 3405724.58    R 3405775.90    R 3405752.94    3.337m
2D Position                                     4.306m          0.032m
3D Position                                     9.225m          0.069m
HDOP            0.9             3.6             1.51
SVs Tracking    4               8               7.03



The below plot positions were collected for 12  hours 32 minutes.  The RTCM output of the Reference Station was at a rate of one message every 30 seconds.  SAWatch calculated the 95th percentile at 14 meters.  The scale of the outer ring is 15 meters.

SA Watch Statistics (editied)
 
First sample: 00:01:21  2.Feb.1998 — Last sample: 12:33:35  2.Feb.1998 — UTC
Elapsed time: 0:12:32:14 — Logging time: 0:12:32:14 for 22,569 records
Logging interval:  100%@2sec
Sample interval is all samples yielding 22,568 samples
 
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:58.008N   30:47:00.024N   30:46:58.964N   0:00:00.1832
Longitude       81:40:31.710W   81:40:29.724W   81:40:30.828W   0:00:00.1766
Elevation       -97.80m         119.50m         6.79m           14.068m
UTM Easting     17 0435368.64   17 0435421.80   17 0435392.26   4.691m
UTM Northing    R 3405723.89    R 3405785.64    R 3405753.18    5.663m
2D Position                                     7.354m          0.049m
3D Position                                     15.874m         0.106m
HDOP            1.0             3.2             1.39
SVs Tracking    5               8               7.00


Garmin 45 Zero Baseline Test

The below plot positions were collected for 8 hours 24 minutes.  The RTCM output of the Reference Station was at a rate of one message every 1 second.  SAWatch calculated the 95th percentile at 14 meters.  The scale of the outer ring is 30 meters.

SA Watch Statistics (edited)

First sample: 03:16:12  19.Jan.1998 — Last sample: 11:40:50  19.Jan.1998 — UTC
Elapsed time: 0:08:24:38 — Logging time: 0:08:24:38 for 14,591 records
Logging interval:  98%@2sec  1%@5sec
Sample interval is all samples yielding 14,590 samples
Note: Garmin 45 Zero Baseline
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:56.400N   30:47:00.180N   30:46:58.947N   0:00:00.2093
Longitude       81:40:32.640W   81:40:29.640W   81:40:30.864W   0:00:00.2024
Elevation       -46.90m         244.00m         27.35m          15.413m
UTM Easting     17 0435343.62   17 0435424.06   17 0435391.29   5.377m
UTM Northing    R 3405674.54    R 3405790.42    R 3405752.67    6.472m
2D Position                                     8.415m          0.070m
3D Position                                     17.560m         0.145m
EPE             8.3             108.8           13.27
HDOP            1.0             9.6             1.58
SVs Tracking    3               8               5.96


Garmin GPS-20 Zero Baseline Test

The below plot positions were collected for 6 hours.  The RTCM output of the Reference Station was at a rate of one message every 1 second.  SAWatch calculated the 95th percentile at 18 meters.  The scale of the outer ring is 30 meters.

SA Watch Statistics (edited)

First sample: 17:13:45  19.Jan.1998 — Last sample: 23:14:04  19.Jan.1998 — UTC
Elapsed time: 0:06:00:19 — Logging time: 0:06:00:19 for 20,698 records
Logging interval:  99%@2sec  1%@5sec
Sample interval is all samples yielding 20,697 samples
Note: Garmin GPS-20 Zero Baseline
 
Element         Minimum         Maximum         Mean            St Dev
Latitude        30:46:57.582N   30:47:00.174N   30:46:58.984N   0:00:00.2271
Longitude       81:40:32.004W   81:40:29.964W   81:40:30.831W   0:00:00.2170
Elevation       -201.90m        105.20m         25.14m          16.362m
UTM Easting     17 0435360.74   17 0435415.45   17 0435392.19   5.764m
UTM Northing    R 3405710.82    R 3405790.29    R 3405753.81    7.022m
2D Position                                     9.085m          0.063m
3D Position                                     18.715m         0.130m
EPE             17.9            158.4           30.68
HDOP            1.1             9.7             1.85
SVs Tracking    3               6               4.73
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