Introduction

The goal of the SSR computation module is to compute the orbits and clocks corrections that are necessary for the  PPP (user side), and to encode these corrections in a well-defined standard (here the RTCM 3.1 standard). This module mainly uses as input the real-time measurements provided by a GNSS network, the computation is done by means of a Kalman filter.

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 is  represented on the following figure (about 70 stations) :


network snapshot

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 data flow

SSR products

The following table describes the different parameters available on the CNES streams:

 



Parameter Nature

RTCM message

Occurrence (sec)

CNES streams

GPS orbits/clocks

1060

5

CLK91, CLK93

Glonass orbits/clocks

1066

5

CLK91, CLK93

Galileo orbits/clocks

1243

5

CLK93

GPS code biases

1059

5

CLK91, CLK93

Glonass code biases

1065

5

CLK91, CLK93

Galileo code biases

1242

5

CLK93

GPS phase biases (L1, L2)

1265

5

CLK91, CLK93

GPS phase biases (L5)

1265

5

CLK93

Ionosphere VTEC

1264

60

CLK93