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 welldefined standard (here the RTCM 3.1 standard). This module mainly uses as input the realtime measurements provided by a GNSS network, the computation is done by means of a Kalman filter.The realtime GNSS stations
network
The SSR computation module relies upon a realtime GNSS networks of stations, made available to CNES thanks to its participation to the realtime IGS pilot project. Indeed, one of the goal of this pilot project is to collect and broadcast in realtime 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 realtime with the BNC tool. A typical network of realtime available stations (about 100) is represented on the following figure:
The other input interfaces
In addition to the realtime 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
ultrarapid IGU orbits to feed the Kalman filter.The Kalman filter
The
core of the realtime implementation is the Kalman filter working in
mixedmode (with both real and integervalued 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 
Overview of the SSR module
The
following figure represents an overview of the SSR module (with its
interfaces):SSR products
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 