For the GBT a session can be thought of as the longest period of contiguous time that the proposed observations can be scheduled. There are several scientific and technical reasons why one should separate the proposed observations into different or repeating sessions. Some of these reasons include:

1. The time needed on source is greater than the time the source is above the horizon.
2. The sources in a proposal are grouped in widely separated declinations.
3. The sources in a proposal are grouped in widely separated right ascensions.
4. A prime focus receiver change is required for all of the observations to be completed. The prime focus receiver changes require the receiver to be physically changed on a maintenance day. Changing the Gregorian receivers just requires a rotation of the Gregorian feed mount (which is performed by the telescope operator).
5. The proposals are for monitoring observations.

The following examples provide guidance on how you should break up your proposed observations into sessions.

• Example 1: A proposer would like to observe galaxies that are members of two different galaxy clusters. The galaxy clusters are well separated in right ascension, and each is up for 8 hours each day. There are approximately 30 galaxies per cluster. This means that a session will be 8 hours long. Since there is a large break/gap in the LST ranges in this survey, there should be two different sessions, one for each galaxy cluster. Now we will let each session contain all the sources within that galaxy cluster. This project then has two sessions of 30 sources with a length of eight hours each. Each session will likely need to be repeated. The time between the scheduling of sessions can be arbitrary.
• Example 2: A proposer would like to observe approximately 100 extragalactic sources. They will need to look at each source for 1 hour, giving a total proposed observing time of 100 hours. The sources are randomly distributed across the sky.  Since it is possible to observe this project for 100 contiguous hours this proposal should contain one session with 100 sources.
• Example 3: Now let's consider a survey where the sources are in a narrow range of right ascensions but are clumped into different declination ranges. To illustrate the point let's say half the sources are circumpolar and the other half are low elevation sources in the South. In this case there should be two sessions, breaking the sources up via their declination ranges. This is because they put different pressures on the scheduling of the telescope. The circumpolar sources can be done at any LST while the southern sources have a narrow range of LSTs for which they can be observed. The session containing the circumpolar sources will not need to be repeated since the observations of all the sources could be done continuously. The session with the low declination sources will likely need to be repeated.
• Example 4: An observer would like to determine the properties of a single source over many different frequencies (i.e., receivers). Let's assume that the observer only needs one hour per receiver to make their measurement. If we assume that all the required receivers are available on the telescope (i.e., no receiver of feed change is needed) then there is no technical reason to have this observing be in multiple sessions at the proposal stage. This is an example of a session with a single source and multiple resources. If the requested observations are known to require a receiver change, such as using multiple prime focus receivers, then there should be multiple sessions. There should be a session for each set of observations that could be scheduled at the same time. Should the time required to make all the observations be more than the time that the source is visible in one day, then the project should be broken up into multiple sessions or repeats of the same session.
• Example 5: It is common for pulsar timing observations to use up to three different pulsar back ends at the same time with the GBT. The observers may also be timing pulsars in globular clusters, which means that they are timing multiples sources at the same time also. This is an example where you would want to have a single (or multiple) source(s) with multiple resources (i.e. back ends) in a single session.
• Example 6: A source has a high proper motion across the sky. Currently it is behind a large cloud and molecular absorption lines are seen against the source. The source will soon emerge from behind the cloud. The proposer would like to monitor the change in the molecular absorption lines as the source emerges from behind the cloud by observing the source once a month for a whole year. This is an example of where a single source is observed within a session with the session being repeated twelve times.
• Example 7: A proposer wishes to obtain a timing solution on a newly discovered pulsar by using two different frequencies which are widely separated, requiring different receivers. For the lower frequency the timing measurements need to be done once a month. For the higher frequency they need to be done once every two months. The timing solution can be determined after monitoring the pulsar for one year. In this example we have two different sessions because the time between repeats for each resource (receiver) is different. The first session is repeated twelve times, once a month for a year, and uses the lower frequency receiver. The second session is repeated six times, running once every other month, and used the higher frequency receiver.
• Example 8: The Arecibo radar is being used to illuminate an object beyond Saturn. The round trip time is longer than the time the object is viewable at Arecibo so the GBT is being used to receive the return radar signal. This observing run must be coordinated with other telescopes and must occur on a specific date due to the property of the source being measured. The observations must occur on two separate days. This is an example of a coordinated observation. The observation must be carried out on given UTC dates at specified times. Because the coordinated observations must occur on separate but specific dates these should be in different sessions.
• Example 9: A new supernova has gone off. A proposal is received to measure its flux versus frequency between 300 MHz and 2 GHz in order to look for synchrotron self-absorption. This proposal has one source and six receivers: three Prime Focus 1 receiver bands, the Prime Focus 2 receiver, the L-band receiver, and the S-band receiver. However, only one of the three Prime Focus 1 receiver bands and the Prime Focus 2 receiver can be on the telescope at one given time. This then is a scenario where multiple sessions are needed due to technical reasons - receivers or feeds will have to be physically removed/placed on the telescope.
• Example 10: The proposers want to observe a source at a very low declination for four different epochs separated by about three months each to look for variations. Since the source reaches an elevation of only 6 degrees above the horizon and is visible for only 80 minutes per day, the source must be observed for five consecutive days to reach the desired sensitivity for a single epoch of observation. This requires 20 repeats of the same session. However, there are two time scales involved. The observer should request the minimum timescale. In the constraints to the session they should give a full written explanation of how the observations should be scheduled.

For the VLA, a session can be thought of as the longest period of contiguous time over which the proposed observations can be scheduled. There are several scientific and technical reasons why one should separate the proposed observations into different or repeating sessions. Some of these reasons include:

1. More than one array configuration is required for all of the proposed observations to be completed. Changing between array configurations requires that some antennas be transported from one antenna pad to another. Such changes occur during pre-set reconfiguration periods that span one to three weeks, depending on the starting and ending configurations of the array. Note that even if a proposal requests only a main configuration (e.g., D) plus its associated hybrid configuration (e.g., DnC), separate sessions are still required.
2. The time needed on a source is greater than the time the source is above the elevation limit of 8 degrees, requiring repeated sessions.
3. The sources in a proposal are grouped in widely-separated declinations.
4. The sources in a proposal are grouped in widely-separated right ascensions.
5. The proposal is for monitoring observations.

The following examples provide guidance on how you should break up your proposed observations into sessions.

• Example 1: The proposers would like to observe galaxies that are members of two different galaxy clusters. The galaxy clusters are well separated in right ascension and have positive declinations. For each cluster, its approximately 20 galaxies are to be observed for 8 hours including calibration and slew overheads. The same VLA configuration is required for both clusters. Since there is a large break/gap in the LST ranges in this survey, there should be two different sessions, one for each galaxy cluster. Each session will contain all the sources within that galaxy cluster, so the proposers might want to place that cluster's sources into a source group. This proposal then has two sessions, each involving about 20 sources and a total session time of 8 hours. The resource(s) for each session will involve the same VLA configuration.
• Example 2: The proposers would like to observe approximately 20 extragalactic sources in the same VLA configuration. They will need an average of about 5 hours per source, giving a total proposed observing time of 100 hours including calibration and slew overheads. The sources are randomly distributed across the northern sky. Since it is possible to observe this project for 100 contiguous hours in one VLA configuration, this proposal could contain one session with 20 sources and a total time of 100 hours. The proposers might want to use a source data file to read in the source information and place the sources into one source group. In the scheduling constraints field, the proposers might want to note that it would be acceptable to split the session into four, each lasting about one day and involving about a quarter of the sources, with no preference regarding the separation between the four repeats.
• Example 3: Now let's consider a survey where the sources are in a narrow LST range but are clumped near different declinations. To illustrate the point let's say half the sources are near a declination of 70 degrees (thus circumpolar) and the other half are near a declination of -15 degrees. The same VLA configuration suffices for all sources. In this case there should be two sessions, breaking the sources up according to their declinations. This is because they put different pressures on the scheduling of the telescope. The circumpolar sources can be done at any LST, while the southern sources have a narrower range of LSTs during which they can be observed. The session containing the circumpolar sources might not need to be repeated, since the observations of all the sources could be done continuously. The session with the southern sources might need to be repeated, because those sources are above the elevation limit only for about 9 hours per day. In the comments field, the proposers might want to note that the associated hybrid configuration would also be acceptable for their southern sources.
• Example 4: The proposers would like to determine the properties of a single source using one VLA configuration and the seven receivers that are always (sequentially) accessible. This is an example of a single VLA configuration, with a single source, and with multiple resources due to multiple receivers. Since multiple source/resource pairs can be added to one session, there is generally no reason to request multiple sessions. But if the total session time is more than the time per day that the source is above the elevation limit, then the request should be broken up, either as repeats involving the same set of receivers or as multiple sessions involving different sets of receivers.
• Example 5: The proposers would like to determine the properties of a single source using a single receiver and two VLA configurations. This is an example of a single source and a single receiver, and with multiple sessions needed because of multiple configurations. In this case, two sessions are generally needed, one for each VLA configuration. If the total time per session is more than the time per day that the source is above the elevation limit, then each request should be broken up as repeats.
• Example 6: The proposers would like to determine the spectral index of a single source at a matched angular resolution, thus using two receivers and two appropriately-scaled VLA configurations. This is an example of a single source, with a single receiver per configuration, and with multiple sessions needed because of multiple configurations. In this case, two sessions are generally needed, one for each VLA configuration with its single receiver. If the total time per session is more than the time per day that the source is above the elevation limit, then each request should be broken up as repeats.
• Example 7: A source has a high proper motion across the sky. Currently it is behind a large cloud and molecular absorption lines are seen against the source. The source will soon emerge from behind the cloud. The proposers would like to monitor the change in the molecular absorption lines as the source emerges from behind the cloud by observing the source monthly throughout a year that does not involve the D configuration. The simplest approach would be to have this single source be observed within a single session, with the session repeated 12 time separated by 30 days. The resource(s) would specify any configuration and the scheduling constraints field would note that the D configuration is not acceptable. Alternatively, three sessions could be requested, involving resource(s) specifying the A configuration, the subsequent B configuration, and the subsequent C configuration. The session for each configuration would involve 4 repeats separated by 30 days.
• Example 8: The proposers would like the VLA to observe during Suzaku time already awarded. This joint observation must be coordinated with another telescope and must occur on separate days, with the specific dates and times established by that telescope. Any VLA configuration is acceptable. This is an example of coordinated observations. The observations must be carried out on given UTC dates at specified times. Because the coordinated observations must occur on separate but specific dates, these should be in different sessions. For each session, the resource(s) should specify any configuration and the scheduling constraint should note the date and times for the coordinated observing.
• Example 9: The proposers want to observe a source at a declination of -45 degrees to look for variations between observations during two BnA configurations separated by the nominal 16 months for the configuration cycle. Since the source is above the elevation limit for only 90 minutes per day, it must be observed daily for five days to reach the desired sensitivity during each BnA configuration. The proposers should request two sessions, one for the first BnA configuration (in 2007, say) and one for the second BnA configuration (in 2008, say). Each session will involve 5 repeats, separated by 3 days; the proposer would note in the constraints field that any separation between repeats is fine, so long as the first and last observations occur within two weeks of one another (if the source is expected to have interesting variations only on longer timescales).
• Example 10: The proposers wish to monitor the evolution of the next bright nova, using logarithmic time sampling and selecting the receivers used in each epoch according to the current source spectrum and flux density. Each observation should be about an hour, with a maximum of 20 observations. The proposers are also requesting similar RXTE observations, and would like the VLA and RXTE data to be taken quasi-simultaneously. In addition, the proposers wish to obtain a three-hour HI absorption spectrum if the source rises above 100mJy at 1.4GHz, as well as a 20-hour observation at 5 or 8GHz to obtain a deep limit on both polarization and extended low surface-brightness emission after 100 days. The proposers should request three sessions: one for the flux density monitoring, one for the HI absorption observation, and one for the deep polarization limit. The flux density monitoring session should have a single source (Bright Nova 1) and multiple resources (one for each VLA receiver that might be used). This session will involve 20 repeats, with a placeholder'' separation of 1 day. The time per session should be entered manually, since the actual time per receiver will vary as the source evolves. The scheduling constraints field should note that the actual separation should be 1, 3, 10, 30, 100, etc. days, with the first observation occurring as soon as possible after an appropriate nova is discovered. The constraints field should also note the desire to coordinate with Swift, insofar as possible. The HI absorption session should have a single source and receiver, and request a single 10-hour observation. The constraints field should be used to specify that this observation should occur as soon as possible after the nova is observed with a flux density of at least 100mJy at 1.4GHz. Note that a single 10-hour observation is requested, even though the nova may appear at such a low declination that it is observable for only a couple hours a day. Finally, the deep continuum observation should have a single source and multiple receivers, and request a single 20-hour observation. The constraints field should request observations between 90 and 120 days after the triggering nova explosion. The proposers should also note here that multiple observing runs are fine if that makes scheduling easier, but that each individual observation must be at least 5hours long, to ensure adequate uv-coverage and to cover a wide range of parallactic angles. The comments field should note that only one of the requested receivers will actually be used, depending on the nova characteristics and the VLA array configuration (e.g., one might prefer 5GHz observations in B configuration, but 1GHz observations in A configuration, for sensitivity to similarly extended emission). All of this should of course be discussed and justified in the scientific justification.