The real-time GNSS stations network

The SSR computation module relies upon a real-time GNSS networks of stations, made available to CNES thanks to its participation to the real-time IGS pilot project. Indeed, one of the goal of this pilot project is to collect and broadcast in real-time the measurements of some of the IGS network of GNSS stations.

 

In this framework, the CNES uses mainly the measurements collected by 2 'casters', or broadcast relays :

• The GFZ caster, which proposes more than 80 streams.
• The NrCan caster, which proposes some additional streams.
• The CNES own caster, which proposes streams operated by CNES.

The measurements are collected in real-time with the BNC tool. A typical network of real-time available stations (about 160) is represented on the following figure:

The other input interfaces

In addition to the real-time measurements, the SSR module uses a daily interface with the  IGS to recover the measurements necessary for the widelane clock computation. Another IGS interface allows to recover ultra-rapid IGU orbits to feed the Kalman filter.

 

The Kalman filter

The core of the real-time implementation is the Kalman filter working in mixed-mode (with both real- and integer-valued phase ambiguities). A preprocessing step allows to detect cycle slips and to solve for the integer widelane ambiguities. The N1 ambiguity fixing is performed directly in the Kalman filter using network connectivity considerations. The typical rate of the Kalman filter is 5 secondes, for a total latency of 5 seconds. The parameters estimated in the filter are detailled in the following table:

 

Parameter nature	Quantity	Typical number
satellite phase clock	1 per satellite	34
station phase clock	1 per station	50
code/phase satellite clock bias	1 per satellite	34
code/phase station clock bias	1 per station	50
zenith troposphere delay	1 per station	50
station coordinates corrections	3 per station	50*3
satellite orbit corrections (R,T,N)	3 per satellite	34*3
Phase ambiguities	12 per station (max)	50*12
		1070

 

 

The ouput of the filter contains mainly the corrected orbits and the complete set of clocks (widelane, pseudo-range and phase), that are sent to the caster with the BNC software. 

 

Overview of the SSR module

The following figure represents an overview of the SSR module (with its interfaces):

SSR products

The following table describes the different parameters available on the CNES streams (CLK91 or CLK93):

Constellation	Nature	RTCM Message	Occurrence (sec)
GPS	orbits/clocks	1060	5
GPS	code biases	1059	5
GPS	phase biases (L1, L2, L5)	1265	5
Glonass	orbits/clocks	1066	5
Glonass	code biases	1065	5
Galileo	orbits/clocks	1243	5
Galileo	code biases	1242	5
Galileo	phase biases (E1, E5a, E5b, E6)	1267	5
Beidou	orbits/clocks	1261	5
Beidou	code biases	1260	5
Beidou	phase biases (B1, B2, B3)	1270	5
	Ionosphere VTEC	1264	60