For Galileo processing, GAPS uses both the Galileo in-orbit-validation (IOV) and full-operational-capability (FOC) satellites. With the exception of PRN E20 (which transmits only the E1 observable), all available IOV and FOC satellites are used in processing, including PRNs E14 and E18. Following an orbit-correction campaign after their initial orbit injection failure, PRNs E14 and E18 can now be successfully used in GAPS' algorithm.

For BeiDou processing, GAPS currently uses all available inclined geostationary orbit (IGSO), medium earth orbit (MEO), and geostationary orbit (GEO) satellites. While global users will have access to observations from MEO satellites, only those located in and around Asia and Australia will be able to observe IGSO and GEO satellites at effective elevation angles.

In either case, GAPS users will have access to an increasing number of Galileo and BeiDou satellites as these constellations approach completion. For more information on the status of Galileo or BeiDou, please refer to the IGS MGEX page.

In order to use Galileo and/or BeiDou observables, GAPS uses orbit and clock products made available through the IGS Multi-GNSS Experiment (MGEX) campaign. Unlike standard IGS orbits and clocks, MGEX products are un-averaged solutions representative of just one IGS AC. To provide the user with several product options, GAPS allows user selection of which MGEX product source will be used for processing. It is important to note that products from each provider differ in several ways. For example, the orbits and clocks provided by CODE are available at a 15-minute and 5-minute interval, respectively, whereas the orbits and clocks from GFZ are available at 5-minutes and 30-seconds, respectively. A difference in product latency can also be noticed where one product source may provide a general latency of approximately 2 weeks while another may wait several weeks before updating their contributions.

It is also important to note that issues with IGS MGEX products may lead to processing failure. First and foremost, the user should be sure to select a product that provides orbits and clocks for the selected GNSS. For example, only the GFZ, CODE, and Wuhan ACs currently provide orbits and clocks for BeiDou. Also, inconsistencies in file formats may cause errors. For instance, certain periods of Wuhan products contain multiple "END OF HEADER" strings, creating a scenario where orbits and/or clocks cannot be successfully read.

Finally, please be aware that GPS and Galileo/BeiDou products must be used homogeneously. For example, when using the IGS Ultra-Rapid ephemeris, the user must also select use of the GFZ Ultra-Rapid product for use with Galileo/BeiDou. Use of any other product(s) while an Ultra-Rapid product has been selected will result in processing failure.

When using the GAPS "Advanced Mode" and "Basic Mode" submission pages, the user is provided with options for which GNSS constellations will be used during processing. Users can select combinations including GPS-only, Galileo-only, BeiDou-only, GPS + Galileo, GPS + BeiDou, and GPS + Galileo + BeiDou. Please note that a Galileo + BeiDou combination is not currently available. Also, due to the fact that neither Galileo nor BeiDou are "completed" systems, users will notice a lack of sufficient simultaneously-observable satellites (4 or more) with these constellations. Such limited simultaneous-observability will significantly degrade the achievable accuracy of the PPP solution. For more reasonable solutions in these situations, the user is advised to select a window for which a minimum of 4 satellites from the selected constellation are simultaneously observable. In any event, the user is advised to ensure that the submitted RINEX file contains observables from the desired constellation prior to submission.

Users of the GAPS "Advanced Mode" submission page are provided with the option to select from various available observables for both Galileo and BeiDou. While options for BeiDou primarily rely on which observables a particular receiver will have logged, Galileo users usually have several options. These options mainly include selection of the E5 pseudorange and carrier-phase observables including use of the "standard" E5a, E5b, or full E5 carrier-phase and pseudorange observables.

While not currently an option available to GAPS users, different sets of phase centre offsets (PCOs) can be applied within GAPS' processing algorithm. As Galileo and BeiDou are not yet finalized GNSSs, the use of estimated PCOs has not yet been used. While the IGS MGEX campaign recommends use of "standard" PCOs for each system, some ACs alternatively use their own PCOs estimated from actual system observables. In GAPS, standard IGS MGEX PCOs are used for all available Galileo satellites (whether IOV or FOC). For BeiDou processing using CODE products, standard IGS MGEX PCOs are also used for both IGSO and MEO satellites. However, if use of the GFZ products has been selected, appropriate PCOs (according to Dilssner, 2014) are applied, allowing for much more accurate processing estimates. Similarly, selection of the Wuhan products will result in the use of estimated PCOs (according to Guo, 2016).

Dilssner F., Springer T., Schönemann E., and Enderle W. (2014) Estimation of satellite antenna phase center corrections for BeiDou. IGS Workshop, Pasadena, California, USA. (available via IGS Workshop, 2014)

Guo, J., Xu, X., Zhao, Q., and Liu, J. (2016). Precise orbit determination for quad-constellation satellites at Wuhan University: Strategy, result validation, and comparison. Journal of Geodesy, 90(2), 143-159. doi:10.1007/s00190-015-0862-9.