EWR 127-1 (Paragraph 2.5.4.1.1) establishes different IIP error standards for the WR and ER. The ER believes that the current limits are overly restrictive and has proposed relaxing them in the next version of EWR 127-1 (Campbell, 1999). For the ER, crossrange and downrange error in IIP must be no more than 100 feet (three sigma)¹ until IIP clears the launch area (as currently authorized) or no more than 300 feet (per the proposed change). The proposed change also requires that error in vehicle position not exceed 140 feet in the vicinity of the launch area. At the WR, the allowable error is 1,000 feet. Once the vehicle clears the launch area, IIP crossrange error at the ER must not exceed 0.5 percent of the IIP range, and downrange error may not exceed 5 percent of the IIP range. At the WR, crossrange and downrange errors must not exceed 1 percent of the IIP range. The ER bases estimates of IIP error on static error sources, excluding time lags caused by data transfer and processing delays. The WR incorporates lag errors in its methodology. This may explain the large differences in accuracy requirements between the ranges.

It is essential to account for lag errors in predicting the accuracy of IIPs because range radars cannot satisfy the IIP accuracy requirements in real time. In addition to mechanical pointing lags and various delays in relaying the data to the ROCC (Range Operations Control Center), sequential-difference data derived from the azimuth, elevation, and range data must be “smoothed” to develop the velocity estimate needed to calculate the IIP. This smoothing typically requires several seconds of data. The accuracy of the real-time IIPs is highly sensitive to the velocity value used, and a sudden change in the vehicle thrust vector takes time for the tracking system to detect and display; meanwhile, the actual IIP may be hundreds of feet from the displayed location.

In practice, however, small errors in IIP are not significant. The debris pattern after an explosive flight termination is many thousands of feet across even for an accident 10 to 15 seconds after liftoff, and pinpointing the center of the pattern is not necessary to ensure safety. The committee supports the proposal to relax the tight IIP accuracy requirements currently imposed at the ER for launch vehicles that have not yet cleared the launch area. Satisfying IIP accuracy requirements outside the launch area is well within the capabilities of both radar and GPS tracking systems.

The ER and WR each have a network of 10 C-band radars. Several of these radars are located at downrange facilities. Each network consists of one phased-array multiple-object tracking radar and nine, generally aging, C-band single-object tracking radars (see Figure 4-1). Ongoing modernization of both ranges will eliminate the need for most of these radars. As currently planned, the modernized ranges will use differential GPS tracking systems supplemented by two radars at the WR and seven radars at the ER. Three of the seven radars at the ER will be necessary only to support launches of the space shuttle, and three others will be located at downrange facilities to support ballistic missile tests and space object identification.

The WR launches vehicles into polar orbits using initial launch azimuths between 158^o and 201^o. Downrange assets are not needed for these launches because, by the time uprange facilities lose contact with launch vehicles, they no longer pose a threat to inhabited landmasses.

The ER uses initial launch azimuths of 37° to 114°. Northerly trajectories parallel the U.S. and Canadian coasts. Many launch vehicles on easterly trajectories do not achieve orbit prior to flying over Europe or Africa, and African overflight is common for missions with large payloads headed for geo-synchronous orbit. Currently, the ER uses downrange facilities to track vehicles to orbital insertion.²

EWR 127-1 specifies that the ground segment of the tracking system must have a reliability of at least 0.999 for a one-hour duration during the period of range safety responsibility (Paragraph 2.5.4.1.3). EWR 127-1 also says that the reliability requirements for vehicle-based range tracking systems are 0.995 for the C-band transponder systems and 0.999 for GPS-based systems (Paragraph 4.10.3a). A proposed change to EWR 127-1 would establish a slightly lower reliability standard (of 0.96) for each of the two independent sources of tracking data used in a GPS-based system (Cather, 1999).³ The requirement to have two independent tracking sources would not be changed.

Telemetered data are routinely collected during launches, and selected items are provided in real time to the range user and to range safety personnel in the ROCC. Data of particular interest to range safety are guidance data, command receiver status, and steering commands.

TMIG data must be used as a tracking source for launch vehicles equipped with an inertial guidance system (EWR 127-1, Section 2.5.5.1). TMIG data are important because they provide vehicle state vectors (which indicate vehicle location and velocity) to determine IIPs with a minimum of data processing. Compared to IIP displays based on