Other factors will affect the accuracy of GPS receivers. One of these is that the speed of light varies as it goes through the atmosphere. This might result in a receiver error of up to 12 feet. The clock error alone might add up to 2 feet. Built-in receiver errors might come to another 4 feet. The worst case satellite selection errors might add up to another 25 feet, and the ephemeris error will result in 2 more feet. AU of this adds up to somewhere between 10 and 30 feet of error. One other error has until recently prevented GPS from being used for precision IFR approaches: selective availability (SA). Two frequencies are used by the satellites. The precision code (P-code) is used by military and the course acquisition code (C/A code) is used by the public. The military can arbi trarily induce an error into the C/A – code, which can impart an error of up to 300 feet with out the user being aware of it. Naturally, an error of this magnitude would prevent the pilot from using GPSs for a precision approach. To overcome this, ground stations are being provided that know their position precisely. This ground station will then derive its own GPS position, recognize any error, and then transmit that difference to the user＇s receiver so that the receiver’s computer will then be able to give a precise location with repeatable accuracy of within 3-10 meters (roughly 10-30 feet.) This is called differential GPS (DGPS). It is this precision that is enabling aircraft to make fully-occupied autopilots landings, and which, someday, will make all other types of navigation obsolete. ARNAV Systems, Inc. has two types of GPS receivers available. The first type uses a separate receiver (GPS-506) that interfaces with the FMS-5000 or R-5000 receiver. This combination will provide the pilot with either a loran or GPS position, whichever signal is deemed most accurate. The pilot can also manually select either mode of navigation. The three-dimensional accuracy in the GPS mode (without SA) is within 15 meters horizontally and 90 feet vertically. The second GPS available from ARNAV is the pure GPS receiver, the STAR 5000. The only physical difference between this receiver and the FMS-5000 is appar ently the name on the lower right hand side, the letters GPS on the upper left hand side, and the absence of the XP light on the STAR 5000. 3.From the passage, we can infer that （ ） .