If Time is Critical, GPS Receivers Need to Be Adjusted for Expected Motion to Maintain Accuracy ITS has just released programmable dynamics modes in their GPS receivers. These unique receivers are included in the 6115G and 6055B series of IRIG time code generators and video inserters. Dynamics modes enhance the capability of the company's
GPS receivers to maintain GPS lock and timing accuracy of less than 70 nanoseconds even in aircraft moving at velocities of 500 knots and when maneuvering at accelerations up to 1 g. Without dynamics mode capabilities, many GPS receivers will lose lock and the resultant synchronization will fall back to the accuracy and drift characteristics of internal system clocks. These new receivers deliver timing accuracy
of less than 25 nanoseconds when stationary.
In general GPS receivers targeted at generating accurate time assume that the receiver is not moving. In fact, when the receiver is not moving the most accurate time is calculated. If the receiver assumes it is in a fixed location,
changes in satellite distances will be very predictable as they are solely due to earth's rotation and satellite orbital motion prescribed in its ephemeris. The ITS GPS receiver module in Dynamics Mode 1 (a setting that tells the receiver that a fixed location can be assumed) uses specialized firmware that is designed specifically for GPS time synchronization.
It will automatically enter a self survey mode on power up and can take up to 20 minutes to complete. After the self survey period, the receiver will automatically calculate an average of the geographical position and fix this value in the receiver firmware. With a stored known "fix" (position) the receiver no longer has to
calculate a positional fix to update the 1PPS phase. Having a stored fix, GPS lock on new "warm" starts can be as short as 3 seconds. Using the unique ITS GPS receiver 1PPS phase error is less than 25 nanoseconds. If satellites fall from view (as few as one can maintain timing accuracy after initial lock) the ITS system uses a real-time phase error correction algorithm to discipline the internal crystal to correct for frequency error (tick rate), phase error (when ticks occur) and crystal drift (age component). With sufficient history acquired by the disciplining algorithm, at a constant ambient temperature, free run (no GPS lock) time ticks will maintain a low phase error; typically less than a 10 millisecond cumulative error in a 24 hour day.
The calculations to achieve all of this are rather complex. Combinations of four satellites are used to compute a geometric dilution of precision (GDOP) vector. The more satellites a receiver can track the more four-satellite GDOP values can be generated. The final time and position result is an RMS of GDOP values. The ITS receiver is a 12-channel device so it can track twelve satellites simultaneously. The result is a robust array of GDOP values culminating in a highly accurate time value. Position can be reported to an accuracy of less than 5 meters.
The data samples from the satellites are processed through a Kalman filter. The Kalman filter has two distinct phases: predict and update. The predict phase uses the time delay estimate from the previous time-step to produce an estimate of the time delay at the current time-step from each satellite. In the update phase, measurement information at the current time-step is used to refine this prediction to arrive at a new, presumably more accurate time delay estimate, again for the current time-step. The "slower" the solution rate setting of the filter, the more integration (smoothing) of the data stream results. These elements of the Kalman filter are important concepts when trying to understand what happens when the GPS receiver is moving.
When the receiver or the antenna is actually not stationary, the measurements are constantly changing and the self survey fix can no longer be relied upon to generate low phase error to the time mark.
In fact the more quickly the receiver moves in space the greater the error would be. Acceleration is most critical. Each sample is assumed to be received at the surveyed location. When the time delay is estimated the satellite position will seem wrong compared to where receiver expected it to be. That is, the measurement values will differ greatly from the values predicted by the Kalman filter. Measurements excluded are those that exceed or do not track anticipated values. As the number of measurements discarded grows, too few samples will remain to result in a valid solution. Asmeasurements present greater and greater error from that predicted, unlock will be declared by the receiver.
The dynamics modes of the ITS GPS receiver changes the configuration of the Kalman filter to employ a prediction based on the previous velocity and acceleration solution. The range of new measured values would fit inside the error criteria set for the dynamics selected.
The solution rate is also a critical parameter. If a slow solution rate is used (high integration) high dynamics will be filtered too much resulting in very large solution errors. The most accurate solutions are achieved when the solution rate and expected range of dynamics are well matched to what is physically happening from the receiver's perspective. To that end, the ITS receiver incorporates the ability to set dynamics modes that adjust the system parameters and algorithms to maximize accuracy of the solution for several bands of operations, fixed (1), man walking (2), land vehicle (3), marine (4), aircraft (5).
Selecting the highest dynamics expected is important. Simulation testing revealed that if aircraft dynamics in the range of 0.8g are experienced, but the dynamics mode selected were 1 (stationary) timing errors would exceed 1 mS and the receiver could not reliably maintain a fix (know exactly where it was). If a fix can not be maintained or the number of samples discarded grows, the receiver will declare an unlocked condition or may, in fact, truly lose lock and have to restart the survey process.
On the other hand, as the dynamics mode is set higher and higher the prediction error allowed grows with it. As a consequence the time error will grow as well. So setting for aircraft (dynamics 5) for a man walking situation (dynamics 2) would result in a larger than necessary allowable time phase error. The exact time phase error will depend on what is actually going on. In this scenario, time phase error could be larger than desired even though unlock states would not likely ever occur (unless all of the satellites were out of view).
These new ITS GPS products have recently been used in a range of military and civilian jet aircraft. Customers have confirmed that the receivers successfully maintained lock and time accuracy even in these fast moving environments.
ITS is a leader in the design and manufacture of video synchronization, video insertion, time code generation and surveillance controller products serving the government, aerospace and industrial markets. ITS products are sold worldwide through a network of sales representatives and distributors. To learn more about, GPS Dynamics Modes, ITS and to explore the full range of products available, please browse www.ITSamerica.com, contact the company at (818)886-2034 or email us at sales@ITSamerica.com.
ITS has released a more informative white paper on this subject. If you would like to receive a copy, schedule an interview, obtain more information about ITS or the company's products, please contact Paul Hightower directly at (818) 886-2034 or mail him at phightower@ITSamerica.com.
Tel: (818) 886-2034