SSR vs OSR :
A revolution opening high-precision GNSS to the industrial world
High-precision geolocalisation is currently experiencing a real boom. GNSS (Global Navigation Satellite System) is omnipresent in new technologies and the demand is exploding. It is in this climate that a new technological “Revolution” is emerging.
Autonomous vehicles, drones, use via smartphones, connected objects…, the list of fields of application requiring centimetric precision positioning is endless. However, is the technology enabling this level of performance ready for development in the industrial world?
Previously reserved for niche markets where high precision was required, the technology has been able to innovate for the mass market through SSR technology.
What is State Space Representation (SSR)? What is the difference with OSR which is commonly used today?
GNSS signals are received by the antenna and then processed by the receiver to measure pseudo-distances, the distance between a GNSS satellite and the user’s receiving antenna. These are impacted by various errors listed below:
Code and phase bias,
The accumulation of errors on the pseudorange results in a position with an accuracy of several meters. Therefore, in order to obtain a positioning with a high centimetric accuracy, it is necessary to correct the above errors.
For this purpose, there are real-time GNSS correction services such as TERIA. The latter will solve the errors by providing different types of corrections to correct pseudo-distance measurements and to arrive at centimetric measurements.
There are several correction services which are technically different:
- DGPS (Differential Global Positioning System)
- RTK (Real Time Kinematic)
- NRTK ( Network Real Time Kinematic)
- PPP (Precise Point Positionning)
- SBAS (Local Based Augmentation System)
The above-mentioned corrections can be separated into two categories according to their nature. On the one hand, corrections are made over the entire pseudorange (OSR), on the other hand, errors are corrected individually/separately (SSR).
OSR – « Observation State Representation »
Observation Space Representation, OSR, is the principle historically used by providers of correction services.
In summary, in NRTK mode, the user will send his approximate position to the correction/calculation centre, these GNSS “Observations” will then be corrected and sent back to it directly through raw data from a physical or virtual reference base station with corrected pseudo-distances of centimetric precision.
This method provides a customized correction for each user through two-way communication (via the Internet).
The OSR thus enables centimeter positioning in real time.
However, this calculation method based on an exchange between the calculation centre and the user requires a bidirectional Internet link using the NTRIP communication protocol. This solution has not been developed for the management of a large number of users because the total bandwidth used is a function of the number of simultaneous connections.
SSR – « State Spatial Representation »
State Space Representation, SSR, is the latest generation of GNSS corrections.
SSR is based on the use of a network of reference stations providing a network covering a large geographical area.
This network of reference stations will enable errors to be modelled over a large area (“state”) and transmitted to all users in the coverage area via the Internet and/or satellite. Thus, each receiver (user) will be able to calculate its position, using the data received to model GNSS errors locally and apply them to its own observations.
The more sources of errors that are corrected, the more efficient the positioning service will be.
The SSR provides the user with real-time centimetric positioning throughout the correction provider’s coverage area.
The SSR compression method is so efficient that it requires very little bandwidth, which opens up a whole range of possibilities for new applications.
In addition, the link can be unidirectional. This is because the same data is transmitted to all users and processed individually by the mobile receiver.
Comparison of benefits and limitations between OSR - SSR
The OSR and the SSR operate on different methods from each other. As a result, certain limitations appear by their nature.
In contrast to SSR, in OSR the centimetric accuracy acquired is via a single physical or virtual reference station (NRTK).
In addition, the OSR system is based on the exchange of data between the user and the computer centre. The size of the packets exchanged is very important. This is why the bandwidth usage is also substantial (see table).
If the station is physical (DGPS, RTK), the user is dependent on this station. The further away from its position, the more accuracy will be lost.
Similarly, if the user moves, he will either have to reconnect to a new station or in case the station is virtual (NRTK) re-generate a new virtual station. Depending on the correction provider, this change may be faster or slower and may result in a micro-cut of a few seconds.
For this reason, this correction method is preferred for local use.
These constraints make it impossible to see such a system develop in industrial applications with mass use. And this is the major difference with SSR.
The SSR is a real market changer, as it has virtually no constraints for industrial development.
Indeed, since its origin, it was developed with the idea of being extended on a large scale and to go beyond the limits encountered by the OSR.
The user benefits from centimetric precision over a wide coverage, using a low bandwidth and without having to emit any information (use without SIM card for example).
There is therefore a one-sided flow of information.
We can still find a limitation on the SSR. Since it is the latest generation of GNSS corrections, there is not yet a fully SSR compatible receiver. Each correction provider offers its own proprietary protocol and format for using SSR. The lack of a standard format is delaying the development of this technology on a large scale.
Various projects are moving in this direction in order to define an open standard for high-precision GNSS positioning. However, we are not there yet.
From a user point of view, in order to benefit from this type of service, it will be necessary to invest in one of the new receivers that will only be available in a few years’ time.
SSR is very promising. However, we will have to wait a little longer to really benefit from all its advantages, and see high-precision GNSS enter the industrial world.
$ 99Per Month
- Centimetric Accuracy
- Local Coverage
- Bi-lateral communication
- High bandwidth requirements
- Calculation of single-point corrections
$ 99Per Month
- Centimetric Accuracy
- Global Coverage
- Unilateral Communication
- Low bandwidth required
- Calculation of corrections over the entire coverage area
- No 100% compatible hardware to date
TERIA network, a network using both OSR and SSR. The advantage of TERIAsat.
TERIA offers both OSR and SSR services according to the demand and needs of its customers.
The advantage of the OSR services (DGPS -RTK – NRTK) with TERIA is that the density of stations in the area covered by the network is such that service availability is almost permanent (> 99% since 2012). In addition, the centimetric accuracy is guaranteed regardless of the user’s position.
TERIA also offers its users SSR services (PPP – PPP-RTK) using satellite transmission of corrections in particular, thanks to its TERIAsat technology.
In order to receive satellite corrections and avoid white zones, the user’s receiver must be L-band compatible. However, this requires an additional investment if his equipment is not.
To facilitate access to this technical innovation, TERIA is launching the TERIAsat Terminal.
The TERIAsat Terminal includes a satellite antenna, an Internet connection via SIM card and a Wi-Fi/Bluetooth antenna. The client receiver connects via Wi-Fi/Bluetooth to receive the terminal’s corrections, which are themselves received either via satellite or the Internet.
The user benefits from the advantages of both OSR and SSR methods on their current equipment, with the device providing the best available service to their mobile receiver without changing configuration.