Selection of GNSS correction source
In order to achieve full accuracy, the GPS2 must receive RTCM v3.x differential corrections from one of the following two data sources.
1. Radio modem link from a fixed base station that is also situated at the test track.
2. From an internet-based correction service, via. a GSM data link.
In both cases, only one source of corrections is required to service multiple units, since multiple GPS2 receivers can receive corrections from a single base station, and when using GSM corrections these can be forwarded using radio modem to further GPS2 units. The pros and cons of the two methods are as follows:
Base station via. radio modem.
The advantage of this setup is that no GSM connectivity is required, and no subscriptions to external services are required, so testing can be performed completely autonomously at a track with no external dependencies. It is particularly convenient at a track where a permanent base station is already available, or at a regular test location where a surveyed base station location can be kept between testing sessions, and is the only possibility at tracks where GSM coverage is not available.
The downside to the base station setup is the need for significant extra hardware; a base station and two radio modems are required. There is also extra configuration effort required in order to ensure that the base station is properly configured, and that the message protocol used is compatible. It is also unsuitable for wide area or open road testing beyond the range of the radio modems, or for testing at any location where the security of the base station cannot be ensured.
Full details of setting up the GPS2 to use base station corrections can be found here.
Internet corrections via. GSM.
The advantage of this setup is that once a data SIM card and a correction service subscription have been sourced, the GPS2 unit can then run differentially without any further hardware wherever a GSM connection is available. Note that continuous corrections are not required; the differential lock is robust to GSM outages of up to approximately 30s at a time once lock has been established, so intermittent GSM outages whilst moving are not a problem. Hence this setup is the most suitable for open road testing where a base station may not be able to be sited or kept within range. It is also perfectly suitable for testing at a circuit where GSM reception is available as a simpler alternative to the base station method.
The downside to the GSM setup is the requirement for GSM reception at the test location, and the need to pay for a data SIM connection and for an NTRIP service. Although free services are available in some locations
Full details of setting up the GPS2 to use internet corrections over GSM can be found here.