GPS receivers — what clock type do they actually use? In practical Global Positioning System (GPS) field equipment, which type of timing reference is used inside the receiver unit to time-stamp and correlate the satellite codes (while the satellites themselves carry ultra-stable standards)?

Introduction / Context:
Precise timing is the heart of satellite navigation. GPS positioning works by measuring how long a coded radio signal takes to travel from a satellite to the receiver. Although nanosecond-level timing stability is required in the space segment, the design choices for user receivers are very different because of cost, size, and power constraints.

Given Data / Assumptions:

• GPS satellites carry highly stable time references.
• User receivers correlate pseudorandom (PRN) codes to estimate signal travel time.
• Receivers must be economical, compact, and low power for mass deployment.

Concept / Approach:

Satellites house atomic clocks (cesium, rubidium) that broadcast time-tagged signals. Receivers do not need on-board atomic clocks; instead they use temperature-compensated crystal oscillators (TCXO/OCXO) based on quartz. Any residual receiver clock bias is solved as an unknown in the navigation solution alongside the three position coordinates, which is why at least four satellites are required for a stand-alone fix.

Step-by-Step Solution:

Verification / Alternative check:

Receiver specification sheets list TCXO/OCXO quartz oscillators. Survey-grade units may offer better-stability quartz or external disciplining (e.g., to GNSS time) but still not internal atomic clocks in normal operation.

Why Other Options Are Wrong:

• Atomic clocks are on satellites and in specialized timing labs, not standard field receivers.
• “Electronic clocks” is vague and non-specific.
• Mechanical clocks cannot provide the required frequency stability.

Common Pitfalls:

• Assuming receivers must mirror satellite atomic stability; they exploit multi-satellite geometry to estimate clock bias instead.
• Confusing disciplining to GPS time with housing an atomic standard.

Final Answer:

Quartz clocks.