33rd Annual Precise Time and Time Interval (PTTI) Meeting

FAST “DIRECT-P(Y)” GPS SIGNAL ACQUISITION USING A SPECIAL PORTABLE CLOCK

Hugo Fruehauf Zyfer Inc., an Odetics Company

1585 S. Manchester Ave. Anaheim, CA 92802-2907, USA

Tel: 714-926-5134; E-mail: hxf@zyfer.com

Abstract

“What goes around comes around,” we have heard it said, and it seems that this is the case for precision Portable Clocks as well. Twenty years ago, the only effective way to transfer Universal Coordinated Time (UTC) from one geographic location to another was with a Portable Clock. Since the advent of GPS however, UTC dissemination is done via satellites, virtually eliminating Portable Clocks. But now a new element is looming on the horizon, which may breathe new life into applications of such devices. This relates to the GPS P(Y)-Code, which is the secure military crypto-keyed signal providing what is referred to as the “Precise Positioning Service” (PPS). More specifically, however, is what a Portable Clock can do to enhance the new functionality of the P(Y)-Code signal acquisition called “Direct-P(Y)”, in an environment where the civil C/A-Code signal is not available. Direct-P(Y) refers to the ability for the military receiver to come online without the aid of the civil (in the clear) CIA-Code signal.

The Portable Clock can play an important role in quick acquisition of the P(Y)-Code signal, a signqiant crew safety consideration for our soldiers in the f i ld. No matter what the operational scenario may be, the Portable Clock proves to be an invaluable tool for Direct-P(Y) terminals. For those about to enter hostile territory, “no-one should leave home without one.”

THE NEW WARFARE REALITIES

With the civil GPS market fully entrenched and fast becoming a multi-billion dollar commercial business, the government recently turned off SA. SA (Selective Availability) is a deliberate degradation of accuracy of the civil C/A signal. SA, which can be set to any level desired by U.S. military planners, had been set to approximately 100 meters navigation uncertainty and 1 microsecond time error for the past 20 years. With SA set to zero, the full civil navigation and time accuracy can now be realized, being about 10 meters or less and 100 nanoseconds or less most of the time. In the unlikely event of a strategic conflict, SA helps to protect our forces against the use of commercial GPS receivers by our enemies. In such a conflict, military planners would simply crank-up the SA to 500 meters or more. This makes the C/A signal useless for commercial navigation, but still available to aid in the acquisition process of the military crypto P(Y)-Code signal.

Today, however, strategic conflicts are less likely, but tactical ones are high on the probability list. In such conflicts, it is simpler to jam the civil C/A signal in the local area, affecting only local commercial GPS receivers. This is a very effective way of dealing with the problem, because activating SA (in place

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1. REPORT DATE NOV 2001 2. REPORT TYPE

3. DATES COVERED 00-00-2001 to 00-00-2001

4. TITLE AND SUBTITLE Fast ’Direct-P(Y)’ GPS Signal Acquisition Using a Special Portable Clock

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) Zyfer Inc., and Odetics Company,1585 S. Manchester Ave,Anaheim,CA,92802-2907

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13. SUPPLEMENTARY NOTES See also ADM001482. 33rd Annual Precise Time and Time Interval (PTTI) Systems and ApplicationsMeeting, 27-29 Nov 2001, Long Beach, CA

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Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18

the receiver was online. Using a Portable Clock for initialization brings the added advantage of the latest Almanac it has in its memory. Its dataset will most likely be only a day or so old, again, as recent as the last time the Portable Clock was in the standby mode receiving the C/A signal. Therefore, the Portable Clock loads accurate time via 1 PPS (Item #4 of Figure 7) and the Almanac via RS232 (Item #5).

THE SPECIAL PORTABLE CLOCK

For the Terminal activator and operator in a hurry, a special Portable Clock will certainly come in handy-a clock that can hold UTC time accuracy to within 1 millisecond in a 24-hour period. As can be seen in Figure 5, with time errors in the millisecond range, Direct-P(Y) acquisition is virtually instantaneous. With time errors in the seconds range, TTFF may take dozens of minutes, even an hour or more, depending on jamming levels. The Portable Clock, therefore, is an indispensable crew safety tool, in that a terminal being erected in hostile territory will cold-start instantly, even in a heavy jamming environment and with no civil C/A signal present. In order for the receiver to come online quickly, the Portable Clock also loads the most recent GPS Almanac. If the Direct-P(Y) receiver’s Almanac data are more recent, it will reject the input from the Portable Clock.

To put things in perspective, for the purpose of this paper, “Special Portable Clock” is defined as a low- cost unit that can easily be carried; 20 pounds or less, with a precision quartz-crystal oscillator as the source for frequency and time generation. This is in contrast to the original cesium-beam (Cs) Portable Clocks that existed in years past, weighing over 100 pounds and cost in excess of $40,000 dollars. Although highly accurate for several hours after calibration, the sheer mass and volume of these original Cs units made them impractical for field operations then, and certainly for today as well, Also, the time accuracy requirements for a Portable Clock serving SAASM Direct-P(Y) applications is less stringent than what was needed for time transfer in days past. Earlier clocks attempted to achieve nanosecond to microsecond accuracy levels, where SAASM requires only milliseconds for fast acquisition. For this reason, a Portable Clock specifically designed to meet SAASM requirements should keep UTC time uncertainty to within milliseconds at the end of 24 hours, including road-travel environments and temperature swings from 0 to 40 degrees C. This accuracy can be achieved with a well-designed GPS- disciplined circuit and holdover algorithm, working in conjunction with a precision quartz crystal oscillator. When powered by AC with the C/A signal present, the portable clock assumes it is in a stationary mode, enabling its internal GPS C/A receiver to steer its internal oscillator to remain within 100nanoseconds of UTC time. When powered by 12 VDC, it assumes it is in a moving mode and disables the C/A signal even if present. This is because the moving mode may not discipline the internal oscillator correctly and the clock will most likely keep better time from just its free-running oscillator. When all external power is removed, the clock goes into the portable mode, powering down the GPS receiver plus all nonessential circuitry and uses its internal batteries to power the clock. The oscillator stability, coupled with the holdover algorithm (which memorized the oscillator’s internal aging and response to temperature changes while it still had C/A), holds accurate time, slowly drifting from its 100- nanosecond starting point to the specification value over the 24-hour period. As discussed previously, the most recent Almanac received while the Portable Clock’s GPS receiver was still locked to the-satellites is placed in memory, available for transfer to its host Direct-P(Y) receiver. A typical Portable Clock should have the following functionality, as shown in Figure 8.

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Figure-9, The SAASM Mandate

I C ..... Per QCSI 6140.01 (Original release OCT. 22, 1998)

SAASM is the next generation GPS security

After JAN.l, 1999 - Procure SAASM only for all new handheld applications; (Red & Black Keys allowed)

After JAN. 1,2000 - Procure SAASM only for all new user equipment; (Red & Black Keys - allowed)

After OCT. 1,2002 - Procure SAASM only, use Black Keys only, cease fielding non-SAASM equipment

YY . . ...

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