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是否所有 GPS 接收器都能夠朔源標準時間?
Even though the GPS broadcasts are continuously monitored by NIST, not all GPS receivers are suitable for use as traceable frequencystandards. Remember that the definition of traceability states thattraceability is the property of "the result of a measurement." Theuncertainty relative to NIST can vary widely depending upon the GPSreceiver used to perform the measurement. The uncertainty is alsodependent upon the method or procedure used to perform the measurement.
To explain why different GPS receivers behave differently requiresdiscussing the similarities and differences of the commerciallyavailable models. Hundreds of companies sell GPS products, and at leasta dozen manufacturers advertise their units as time and frequencystandards. Most receivers are designed using an OEM GPS "engine." Theseengines are small circuit boards or chipsets that require a powersupply, antenna, and control software to use. They typically have acomputer interface and provide a 1 pps output.
Commercial GPS receivers share several characteristics:
- Most GPS receivers automatically select the satellites used inthe timing solution. This makes them easy to use. Often, you simplyturn the receiver on and wait for a signal to be acquired. However,different algorithms are used to select satellites, and each receiverhas its own thresholds at which it decides to keep, drop, or acquire asatellite. Some algorithms choose the satellites that provide the bestgeometric dilution of precision (GDOP). Others choose the satelliteshighest in the sky after a fixed position has been entered. Somealgorithms limit the timing solution to just one, or just a fewsatellites. Others can use as many as 12 satellites in the solution.For this reason, two GPS receivers can obtain very different resultseven when connected to the same antenna in the same location.
- MostGPS receivers have poor short term stability. The models with the bestshort term stability typically discipline an oven controlled quartzoscillator (OCXO) or a rubidium oscillator to the GPS signal. However,many receivers do not discipline an oscillator at all. Instead, theydivide the output of a small temperature controlled crystal oscillator(TCXO) to 1 pps, and then synchronize the 1 pps to the GPS signal. TheTCXO free runs and the receiver accumulates time errors until the totaltime error approaches a threshold (a multiple of the half period of theoscillator), and then generates a phase step that reduces the timeerror to a minimum. Some receivers step phase in increments of 100 nsor less, but some use increments of 1 msor larger. If the TCXO is offset in frequency by 1 x 10-7 (typical), a100 ns phase correction is needed every second. As a result, the shortterm stability of these models is very poor, but their long termperformance may be equivalent to models that discipline a quartz orrubidium.
- Some GPS receivers are suitable for timingapplications, but are not suitable as a frequency reference. Forexample, a receiver that produces a 1 pps output for timingapplications, might do a poor job of producing standard frequencieslike 5 and 10 MHz. In some cases, the 1 pps output is not in phase withthe standard frequency outputs. You might find receivers with aspecification for frequency uncertainty (5 and/or 10 MHz) of about 1 x10-9, even if the specification for their 1 pps output is 100 ns.
- Mostreceivers allow the use of a fixed position, after which no furtherpositions are computed. However, some receivers cannot turn offposition fixes, which makes them a poor choice for a frequencystandard. Even though the receiver is stationary, it will appear to bemoving, and the position errors will contribute large fluctuations tothe frequency.
- Since each GPS satellite is visible at a givenlocation for a limited time, all GPS receivers must add and removesatellites from the group used to obtain time and frequencyinformation. Often, adding and/or removing a satellite from the timingsolution causes an instantaneous frequency change. Some receivers havemuch better "handoff" algorithms than others.
- Differentreceivers handle GPS broadcast errors differently. For example, if asatellite is broadcasting bad data (such as PRN 5 on March 18, 1997),some receivers fail, and others do not. Some receivers have built-insoftware designed to remove "bad" data, but even these receivers mightfail under certain conditions.
If requested by a manufactureror end user, NIST can evaluate a particular GPS receiver for itssuitability as a frequency standard. For a predetermined fee, NIST willissue a report stating the frequency uncertainty of the receiver undertest relative to NIST. The uncertainty will be stated for each outputfrequency over a given measurement interval. The results of thisevaluation are kept confidential and not published by NIST.
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