GPS Rally Computer

GPS Rally Computer is a Universal app designed to simplify computations necessary to navigate a traditional time-speed-distance (TSD) automobile rally. The design and implementation is based on specifications outlined by Clarence Westburg and Rich Bireta, commonly know as the Simple Rally Computer or by its acronym SRC. The design mimics in digital form the functionality of the classic Zeron 660 and 880 rally computers.

A TSD rally is comprised of stages set along a route defined by written instructions. The first stage is the Odometer Check stage that allows you to calibrate your car’s odometer with that of the Rally Master’s odometer.

In later stages you will follow a course at a particular average speed that will change from time to time at places that are generally unknown in advance.

In order to drive the course at the prescribed speeds. Odometer measurement is obtained from Apple’s GPS receiver or third-party devices via Bluetooth.

The GPS Rally Computer allows you to do all this and much, much more. For more documentation, download the User's Guide.

Features of the GPS Rally Computer:

  • Clock time in either minutes and seconds to tenths of a second or decimal minutes to thousandths of a minute
  • The clock is a 24-hour clock to simplify computations
  • Distance measurements in either miles or kilometers to either two- or three-digit precision
  • The car's odometer distance measurement may be the same or different than the official rally odometer distance measurement
  • One button click to reset default conditions for starting a rally
  • The internal computer clock can be adjusted to match the official rally clock
  • The distance between the GPS receiver and the position on the car where the rally measurements are taken (the front wheels, for example) can be added for improved distance precision
  • A "sprocket" button in the top menu bar allows access to the app settings from within the running application including settings for display of certain items
  • Calibrate the car's odometer to the official rally odometer on the odometer check stage via a popup
  • Change average speed (CAST) on a custom popup keypad or button input
  • Preselect the next CAST to simplify the process of changing speed on each stage and setting CAST to zero for transit zones
  • As time passes and as the car's odometer is incremented, the actual speed over the course is compared with the expected speed. The difference in seconds or hundredths of a minute is displayed along with an indication of either "Early" or "Late" to stay on pace
  • Time errors may be displayed in either of two modes selectable in the app Settings
  • Pause or Gain time or take a Time Allowance on any stage or within a transit zone
  • Option to Pause or Gain at user-defined preset seconds or cents
  • Handle off-course corrections to time and distance with Reverse button
  • Record "splits" of time and error at any time with a Split button
  • Record "control" and reset time error and distance to zero and increment next starting time 5 minutes later (distance and time are modifiable) with a Control button
  • Log table that records time, elapsed time, speed, distance, and error for various actions such as splits and CAST changes
  • Set next starting time for the Next CAST and have the time tick down on the master clock and autostart CAST, or override with a manual button click
  • Optionally, use a Dual GPS receiver via Bluetooth to supplement or substitute for the Apple internal GPS

Note: Use of the GPS Rally Computer during any particular rally will be subject to the rules and instructions governing the rally and will likely define your category or class among participants. Some rallies may, in fact, prohibit use of this app. Also, some rallies may have classes strictly for those with Simple Rally Computer (SRC) apps.

For better GPS reception and improved accuracy while rallying, third-party wireless GPS receivers are recommended. This app has been tested with Bluetooth connectivity to a Dual XGPS150A Universal GPS Receiver.

Disclaimer: Continued use of GPS running in the background can dramatically decrease battery life.



changes in version 5.2
  • Recalibrated filter rules to minimize distance creep while stopped or nearly stopped
  • Fixed bug in odometer correction factor routine and improved its functionality
  • Added 0 and 00 as allowable digits when initiating or editing odometer correction factor
  • Fixed bug that prevented Pause or Gain during a Transit Zone after adjusting the odometer with the "bump" feature
  • Improved the computation of distance estimates between GPS signals
  • Minor GUI changes
changes in version 5.1
  • The major code improvements and additions are designed to adjust the GPS coordinates of a moving vehicle owing to the time between reception of new GPS signals and to the time lag between the time the signal was generated from satellites and the time when processing actually occurs in the app
  • Users may now adjust the location coordinate by the distance between the GPS receiver in the car (either the iPhone or iPad or some third-party GPS receiver) and the position on the car where a rally may measure time or location at timed controls (sometimes this is the front of the car or the center of the front wheels). This optional adjustment is available in the Reset popup
  • All three of these potential errors in coordinate measurement are discussed at length in a 6-part thread in the Facebook "Road Rally Tech Talk" forum
  • If clock and GPS device distance adjustments are non-zero, an indication of the adjustments selected will be shown in the upper center of the display beneath the app title
  • If available to the app, compass coordinates are available to verify location when the car is stopped. The compass coordinates and bearing are available from a "compass" button on the "Map" popup
  • The "gps" information display button and contents have been relocated to the "Map" popup. Also, there are two new pieces of information on the display: Time Lag, the difference between the time the GPS signal was generated and the time when the new GPS location is actually processed in the app; and Signal Interval, the time between each receipt of new GPS information from the GPS receiver, either Apple's internal receiver/processor or a third-party GPS receiver
  • Various improvements to the processing of information from the Dual 150 and 160 model GPS receivers
  • Changes to processing of Apple internal GPS signals to account for spurious readings when horizontal accuracy is poor or when the car is stopped or moving very slowly
  • Odometer synchronization has been removed from the "Odo" popup. Rather, you may use the "Adjust Odo" option to set, reset, or adjust the odometer as needed
  • Various, relatively minor GUI changes
  • Coding for displays has been reduced and simplified where possible
changes in version 5.0.6
  • At the start, after the odometer check leg, the 1st CAST happens at a particular point or outmarker, typically at zero distance, and at a particular time. Similarly, after a timed checkpoint or control, the start of the next stage will occur at a particular CAST and time. So, to make certain that teams input the correct CAST and Start Time, the CAST input popup will insert "req'd" in the CAST and Start Time fields. To continue, each field must have a valid input
  • Timed CAST at 1st CAST and at Control must be between T.O.D. -30 minutes and T.O.D. +30 minutes, or 0 minutes for Manual CAST by Unlocking & Clicking. Next CAST must be 0 minutes for Manual CAST or T.O.D. + minutes/seconds only for Timed CAST
  • On the CAST xx.xx button the color is now green to indicate the locked position for the 1st CAST and Curr CAST. When to button is unlocked, the color will change to red and importantly the CAST xx.xx will be the Next CAST. This change was done to allow the driver and navigator to see the Current CAST at a glance and the Next CAST only when unlocked
  • To further emphasize the locked and unlocked positions of the CAST xx.xx button, the words "locked" and "unlocked" have been added beside the icon button
  • The Settings have been relocated to the center of the display. Also, the settings themselves are now saved to a file so that settings you have made will persist even after closing and restarting the app
  • The color of the Control button has been changed to red to reinforce the notion that a Control or Checkpoint is an end of a leg
  • Fixed bug when Start Time is precisely 60:00 or 60.00 minutes
  • GUI tweaks and some added text notes on displays

revised: March 12, 2020