10.4.1 Introduction
In 1983, the U.S. Institute of Navigation asked the Radio Technical Commis- sion for Maritime Services (RTCM) to develop recommendations for transmit- ting differential corrections to users of GPS. A special committee, No. 104, was established, and it published version 2.0 of the RTCM Correction Format in 1990. A further developed version—version 2.1—was published in 1994 [1];
it contains additional formats that support real-time kinematic applications.
RTCM version 2.1 provides 63 different message types, of which 26 were defined. Some of them are still tentative or reserved, while eight of the message types are fixed (see Table 10.1). Each type starts with a header, and the techni- cal information is then allocated to a variable number of 30-bit words.
The RTCM SC-104 DGPS format is widely used by GPS manufacturers, and a significant effort was made in deriving that format. However, the issues dealt with by the RTCM committee are somewhat different than those that must be addressed when a data channel with relatively limited data transmis- sion capacity such as RDS is used. The underlying principles are, of course, the same, and the RTCM format can therefore be used as a guideline to determine, for example, the RDS-ODA format to be used for implementing DGPS within an RDS data stream.
The RTCM format itself is unsuitable for RDS due to its excessive band- width. Therefore, the correction elements required will have to be compressed for transmission via the RDS data channel. The RTCM format then needs to
192 RDS: The Radio Data System
Table 10.1 Defined RTCM Message Types Type Function
1 Differential GPS corrections 2 Delta differential GPS corrections 3 Reference station parameters 5 Constellation health 6 Null frame 7 Beacon almanacs
9 Partial satellite set differential corrections 16 Special message
be reconstructed within a special service-provider-specific DGPS RDS FM subcarrier receiver before the correction data is delivered to the navigator. For example, to achieve a±5m accuracy, 20 to 50 bps within RDS-ODA to carry the correction data will be sufficient. Remember, as explained in Chapter 9, one ODA group type A (e.g., type 11A group) can carry 37 bits for the applica- tion under consideration.
10.4.2 Required Data Elements
Differential GPS requires that certain types of information be available to the DGPS navigator [2]. In the following, some of the most important elements commonly used by DGPS service providers and based on RTCM formats are briefly explained.
10.4.2.1 Pseudo-Range Correction (PRC)
The pseudo-range correction is the most fundamental data item needed by the navigator. It describes the correction for a satellite that should be applied at a certain point of time (the time at which the pseudo-range measurements were made at the reference station). The PRCs are strictly only valid for the epoch when they are computed at the reference station. When the PRC is transmitted to a mobile user, time goes by, and the values grow older and deviate more and more from the actual value. To overcome this, a reference station makes a pre- diction about the rate of change of the PRC, which is called the range rate cor- rection (RRC).
10.4.2.2 Range Rate Correction (RRC)
The RRC is used by the navigators to propagate the PRC forward to the point in time at which it can be applied by the navigator. Since the PRC was gener- ated at a time in the past and a variable time was required for it to be transmit- ted and received by the navigators, the rate term provides the means for the correction to be made current at the navigator.
The composite PRC computed by the navigators from the PRC and RRC terms is in error to the extent that unmodelled effects cause the actual pseudo- range correction to differ from the computed pseudo-range correction. The most significant source of error is selective availability.
10.4.2.3 Age of Correction Data: Time of Correction with the Modified Z-Count
The correction PRC and correction rate RRC for a satellite are useless unless the times for which they were computed are also conveyed to the navigators.
Differential GPS 193
In RTCM, the list of PRCs and RRCs for all satellites is therefore con- nected with the epoch of their creation, called “modified z-count,” which counts the seconds of the current hour. A user DGPS receiver calculates the pseudo-ranges for the epoch in which it can use them. The predicted and broadcast values of RRC grow worse the older they are, such that accuracy degrades significantly with values older than 20–25 seconds.
10.4.2.4 Satellite Identification (SATID)
The correction information for each satellite is different, and there is no assur- ance that corrections are received by all navigators. Therefore, it is necessary to identify the satellite whose corrections are being transmitted.
10.4.2.5 Ephemeris Parameters
The ephemeris is a set of parameters that describe the orbits of the satellites.
The ephemeris information for each satellite is changed periodically (approxi- mately every one to two hours). The DGPS correction information is based on a particular ephemeris, available to the reference station when the correction is generated. It is necessary to advise the navigators of the ephemeris in use at the reference station when the corrections were computed, because the corrections may be properly used by the navigator only if the same ephemeris is employed.
The issue of data ephemeris (IODE) is the key that ensures that the user equipment calculations and the reference station corrections are based on the same set of orbital and clock parameters.
10.4.2.6 User Differential Range Error (UDRE)
It is important for the reference station to advise the navigators of the validity and approximate error in the differential corrections. This information permits the navigators to decide whether or not to use the correction from a particular satellite and how the satellite may be weighted in the position computation.
10.4.2.7 Reference Station Location Data: Station ID and XYZ Coordinates In general, it is useful for the navigators to know the location of the reference station. This permits the navigators to use ionospheric and tropospheric models to estimate the amount of error in the pseudo-range measurements mode at both the reference station and the navigator.
This cannot be done at the reference station since the navigator locations are unknown to the reference station and there is no consistent model available for either troposphere or ionosphere that is accepted by all GPS manufacturers.
For this reason, no ionospheric or tropospheric models are used at the reference station.
194 RDS: The Radio Data System
In the RDS case, it is assumed that the reference station is relatively close to the FM transmitter. The limited range of most FM stations (about 100 km) implies that the navigators will not be far from the transmitter. Hence, a user close enough to receive the FM station will generally be well within the range at which the differential corrections are accurate. These arguments indicate that there may not be a strong need for the navigators to know the location of the reference station.
However, the reference station altitude may be useful, because the atmos- pheric models that are commonly used require the altitude of both the reference station and the navigator. The tendency to site FM transmitters at relatively high locations to improve signal coverage tends to increase the need for reference station altitude.
10.4.2.8 Service Quality Parameter: SQ/Station Health
This DGPS format offers positioning service providers the possibility for serv- ices with different accuracies, depending on the frequency at which corrections are supplied. An indication in the messages is given to advise navigators of the expected accuracy of the service. The service quality parameter, Station Health provides this capability.