Pulsar Topocentric Calculator

Estimates the topocentric (observed) pulse frequency for a selection of strong pulsars, given the observer's Latitude and Longitude, and UTC time. See below for instructions.

Pulsar Topocentric Pulse Frequency Calculator
UTC (DD/MM/YYYY hh:mm:ss):
RA (hh mm ss.s):
DEC (±dd mm ss.s):
Latitude (±dd mm ss.s):
Longitude (dd mm ss.s):   E W
Barycentric Pulse Frequency (Hz):
 Topocentric Velocity (±km/s):
Doppler Shift (±ppm):
Topocentric Pulse Frequency (Hz): (±2 ppm)
Topocentric Pulse Period (ms): (±2 ppm)

Set Pulsar:
Set City/Town/Observatory:

Instructions: Via the drop-down 'Preset' menus provided, set the target pulsar to one of the relevant pulsar candidates for amateur observers. Pulsar candidates are limited to the pulsar drop-down list entries as they need to be associated with their individual ephemeris parameters. (Note: other pulsars can be added if requested via the email address in 'Contact').

The latitude/longitude for a selection of main cities/towns/observatories can be selected from the 'Presets' menu, or can be manually entered in free format sexagisimal - (note: preset public observatory locations are exact, but the location accuracy of preset private stations is limited to the closest town to preserve privacy). Only fields marked green are editable.

Time (UTC) can be set by the 'UTC Now' button or entered manually in the format of DD/MM/YYYY hh:mm:ss (observe format exactly).

Available Calculations: Four different calculation modes are provided...

  1. "Now" shows a single result for the set UTC time.
  2. "Today" opens a new window with a table covering 24 hours starting from the hour of the set UTC time with an increment of 1 minute.  The fields are Date and Time (UTC), Topocentric Frequency and Period, and Azimuth and Elevation. The time row where the pulsar transits is highlighted in green.  The rows where the pulsar is below the horizon are marked light red.
  3. "This Month" opens a new window with a table containing transit time parameters for the current month.  The fields are Date and Transit Time (UTC) and Topocentric Frequency and Period at transit.
  4. "Next Month" opens a new window with a table containing transit time parameters for the following month.  The fields are Date and Transit Time (UTC) and Topocentric Frequency and Period at transit.

 The tables in modes 1., 2. and 3. can be printed - just be sure to enable 'background graphics' to print the highlighting.

Doppler Effect: For +ve velocities (moving away) the doppler shift is -ve. Conversely, for -ve velocities (moving towards) the doppler shift is +ve.

Derived from: Old FORTRAN code ("DOP") to C# to Javascript plus JS code from many sources. Please report errors or discrepancies with other sources.

Pulsar Ephemeris Data: ATNF Pulsar Database (Manchester, R. N., Hobbs, G.B., Teoh, A. & Hobbs, M., AJ, 129, 1993-2006 (2005)) - except where later epoch data is available.


  • Pulsar timing is referenced to a barycentric (centre of gravity of all objects in the solar system) frame of reference. If use is made of the pulsar parameters from, say, the ATNF Pulsar Database, a velocity correction is needed to convert to a topocentric (earth-based observer) pulse timing for accurate epoch-folding.  Strictly speaking, the calculation above is the correction from heliocentric (centre of the sun), but the difference (< 0.05 ppm) between barycentric and heliocentric frames of reference is negligible for amateur purposes and in any case the difference is less than the accuracy of the algorithm.
  • The young Vela and Crab pulsars are 'glitchers'. Vela can suddenly spin-up in frequency by as much as 3 ppm - although this happens, on average, only every 2.5 years. In the case of Vela, the calculation here adds in the average glitch behaviour to produce a result which, over time, has minimal excursions from the actual spin frequency. Consequently, the result will swing either side (±1.5 ppm) of the actual spin frequency dependent on when the last glitch has occurred. Note that the TEMPO Vela result can be more than 3 ppm out depending on whether the TEMPO ephemeris data used is updated after the last glitch(es).  For the Crab pulsar no glitch correction is applied, but the ephemeris data will be updated periodically (once or twice a year) to the latest Jodrell Bank data while it is available. Note that both pulsars are slowing down quite rapidly - the Vela pulsar by about 3.3 ppm/month, and the Crab Pulsar even more so at about 32 ppm/month.
  • Note that the precision in the results exceeds the accuracy.  This deviation from mathematical purity is a programming decision which avoids the need to provide a variable number of significant digits depending on the period calculated.
  • While the calculation of a pulsar topocentric correction velocity is exactly the same as the calculation of radial velocities associated with VLSR, it excludes the velocity of the sun.  Therefore, this calculator is not suitable for determining doppler correction for emission lines (HI, CH3OH, OH, etc) - which requires the sun velocity.