Application Note – 20
GPS Timing Receivers OR How Long is a Piece of String ?
Introduction
One of the most commonly asked questions relating to GPS Receivers is, “How long does it take to lock?”. Unfortunately there is no simple answer to such a question, because there are numerous factors that affect the time to lock, some that are within control of the user, and others that are completely outside of the control of the user.
This paper endeavors to address some of the more common factors affecting the lock time question, and while it is not fully inclusive, it should at least give the user some help in providing the best conditions for minimum lock time, and some insight into what to expect.
Definition of “Time to Lock”
First, we need to define what we mean by time to lock. The definition can vary, dependent upon the particular piece of equipment that is being referred to, however for our purposes we will define it as it relates to a GPS based time and frequency receiver instrument as follows;
“Time to lock is the time required to “acquire and track” satellites and provide time, timing, and frequency outputs locked to the GPS signals.
This definition includes not only the time required to obtain a fix and track satellites in view, but also takes into account that most equipment also includes some form of internal oscillator to provide the actual useable output signals, whether quartz based or atomic based, and this needs in turn to be locked to the received GPS signals once they are valid.
The reason for including this second phase of locking, is that in practice the user really wants to know when the output signals are valid, and therefore knowing that the front end receiver is locked is just part of the story.
Factors Affecting “Time to Lock”
The section is divided into the two parts mentioned above, namely the GPS Receiver and the Internal Oscillator.
- GPS Receiver
The first item to mention here is that the Time to Lock of the receiver (often referred to as Time to First Fix or TTFF) is very dependent on the state in which the receiver was powered off, and the time for which the receiver has been powered off.
This becomes evident by describing the steps the receiver must go through to start generating useful information received from the GPS satellites, which are as follows;
- Receive reliable signals from at least one satellite
This initial requirement should not be underestimated!
While there are currently approximately 30 satellites on orbit (see ftp://tycho.usno.navy.mil/pub/gps/gpstd.txt for exact current status of the constellation) it should be remembered that the satellites are a distance of a little over 10,000 miles away from the Earth’s surface and due to their journey through the earth’s atmosphere, by the time the signals reach the Earth they are quite small (approximately -130dBm).
In this paper we will consider only the civilian use signals (referred to as the Coarse Acquisition or C/A code signals). The nature of the signal transmitted is called a Pseudo Random Noise signal (PRN) and for the C/A code this signal which is transmitted at an L1 carrier frequency of 1575MHz is a unique code transmitted for each satellite.
The mechanism for detecting a specific satellite is to try to correlate received signals with an internally held set of patterns within the receiving equipment.
In this respect, an “Almanac” is used which provides information on when and where satellites can be expected, based on receiver position. The Almanac is one of the data subsets in the message broadcast by the satellites.
If the receiver already has its position and approximate time, it knows which satellite signals to expect, and can therefore attempt correlation of its internally held signal patterns to just a few patterns/satellites. If however, the receiver has no previous knowledge of position or time, and does not have a current “Almanac”, it is forced to search a complete internal database of possible correlation patterns to match the signals being received in order to determine which satellites are in view.
The individual codes are 1,023 bits long and therefore with a large number of codes and large number of possible satellites, the process of homing in on one particular satellite can be quite time consuming and take several minutes. This problem can be exacerbated if satellite view is poor, as satellites may come and go in the time it takes to identify one correctly.
Once at least one satellite is identified, the receiver must then load a current Almanac, if a current Almanac is not already stored. The Almanac applies to all satellites in the constellation and is broadcast by each satellite repeatedly. It takes approximately 12.5 minutes to download the complete Almanac.
- Acquire and track at least four satellites
Once a satellite is acquired and the receiver is armed with a current almanac, it must then obtain detailed orbital information, called the “ephemeris” data, from each satellite in view. Each satellite broadcasts its own detailed ephemeris data every 30 seconds within the navigation message. Once this data is acquired from at least four satellites by the receiver, it is then able to begin to determine detailed positional and then time information based on the data.
- Calculate an accurate position and time estimate
Armed with the ephemeris data and using the transmitted navigation message from at least four satellites, the receiver is then able to determine first exact position, and then exact time.
Typically, to mitigate some of the noise effects, a GPS receiver used for precise timing applications will calculate position based on the average of anywhere from 10 to many hundreds of readings (dependent upon the accuracy required).
At this point, the output timing “mark” (typically output at the rate of one pulse per second – 1PPS) from most GPS receivers will be in the accuracy range of 100 to 200 nano seconds.
- Internal Oscillator
Most time and frequency receivers include an internal oscillator that is phase locked (or disciplined) to the GPS timing signal. The purpose of this is generally two fold;
- Provide a degree of “Fly wheeling” to reduce the noise inherent in the received GPS signal
As mentioned above, there is a degree of noise inherent in the received GPS signal due not only to the very low signal levels by the time the signal has reached the Earth’s surface, but also because of certain atmospheric affects creating variability in the actual transit time from satellite to Earth.
As this “noise” is fairly short term, it is possible to gain substantial improvement in output stability and short term accuracy by averaging the noise over a period of time. This is typically done by using either a quartz oscillator (TCXO or OCXO) or rubidium atomic oscillator. The effective averaging times for these vary from around 100 seconds (TCXO) to 24 hours for a rubidium oscillator.
- Provide “Holdover” in the event that GPS signals are lost.
The second reason for including an internal oscillator is for circumstances where there may be poor visibility of satellites, resulting in loss of satellite view for extended periods of time. With an internal oscillator, accurate frequency and timing signals can be maintained in the absence of GPS satellite view for many hours, or even several days dependent upon the demands of the application.
Dependent on the type of oscillator employed, there will be a period of time following acquisition and tracking of satellites when the internal oscillator must be phase locked to the GPS signal. Dependent upon the loop time constant this could take a considerable amount time.
In addition to this, there is also a finite time for the internal oscillator to first warm up in order that it can be used in the control loop. If this is an “initial” acquisition, the internal oscillator will almost certainly warm up in parallel with the receiver endeavoring to acquire satellites. For a rubidium oscillator this will generally take between 6 to 10 minutes, dependent upon ambient temperature.
Summary
The above factors to be considered in estimating “time to lock” can be summarized as follows;
- Is this the first time the Receiver has been initialized, or initialized in this location ?
- In other words do we need to first acquire Almanac and Ephemeris information. If so, with ideal satellite view this is going to take at least 15 minutes.
- Does the receiver have an accurate position stored?
- In most time and frequency receivers, there is the ability to store the last “surveyed” position. With accurate position information available, time to lock can be reduced considerably as only one satellite is required in order to provide precise timing information.
- How good is the satellite view?
- Due to the orbital paths of the satellites, satellite view is dependent not only upon a clear view of the sky, but is also dependent upon where on the Earth’s globe the receiver is situated. Nominally the complete constellation provides good coverage of the complete globe, however as a general rule it is recommended that locations situated in the Northern Hemisphere insure a good southern view without obstruction, whereas in the Southern Hemisphere, an unobstructed view North is recommended.
- For the same reason, as the satellites are constantly moving relative to the receiver fixed position, satellite view is also dependent upon the time of day. In some areas there may be more satellites visible in morning hours than in the afternoon, and vice versa.
- Are there any sources of interference nearby?
- Due to the nature of some applications there can be sources of interference with RF signals in the same frequency band region as the L1 GPS signal carrier around 1575 MHz. In this case it is important to shield the GPS antenna from these signals as they can have the same effect as “jamming” the GPS receiver. This is the situation where the front end of the receiving electronics becomes overloaded with the interfering signal and is therefore unable to detect the required GPS L1 signal at the same frequency.
Conclusion
So back to the original question “How long does it take to lock?” the answer is;
first time the unit is powered on ;
Best (good location, good satellite view) 15 to 30 minutes
Poor(poor location or poor satellite view) several hours
Worst(very intermittent satellite view) Never !
(Actually can happen in some “urban canyons”, e.g. downtown Tokyo)
unit has been previously powered on and has current time/position/almanac;
Best (good location, good satellite view) 30 seconds to 5 minutes
Poor(poor location or poor satellite view) 30 minutes plus
Note* If the unit has been moved more than 100 kilometers since the last surveyed position, it will be necessary to re-survey position in order to effectively acquire satellites.
This paper has endeavored to highlight some of the more common factors associated with using and obtaining the best response from a GPS receiver. It is by no means fully exhaustive, however if the reader feels that a critical factor may have been omitted please contact Precise Time and Frequency, Inc. at the link below.
Please contact ; http://www.ptfinc.com/contactPage01.htm