28 October 2014

DGPS: the new frontier of DXing ?

Differential Global Positioning System (DGPS) is an enhancement to Global Positioning System that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm (!) in case of the best implementations.
DGPS uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. The digital correction signal is typically broadcast locally over ground-based transmitters of shorter range. Just these stations are called DGPS Beacons.

DGPS serving marine navigation

DGPS serving inland users

Differential correction techniques are used to enhance the quality of location data gathered using global positioning system (GPS) receivers. Differential correction can be applied in real-time directly in the field or when postprocessing data in the office. Although both methods are based on the same underlying principles, each accesses different data sources and achieves different levels of accuracy. Combining both methods provides flexibility during data collection and improves data integrity.


Real-time DGPS
occurs when the base station calculates and broadcasts corrections for each satellite as it receives the data. The correction is received by the roving receiver via a radio signal if the source is land based or via a satellite signal if it is satellite based and applied to the position it is calculating. As a result, the position displayed and logged to the data file of the roving GPS receiver is a differentially corrected position.

Postprocessing Correction
Differentially correcting GPS data by postprocessing uses a base GPS receiver that logs positions at a known location and a rover GPS receiver that collects positions in the field. The files from the base and rover are transferred to the office processing software, which computes corrected positions for the rover's file. This resulting corrected file can be viewed in or exported to a GIS.


postprocessing


These signals can be found on LF, on the channels listed in the Marine Beacon Bandplan in Section Nine; in Europe the band covers 283.5 to 315 kHz, but in some other parts of the world 315 to 325 kHz are also used.  DGPS beacons are heard using G1D modulation with Minimum Shift Keying (MSK), a frequency shift keying mode with very small bandwidth, and their sound resembles a RTTY/Navtex signal. 


DGPS spectrum [1]
The baud rate in many cases will be 100 bps though there are still quite a lot of 200 bps beacons in some parts of the world (especially North America). Baud rate setting may be set manually or automatically by the decoder.
tuning a DGPS beacon on 286.5 KHz
You can use software such as DSCdecoder or Multipsk to decode DGPS signals and see where they are coming from: DSCdecoder my be downloaded from the following site , it has a 21 days test period and costs Euro €25 (plus VAT for EUresidents) for personal use. Personally I use Multipsk and SkySweeper (see below).

Pay attention to the false decodes which return "exotic" beacons. The reasons for these being created are more complex, but sometimes not being tuned in properly, or even loud static bursts can start the decoder going and ‘invert’ signals , and this can be a problem when unattended monitoring is being attempted, and the user can’t see what is causing it. Moreover, most Message Types used by DGPS beacons fall into a limited category, so anything outside of these should be treated with caution, especially if only one decode ‘frame’ is received, and not multiple identical decodes. 

As David GM8XBZ say:
"The station details that a decoding programme gives are from a lookup table that it holds. When it gets a station reference, it prints out the info it has in the software. All you receive is the station number. If that is an error, the software doesn't know.
The big clue, besides the range and time, is the Z-count value. In a 'good' decode, this should be the same as the time-stamp from the PC.  for example, at 21:12:30, the Z-count should be close to 1230 (12 min 30secs)."






DGPS Message Types
There are a number of different ‘Message Types’ broadcasted by the various DGPS beacons, and below is a list of what these are in my log and what they mean:

Message Type: 1 Differential GPS Correction
Message Type: 3 GPS Reference Station Parameters

Message Type: 5 GPS constellation health
Message Type: 6 GPS null frame
Message Type: 7 DGPS Radiobeacon Almanac
Message Type: 9 GPS Partial Correction Set

but there are up to 63 message types:

DGPS message types [1]
DGPS decoders and reception
Below a DGPS transmission received just some minute ago from station number 469 (Porquerolles FRA 286.5 Khz TXID 339 100bps): the same transmission has been decoded with Multipsk and SkySweeper (this one showing local time, UTC -1):

working 469 DGPS beacon with Multipsk


working 469 DGPS beacon with SkySweeper

When loggin a DGPS beacon, its "reference ID" is indicated as "station number" by decoders: this number is usually taken as its callsign while the TX ID number is noted in details within the its baud rate. In the above case I'll log:

00286.5 469: DGPS Porquerolles, FRA 1243 TXID #339 100bps

The "station number" helps to identify the received beacon. As seen, two numbers exits (see the table below)

1. GPS reference station number
2. DGPS broadcast station number (see the table below)

The numbers itself are not part of the RTCM standard, but are assigned by IALA. Some authorities stick to the RTCM standard and send the reference station number, others use the broadcast station number.
DGPS beacons in the UK, Norway and Denmark, for example, transmit reference station numbers, while those in The Netherlands, Germany, Sweden and Finland send the broadcast station number.
This confusion has not been resolved so far.



Stations Numbers [1]

European Differential Beacon Transmitters (European DGPS Network)

Trinity House have changed the frequency of many of the UK DGPS beacons, see:
http://www.trinityhouse.co.uk/pdfs/gps_ukireland.pdf 
  
 



Happy DGPS DXing !