IPA IP 2020 Question 91
A question IPA Japan published in 令和2年度 10月 ITパスポート試験 公開問題 問91, translated into English. ITPEC is based on the same exams.
Which of the following is an appropriate mechanism by which a terminal such as an IoT device or a smartphone obtains its own location information by using the GPS function installed in the terminal?
Explanation
Receiving signals from three or more satellites and timing them — GPS is receive-only: the terminal listens to the satellites and works out its own position.
Each GPS satellite continuously broadcasts radio signals carrying the exact time of transmission and its own orbital position. The receiver notes when each signal arrives; the delay times the speed of radio waves gives its distance from that satellite. With the distances to three or more satellites at known positions, the receiver finds the one point that fits them all (in practice a fourth satellite also corrects the receiver's clock). The satellites never learn where the terminal is.
Answer (c)
Why not others:
- •(a) — a GPS terminal transmits nothing to the satellites; there is no request and no reply, which is why any number of receivers can use the system at once
- •(b) — asking the mobile network for a position is base-station positioning, not the GPS function, and again the GPS receiver sends no request
- •(d) — measuring signals from ground base stations is terrestrial positioning; GPS signals come from satellites
Key rule: GPS positioning is one-way: the terminal receives time-stamped signals from at least three satellites and computes its position from the arrival delays.
Networks · AI-written, may contain errors
This question comes from the past questions IPA Japan publishes, translated into English by ITPEC Practice. ITPEC Practice is an independent study tool and is not affiliated with IPA or ITPEC. See the original paper (PDF) or Report an issue.