UAV Navigation in depth: GPS Week Overflow

INTRODUCTION

1.   The Global Positioning System (GPS) system is designed so that from any point on the surface of the planet at least four satellites of the GPS constellation are always visible.

2.   The GPS system is based on the known position of these satellites in space in addition to precise timing signals, based on stable atomic clocks, which they transmit to the Earth's surface. The clocks inside typical GPS receivers, however, are less precise and the error that they accumulate, however small, must be corrected in order for them to maintain position accuracy.

3.   Each GPS satellite reports information about its position and the precise current time at regular intervals. These signals are transmitted as electromagnetic waves and therefore travel at the speed of light. Position can therefore be calculated by triangulation between at least three satellites and a receiver.

4.   Although GPS is mainly used as a positioning system, it may be also used as a precise timing reference for critical systems such us the electrical network, communications and the financial market.

GPS WEEK ROLL-OVER

5.   GPS Time (GPST) is determined from a clock ensemble composed of the caesium and rubidium atomic clocks within GPS ground stations and the atomic clocks onboard the GPS satellites.

6.   As with UTC, GPST is a weighted mean average time, but with two differences: GPST is available in real time, plus it is a continuous time without leap seconds. It is steered to be as synchronous as possible with UTC. For example, in recent years the deviation from UTC was within about 10 ns. Since January 2009 the difference between UTC and GPST was approximately 15 seconds.

7.   This time difference is transmitted in the GPS navigation data message, so that each receiver can calculate UTC.

8.   The GPS constellation also transmits the number of weeks which have passed from the initiation of the system to every receiver.

9.   The first epoch of GPST started at midnight between Saturday, January 5, and Sunday, January 6, 1980; hence 00:00:00 UTC 06 January 1980.

10.   GPST is defined as a week number together with the number of seconds from the start of the week as it is reset every Sunday at 00:00 hours. This allows the correct translation of the date and hour to a friendly format: day, month, year and hour.

11.   Since the GPS week is represented by 10 bits in the navigation message, this limits the number of weeks to a range from 0 to 1023, which equals 1024 weeks in total.

12.   The first overflow and reset to week 0 therefore took place between 21 and 22 August 1999. The next time the counter will reach week 1023 is on 6 April 2019.

Week beginning at 0000 GPS Time on

GPS Week Number broadcast by satellites

08 Aug 1999 1022
15 Aug 1999 1023
22 Aug 1999 0
29 Aug 1999 1
24 Mar 2019 1022
31 Mar 2019 1023
07 Apr 2019 0
14 Apr 2019 1

Ref: U.S. Naval Observatory

13.   This phenomenon is known as GPS week roll-over and may provoke the failure of inferior equipments which are not adequately prepared.

UAV NAVIGATION SYSTEM SOLUTION

14.   The GPS receivers used inside UAV Navigation's systems are designed to overcome the GPS week roll-over issue. The problem is resolved by assuming that every subsequent week number must be bigger than the week number which is used as the reference.

15.   This rollover reference week is coded in the product firmware and is set a number of weeks before shipment of the product and its firmware to a client. In addition, this value can be updated by the autopilot if necessary.

16.   The GPS receivers used by UAV Navigation in its products will return the correct date for 20 years (1024 bits / 52 weeks/bits) from compilation of the firmware.

17.   The following is an example of how this works in practice:

  • Assume that the reference number set in the GPS receiver is 1524.
  • As previously explained, the GPS system reset in 1999, so this week was transmitted by the satellites as 500 and corresponds to a week in the year 2009.
  • In this case, if the receiver acquires a number of weeks in a range between 500 and 1023, these number will be interpreted as weeks between 1524 to 2047 so the week will be between the years 2009 and first weeks of 2019 (April 2019). If, on the other hand, it receives transmissions with week numbers from 0 to 499, they will be represented as the number of weeks 2047 to 2546 so the week will correspond to a year from 2019 to 2029.
  • If the number received is 625, the year represented will be between 2009 and 2019; to be precise, week 21 of the year 2011:

625 = 500 + 125 [range 500 to 1023]

113 = 52 (weeks/year) + 52 (weeks/year) + 21 (weeks/year)

  • However, if the number received is 426, the year will be between 2019 and 2029; to be precise, week 10 of the year 2027:

426 = 52 (weeks/year) x 8 + 10 (weeks/year)

2019 + 8 = 2027

GNSS Timeline Schema

18.   UAV Navigation uses a week in the year 2014 as the reference, so the GPS week roll-over phenomenon will not cause a problem until 2034 at the earliest.

SUMMARY

19.   Implementation of GPS within the UAV Navigation system is robust and well-designed; it features protection against the GPS week roll-over effect.

20.   Other, inferior, products from competing manufacturers may not feature such protection and will be prone to failure on 6 April 2019.

21.   Protection against this issue has been built into UAV Navigation products up to the year 2034. However, the Company will continue to develop innovative solutions and to provide robust systems which platform manufacturers can rely upon well beyond that date.